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New mapping of the giant planet’s upper atmosphere reveals a likely reason why it’s so hot.
NASA Jet Propulsion Laboratory and University Communications
April 6, 2020
The upper layers in the atmospheres of gas giants – Saturn, Jupiter, Uranus and Neptune – are hot, just like Earth’s. But unlike Earth, the sun is too far from these outer planets to account for the high temperatures. Their heat source has been one of the great mysteries of planetary science.
New analysis of data from NASA’s Cassini spacecraft finds a viable explanation for what’s keeping the upper layers of Saturn, and possibly the other gas giants, so hot: auroras at the planet’s north and south poles. Electric currents, triggered by interactions between solar winds and charged particles from Saturn’s moons, spark the auroras and heat the upper atmosphere. As with Earth’s northern lights, studying auroras tells scientists what’s going on in the planet’s atmosphere.
The work, published today in Nature Astronomy, is the most complete mapping yet of both temperature and density of a Saturn’s upper atmosphere – a region that has been poorly understood.
“Understanding the dynamics really requires a global view. This dataset is the first time we’ve been able to look at the upper atmosphere from pole to pole while also seeing how temperature changes with depth,” said Zarah Brown, lead author of the study and a graduate student in the University of Arizona Lunar and Planetary Laboratory.
By building a complete picture of how heat circulates in the atmosphere, scientists are better able to understand how auroral electric currents heat the upper layers of Saturn’s atmosphere and drive winds. The global wind system can distribute this energy, which is initially deposited near the poles toward the equatorial regions, heating them to twice the temperatures expected from the sun’s heating alone.
“The results are vital to our general understanding of planetary upper atmospheres and are an important part of Cassini’s legacy,” said study co-author Tommi Koskinen, a member of Cassini’s Ultraviolet Imaging Spectograph team. “They help address the question of why the uppermost part of the atmosphere is so hot, while the rest of the atmosphere – due to the large distance from the sun – is cold.”
Managed by NASA’s Jet Propulsion Laboratory in Southern California, Cassini was an orbiter that observed Saturn for more than 13 years before exhausting its fuel supply.
NASA JPLNASA/ESA/ASI Cassini-Huygens Spacecraft
The mission plunged it into the planet’s atmosphere in September 2017, in part to protect its moon Enceladus, which Cassini discovered might hold conditions suitable for life. But before its plunge, Cassini performed 22 ultra-close orbits of Saturn, a final tour called the Grand Finale.
It was during the Grand Finale that the key data was collected for the new temperature map of Saturn’s atmosphere. For six weeks, Cassini targeted several bright stars in the constellations of Orion and Canis Major as they passed behind Saturn. As the spacecraft observed the stars rise and set behind the giant planet, scientists analyzed how the starlight changed as it passed through the atmosphere.
Measuring the density of the atmosphere gave scientists the information they needed to find the temperatures. Density decreases with altitude, and the rate of decrease depends on temperature. They found that temperatures peak near the auroras, indicating that auroral electric currents heat the upper atmosphere.
Density and temperature measurements together helped scientists figure out wind speeds. Understanding Saturn’s upper atmosphere, where planet meets space, is key to understanding space weather and its impact on other planets in our solar system and exoplanets around other stars.
“Even though thousands of exoplanets have been found, only the planets in our solar system can be studied in this kind of detail. Thanks to Cassini, we have a more detailed picture of Saturn’s upper atmosphere right now than any other giant planet in the universe,” Brown said.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA’s Jet Propulsion Laboratory, or JPL, a division of Caltech in Pasadena, manages the mission for NASA’s Science Mission Directorate in Washington. JPL designed, developed and assembled the Cassini orbiter.
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Alana Johnson
NASA Headquarters, Washington
202-358-1501
alana.r.johnson@nasa.gov
In this image captured by NASA’s Cassini spacecraft in 2007, the plumes of Enceladus are clearly visible. The moon is nearly in front of the Sun from Cassini’s viewpoint. Credits: NASA/JPL-Caltech
NASA/ESA/ASI Cassini-Huygens Spacecraft
New kinds of organic compounds, the ingredients of amino acids, have been detected in the plumes bursting from Saturn’s moon Enceladus. The findings are the result of the ongoing deep dive into data from NASA’s Cassini mission.
Powerful hydrothermal vents eject material from Enceladus’ core, which mixes with water from the moon’s massive subsurface ocean before it is released into space as water vapor and ice grains. The newly discovered molecules, condensed onto the ice grains, were determined to be nitrogen- and oxygen-bearing compounds.
On Earth, similar compounds are part of chemical reactions that produce amino acids, the building blocks of life. Hydrothermal vents on the ocean floor provide the energy that fuels the reactions. Scientists believe Enceladus’ hydrothermal vents may operate in the same way, supplying energy that leads to the production of amino acids.
This illustration shows the process of organic compounds making their way onto ice grains emitted in plumes from Saturn’s moon Enceladus, where they were detected by NASA’s Cassini spacecraft. Credits: NASA/JPL-Caltech
“If the conditions are right, these molecules coming from the deep ocean of Enceladus could be on the same reaction pathway as we see here on Earth. We don’t yet know if amino acids are needed for life beyond Earth, but finding the molecules that form amino acids is an important piece of the puzzle,” said Nozair Khawaja, who led the research team of the Free University of Berlin. His findings were published Oct. 2 in the Monthly Notices of the Royal Astronomical Society.
Although the Cassini mission ended in September 2017, the data it provided will be mined for decades. Khawaja’s team used data from the spacecraft’s Cosmic Dust Analyzer, or CDA, which detected ice grains emitted from Enceladus into Saturn’s E ring.
The scientists used the CDA’s mass spectrometer measurements to determine the composition of organic material in the grains.
The identified organics first dissolved in the ocean of Enceladus, then evaporated from the water surface before condensing and freezing onto ice grains inside the fractures in the moon’s crust, scientists found. Blown into space with the rising plume emitted through those fractures, the ice grains were then analyzed by Cassini’s CDA.
The new findings complement the team’s discovery last year of large, insoluble complex organic molecules believed to float on the surface of Enceladus’ ocean. The team went deeper with this recent work to find the ingredients, dissolved in the ocean, that are needed for the hydrothermal processes that would spur amino acid formation.
“Here we are finding smaller and soluble organic building blocks — potential precursors for amino acids and other ingredients required for life on Earth,” said co-author Jon Hillier.
“This work shows that Enceladus’ ocean has reactive building blocks in abundance, and it’s another green light in the investigation of the habitability of Enceladus,” added co-author Frank Postberg.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency (ESA) and the Italian Space Agency. NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages the mission for NASA’s Science Mission Directorate, Washington. JPL designed, developed and assembled the Cassini orbiter. The radar instrument was built by JPL and the Italian Space Agency, working with team members from the U.S. and several European countries.
