Lights and swirls and rays fill the sky, dancing and darting here and there…
Little Puffles and Honey are off on a trip of a lifetime to see the Aurora Borealis, or Northern Lights. Said lights better make an appearance or little bears will be very cranky!
Earth is constantly bombarded with debris, radiation and other magnetic waves from space that could threaten the future of life as we know it. Most of the time, the planet’s own magnetic field does an excellent job of deflecting these potentially harmful rays and particles, including those from the sun.
Particles discharged from the sun travel 150 million kilometres toward Earth before they are drawn irresistibly toward the magnetic north and south poles. As the particles pass through the Earth’s magnetic shield, they mingle with atoms and molecules of oxygen, nitrogen and other elements that result in the dazzling display of lights in the sky.
Gaseous particles collide in the sky to create the most amazing light show you’ll ever see. The colours change and dance before your eyes. Green, yellow and blue are among the most common Aurora shades. Sometimes there’s a quick flash, other times there’s a soft, long glow along the horizon.
You need darkness to see the northern lights, and places in the auroral zone have precious little of it during the summer months. The lights are always present, even in summer, but you won’t see them if the sun isn’t at least 10 degrees below the horizon.
You also want clear skies. Winter and springtime are generally less cloudy than autumn in and around the northern auroral zone, so a trip between December and April makes sense. Ideally, time your trip to coincide with the new moon, and make sure to get away from city lights when it’s time to look up. Although Reykjavik is one city that turns off its lights to allow people to look up and see the northern lights. The aurora is a sporadic phenomenon, occurring randomly for short periods or perhaps not at all. At the moment, we are not entertaining the not at all option!
The sun is a slightly variable star, so every 11 years or so the magnetic activity increases until it reaches maximum, after which it slowly fades out. A new solar cycle then begins. During maximum, the sun is often pockmarked by dark regions called sunspots. During minimum, hardly any are seen for weeks, even months. When the activity is low, bright and dynamic auroras are seen less frequently because there are fewer powerful coronal mass ejections, which is basically a cloud of solar wind gushing away from the sun. The last maximum was in 2014. Magnetic activity should reach minimum around 2020 and maximum again around 2026. Some say the best time for auroras is usually two or three years after the maximum, not during it. So here’s hoping for 2017! I wish I could say this was planned, but it will be more a case of sheer damn luck! The lucky little bears effect 🙂 And they are lucky!
To maximize the chances of catching this sporadic phenomenon, little Puffles and Honey are going to some of the best places to see the Aurora Borealis.
Tromsø, the buzzing hub of Norway’s far north, is the coolest vantage point from which to see them. Located 450km north of the Arctic Circle, Tromsø is right beneath the auroral oval – the ring around the North Pole within which the lights can be seen. The sun doesn’t rise from mid-November to mid-January, so you don’t have to wait until midnight to experience the display. The best viewing spots are usually a little way out of town.
80 kilometres west of Tromsø is Lyngen Lodge where we’ll start our adventure. It better look like this when we get there!
The northern towns of Svalbard, Finnmark, Alta and Svovær (in the Lofoten Islands) are also regarded as among the best places to try.
The lack of city illumination means Abisko, a remote town near the border of Norway, is Sweden’s top spot to see the northern lights. Abisko National Park is known for its ‘blue hole’ – a patch of permanently clear sky – above the park’s 70km long lake. This area has a unique micro-climate that makes it an ideal viewing spot for the Aurora Borealis.
Iceland is also close to the auroral oval where the northern lights form. If the sky is clear, the lights are almost always visible. One of the best spots is the edge of Europe’s largest glacier, Vatnajökull. And be sure to visit the magnificent glacial lagoon, where you might spot seals playing around newly formed icebergs.
Iceland’s Thingvellir National Park is another good location… but in fact almost anywhere outside the capital Reykjavik will give you a good chance. Street lighting in Reykjavik is switched off to allow a better view of the Aurora Borealis when it’s on display.
Rovaniemi and Levi in the Finnish Lapland are also good locations. The 24-hour polar night, a twilight period in which the sun doesn’t rise above the horizon, can last up to 51 days here. During this time without sun, the northern lights brighten the day.
And bonus, Santa lives in Rovaniemi! Puffles and Honey have their present list ready 🙂
Kirsti Kauristie, head of the Northern Lights department at the University of Lapland in Rovaniemi, says on clear nights the probability of seeing the aurora is roughly 50 per cent at latitudes near the Arctic Circle and to stay at least two nights to double your chances. Altogether, little bears are spending 16 nights in auroral locations. I’m feeling cold just thinking about it! And that’s no easy feat considering we’ve had a week of 30-36 degrees Celsius. It’s been the warmest opening week of November on record, after the hottest winter on record in Western Australia. Not good.
Puffles and Honey are well prepared for a cold winter in the northern hemisphere. And Santa sent a little helper to fly the sleigh 🙂
Witnessing an aurora first-hand is a truly awe-inspiring experience. The natural beauty of the northern or southern lights captures the public imagination unlike any other aspect of space weather. But auroras aren’t unique to Earth and can be seen on several other planets in our solar system.
An aurora is the impressive end result of a series of events that starts at the sun. The sun constantly emits a stream of charged particles known as the solar wind into the depths of the solar system. When these particles reach a planet, such as Earth, they interact with the magnetic field surrounding it (the magnetosphere), compressing the field into a teardrop shape and transferring energy to it.
Because of the way the lines of a magnetic field can change, the charged particles inside the magnetosphere can then be accelerated into the upper atmosphere. Here they collide with molecules such as nitrogen and oxygen, giving off energy in the form of light. This creates a ribbon of colour that can be seen across the sky close to the planet’s magnetic north and south poles – this is the aurora.
Using measurements from spacecraft, such as Cassini, or images from telescopes, such as the Hubble Space Telescope, space physicists have been able to verify that some of our closest neighbours have their own auroras. Scientists do this by studying the electromagnetic radiation received from the planets, and certain wavelength emissions are good indicators of the presence of auroras.
Each of the gas giants (Jupiter, Saturn, Uranus, and Neptune) has a strong magnetic field, a dense atmosphere and, as a result, its own aurora. The exact nature of these auroras is slightly different from Earth’s, since their atmospheres and magnetospheres are different. The colours, for example, depend on the gases in the planet’s atmosphere. But the fundamental idea behind the auroras is the same.
For example, several of Jupiter’s moons, including Io, Ganymede and Europa, affect the blue aurora created by the solar wind. Io, which is just a little larger than our own moon, is volcanic and spews out vast amounts of charged particles into Jupiter’s magnetosphere, producing large electrical currents and bright ultraviolet (UV) aurora.
On Saturn, the strongest auroras are in the UV and infrared bands of the colour spectrum and so would not be visible to the human eye. But weaker (and rarer) pink and purple auroras have also been spotted.
Mercury also has a magnetosphere and so we might expect aurora there too. Unfortunately, Mercury is too small and too close to the sun for it to retain an atmosphere, meaning the planet doesn’t have any molecules for the solar wind to excite and that means no auroras.