Jet Propulsion Laboratory (JPL)) is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. Although the facility has a Pasadena postal address, it is actually headquartered in the city of La Cañada Flintridge, on the northwest border of Pasadena. JPL is managed by the nearby California Institute of Technology (Caltech) for the National Aeronautics and Space Administration. The Laboratory’s primary function is the construction and operation of robotic planetary spacecraft, though it also conducts Earth-orbit and astronomy missions. It is also responsible for operating NASA’s Deep Space Network.
JoAnna Wendel
NASA Headquarters, Washington
202-358-1003
joanna.r.wendel@nasa.gov
This near-infrared, color view from Cassini shows the sun glinting off of Titan’s north polar seas. Image credit: NASA/JPL-Caltech/Univ. Arizona/Univ. Idaho
October 30, 2014- This near-infrared, color mosaic from NASA’s Cassini spacecraft shows the sun glinting off of Titan’s north polar seas. While Cassini has captured, separately, views of the polar seas (see PIA17470) and the sun glinting off of them (see PIA12481 and PIA18433) in the past, this is the first time both have been seen together in the same view.
NASA/ESA/ASI Cassini-Huygens Spacecraft
On its final flyby of Saturn’s largest moon in 2017, NASA’s Cassini spacecraft gathered radar data revealing that the small liquid lakes in Titan’s northern hemisphere are surprisingly deep, perched atop hills and filled with methane.
PIA20021: Mystery Feature Evolves in Titan’s Ligeia Mare
The existence of oceans or lakes of liquid methane on Saturn’s moon Titan was predicted more than 20 years ago. But with a dense haze preventing a closer look it has not been possible to confirm their presence. Until the Cassini flyby of July 22, 2006, that is.
The new findings, published April 15 in Nature Astronomy, are the first confirmation of just how deep some of Titan’s lakes are (more than 300 feet, or 100 meters) and of their composition. They provide new information about the way liquid methane rains on, evaporates from and seeps into Titan – the only planetary body in our solar system other than Earth known to have stable liquid on its surface.
Scientists have known that Titan’s hydrologic cycle works similarly to Earth’s – with one major difference. Instead of water evaporating from seas, forming clouds and rain, Titan does it all with methane and ethane. We tend to think of these hydrocarbons as a gas on Earth, unless they’re pressurized in a tank. But Titan is so cold that they behave as liquids, like gasoline at room temperature on our planet.
Scientists have known that the much larger northern seas are filled with methane, but finding the smaller northern lakes filled mostly with methane was a surprise. Previously, Cassini data measured Ontario Lacus, the only major lake in Titan’s southern hemisphere. There they found a roughly equal mix of methane and ethane. Ethane is slightly heavier than methane, with more carbon and hydrogen atoms in its makeup.
“Every time we make discoveries on Titan, Titan becomes more and more mysterious,” said lead author Marco Mastrogiuseppe, Cassini radar scientist at Caltech in Pasadena, California. “But these new measurements help give an answer to a few key questions. We can actually now better understand the hydrology of Titan.”
Adding to the oddities of Titan, with its Earth-like features carved by exotic materials, is the fact that the hydrology on one side of the northern hemisphere is completely different than the that of other side, said Cassini scientist and co-author Jonathan Lunine of Cornell University in Ithaca, New York.
“It is as if you looked down on the Earth’s North Pole and could see that North America had completely different geologic setting for bodies of liquid than Asia does,” Lunine said.
On the eastern side of Titan, there are big seas with low elevation, canyons and islands. On the western side: small lakes. And the new measurements show the lakes perched atop big hills and plateaus. The new radar measurements confirm earlier findings that the lakes are far above sea level, but they conjure a new image of landforms – like mesas or buttes – sticking hundreds of feet above the surrounding landscape, with deep liquid lakes on top.
The fact that these western lakes are small – just tens of miles across – but very deep also tells scientists something new about their geology: It’s the best evidence yet that they likely formed when the surrounding bedrock of ice and solid organics chemically dissolved and collapsed. On Earth, similar water lakes are known as karstic lakes. Occurring in in areas like Germany, Croatia and the United States, they form when water dissolves limestone bedrock.
Alongside the investigation of deep lakes, a second paper in Nature Astronomy helps unravel more of the mystery of Titan’s hydrologic cycle. Researchers used Cassini data to reveal what they call transient lakes. Different sets of observations – from radar and infrared data – seem to show liquid levels significantly changed.
The best explanation is that there was some seasonally driven change in the surface liquids, said lead author Shannon MacKenzie, planetary scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. “One possibility is that these transient features could have been shallower bodies of liquid that over the course of the season evaporated and infiltrated into the subsurface,” she said.
These results and the findings from the Nature Astronomy paper on Titan’s deep lakes support the idea that hydrocarbon rain feeds the lakes, which then can evaporate back into the atmosphere or drain into the subsurface, leaving reservoirs of liquid stored below.
Cassini, which arrived in the Saturn system in 2004 and ended its mission in 2017 by deliberately plunging into Saturn’s atmosphere, mapped more than 620,000 square miles (1.6 million square kilometers) of liquid lakes and seas on Titan’s surface. It did the work with the radar instrument, which sent out radio waves and collected a return signal (or echo) that provided information about the terrain and the liquid bodies’ depth and composition, along with two imaging systems that could penetrate the moon’s thick atmospheric haze.
The crucial data for the new research were gathered on Cassini’s final close flyby of Titan, on April 22, 2017. It was the mission’s last look at the moon’s smaller lakes, and the team made the most of it. Collecting echoes from the surfaces of small lakes while Cassini zipped by Titan was a unique challenge.
“This was Cassini’s last hurrah at Titan, and it really was a feat,” Lunine said
The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages the mission for NASA’s Science Mission Directorate, Washington. JPL designed, developed and assembled the Cassini orbiter. The radar instrument was built by JPL and the Italian Space Agency, working with team members from the U.S. and several European countries.
Jet Propulsion Laboratory (JPL)) is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. Although the facility has a Pasadena postal address, it is actually headquartered in the city of La Cañada Flintridge, on the northwest border of Pasadena. JPL is managed by the nearby California Institute of Technology (Caltech) for the National Aeronautics and Space Administration. The Laboratory’s primary function is the construction and operation of robotic planetary spacecraft, though it also conducts Earth-orbit and astronomy missions. It is also responsible for operating NASA’s Deep Space Network.
Cassini’s flagship mission to the ringed planet Saturn ended over a year ago, but data from the craft’s Grand Finale tour of the Saturnian system continues to allow scientists to unlock the mysteries surrounding the sixth planet in our solar system.
Among the discoveries after the end of Cassini’s mission include a firm understanding of the planet’s rotational period, the the age of the planet’s iconic rings, observations of dust storms on its moon Titan, and numerous other scientific revelations.
Grand Finale science returns:
Cassini’s remarkable tenure at Saturn came to an end on 15 September 2017 after numerous mission extensions and scientific returns that scientists could only have dreamed of before the craft’s launch nearly 20 years prior on 15 October 1997.
In an effort to execute planetary protection to ensure Cassini did not accidentally contaminate one of the potentially life harboring moons of Saturn, the craft was plunged into the ringed planet’s atmosphere for a destructive entry after a final set of 22 close orbits of the planet that constituted the mission’s Grand Finale.
Knowing this was the end, Cassini scientists did something with their craft that no spacecraft had done before; they flew Cassini through the rings of Saturn and into Saturn’s upper atmosphere multiple times.
The daring nature of the Grand Finale paid off handsomely, with a host of returns not possible during the craft’s more stable orbits of the planet in the years prior.
In all, the Grand Finale revealed complex organic compounds embedded in water nanograins (very fine, small particles) raining down from Saturn’s rings into the planet’s upper atmosphere, observed how the rings interact with the planet and how inner-ring particles and gases fall directly into Saturn’s atmosphere, and revealed what the material looks like in the gap between the rings and a planet’s atmosphere.
In terms of the organic compounds observed in water nanograins, scientists saw methane, ammonia, carbon monoxide, nitrogen, and carbon dioxide – organic compositions far different from the organic compounds found emanating from the icy moon Enceladus and on the methane-rich moon Titan.
This result revealed the presence of at least three distinct groupings of organic molecules in the Saturnian system – something not expected.
Additionally, Cassini observed ring particles and gases from the innermost ring raining down into the planet’s atmosphere. Inner-ring particles can take on electric charges and spiral along Saturn’s magnetic field lines, eventually falling into Saturn’s atmosphere at high latitudes in a phenomenon known as “ring rain.”
But Grand Finale data revealed that some inner-ring particles fall from the rings and are dragged quickly into Saturn’s atmosphere at its equatorial latitudes at an impressive rate of 22,000 pounds (10,000 kg) of material per second.
Moreover, the area between Saturn and its rings revealed even more surprises for scientists when Cassini was finally able sample the material in the ring gap. This sampling showed nanometer size particles, like smoke, residing in the region, suggesting an as-yet-unknown process grinding up ring particles into a smoke-like consistancy.
A few of the findings from Cassini’s direct sampling of the rings and atmosphere during the Grand Finale. NASA/JPL Caltech
Additional data from the Grand Finale also revealed that Saturn and its rings are more interconnected than scientists thought, with Cassini showing previously unknown electric currents from the rings to the top of Saturn’s atmosphere.
For NASA, Cassini’s daring and risky Grand Finale has been more than justified with the host of data returned, with Cassini Project Science Linda Spilker saying, “Almost everything going on in that region turned out to be a surprise. That was the importance of going there, to explore a place we’d never been before. And the expedition really paid off – the data is tremendously exciting.
“Many mysteries remain as we put together pieces of the puzzle. Results from Cassini’s final orbits turned out to be more interesting than we could have imagined.”
When in doubt, look to the rings:
For decades, a persistent mystery has puzzled scientists about Saturn: how long is a day on the ringed planet?
It might seem like an easy question to answer, after all, one needs only observe the rotational rate of the planet. But therein lies the problem. Saturn has no solid surface and no defined feature in its gaseous atmosphere to track as the planet rotates.
Moreover, the planet’s unusual magnetic field masks the planet’s rotation rate.
But as it turned out, the answer to Saturn’s rotation rate, and therefore the length of its day, resided in the planet’s iconic rings.
Dive between Saturn and innermost rings. NASA/JPL Caltech
During the Grand Finale, Cassini flew through Saturn’s rings multiple times, observing the icy and rocky components in unprecedented detail – observations that allowed scientists studying wave patterns in the rings to find that the rings respond to vibrations within Saturn’s interior.
According to Christopher Mankovich, a graduate student in astronomy and astrophysics at the University of California, Santa Cruz, the vibrations within the rings acted as seismometers measuring vibrations in Saturn’s interior.
“Particles throughout the rings can’t help but feel these oscillations in the gravity field. At specific locations in the rings, these oscillations catch ring particles at just the right time in their orbits to gradually build up energy, and that energy gets carried away as an observable wave.”
Tracking these observable waves led Mankovich to develop models of Saturn’s internal structure that’s allowed him to track the movements of the interior and thus calculate Saturn’s rotation rate.
Using this data, Mankovich was able to determine that Saturn’s rotation rate is 10 hours 33 minutes 38 seconds.
But Saturn’s rings themselves also held a tantalizing surprise for scientists that was not revealed until the Grand Finale dives through this icy and rocky region.
____________________________________
Saturn’s rings are its most famous feature and make Saturn the most recognizable and exotic planet in the solar system. But new information from Cassini’s Grand Finale shows that the rings formed just 10 to 100 million years ago, meaning the planet’s most iconic feature is also one of its youngest.
Understanding the age of Saturn’s rings is a matter of determining the mass of the planet, the gravitational pull of Saturn on the rings, and the mass of the rings.
In order to directly sample the needed data points, Cassini had to fly between Saturn and the rings, something not permitted during the craft’s primary mission but that became a feature of its Grand Finale.
By flying directly between Saturn and its rings, Cassini was able the return data that allowed scientists to calculate how much gravity was pulling on Cassini, causing it to accelerate, down to a fraction of a millimeter per second. With that information, scientists could understand the mass of the planet and the mass of the rings.
“Only by getting so close to Saturn in Cassini’s final orbits were we able to gather the measurements to make the new discoveries,” said Cassini radio science team member and lead author Luciano Iess, of Sapienza University of Rome. “And with this work, Cassini fulfills a fundamental goal of its mission: not only to determine the mass of the rings, but to use the information to refine models and determine the age of the rings.”
Cassini data showed the rings have a low mass, corresponding to a younger age of between 10 to 100 million years. Corroborating this is the appearance of the rings, which are bright and clean. An older ring structure would be dark and contaminated by debris.
____________________________________
However, it has not just been data gathered during the Grand Finale that has continued to illuminate the Saturnian system; information collected throughout Cassini’s mission continues to be analyzed and continues to reveal new and exciting elements of the system.
Dust storms on Titan:
Titan is a complex world, to say the least, and the only moon in the solar system with a thick and complex atmosphere and with a hydrologic cycle. Now, the impressive moon enters a category previously reserved for only two other bodies in the solar system: the presence of dust storms on its surface.
As on Earth and Mars, weather patterns on Titan vary from season to season, and data from Cassini throughout its mission has revealed the changing nature of Titan’s weather, including the emergence of powerful methane storms near the moon’s equator during the northern equinox in 2009.