On Venus and Mars, the story is different. While neither of these planets has a large-scale magnetic field, both have an atmosphere. As the solar wind interacts with the Venusian ionosphere (the layer of the atmosphere with the most charged particles), it actually creates or induces a magnetic field. Using data from the Venus Express spacecraft, scientists found that this magnetic field stretches out away from the sun to form a “magnetotail” that redirects accelerated particles into the atmosphere and forms an aurora.
Mars’s atmosphere is too thin for a similar process to occur there, but it still has aurora created by localised magnetic fields embedded in the planet’s crust. These are the remnants of a much larger, global magnetic field that disappeared as the planet’s core cooled. Interaction between the solar wind and the Martian atmosphere generates “discrete” auroras that are confined to the regions of crustal field.
A recent discovery by the MAVEN mission showed that Mars also has much larger auroras spread across the northern hemisphere, and probably the whole planet too. This “diffuse” aurora is the result of solar energetic particles raining into the Martian atmosphere, rather than particles from the solar wind interacting with a magnetic field.
If an astronaut were to stand on the surface of Mars, they might still see an aurora but it would likely be rather faint and blue, and, unlike on Earth, not be necessarily near the planet’s poles.
Most planets outside our solar system are too dim compared to their parent star for us to see if they have auroras. But scientists recently discovered a brown dwarf (an object bigger than a planet but not big enough to burn like a star) 18 light years from Earth that is believed to have a bright red aurora. This raises the possibility of discovering other exoplanets with atmospheres and magnetic fields that have their own auroras.
Such discoveries are exciting and beautiful, but they are also scientifically useful. Investigating auroras gives scientists tantalising clues about a planet’s magnetic and particle environment and could further our understanding of how charged particles and magnetic fields interact. This could even unlock the answers to other physics problems, such as nuclear fusion.
Some of the most wonderful pictures taken by astronauts from space are of aurora dancing over our planet. Now the photos are more than just pretty pictures thanks to an ESA project that makes them scientifically usable.
Aurora offer a visual means to study space weather, the conditions in the upper regions of our atmosphere. These colourful displays are produced when electrically charged particles from the Sun in the solar wind are channelled along Earth’s magnetic field lines and strike atoms high in the atmosphere.
Just as the Sun is instrumental to the weather on Earth, solar activity influences space weather, which in turn can interfere with radio transmissions, satellites and even our electricity supply.
Scientists study space weather and aurora using satellites such as ESA’s Cluster and Proba-2 but also through a network of cameras on the ground. These cameras are often obscured by cloud or snow and coverage from the southern hemisphere is poor because there is not much land at the best latitudes for observing the aurora.
Pictures taken from the International Space Station can provide context and add information by improving estimates of the height and length of aurora. Some reach 500 km high – meaning the Station sometimes flies right through them.
First, the images need to be turned into something that scientists can use. Most important is to know the exact time and where the camera was pointing.
The images are downloaded in the highest resolution and faulty camera pixels from cosmic radiation are removed. Software corrects distortion from the camera lens.
Just like 19th century explorers before navigation satellites existed, ESA’s team looked to the stars for reference, using software to identify the stars in the image, and from there calculate the precise position of each pixel and its scale.
Last, the image time is determined by linking cities with their calculated locations and the horizon.
Software engineer and ESA young graduate trainee Maik Riechert, who worked on the project, explains: “The ideal images for processing are pictures showing Earth and the stars with the horizon just above the middle.”
When all the images are processed, the timelapse videos offer a way to check the process went smoothly. Any jitter or changes in star tracking will show up in the final video, so a smooth run proves that the individual images are ready for analysis. Then it is over to the scientists who can use the extra information in their research.
When it’s completely tranquil and quiet, and the weather conditions are good, you can detect the mysterious sound of the Northern Lights – a natural light display that can be seen 200 nights a year in the skies of Finnish Lapland. The stories about whispers and crackling of Aurora Borealis have been told for centuries, but since there was no hard scientific evidence of the phenomenon, many people have taken the observations as folklore. In the last few years Finnish scientists have finally managed to confirm that the Northern Lights do cause faint crackling, which is detectable by human hearing.
Witnesses say the sounds are comparable to radio static, like a faint crackling, light rustling, or hissing heard for a few minutes during a strong display. Finnish scientists have not only shown that they really happen but now the team thinks they know why.
The answer can be traced to charged particles trapped in a layer of the atmosphere that forms during cold nights. These particles rapidly discharge when bursts of material from the sun slam into Earth, producing clapping sounds and other noises.
Charged particles are constantly streaming from the sun in the solar wind, and auroras occur when these particles interact with Earth’s magnetic field. The particles are funnelled toward the poles, where they slam into the atmosphere and set off colourful light shows.
Sometimes, the sun flings off major bursts of particles that, when aimed at Earth, can set off disturbances in the planet’s magnetic field known as geomagnetic storms. These storms can interfere with orbiting satellites and even the electrical grid, but they also produce the most dramatic auroral displays.
Previously, one of the leading theories for aurora noise suggested that tree needles or pine cones may be involved. During geomagnetic storms, the atmosphere can hold abnormally high electric fields, creating a difference in charge between the air and objects on the ground.
Anything pointy, like leaves and pine cones, would offer the perfect surface for electricity to discharge, like a static shock jumping from a doorknob to your finger, and that might set off an audible cracking sound.
But back in 2012, Aalto University researcher Unto K. Laine was able to prove that auroral sounds were emanating from above the treetops — 70 meters above Earth’s surface — during the times of the most intense displays.
Now, his team’s follow-up study proposes a specific explanation for the auroral snap, crackle, and pop. The key is something called an inversion layer, a region of the atmosphere where the air temperature increases with altitude instead of experiencing the usual decrease. Such layers can develop after calm, sunny days, says Laine. After sunset, warmer air rises while the surface cools, and continuing calm conditions mean the two temperature regions don’t mix.
According to Laine and his team, this inversion layer then acts like a lid, trapping negative electrical charge in the region below it and positive charge in the air above. When a geomagnetic storm hits Earth, the lid breaks and the charge is released, creating the weird sounds.
This theory matches nicely with the team’s previous observations. They showed that 60 of the loudest recorded sounds originated about 75 meters above the ground. That’s the same altitude as a typical inversion layer, according to independent measurements conducted by the Finnish Meteorological Institute.
“Auroral sound is a phenomenon that is dismissed by many people, scientists and otherwise, as originating in the imagination of the observer,” says Dirk Lummerzheim, an aurora researcher at the University of Alaska, Fairbanks.
“I think this is the first time that the sounds are not only observed to actually be an acoustic signal — as opposed to something that is manufactured in the human brain, for example, similar to synaesthesia — but Doctor Laine also has proposed a physical process that provides a good explanation.”
Marie Curie a woman with an extensive list of “firsts” to her name. Not only was she the first woman to win a Nobel prize, she was also the first person and only woman to win the Nobel prize twice, for achievements in two distinct scientific fields. Curie became the first woman to receive a PhD from a French university, as well as the first woman to become a professor at the University of Paris. Even 60 years after her death, Curie’s heritage was such that she became the first woman to be entombed on her own merits in the Panthéon in Paris in 1995 – a rare honour reserved for the country’s most esteemed figures. (There are now five women buried in the Panthéon. And 76 men.)