During this period, Sebastien Rodriguez, an astronomer at the Université Paris Diderot, France, and his team noted unusual equatorial bright spots in infrared data returned from Cassini, brightenings they interpreted at the time to be the same kind of methane clouds and storms observed on other areas of the moon during the northern equinox.
But subsequent investigations have revealed that these brightenings were in fact something very different. They were not methane storms. They were dust storms.
Image showing dust storms. NASA/JPL Caltech, U Arizona, University Paris Diderot, IPGP, S.Rodriguez et al. 2018
“From what we know about cloud formation on Titan, we can say that such methane clouds in this area and in this time of the year are not physically possible,” said Rodriguez. “The convective methane clouds that can develop in this area and during this period of time would contain huge droplets and must be at a very high altitude – much higher than the 6 miles (10 kilometers) that modeling tells us the new features are located.”
Modeling and investigation revealed the features seen in 2009 where in the atmosphere but close to Titan’s surface and likely formed “a very thin layer of tiny solid organic particles.” The bright spots appearances directly over the dune fields around Titan’s equator left only a single explanation for what they actually were: clouds of dust raised from the dunes.
“The near-surface wind speeds required to raise such an amount of dust as we see in these dust storms would have to be very strong – about five times as strong as the average wind speeds estimated by the Huygens measurements near the surface and with climate models,” noted Rodriguez.
The presence of strong winds and confirmation of dust storms on the surface of Titan means that Titan’s dunes are still active and continually changing. Moreover, winds on Titan could be carrying dust gathered from across large distances, “contributing to the global cycle of organic dust on Titan and causing similar effects to those that can be observed on Earth and Mars”
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30 August 2018
Laurent Lamy
Observatoire de Paris
Paris, France
Tel: +33 145 077668
Email: laurent.lamy@obspm.fr
Mathias Jäger
ESA/Hubble, Public Information Officer
Garching, Germany
Tel: +49 176 62397500
Email: mjaeger@partner.eso.org
Astronomers using the NASA/ESA Hubble Space telescope have taken a series of spectacular images featuring the fluttering auroras at the north pole of Saturn. The observations were taken in ultraviolet light and the resulting images provide astronomers with the most comprehensive picture so far of Saturn’s northern aurora. Image credit: NASA, ESA & L. Lamy
In 2017, over a period of seven months, the NASA/ESA Hubble Space Telescope took images of auroras above Saturn’s north pole region using the Space Telescope Imaging Spectrograph.
NASA/ESA Hubble Space Telescope Imaging Spectrograph
The observations were taken before and after the Saturnian northern summer solstice. These conditions provided the best achievable viewing of the northern auroral region for Hubble.
On Earth, auroras are mainly created by particles originally emitted by the Sun in the form of solar wind. When this stream of electrically charged particles gets close to our planet, it interacts with the magnetic field, which acts as a gigantic shield.
Magnetosphere of Earth, original bitmap from NASA. SVG rendering by Aaron Kaase
While it protects Earth’s environment from solar wind particles, it can also trap a small fraction of them. Particles trapped within the magnetosphere — the region of space surrounding Earth in which charged particles are affected by its magnetic field — can be energised and then follow the magnetic field lines down to the magnetic poles. There, they interact with oxygen and nitrogen atoms in the upper layers of the atmosphere, creating the flickering, colourful lights visible in the polar regions here on Earth [1].
However, these auroras are not unique to Earth. Other planets in our Solar System have been found to have similar auroras. Among them are the four gas giants Jupiter, Saturn, Uranus and Neptune. Because the atmosphere of each of the four outer planets in the Solar System is — unlike the Earth — dominated by hydrogen, Saturn’s auroras can only be seen in ultraviolet wavelengths; a part of the electromagnetic spectrum which can only be studied from space.
Hubble allowed researchers to monitor the behaviour of the auroras at Saturn’s north pole over an extended period of time. The Hubble observations were coordinated with the “Grand Finale” of the Cassini spacecraft, when the spacecraft simultaneously probed the auroral regions of Saturn [2].
NASA/ESA/ASI Cassini-Huygens Spacecraft
The Hubble data allowed astronomers to learn more about Saturn’s magnetosphere, which is the largest of any planet in the Solar System other than Jupiter.
Saturn’s magnetosphere via Cassini spacecraft mission
The images show a rich variety of emissions with highly variable localised features. The variability of the auroras is influenced by both the solar wind and the rapid rotation of Saturn, which lasts only about 11 hours. On top of this, the northern aurora displays two distinct peaks in brightness — at dawn and just before midnight. The latter peak, unreported before, seems specific to the interaction of the solar wind with the magnetosphere at Saturn’s solstice.
The main image presented here is a composite of observations made of Saturn in early 2018 in the optical and of the auroras on Saturn’s north pole region, made in 2017, demonstrating the size of the auroras along with the beautiful colours of Saturn.
Hubble has studied Saturn’s auroras in the past. In 2004, it studied the southern auroras shortly after the southern solstice (heic0504) and in 2009 it took advantage of a rare opportunity to record Saturn when its rings were edge-on (heic1003). This allowed Hubble to observe both poles and their auroras simultaneously.
Notes
[1] The auroras here on Earth have different names depending on which pole they occur at. Aurora Borealis, or the northern lights, is the name given to auroras around the north pole and Aurora Australis, or the southern lights, is the name given for auroras around the south pole.
[2] Cassini was a collaboration between NASA, ESA and the Italian Space Agency. It spent 13 years orbiting Saturn, gathering information and giving astronomers a great insight into the inner workings of Saturn. Cassini took more risks at the end of its mission, travelling through the gap between Saturn and its rings. No spacecraft had previously done this, and Cassini gathered spectacular images of Saturn as well as new data for scientists to work with. On 15 September 2017 Cassini was sent on a controlled crash into Saturn.
The universe around us.
Astronomy, everything you wanted to know about our local universe and never dared to ask.
Thanks, Manu, ESA did not bother to put this up in English.
11.7.17
Gaël Choblet
Université de Nantes, France
Email: Gael.Choblet@univ-nantes.fr
Gabriel Tobie
Université de Nantes, France
Email: gabriel.tobie@univ-nantes.fr
Nicolas Altobelli
ESA Cassini–Huygens Project Scientist
Tel: +34 91 813 1201
Email: nicolas.altobelli@esa.int
Markus Bauer
ESA Science Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int
Oficina de Comunicación de ESAC
Email: comunicacionesac@esa.int
Image Credit: NASA / JPL-Caltech / Space Science Institute; Inside: LPG-CNRS / U. Nantes / U. Angers. Graphic composition: ESA
1. passive flow of cold water from the ocean salt to a porous rock core.
2. The heated water rises in the core in tight tufts and interacts with rocks.
3. Hotspots on the seabed.
4. Transport of heat and rocky material across the ocean.
5. Localized heating in the ocean-ice thins the ice.
6. jets of water vapor and particles erupt through cracks.