The Curies received another honour in 1944 with the discovery of the 96th element on the Periodic Table of the Elements, which was named curium.
Today is the 150th anniversary of the birth of Marie Curie (born Maria Salomea Skłodowska). With her husband, Pierre, the Polish-born Frenchwoman pioneered the study of radioactivity until her death in 1934. Today, she is recognized throughout the world not only for her ground-breaking Nobel Prize-winning discoveries, but also for having boldly broken many gender barriers during her lifetime.
In her honour, the United Nations named 2011 the International Year of Chemistry. The year marked the 100th anniversary of her second Nobel Prize, the first time anyone had achieved such a feat. (Three more people, all men, have won the Nobel prize twice since then.)
Marie Curie’s birthday is the day the International Organization for Medical Physics (IOMP) organizes annually the International Day of Medical Physics (IDMP). The objective is to raise the visibility and awareness of medical physicist in the global community. The event started in 2013.
Albert Einstein once remarked that: “Marie Curie is, of all celebrated beings, the only one whom fame has not corrupted”.
This is Chapter 4, written by Abraham Pais, from Out of the Shadows: Contributions of Twentieth-Century Women to Physics, 2006, edited by Nina Byers and Gary Williams, University of California, Los Angeles.
Marie Curie (November 7, 1867 – July 4, 1934), baptized Maria Sklodowska, was born in Warsaw, the fifth child of Wladyslaw Sklodowska, a physics teacher, and Bronislava née Boguska. In her high-school years she read French, German and Polish poets, all in the original language, gave lessons to earn money, and in 1886 became a governess for three years. In 1891, she moved to Paris to study physics at the Sorbonne, graduating in 1893 at the top of her class. In April 1894, she met the physicist Pierre Curie (1859–1906), whom she married in July 1895. By then his career was already well underway. He had done important work on piezo-electricity, on symmetries of crystals and on magnetism. They had two daughters, Iréne (1896–1956) and Ève (1894–2007). Marie was a devoted mother, bathing her two babies every day herself.
Soon after Marie had completed her first paper, dealing with magnetism of tempered steels, she heard of Becquerel’s discovery (1896) of “uranic rays,” which she discussed with her husband. “The study of this phenomenon seemed to us very attractive . . . I decided to undertake the study of it . . . In order to go beyond the results reached by Becquerel, it was necessary to employ a precise quantitative method.” For this purpose she developed a new tool, an early form of the parallel plate ionization chamber: two condenser plates, 3 cm apart, each 8 cm in diameter. A modest 100 volt potential difference between the plates and a sensitive electrometer was all she needed for her work. Her first paper on this new subject contains two major new points.
(1) She not only reconfirmed Becquerel’s earlier findings for uranium but also discovered a new active substance: thorium. “Thorium oxide is even more active than metallic uranium.” While she discovered the activity of thorium independently, she was not the first to do so. Unbeknownst to her, the German physicist Gerhard Carl Schmidt, from Erlangen, had reported the same result three weeks earlier. The appellation “uranic” for the new rays was certainly too narrow. In the title of her next paper the term “radioactive substance” makes its first appearance in the world literature. I note right away that in a sequel to this paper, one finds the following remark: “One of us [M.C.] has shown that radioactivity is an atomic property.” It is the first time in history that radioactivity is explicitly linked to individual atoms. Twenty years later we find a remark in a textbook about “Madame Curie’s theory that radioactivity is an intrinsic property of the atom.”
(2) Marie’s first paper clearly shows that her insight into what constituted a “normal” increase of radioactivity with increasing uranium content was sufficiently quantitative to note that two minerals, pitchblende (rich in uranium oxide) and chalcite (rich in uranyl phosphate), behaved anomalously: “[They are] much more active than uranium itself. This fact is very remarkable and leads one to believe that these minerals contain an element which is much more active than uranium.” With this conjecture she introduced another novelty into physics: radioactive properties are a diagnostic for the discovery of new substances, the second of the major points of her first paper on radioactivity.
Her next assignment was obvious: to verify whether her idea of a new element was indeed true. “I had a passionate desire to verify this hypothesis as rapidly as possible. And Pierre Curie, keenly interested in the question, abandoned his work on crystals … to join me in the search.” He was never again to return to his crystals. Jointly, they investigated pitchblende by ordinary chemical methods. In July, 1898, they announced that by precipitating with bismuth they had been able to isolate a product about 400 times as active as uranium. “We believe that [this product] … contains a not yet observed metal … which we propose to call polonium, after the country of origin of one of us.”
On October 17, Marie Curie wrote in a private notebook: “Iréne can walk very well and no longer goes on all fours.”
The analysis of pitchblende continued, Gustave Bemont, a laboratory chief at Pierre’s school, aiding the couple in their labors. Meanwhile, the Austrian government had presented them with a gift of 100 kg of residues of pitchblende, material (believed to be without commercial value) obtained from mining operations in Joachimsthal. Separation by chemical methods remained their main procedure. They worked “in an old, by no means weatherproof, barrack in the yard of [Pierre’s] school.” These were no longer table-top operations. As Marie wrote later: “It was exhausting work to move the containers about … to transfer the liquids, and to stir for hours at a time, with an iron bar, the boiling material in the cast iron basin.” On December 26, 1898, they announced that, by precipitating with barium, they had found yet another radioactive substance in pitchblende: radium, a discovery that led Rutherford to remark: “The spontaneous emission of radiation from this element was so marked that not only was it difficult at first to explain but also, what was more important, still more difficult to explain away.”
On January 5, 1899, Marie wrote in her notebook: “Iréne has fifteen teeth!”
Ever since Becquerel’s initial discovery in 1896, those in the know had been surprised at the persistence with which “uranic rays” kept pouring out energy. In 1910, Marie Curie reminisced as follows about those early days: “The constancy of the uranic radiation caused profound astonishment to those physicists who were the first to be interested in the discovery of H. Becquerel. This constancy appears in fact to be surprising: the radiation does not seem to vary spontaneously with time …” In order to appreciate this statement fully, three facts should be borne in mind:
The radiation emitted by uranium when unseparated from its daughter products does indeed represent, to a very high degree, a steady state of affairs.
It took two years from Becquerel’s initial discovery until the first parent–daughter separation was affected.
It took another two years until it was firmly established that radioactivity does diminish with time.
Speculation on the origin of radioactive energy started with Marie Curie’s very first paper on radioactivity (the one in which she announced her discovery of the activity of thorium, in 1898). There, cautiously, she suggests the possibility that the energy might be due to an outside source: “One might imagine that all of space is constantly traversed by rays similar to Roentgen rays, only much more penetrating and being able to be absorbed only by certain elements with large atomic weight, such as uranium and thorium.”
In that same year, 1898, Marie Curie discovered polonium, for which the liberated energy per unit weight of separated material was even larger than for uranium and thorium. Thus, the question of the origin of this energy became an even more burning one and she returned to it, listing a number of possible answers. Here we find the first mention that one might have to face a contradiction with the law of conservation of energy. Furthermore, she emphasized that the assumption of an external source would be nothing but an evasion of energy nonconservation – unless the nature of the external source were determined: “Any exception to Carnot’s principle [i.e., the law of conservation of energy] can be evaded by the intervention of an unknown energy which comes to us from space. To adopt such an explanation or to put in doubt the generality of the Carnot principle are in fact two points of view which to us amount to one and the same as long as the nature of the energy here invoked stays entirely ‘dans le domaine de l’arbitraire.’” She also pointed out that the interior of the atom could be the energy source. “The radiation [may be] an emission of matter accompanied by a loss of weight of the radioactive substances.”