If the core of Enceladus was porous, tidal friction could generate enough to cause hydrothermal activity inside for thousands of millions of years heat, which would increase their chances of habitability.
This is what emerges from a new study, published yesterday in Nature Astronomy, which has a first concept that would explain the key features of Enceladus, the Saturnian moon of 500 km diameter observed by the international Cassini spacecraft during its mission , completed last September.
NASA/ESA/ASI Cassini-Huygens Spacecraft
Encélado house a salty overall ocean under an ice layer having an average thickness of 20 to 25 km, which would only 1-5 km in the south polar region. There, through fissures in the ice jets water vapor and ice grains they are expelled. The composition of ejecta measured by Cassini, includes salts and silicon powder, suggesting that would be formed by the interaction of warm water at least 90 ° C, with the porous rock core.
Enceladus plumes.
Credit: NASA / JPL / SSI.
That would require a huge source of heat, a hundred times greater than that could generate natural decay of radioactive elements in rocks your core and medium focalizase activity at the South Pole.
It is believed that the tidal effect on Saturn is responsible for the eruptions that deforms the ice Enceladus by movements of attraction and repulsion along its elliptical path around the giant planet. However, the energy produced by tidal friction on the ice would be too weak by itself to offset the heat loss from the ocean: the moon would have frozen after 30 million years. However, as Cassini has shown, the moon is still extremely active, suggesting that something else is happening.
“Although it has never been clear what the source of that Enceladus gets the energy to stay active, we have now seen in more detail how the structure and composition of its rocky core could have a key role in generating the energy needed” says lead study author Gaël Choblet, University of Nantes (France).
In the new simulations, the core is formed of deformable porous unconsolidated rock, water can readily permeate. Thus, the liquid cold ocean water can seep into the core and gradually heated as it penetrates due to tidal friction between moving rock fragments.
Water circulates through the core and then rises again because it is hotter than the surrounding material. Ultimately, this process transfers heat to the ocean floor in thin columns that interact closely with rocks. On the ocean floor, these columns reach the colder ocean.
Last observation feathers Enceladus by Cassini .. Credit: NASA / JPL / SSI.
It is estimated that one hot spot on the ocean floor up to 5 GW release energy equivalent to geothermal energy consumed annually in Iceland. These hot spots seafloor generated columns totaling several centimeters per second. Not only the columns make the icy crust there above is based, also transported for weeks and months, from the ocean floor, small particles which are then released into space in the form of icy jets.
Also, computer models of the authors show that most of the water is expelled in the polar regions of the moon, with a chain process causing hot spots in localized areas and consequently, a smaller thickness in the ice fair over something that matches interpreted by Cassini.
“Our simulations can explain both the existence of a global ocean due to heat transport large scale between the depths of the inside and the ice, and the concentration of activity in a region relatively small around the south pole, justifying the main phenomena observed by Cassini, “says study co-author Gabriel Tobie, also of the University of Nantes.
Scientists say that efficient rock-water interactions in a porous core caused by tidal friction could generate up to 30 GW of heat over tens of millions or even thousands of millions of years.
“Future missions able to analyze organic molecules columns Enceladus more accurately than Cassini would be able to confirm whether the maintenance of hydrothermal conditions could have permitted the emergence of life,” said Nicolas Altobelli, Cassini project scientist at ESA .
A future mission equipped with a radar to penetrate the ice, may also limit the thickness of the ice and additional overpasses or orbiter improve models interior, also verifying the presence of active hydrothermal columns.
“In the next decade, with the Juice mission Jovian moons send new generation instruments, including a ground penetrating radar.
ESA/Juice spacecraftESA /JUICE schematic
This mission is specifically devoted to assess the potential for habitability of ocean worlds in the outer solar system, “adds Nicolas.
Additional Information.
The article “Powering prolonged hydrothermal activity inside Enceladus,” Choblet G. et al., Was published in Nature Astronomy on November 6, 2017, article online.
The Cassini-Huygens mission is a cooperative project between NASA, ESA and the Italian space agency ASI.
Sept. 11, 2017
Elizabeth Zubritsky
elizabeth.a.zubritsky@nasa.gov
NASA’s Goddard Space Flight Center, Greenbelt, Md.
When NASA’s Cassini spacecraft plunges into the atmosphere of Saturn on Sept. 15, ending its 20 years of exploration, astronomers will observe the giant planet from Earth, giving context to Cassini’s final measurements.
“The whole time Cassini is descending, we’ll be on the ground, taking data and learning about conditions on Saturn,” said Don Jennings, a senior scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and a co-investigator for a Cassini instrument called the Composite Infrared Spectrometer.
The aftermath of a massive storm that erupted in Saturn’s northern hemisphere in December 2010 continues to be tracked by researchers, including observations planned using the new high-resolution iSHELL instrument at NASA’s Infrared Telescope Facility. Credits: NASA/JPL-Caltech/SSI
This farewell is fitting for a mission that has been supported by similar observations throughout its lifetime. NASA’s Infrared Telescope Facility, or IRTF, and the W. M. Keck Observatory, in which NASA is a partner, have provided crucial contributions from the summit of Maunakea in Hawaii. Other U.S. and international telescopes also have investigated the Saturn system, complementing and enhancing the mission.
NASA Infrared Telescope facility Mauna Kea, Hawaii, USA, 4,207 m (13,802 ft) above sea levelKeck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft) above sea level
“IRTF and other facilities have provided direct support to the Cassini–Huygens mission and made it possible to link that data to decades’ worth of earlier and ongoing ground-based studies,” said IRTF director John Rayner.
NASA/ESA/ASI Cassini-Huygens Spacecraft
“Through its daytime observing capabilities IRTF is able to provide almost year-round monitoring of planets in support of NASA missions.”
Ground-based observations of Titan, the giant planet’s largest moon, helped with preparations for the Huygens probe mission early in Cassini’s exploration of the Saturn system. The probe was released after Cassini entered Saturn orbit and descended through Titan’s thick atmosphere to land on the surface.
A coordinated ground campaign was organized to study Titan’s atmosphere and surface, to measure the wind speed and direction, to look at atmospheric chemistry and to provide global imaging.
Eight facilities worldwide participated, observing before, during and after the Huygens probe mission, led by the European Space Agency. These included the Keck Observatory, which captured high-resolution images of the atmospheric weather patterns on Titan, and the IRTF, which helped determine the direction of Titan’s winds.
“Ground-based observing played a crucial role, because at that time, it was the only way to determine the direction of Titan’s winds, which had the potential to affect Huygens’ descent to the surface,” said Goddard’s Theodor (Ted) Kostiuk, who led those observations at the IRTF and is now an emeritus scientist. “The Voyager flyby provided some information about Titan, but wind direction was one thing it could not tell us.”
IRTF continues to be used for long-term studies of Saturn and Titan and their atmospheres, and to investigate Saturn’s moons, extending and complementing Cassini findings. The facility’s recently installed high-resolution infrared instrument, called iSHELL, will be deployed for ongoing studies of the aftermath of a massive storm that broke out in Saturn’s northern hemisphere in 2010. With its very high spectral resolution, iSHELL has been optimized for the study of planetary atmospheres.
Cassini also has received plenty of aloha from the Keck Observatory, which has provided many sharp images and spectra of Saturn’s most famous feature – its rings. These studies are made possible by the high spatial resolution of Keck’s large aperture combined with a state-of-the-art adaptive optics system to correct for distortions caused by Earth’s atmosphere.
“It’s been exciting to be involved in ground support of the Cassini orbiter over these many years,” said Observing Support Manager Randy Campbell of Keck Observatory. “This mission has given us an opportunity to work together toward a better understanding of some of the most beautiful and enigmatic objects in the night sky, Saturn and its moons.”
During the summer of 2017, the Cassini team used Keck Observatory to take near-infrared spectroscopic data of the regions near Saturn’s equator, just as Cassini was diving between Saturn and its rings during its final orbits. The team also took Keck data of the polar magnetic fields to better understand the planet’s auroras, which are similar to Earth’s northern and southern lights. The Keck Observatory data will be used to verify Cassini’s data to provide a sort of “ground-truth” calibration of some of the on-board instruments of the orbiter.
After Cassini, ground-based studies will continue, building on everything the spacecraft observed, and keeping the discoveries coming.
For more information about NASA’s Infrared Telescope Facility, visit:
NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.
Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.
Artist rendition of the Cassini spacecraft over Saturn. Credit: NASA/JPL-Caltech/SSI/Kevin M. Gill.
“With Cassini, we had a rare opportunity and we seized it,” said Linda Spilker, Cassini Mission Scientist.
And on Friday, September 15, we say goodbye to this incredible spacecraft.
Since 2004, Cassini has been orbiting Saturn, exploring the magnificent gas giant planet while weaving through an incredibly diverse assortment of 60-plus icy moons, and skimming along the edges of the complex but iconic icy rings.
Cassini’s findings have revolutionized our understanding of the entire Saturn system, providing intriguing insights on Saturn itself as well as revealing secrets held by moons such as Enceladus, which should be a big iceball but instead is one of the most geothermally active places in our solar system. And thanks to the Huygens lander, we now know Saturn’s largest moon, Titan is eerily Earthlike, but yet totally alien.
“The lasting story of Cassini will likely be its longevity and the monumental amount of scientific discovery,” Cassini Project Manager Earl Maize told me last year. “It was absolutely the right spacecraft in the right place at the right time to capture a huge array of phenomena at Saturn.”
Diagram of Cassini’s final week, showing some of the milestones as the spacecraft heads for its plunge into Saturn. Credit: NASA/JPL-Caltech
In the upper reaches of Saturn’s atmosphere, the Cassini spacecraft will use its thrusters to point its antenna toward Earth until it breaks up. (Credit: NASA/JPL-Caltech)
For UA scientists who contributed to NASA’s Cassini-Huygens mission, the Grand Finale of humanity’s tour of the Saturn system marks the end of an era.
When NASA’s Cassini spacecraft careens to its final destination, the upper atmosphere of Saturn, it will take with it a sizable chunk of University of Arizona space research history. After a journey of 4.9 billion miles, and one month shy of 20 years in space, the probe is programmed to end its voyage exploring the Saturnian system through a deliberate plunge into the second-largest planet of the solar system.
The spacecraft’s fateful dive on Friday will be the final beat in the mission’s Grand Finale — 22 weekly dives, begun in late April, through the gap between Saturn and its rings. According to NASA, no spacecraft has ever ventured so close to the planet before.
“Cassini-Huygens is a classic example of a ‘flagship’ mission, accomplishing tremendous science in many disciplines over many years,” said Alfred McEwen, a UA professor of planetary sciences, on Monday as he prepared to leave for Pasadena, California. There, at NASA’s Jet Propulsion Laboratory, he would attend the final moments of the mission, along with other UA planetary scientists who have participated in the project.
NASA’s Cassini spacecraft delivered this glorious view of Saturn on Dec. 18, 2012, taken while the spacecraft was in Saturn’s shadow. The cameras were turned toward Saturn and the sun so that the planet and rings are backlit. (Credit: NASA/JPL-Caltech/Space Science Institute)
NASA chose to end the mission by safely disposing of the spacecraft, burning it up in Saturn’s atmosphere rather than allowing it to run out of fuel and committing its fate to an aimless tumble and potential crash onto one of Saturn’s moons. Mission scientists were especially concerned about contaminating Titan or Enceladus, the two Saturnian moons where life as we know it might be possible — a possibility discovered by Cassini’s multiple flybys.
When it launched, Cassini-Huygens was the biggest, most complex interplanetary spacecraft ever flown. In 2004, it arrived in the Saturn system, carrying with it a robotic passenger in form of the Huygens probe, contributed to the mission by the European Space Agency, or ESA. On Jan. 14, 2005, Huygens would make history as the first — and, so far, only — humanmade object to touch down on a world in the outer solar system. Through the eyes of Huygens, an instrument built by UA scientists and engineers, people on Earth could watch as the probe hurtled through the opaque and hazy atmosphere enshrouding Titan.
The probe was equipped with an instrument called DISR, short for Descent Imager/Spectral Radiometer. Led by Martin Tomasko, a now-retired research professor at the Lunar and Planetary Laboratory, UA scientists joined their ESA colleagues in Germany to follow Huygens with six science experiments as it descended through Titan’s thick atmosphere until it touched down on a virtually unseen surface. In addition to images taken with DISR, the lander recorded data that enabled LPL staff scientist Erich Karkoschka to gather surprising clues about Titan’s surface many years after the event.
Cassini-Huygens is a “flagship mission” and has the track record to show it. (Credit: NASA/JPL-Caltech)
Monitoring the Moon Titan
During many flybys, Cassini monitored the dynamic Titan using its camera suite and an instrument called VIMS, a Visual and Infrared Mapping Spectrometer. Built at Jet Propulsion Laboratory under the leadership of Robert Brown, operations for VIMS moved to the UA when Brown assumed a position as professor at LPL. According to Brown, VIMS has been taking spectra over areas of Saturn, its rings and moons so scientists can discover what these objects are made of.
Those observations revealed details about the cycle of methane, which on Titan takes the role of water on Earth — forming clouds, raining down and forming lakes, as well as freezing into ice. In all those observations, Cassini’s cameras played an important role, said McEwen, who is a team member of the craft’s imaging science subsystem. Those cameras, over the years of photographing Saturn, its rings and moons, created some of the most visually beautiful images of the solar system.