That last article was written before, but published after, the discovery of radium by her, Pierre Curie and Bemont. This last development once again brought the issue to the fore. The radium radiation was even more intense than the polonium, about 100,000 times more active than uranium! The question of nonconservation of energy came up once again: “On r´ealise ainsi une source de lumiére a vrai dire trés faible, mais qui fonctionne sans source d’énergie. Il y a lá une contradiction tout au moins apparente avec le principe de Carnot.”
Nonconservation of energy was never a widely held explanation of these effects. In 1902, the Curies again gave a list of possible interpretations, on which this possibility no longer appears. It should also be stressed that such options as nonconservation of energy or external sources were not proposed lightheartedly. The idea that the atom itself is the source was not easily swallowed at that time, since it meant giving up the concept of the atom as an immutable entity. By 1900, the debate over the reality of atoms was well past its peak; but at that time the question was not universally regarded as settled. The Curies were proponents of the existence of real atoms, as their writings make abundantly clear. But to accept the atom itself as the source of the energy could only mean one thing to them: transmutation. And they could not simply accept this, since to them at that time it seemed in conflict with the principles of chemistry as then known – which indeed it was. In 1900, Marie Curie summed up the dilemma in the following way:
Uranium exhibits no appreciable change of state, no visible chemical transformation, it remains, or so it seems, identical with itself, the source of energy which it emits undetectable – and therein lies the profound interest of the phenomenon. There is perhaps a disagreement with the fundamental laws of science which until now have been considered as general … The materialistic theory of radioactivity is very attractive. It does explain the phenomena of radioactivity. However, if we adopt this theory, we have to decide to admit that radioactive matter is not in an ordinary chemical state; according to it, the atoms do not constitute a stable state, since particles smaller than the atom are emitted. The atoms, indivisible from the chemical point of view [author’s italics], are here divisible, and the sub-atoms are in motion … The materialist theory of radioactivity leads us … quite far. If we refuse to admit its consequences, our embarrassment will not lessen. If radioactive matter does not modify itself, then we find ourselves again in the presence of the question: from where comes the radioactive energy? And if the source of energy cannot be found we are in conflict with Carnot’s principle, a principle fundamental to thermodynamics … We are then forced to admit that Carnot’s principle is not absolutely general [and] … that the radioactive substances are able to transform heat from the ambient environment into work. This hypothesis undermines the accepted ideas in physics as seriously as the hypothesis of the transformation of the elements does in chemistry, and one sees that the question cannot easily be resolved. (Cette hypothése porte une atteinte aussi grave aux idées admises en physique que l’hypothése de la transformation des éléments aux principes de la chimie, et on voit que la question n’est pas facile a résoudre.) M. Curie.
It is important to stress at this point that these fascinating puzzles were never any hindrance to progress in those days. If anything, the contrary is true. The field of radioactivity was young when these questions arose, the tasks were enormous. While these problems were given much thought by the Curies, that never inhibited them from continuing their superb research.
In her doctoral thesis, Marie Curie raised yet another puzzlement. This document has a section entitled “Is atomic radioactivity a general phenomenon?” in which she reported on the analysis of substances other than uranium and thorium compounds: “I undertook this research with the idea that it was scarcely probable that radioactivity, considered as an atomic property, should belong to a certain kind of matter to the exclusion of all other.” It is to her credit that of all the materials she examined (including a dozen rare earths) she found no evidence for radioactivity.
I conclude this survey of Marie Curie’s early contributions with her remark made in the course of discussions at the first Solvay Conference, held in Brussels in October, 1911. There she observed that thermal, optical, elastic, magnetic and other phenomena all appear to depend on the peripheral structure of the atom, then she continued:
Radioactive phenomena form a world apart, without any connection with the preceding [phenomena]. It seems therefore that radioactive phenomena originate from a deeper region of the atom, a region inaccessible to our means of influence and probably also to our means of observation, except at the moment of atomic explosions.
Rutherford’s discovery of the nucleus had been announced the preceding May. I do not know whether Marie Curie did not follow up on her wise remark with a comment on the nucleus because she was unaware of this work, or did not believe the results, or failed to see the connection. Nor do I know why Rutherford, who was in the audience, refrained from drawing Curie’s attention to the nucleus. However, what may be the long and the short of the matter is that radioactivity prior to 1911 has turned out to be prenatal nuclear physics.
Radium was to make radioactivity known to the public at large. The attendant fame, which was to disturb the quiet and concentrated existence to which the Curies were so deeply attached, would “deafen with little bells the spirit that would think.”
Pierre Curie’s discoveries of the experimental laws of piezoelectricity and of what became known as the Curie temperature in ferromagnetism are samples of his outstanding contributions to physics. He also labored mightily alongside his wife on the problems of radioactivity. But, insofar as one can rely on the record of published papers, it is Marie Curie, a driven and probably obsessive personality, who should be remembered as the principal initiator of radiochemistry. That is abundantly clear, it seems to me, from her April, 1898 paper, discussed at length earlier.
The year 1898 was heroic in the Curies’ careers. Further important work was to come, yet what followed was to a considerable extent painstaking elaboration of their early discoveries. The finest appreciation of their early work was given by Ernest Rutherford, arguably the greatest experimental physicist of the twentieth century: “I have to keep going, as there are always people on my track. The best sprinters in this road of investigation are Becquerel and the Curies in Paris, who have done a great deal of very important work on the subject of radioactive bodies during the last few years.”
As to their later life: in 1900, Pierre was appointed assistant professor at the Sorbonne; Marie, teacher at a high school for girls. Continued intense research and teaching duties were so exacting that she had no time, until 1903, to complete her Ph.D. thesis, a masterful summary of her work to date. She received her degree on June 25 with the distinction “trés honorable,” an elegant understatement. That day was memorable to her for another reason as well: in the evening she met Rutherford for the first time.
In November of that year, the Curies were in London to share the Humphrey Davy Medal of the Royal Society and were informed that they would share the 1903 Nobel Prize with Becquerel, “in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel.” The Curies’ prize was the subject of the following comment by the New York Times of December 11, 1903: “The discoverers of radium have, it is understood, not profited financially from the work as greatly as might have been expected, and their admirers throughout the world will be delighted to hear of this windfall for them.” They never took out patents for any of their discoveries.
In December, 1904, Nobel laureate Marie Curie was named assistant to Pierre at the Faculté des Sciences of the Sorbonne. Until then she had been working without pay.
The Curies were too unwell and overworked to attend the ceremony in person. When they finally went to Stockholm, in June, 1905, only Pierre delivered a Nobel Lecture, as his wife sat and listened. Meanwhile, in 1904 a special chair had been created for him at the Sorbonne.
It appears that Pierre suffered from radiation sickness during the last years of his life, which came to a cruel end on April 19, 1906. He was not yet 47 years old. What happened on that day has been described by his wife.
[As he] was crossing the rue Dauphine, he was struck by a truck coming from the Pont Neuf and fell under its wheels. A concussion of the brain brought instantaneous death. So perished the hope founded on the wonderful being he thus ceased to be. In the study room to which he was never to return, the water buttercups he had brought from the country were still fresh.
In the introduction to his collected works published in 1908, Marie wrote of the shattered hopes for further research, in the French she mastered so exquisitely: “Le sort n’a pas voulu qu’il en fut ainsi, et nous sommes constraints de nous incliner devant sa decision incompréhensible.”
Marie’s younger daughter Eve has written of her mother’s intense grief after this event and of her determined efforts to avoid speaking of Pierre in later years. George Jaffe has left us his impressions of Madame Curie in 1911, when he spent a year at her laboratory as a Carnegie Scholar: “The stranger saw and admired the power of the intellect. He could not know what was going on behind the air of self-constraint or almost impassiveness which became characteristic of Madame Curie after the death of her husband.”
Shortly after Pierre’s death, Marie was named his successor at the Sorbonne. It was the first time in that venerable institution’s more than 600-year-long history that a woman was appointed to a professorship. The Paris papers treated this as a major event. On Monday, 5 November 1906, at 1:30 in the afternoon, Marie began her inaugural lecture, continuing a discourse on radioactivity at precisely the point where Pierre had left off in his last lecture. In 1910, she declined the Legion of Honour.
Marie Curie’s life in 1911 was marked by two events, one humiliating, one gratifying. In the autumn of that year, news items appeared in the French press, quoting private letters, and alleging the existence of an affair between Mme. Curie, widow, and the physicist Paul Langévin, married, a disgusting public treatment of private matters, probably true. In November, she received word that she had been awarded the Nobel Prize for chemistry for 1911, “in recognition of the part she has played in the development of chemistry: by the discovery of the chemical elements radium and polonium; by the determination of the properties of radium and by the isolation of radium in its pure metallic state; and finally, by her research into the compounds of this remarkable element.” In her Nobel Lecture, given on 11 December 1911, she recalled that many of these discoveries were made “by Pierre Curie in collaboration with me …” The chemical work aimed at isolating radium in the state of the pure salt “… was carried out by me, but it is intimately connected with our common work … I thus feel that … the award of this high distinction to me is motivated by this common work and thus pays homage to the memory of Pierre Curie.” She is the only woman ever to have been awarded two Nobel Prizes.
During World War I, Marie Curie organized and participated in the work of a number of radiology units for diagnostic and therapeutic purposes. She arranged for the equipment of some twenty automobiles with Roentgen apparatus in order that soldiers could be operated on near the battlefield. She also oversaw the installation of some 200 radiological rooms in various hospitals. She trained others as X-ray diagnosticians and often acted as one herself. In 1921, she paid a triumphal visit to the United States, where she was received by President Harding.
In May 1922, she was named member of the League of Nations’ International Committee on Intellectual Cooperation, and took an active part in its work for many years thereafter.
The fifth Solvay Conference, largely devoted to the new quantum mechanics, was held in October 1927. The printed proceedings of this meeting appeared in 1928. They open with a tribute by Marie Curie to Hendrik Lorentz, who had presided over the October conference and who had died shortly thereafter.
Meanwhile, Marie continued her research activities. In all, she published about 70 papers, the last one in 1933. Soon thereafter, at age 66, she died in a sanatorium in the Haute Savoye after a brief illness. The death report reads as follows: “Mme Pierre Curie died at Sancellemoz on 4 July 1934. The disease was an aplastic pernicious anaemia of rapid feverish development. The bone marrow did not react, probably because it had been injured by a long accumulation of radiations.” She was buried at Sceaux, near her husband. She just did not live long enough to hear of the Nobel Prize in chemistry (1935) award to daughter Iréne and her husband Frédéric Joliot (1900–58), for the discovery of positron radioactivity.
George Jaffe, who spent the year 1905 at the Curies’ laboratory in Paris, has written about them:
There have been and there are, scientific couples who collaborate with great distinction, but there has not been a second union of woman and man who represented, both in their own right, a great scientist. Nor would it be possible to find a more distinguished instance where husband and wife with all their mutual admiration and devotion preserved so completely independence of character, in life as well as in science … I was most strongly impressed by their extreme simplicity and modesty together with their extraordinary devotion to their task … There was about both of them an unostentatious superiority … [Pierre’s] disposition made him stand aloof when she entered upon something like a romantic enterprise: the search for an unknown element.
The complete works of Pierre Curie appeared in 1908, those of Marie Curie in 1954. Of particular interest in the latter book are the elegantly written essays on the status of radioactivity at various times. Biographies by Marie Curie of her husband (in which an autobiographical sketch is included), by Ève Curie of her mother and by Robert Reid, also of Marie, are especially important.
Marie Curie took great care for the education and development of interests of both her daughters. Irène followed in her mother’s footsteps and became a scientist while Ève showed more artistic and literary interests and displayed a particular talent for music.
Following in her parents’ sizable footsteps, Irène enrolled at the Faculty of Science in Paris. However, the outbreak of the first World War interrupted her studies. She joined her mother and began working as a nurse radiographer, operating x-ray machines to assist with the treatment of soldiers wounded on the battlefield.
By 1925, Irène had received her doctorate, having joined her mother in the field of the study of radioactivity. Ten years later, she and her husband, Frédéric Joliot, were jointly awarded the Nobel Prize in Chemistry for the breakthroughs they had made in the synthesis of new radioactive elements. Though it had been Marie’s pleasure to have witnessed her daughter and son-in-law’s successful research, she did not live to see them win the award. Irène also died of leukemia in 1956.
Irène was rejected three times by the French Academy of Sciences – which also famously never elected Marie Curie into its ranks. “She said, ‘Okay, I will ask for a seat at the Academy at every occasion and we will see how long it will take,’ ” recalls her daughter, Hélène Langevin-Joliot. Sadly, her mother died in 1956 before that could happen. It was to be another six years before the first woman was elected to the academy – Marguerite Perey, who was one of Curie’s doctoral students.
Ève, like her sister Irène, graduated from the Collège Sévigné in Paris, where she obtained two bachelor’s degrees, in Science and Philosophy, in 1925. After Marie Curie’s death, Ève decided to express her love by writing a biography. In Autumn 1935, she visited her family in Poland, looking for information about her mother’s childhood and youth. The fruit of this work was the biography Madame Curie, simultaneously published in France, Britain, Italy, Spain, the United States and other countries in 1937.
Madame Curie was instantly popular and became a bestseller in many countries.
Ève became more and more engaged in literary and journalistic work. Apart from her mother’s biography, she published musical reviews and articles on theatre, music and film in various newspapers. From the 1960s she committed herself to work for UNICEF, providing help to children and mothers in developing countries. And without the impact of radiation, she lived to 102!
The Curie family legacy continued with the next generation. Irène and Frédéric Joliot had two children of their own, named Hélène and Pierre, in honour of their grandparents. Marie’s grandchildren would both go on to distinguish themselves in the field of science as well.