Cassini’s imaging team leader Carolyn Porco was appointed to the mission while on the faculty at LPL, where she had been working on NASA’s Voyager mission, and was a co-originator of the idea to use Voyager-1 to take portraits of the planets, including the famous Pale Blue Dot image of Earth.
Surface observations on Titan are planned at LPL, and then sent to the Cassini Imaging Central Laboratory for Operations, or CICLOPS, at the University of Colorado, Boulder, which Porco heads as director.
“From there, the necessary commands are sent to JPL and then to the spacecraft,” McEwen explains.
Another one of Saturn’s moons, ice-clad Enceladus, rose to stardom during several flybys over the course of the mission. Enceladus plows along the orbit of the E Ring, Saturn’s second-from-outermost ring, which reaches extremely far out into space, brushing up against the orbit of Titan.
“There was speculation that the moon had something to do with the E Ring,” McEwen says.
During multiple close flybys, Cassini used its full science payload to detect and analyze water-rich plumes erupting from the moon’s south pole far into space, a spectacular discovery that McEwen considers one of the highlights of the entire mission.
“We saw that these plumes are quite large and extensive,” he recalls. “Because we were able to measure their composition with Cassini’s instruments, we could show that (tiny particles from those eruptions) are the source of the E Ring.”
The Last Closest Approach
Evidence for subsurface oceans of water were discovered by Cassini inside both Enceladus and Titan, making them prime targets for future NASA missions.
Cassini made its last closest approach to Titan on Sept. 11 at 12:04 p.m. PDT, at an altitude of 73,974 miles (119,049 kilometers) above the moon’s surface, causing the spacecraft to slingshot into its final approach to Saturn — but not before it would send final images from Titan to Earth, eagerly awaited by scientists, including McEwen.
“Previously, we saw thunderstorms in Titan’s southern hemisphere when it was summer there,” he says, “and because it’s now the northern summer solstice, we are hoping to see cloud activity and perhaps thunderstorms in the northern hemisphere.”
Cassini will be doing science even after being gripped by Saturn’s gravity, pulling it into destruction, by measuring the composition, temperature and other properties of Saturn’s atmosphere.
“The spacecraft will be transmitting data until the very end, and we’ll be there when it stops,” McEwen says. “It won’t go very deep, because it is not a probe designed to go deep, but still deeper than anything else.”
When Cassini arrived at Saturn, where one “year” lasts 29.5 Earth years, the gas giant went through northern winter, and Cassini was there to witness the planet’s change of seasons.
The end of the mission, McEwen says, is “not unexpected,” adding that the plan to end with a solstice mission, followed by a plunge into Saturn, was put in place about seven years ago.
Still, “this mission has been going for so long, it’s a little hard to believe that it’s over,” he says.
The University of Arizona (UA) is a place without limits-where teaching, research, service and innovation merge to improve lives in Arizona and beyond. We aren’t afraid to ask big questions, and find even better answers.
In 1885, establishing Arizona’s first university in the middle of the Sonoran Desert was a bold move. But our founders were fearless, and we have never lost that spirit. To this day, we’re revolutionizing the fields of space sciences, optics, biosciences, medicine, arts and humanities, business, technology transfer and many others. Since it was founded, the UA has grown to cover more than 380 acres in central Tucson, a rich breeding ground for discovery.
Where else in the world can you find an astronomical observatory mirror lab under a football stadium? An entire ecosystem under a glass dome? Visit our campus, just once, and you’ll quickly understand why the UA is a university unlike any other.
A complex coordinated ‘dance’ between ESA and NASA tracking stations is following Cassini during its Grand Finale.
15 July 2017
Daniel
NASA/ESA/ASI Cassini-Huygens Spacecraft
In Cassini’s Grand Finale orbits – the final chapter of its nearly 20-year mission – the spacecraft travels in an elliptical path that sends it diving at tens of thousands of kilometers per hour through the 2400-km space between the rings and the planet where no spacecraft has ventured before.
Ring Crossing: In this still from the short film Cassini’s Grand Finale, the spacecraft is shown diving between Saturn and the planet’s innermost ring. Credit: NASA/JPL-Caltech.
May, June and July have been busy months for Cassini, as a series of complex ground-station tracking passes involving ESA’s Deep Space Antennas (DSA) and NASA’s Deep Space Network (DSN) captured a series of Grand Finale radio science passes.
In Cassini’s Grand Finale orbits – the final chapter of its nearly 20-year mission – the spacecraft travels in an elliptical path that sends it diving at tens of thousands of kilometers per hour through the 2400-km space between the rings and the planet where no spacecraft has ventured before.
May, June and July have been busy months for Cassini, as a series of complex ground-station tracking passes involving ESA’s Deep Space Antennas (DSA) and NASA’s Deep Space Network (DSN) captured a series of Grand Finale radio science passes.
The Cassini mission has performed radio science observations many times during its time at Saturn. Previously, the mission relied solely on the antennas of NASA’s Deep Space Network for these observations.
Now, the addition of new ESA tracking capabilities is helping provide the continuous signal reception needed during Cassini radio science activities. But it means the two agencies’ networks must work closely together.
Hearing the distant shout
ESA deep space ground stations began working with Cassini last year, conducting a series of test ‘passes’ – a ‘pass’ occurs when a spacecraft arcs into line-of-sight visibility above a station and continues until it disappears below the station’s horizon as Earth rotates – using their large, 35-meter-diameter, 630-tonne antennas pointed with exquisite accuracy at Cassini in the sky, listening for the craft’s call from Saturn.
Estrack New Norcia
In an initial test on 10 August 2016, ESA’s tracking station at New Norcia, Western Australia, received signals transmitted by Cassini across 1.4 billion km of space – the most distant ‘catch’ ever for an ESA station.
Now, during the 22 ‘inside-the-rings’ orbits of the Grand Finale, ESA stations are putting into practice the experience learned last year and are receiving Cassini’s signals, focussing on radio science gravity and ring occultation measurements, and delivering the data received to scientists in the U.S. and Europe for scientific analysis.
Tugged by gravity & passing through particles
The gravity experiments aim at measuring Saturn’s gravitational field with an unprecedented level of detail in order to gain insights into the planet’s interior structure, and at constraining the scenario of formation of Saturn’s rings by determining the rings’ mass.
Variations in Cassini’s orbit – even minute ones – from its expected trajectory can be detected by analysing the Doppler shift in the craft’s transmitted signal[1], enabling the tugs due to gravity to be studied and measured.
The radio science occultations aim at analysing the fine-scale structure of the rings and the physical properties of its particles.
Radio science occultations occur when the signals that Cassini transmits to the ground stations pass through the rings – affecting the signals in certain ways that can be studied and analysed.
Dipping and diving
The 22 Grand Finale orbits are bringing Cassini between Saturn and its rings; the spacecraft’s closest approach to Saturn, reached during each passage through the ring plane, ranges between approximately 1655 km and 3910 km with respect to Saturn’s ‘1 bar level.’ (This the place in the atmosphere where air pressure is the same as at sea-level on Earth. It’s also approximately the height of Saturn’s tallest clouds.)
For six of these closest approach passages (the last one to occur on 19 July), the spacecraft had its High Gain Antenna pointed toward Earth to perform radio occultation measurements of the rings and the radio science gravity experiments.
These radio tracking passes run for very long periods – lasting up to 37 hours, meaning that no single ESA or NASA station has visibility of Saturn for the entire pass.
Uninterrupted receipt of signals
For Cassini radio science, receipt of the craft’s signal must be uninterrupted in order to obtain measurements of Saturn’s gravitational effects on the spacecraft without gaps.
To achieve the extra-long passes that Cassini needs, and since we can’t simply slow down Earth’s rotation, a series of very technically challenging, real-time handovers of Cassini’s received signal was planned between multiple ESA and NASA ground stations.
This effort involves antennas located in Argentina (ESA), California (NASA), Spain and Australia (ESA and NASA).
“We are now getting into a new mode of radio science, giving much more accurate measurements of gravitational effects compared to previous observations where the effects of ring density on radio signal propagation were the main topic of study,” Daniel Firre, ESA Service Manager for NASA cross support.
Visibility: why the ESA-NASA station activity is so complex
As mentioned above, a pass starts when a spacecraft rises into line-of-sight view above one local horizon, seen from the station’s location, and continues until it drops out of sight below another horizon, the movement being due to the station rotating with the planet (good example with pictures here from when an ESA station tracked NASA’s Juno Earth flyby in 2013).
Tracking Juno: Here it comes and there it goes
A pair of photos of ESA’s Malargue station that perfectly illustrate how the Agency’s tracking efforts progressed last night.
At left, the huge 35m dish antenna is pointing more or less straight up as Juno approached Earth high above Argentina. Of course, Earth rotates, so the antenna had to be continuously tracked down and rotated.
At right, finally, as Juno dipped out of line-of-sight below the horizon, the station lost contact with the spacecraft with the antenna pointed low toward the East.
The craft continued en route to make closest approach above S. Africa a few minutes after the right-hand image was taken.
ESA Malargüe station pointing almost vertically up as NASA’s Juno spacecraft approaches from deep space over Argentina on 9 October 2013. Credit: ESA
ESA Malargüe station as Juno zooms out of view Credit: ESA
Since the Earth is rotating, a routine tracking pass link from any single ground station to a planetary mission typically can last only a few hours – but Cassini radio science needs much longer.
In comparison, for Earth missions (which typically orbit Earth 14 times per day), passes last just a few minutes!
The handover plan between antennas to receive the spacecraft’s signal has been organised to provide the necessary continuous coverage, and has necessitated very close technical and organisational cooperation between the two networks across multiple continents.
Incredibly, there can be up to seven stations involved in a single pass.
The graphic below illustrates how each ground station tracks Cassini’s signal during these passes. Each station is represented by a different colour. Cassini’s signal is tracked as Saturn (and hence Cassini) rises and sets in the sky above each station.
Typical multi-station, ESA-NASA tracking pass for Cassini Credit: NASA/JPL-Caltech
C – NASA Canberra DSN
NN – ESA New Norcia DSA
M – NASA Madrid DSN
ML – ESA Malargüe DSA
G – NASA Goldstone DSN
Note that the optimum angle for the ground station antennas is between 25 and 30 degrees above the horizon; below this “cut-off angle,” there are too many things that could disturb the communication link with the spacecraft.
As a result, ESA’s stations in the southern hemisphere are in the best position to receive the Cassini’s signal due to Saturn’s location in the sky right now. This can be clearly seen in the diagram, where M(adrid) and G(oldstone) have visibilities of around 30 degrees, while Malargüe and New Norcia are at nearly 80 degrees.
Cassini is a sophisticated spacecraft; it can receive signals in X-band and transmit in S-band, X-band and Ka-band (details on these frequencies here).
For the radio science passes, Cassini is transmitting and the stations on Earth are receiving; every time one receiving station rotates out of view, the succeeding station picks up, and there is a five-second overlap to avoid losing contact (note that the coloured coverage arcs in the graphic above overlap).
Not all the stations can receive all three of Cassini’s frequencies; nonetheless, the station-to-station handovers are so well coordinated that it is always possible to receive at least two of them.
Feeling the pull of a gas giant
Saturn’s gravity field and the mass of the rings are detected by means of what are called “range rate” measurements – basically, measuring the rate at which the distance from the ground station to the spacecraft varies. These measurements are enabled by Cassini’s on-board X-band radio system, along with the five DSN and two ESA stations working in tight coordination.
Gravity field measurements are obtained by comparing the detected speed of the spacecraft (technically, its radial velocity, with accuracies of about 0.05 mm/s) to a model of the spacecraft’s orbit that takes in consideration the effects of the Doppler shift.
Cassini radio science observations will provide crucial clues on how and when Saturn and its rings formed, as well as their relation to its moons: a large ring mass would allow the rings to be as old as the Saturnian system, formed 4.5 billion years ago, while a smaller ring mass suggests that the rings must be much younger, possibly formed by the breakup of a large comet or small moon.
Ring occultations
As described above, occultation observations happen when the rings partially block Cassini’s radio signals enroute to Earth.
During the Grand Finale, these observations are taking advantage of the spacecraft’s ultra-close perspective on the rings, which allows the radio signal to systematically sweep across the ring system from quite close.
The campaign tracked occultations during the six radio science gravity orbits (the so-called “RSS orbits” when Cassini’s radio science subsystem was active) and two “Ring Segment” orbits (orbits 276 and 282 – see the Grand Finale orbit guide for details).
The Grand Finale orbits take Cassini between the planet and its rings. Credit: NASA JPL-Caltech
The radio occultations are short in duration (less than 26 minutes), starting almost immediately after Cassini dives through the ring plane, and they cover the entire main ring system. The technique can measure up to three frequencies, profiling the ring structure and constraining the structures’ physical properties.
Credit: NASA/JPL-Caltech.
The collective ring coverage of the RSS Grand Finale occultations is unprecedented in the Cassini mission. Never before has such a close occultation observation technique been attempted.
“By marshalling the two agencies’ stations together, the overall science return from Cassini is being significantly enhanced, as their sensitive radio ‘ears’ can listen for signals from the craft as it is tugged by gravity or as the signals pass through the rings, providing additional, important information to help us understand this incredible system,” says Nicolas Altobelli, ESA’s Cassini-Huygens project scientist.
These powerful scientific insights are being made possible through intense coordination and cooperation between the ground stations of ESA and its NASA partners – truly a technical and expertise tour de force within a Grand Finale.
The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.
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