Hélène became a nuclear physicist and, at 90 years old, still maintains a seat on the advisory board to the French government. She is a professor of nuclear physics at the Institute of Nuclear Physics at the University of Paris and a Director of Research at the CNRS (Centre national de la recherche scientifique – French National Centre for Scientific Research). She is also known for her work in actively encouraging women to pursue careers in scientific fields. She is Chairperson of the panel that awards the Marie Curie Excellence Award, a prize given to outstanding European researchers.
Pierre is a noted French biologist and researcher for the CNRS who has made contributions to the study of photosynthesis. He is emeritus professor at the Collège de France and a member of the Academy of Science of France.
Yves Langevin, Hélène’s son, is an astrophysicist and planetary scientist with a focus on planet Mars.
It has been five years since the Avengers stopped alien forces in New York, preventing the Chitauri army from invading the planet. Now, they need help in the Infinity War!
And little bears are very helpful 🙂
They are helping themselves to elevenses from Chu Bakery 🙂
Next March they will join the elite support network of S.H.I.E.L.D. agents that helps superheroes like Captain America, Iron Man and Thor protect Earth. Prospective agents can sign up for training at Marvel’s Avengers S.T.A.T.I.O.N., a scientific training centre that will open in March at Federation Square in Melbourne. Inside, the S.H.I.E.L.D. director, Nick Fury, will guide recruits through the process, starting with obtaining an ID badge.
Over the course of their training, recruits will use these badges at interactive labs that will allow them to explore the origins of the Avengers through “classified” documents and scientific experiments. The mission is to delve into the history, science, engineering, genetics, technology and profiles of Captain America, Hulk, Thor and Iron Man and more.
How does Mjolnir, Thor’s hammer, conduct electricity? How do you navigate the HUD interface inside Iron Man’s helmet? How did the supersoldier serum created Captain America? How do your strength and reflexes test against his? How did the gamma radiation turned Bruce Banner into the Hulk?
The exhibit is an interactive experience. The S.T.A.T.I.O.N. tour begins with a mobile device. Trainees can rent a device and enter their information, or download the app to their phone and enter their information, which will be used to create a S.T.A.T.I.O.N. profile. This serves as a personal tour guide for exhibit and is narrated by Friday, Tony Stark’s artificial intelligence introduced in Avengers: Age of Ultron. The device/phone is also used to quiz trainees on Marvel trivia (points are tallied at the end), and lets the guests take photos that are printed in a collage along with the official S.T.A.T.I.O.N. ID card.
The tour begins with a briefing from agent Maria Hill, and when the doors open, trainees are transported into a different world, completely reminiscent of the labs seen in the various Avengers films. Each area has its own theme, with the major rooms focusing on Captain America, The Incredible Hulk, Iron Man and Thor. The A.I. Friday, which knows exactly where the trainee is in the exhibit, briefs trainees on each aspect. As trainees go from room to room, new messages pop up on the device/phone, helping them along to their next stage in training.
Trainees test Captain America’s shield, interact with the Incredible Hulk, help defeat Ultron and experience Iron Man’s Heads Up Display, or HUD for short. The HUD puts trainees into a contraption that scans their brainwaves and tracks their eye movement. They use their eyes to focus on targets and blast them, all while their face is projected onto a screen that makes them appear as if they’re in an Iron Man helmet.
The Avengers S.T.A.T.I.O.N. (Scientific Training and Tactical Intelligence Operative Network) is an exhibit that marries costumes, props and set pieces from the Marvel Cinematic Universe with a program that trains the recruits to be official Avengers operatives.
One of the biggest draws to the exhibit is the artefacts, about 80 percent of which are actual movie props. Marvel supported the project by providing paraphernalia from the 2012 film The Avengers. This includes all the costumes, from Captain America, Hawkeye, Black Widow, Agent Coulson, Scarlet Witch, Ant-Man, Falcon and more, to pieces like Captain America’s shield; the sceptre wielded by Loki, the mischievous god who led the invasion; and a Tesseract, a powerful device that opens portals to other worlds. The station also includes a Chitauri captured in the battle and placed in a hermetically sealed chamber.
A super-powered dose of science and technology has been provided by NASA to enhance the authenticity of the experience and pique visitors’ interest in real-world science and technology.
The station is the brainchild of Victory Hill Exhibitions, which worked with Marvel Entertainment, NASA and the National Academy of Sciences. The Avengers S.T.A.T.I.O.N. got its start in New York City in 2014. Since then it’s been to Seoul, Paris, Las Vegas, Singapore, Beijing, Taipei and Chong Qing. In Las Vegas it has now been established as a permanent attraction. In 2017 Marvel broke records in Brisbane with its Creating the Cinematic Universe exhibition that ran for three-months and saw close to 270,000 visitors (but no bears 😦 ) experience one of the largest displays ever presented.
The station has become an extension of the Marvel Cinematic Universe. With new Marvel films coming out every year, the instillation grows and changes.
In this rare image taken on July 19, 2013, the wide-angle camera on NASA’s Cassini spacecraft has captured Saturn’s rings and our planet Earth and its moon in the same frame. It is only one footprint in a mosaic of 33 footprints covering the entire Saturn ring system (including Saturn itself). At each footprint, images were taken in different spectral filters for a total of 323 images: some were taken for scientific purposes and some to produce a natural colour mosaic. This is the only wide-angle footprint that has the Earth-moon system in it.
The dark side of Saturn, its bright limb, the main rings, the F ring, and the G and E rings are clearly seen; the limb of Saturn and the F ring are overexposed. The “breaks” in the brightness of Saturn’s limb are due to the shadows of the rings on the globe of Saturn, preventing sunlight from shining through the atmosphere in those regions. The E and G rings have been brightened for better visibility.
Earth, which is 1.44 billion kilometres away in this image, appears as a blue dot at centre right; the moon can be seen as a fainter protrusion off its right side. An arrow indicates their location in the annotated version. The other bright dots nearby are stars.
This is only the third time ever that Earth has been imaged from the outer solar system. The acquisition of this image, along with the accompanying composite narrow- and wide-angle image of Earth and the moon and the full mosaic from which both are taken, marked the first time that inhabitants of Earth knew in advance that their planet was being imaged. That opportunity allowed people around the world to join together in social events to celebrate the occasion.
The spacecraft launched on October 15, 1997, from Cape Canaveral, endured a seven-year voyage to Saturn and it arrived at its final destination in 2004. There and then Cassini began revolutionizing our view of Saturn and everything that surrounds it.
Over the course of its voyage Cassini surveyed Saturn’s atmosphere, rings and moons in exquisite detail. In 2005 Cassini’s Huygens probe descended to the surface of Saturn’s moon Titan.
Among its many discoveries, Cassini found liquid methane lakes on Titan and a buried liquid-water ocean on the moon Enceladus that escapes to the surface via geysers. Scientists suspect this underground sea might be capable of hosting alien life. Cassini also uncovered mountainous waves of rubble and “moonlets” in Saturn’s rings and an effect that turns its atmosphere blue in the winter.
No mission has ever explored a planetary system as rich as Saturn’s in such depth for so long. On its moon Titan, we found seas of hydrocarbons and a surface environment whose complexity rivals that of Earth. We observed the meteorology of Saturn’s atmosphere and witnessed the birth, evolution and demise of giant storms. We saw new phenomena in Saturn’s rings that told of the processes involved in the formation of solar systems, including our own. Like the cartographers of old, we mapped the moons of Saturn for future explorers and uncovered new ones, including an entire class of small bodies embedded within the rings themselves. And then there is Cassini’s most profound discovery of all: at the south pole of the moon Enceladus, more than 100 geysers spouting from an underground ocean that could be home to extra-terrestrial organisms. And now that bountiful scientific expedition has come to an end; the Cassini spacecraft ended its mission on September 15, 2017 by making a planned dive into the planet’s atmosphere.
The need for a detailed, comprehensive examination of the Saturn system became clear during the early 1980s, after the two Voyager spacecraft made flybys of the planet. These celebrated events were the opening acts in the story of humanity’s exploration of Saturn. They gave the planet dimension and personality but left behind questions that demanded answers. Voyager found Saturn to be a planet with a complex interior, atmosphere and magnetosphere. In its rings — a vast, gleaming disk of icy rubble — the mission recorded signs of the same physical mechanisms that were key in configuring the early solar system and similar disks of material around other stars. Voyager’s passage through Saturn’s inner system exposed diverse moons with dynamic forces at work. Titan, Saturn’s largest moon, whose surface remained invisible through its thick, ubiquitous haze, nonetheless teased observers with hints of a possible ocean of liquid hydrocarbons. Altogether the Saturn system seemed an ideal destination for further in-depth study and exploration.
Cassini was an international undertaking, led by NASA and the European Space Agency and designed to be, in every dimension, a dramatic advance over Voyager. At the size of a school bus, it was bigger than Voyager and outfitted with the most sophisticated scientific instruments ever carried into the outer solar system. Cassini also carried the Huygens probe — a four meter-wide, aerodynamically shaped device, equipped with a six-instrument payload, that descended to the surface of Titan.
After traversing the solar system, Cassini flawlessly took up residence around Saturn on June 30, 2004. Its trajectory around Saturn was both convoluted and precise, unfurling over the course of its 13-year tour like the opening petals of a blossom. To enable close-up viewing of everything in the inner Saturnian system, its orbits varied in size, tilt and orientation. We also had the luxury of modifying orbits to dive in for another look — in some cases, many looks — at things we had discovered earlier.
The length of Cassini’s stay at Saturn was critical to the success of the mission. Prolonged monitoring is the only way to catch unpredictable processes such as meteoroid impacts on Saturn’s rings. Furthermore, the slow, steady orbital migrations of Saturn’s moons, along with atmospheric changes that arise from the large seasonal variations in solar illumination, required us to collect observations over as lengthy a time span as possible. Cassini’s nominal mission was four years long and slated to end on June 30, 2008. But the spacecraft’s resounding triumphs in that time and the indisputable logic of keeping such a productive asset at work helped us press the case for continuing Cassini’s mission. Our arguments were successful, garnering several extensions and ensuring, for example, that we witnessed the rare illumination conditions of Saturn’s equinox in August 2009, when the sun’s shallow rays on Saturn’s rings revealed the presence of vertical structures protruding above the ring plane that cast long, easily seen shadows.
Ultimately Cassini’s orbital operations ended nearly one half of a Saturnian year (or, on Earth, 13 years and two and a half months) after they began. Cassini arrived a bit past the height of the planet’s southern summer, and the mission closed at the height of its northern summer. This time frame allowed us to observe over almost a full seasonal cycle: we watched Saturn’s and Titan’s southern hemispheres go from summer to winter and their northern hemispheres go from winter to summer. It was somewhat of a cosmic cheat, but it worked.
Before the space age, scientists thought the moons of the outer solar system would be featureless, geologically dead balls of ice. Voyager proved that assumption wrong; Cassini’s mission was to survey Saturn’s horde of satellites and return some understanding of their histories. In some cases, those histories turned out to be remarkable. Take Iapetus. The origin of its two toned appearance — one hemisphere as white as snow and the other deep black — was a long-standing mystery. From Cassini’s high-resolution images, we learned that even on small scales, the moon is a piebald mix of dark and light patches. Together Cassini’s cameras and thermal instrument showed us why this is so. Both the hemisphere-scale colour variations and the local piebald patches are caused by a runaway thermal process found only on the slowly rotating Iapetus. Regions that start out dark get hot enough to sublimate ice and thus become darker and hotter. Regions that start out white are colder and become the sites where those sublimated vapours condense. Over time all the ice in the dark region disappears and reaccumulates in the white regions. How did an entire hemisphere partake in this process? In its orbit around Saturn, Iapetus barrels through a cloud of dark, fine-grained material originating from Phoebe, one of Saturn’s outer irregular satellites. This cloud turns Iapetus’s entire leading hemisphere dark, keeping it warmer and ice-free. Mystery solved.
Another standout moon is Titan. Cassini’s visible and near-infrared cameras as well as its radar instrument were able to cut through Titan’s haze. And the early 2005 descent of the Huygens probe through Titan’s atmosphere for two and a half hours captured panoramic images and measurements of atmospheric composition, transparency, winds and temperature before the probe came to rest on the moon’s surface. In all, what Cassini found on Titan was a world out of science fiction, where the scenery — landforms and clouds — are recognizable but made of unusual substances, where the look of the place is familiar but the feel is not.
Titan, we discovered, has lakes and seas made not of water but of liquid methane. At the moon’s south pole, Cassini’s high resolution camera sighted such a liquid body close to the size of Lake Ontario (and hence named Ontario Lacus) amid a district of smaller similar features. Other Cassini instruments later verified that Ontario Lacus indeed holds liquid methane. We have since found many bodies of liquid methane of varying sizes; for some reason, they mostly inhabit the high northern latitudes. Radar observations have revealed craggy, rocky shorelines that resemble the coast of Maine. In contrast, the equatorial plains, where the Huygens probe landed, are dry and covered with dunes that continue for long stretches, interrupted here and there by higher ground, all the way around the moon.
The lakes and seas of liquid organics on Titan’s surface have naturally raised speculation about whether they might contain life. But the surface temperature on Titan is exceedingly cold: −180 degrees Celsius. It would be surprising to find chemical reactions similar to those we believe are required for water-based biochemistry operating at such temperatures. But should we ever detect truly “alien” biochemistry thriving in methane, it would be a remarkable and historic find.
The site of Cassini’s greatest discovery is without question Enceladus, an icy moon a tenth the size of Titan. There Voyager had laid bare vast, surprisingly smooth stretches that told of a past marked by intense internal activity and maybe even a liquid-water layer buried below its icy shell — both on a moon seemingly too small for such phenomena.
The first inkling we had of any activity on Enceladus came early in the mission, in January 2005, when we discovered a plume of icy particles coming off the south pole. Very soon thereafter other Cassini instruments confirmed that the plume was indeed real. Cassini’s operators responded quickly, altering trajectories to have a closer look. What we learned about Enceladus during that early part of the mission was astounding, but it was not until after 2008, when NASA extended the mission, that significant time and resources to examining were devoted to Enceladus.
Enceladus, we now know, is a moon being flexed and pulled by the gravitational tidal forces of Saturn. This tidal energy produces more than enough internal heat to create a global water ocean, possibly as thick in places as 50 kilometres, buried under an outer layer of ice a few kilometres thick. More than 100 geysers spout from four prominent fractures in the south polar terrain, creating a plume of tiny ice particles and vapour that extends hundreds of kilometres above the surface. Most of the solid mass in this plume falls back to the surface, but a small fraction extends farther to form Saturn’s diffuse but large E ring.
Cassini was able to fly through the plume a dozen times and analyse its material. We found that the particles seen in our images, which were droplets of ocean only hours earlier, bore evidence of large organic molecules and compounds that indicated hydrothermal activity similar to that observed at deep-sea vents on Earth’s seafloor. They also indicated an ocean salinity comparable to Earth’s. The vapour accompanying these particles was mostly water but contained trace amounts of simple organic compounds, as well as carbon dioxide and ammonia — all ingredients important for the sustenance and even origin of life.
Cassini’s results point clearly to a subsurface environment on Enceladus that could contain biological activity. We now must confront the goose-bump-raising questions: Did this small icy world host a second genesis of life in our solar system? Could there be signs of life in its plume? Could microbes be snowing on its surface? No other body so demonstrably possesses all the characteristics we believe are necessary for habitability. It is, at present, the most promising, most accessible place in the solar system to search for life.
The rings, of course, are what make Saturn the glorious spectacle it is, and understanding their intricate workings was a major objective for Cassini. They are the natural end state of the collapse of a rotating cloud of debris, and as such, they are the closest analogue to the rubble disk we think provided the raw ingredients for our own solar system. They are also a model for the protostellar disks from which new solar systems are born and even for the billions of pinwheels of dust and gas we call spiral galaxies. Of all there was to study at Saturn, the rings presented the greatest scientific reach, extending from our local neighbourhood to clear across the cosmos.
Through Cassini’s measurements, we have come to understand the origin of most of the structure in the rings of Saturn. In certain places, we find that the gravitational handiwork of some distant orbiting moon has disturbed the orbits of ring particles, creating sharp edges or wave disturbances that propagate out in a spiral pattern. In others, where moons are embedded in the rings, gravity has nudged particles into beautiful structures. Pan, for instance, a roughly 30-kilometer-wide moon in the Encke ring gap, has done this to the particles in its vicinity; in turn, infalling ring material has reshaped Pan, making the moon look as if it were wearing a tutu.
In regions of the rings where particles are especially dense, we uncovered self-generating waves, with wavelengths ranging from 100 meters to hundreds of kilometres, propagating through the disk. These waves can reflect off sharp discontinuities in particle concentrations and interfere with themselves and one another, creating a chaotic-looking geography. And our understanding of ring structure now includes the gratifying confirmation of a prediction Mark Marley, now at NASA’s Ames Research Centre, and Carolyn Porco made in 1993: that acoustic oscillations within the body of Saturn could also create features in the rings. In this way, Saturn’s rings behave like a seismograph.
Cassini found its most stunning ring surprises during the time surrounding the August 2009 equinox. Along the sharp outer edge of the most massive ring (the B ring), we found an incredible 20,000-kilometer-long continuous string of spiky shadows betraying the presence of “ring mountains” — waves of particles extending three kilometres above the ring plane. These formations might result from the extreme compression of material passing around small “moonlets” that have been caught in the resonance at the ring’s edge like rushing water splashing against a large cliff face on the shore.
In another revelation, we saw a very subtle, tightly wound spiralling pattern continuing without interruption for 19,000 kilometres across the inner C and D rings. Some meticulous sleuthing by Matt Hedman, now at the University of Idaho, and his colleagues revealed that an impact of cometary debris within the inner rings in 1983 likely forced all the ring particles in the impact region into tilted orbits; these orbits precessed like a top, the inner ones precessing faster than the outer ones. Since then, this disturbance has wound up ever tighter, creating a three-meter-high spiral corrugation pattern in the rings. This structure did not even exist during the Voyager flybys. The solar system, we have come to see, is a dynamic marvel, and in their myriad and fluid forms, Saturn’s rings are an object lesson in the universality, scalability and endless complexity of gravity. No artist could do better.
Cassini has also investigated the makeup and behaviour of Saturn’s atmosphere in great detail, uncovering some unexpected features in the process. Its instruments were able to study Saturn’s atmosphere at a wide range of altitudes, revealing its global circulation patterns, composition and vertical structure. The atmosphere is divided into wide bands like Jupiter’s, although Saturn’s bands are less obvious from the outside because of a thick layer of haze lying above the upper ammonia cloud deck. When Cassini probed below the haze and into the troposphere, it revealed that the width of Saturn’s bands alternates with latitude: narrower ones are darker and coincident with rapid jet streams, and the wider bands tend to be brighter, aligned with jets that are slower and maybe even stationary, relative to the general rotation of the planet. Overall, Saturn’s atmosphere seems fairly static over time—even the surprising hexagonshaped jet stream over the north pole has changed little, Cassini showed, since Voyager first sighted it. We are learning that stability is a common feature of large-scale atmospheric systems in the giant planets: with no solid surface underlying the gas, there is no friction to dissipate atmospheric motions. Once started, they endure.
We found that Saturn’s atmosphere is not totally unresponsive to the changing seasons. Above the clouds in the northern winter hemisphere, the planet was putting on quite the unexpected show when Cassini first arrived: it was blue! Because the two Voyager flybys occurred near an equinox and thus returned no views of winter, this extreme coloration came as quite a surprise. Our best guess is that the lower flux of ultraviolet radiation during the winter, along with the sun-blocking effect of the ring shadows on the winter hemisphere, reduces the production of the overlying haze. A clearer atmosphere means better opportunity for Rayleigh scattering, the process that turns our own atmosphere blue, and for methane in the atmosphere to absorb the red rays of the sun. The gorgeous sliver of azure that colours the winter hemisphere in our images of Saturn is, in effect, a slice of Neptune’s atmosphere spliced onto Saturn’s. Who knew?
One distinctive property of Saturn, which has been known for a century, is that on timescales of decades, it is prone to the eruption of colossal storms. Cassini witnessed one such storm in late 2010. Over a period of about 270 days, we watched this thundering, lightning-producing behemoth be born as a small disturbance in the northern hemisphere, then grow, spread clear around the planet until its tail met its head, and eventually fade. This was yet another phenomenon that no spacecraft had ever witnessed. We suspect that water, the constituent of Saturn’s deepest cloud deck, can suppress convection in the lighter hydrogen atmosphere for a period of decades, until finally buoyancy wins out and a large convective outburst ensues.
From its inception in 1990 to its final dramatic conclusion this September, Cassini has been a major, extraordinarily successful component of humanity’s six-decade-long exploration beyond our home planet. Its historic expedition around Saturn has shown us intricate details in the workings of an alluring and remarkably alien planetary system. It has expanded our understanding of the forces that made Saturn and its environs, our solar system and, by extension, other stellar and planetary systems throughout the cosmos what they are today.
Original article by Carolyn Porco in the October 2017 issue of Scientific American.