How Salty is Enceladus’ Ocean Under the ice?

An icy satellite of Saturn, Enceladus, has been a subject of increasing interest in recent years since Cassini captured jets of water and other material being ejected out of the south pole of the moon.  One particularly tantalizing hypothesis supported by the sample composition is that there might be life in the oceans under the ice shells of Enceladus. To evaluate Enceladus’ habitability and to figure out the best way to probe this icy moon, scientists need to better understand the chemical composition and dynamics of Enceladus’ ocean.

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The Same Technology Could Search for Microbes in Mars Rocks or Under the ice on Europa

Ever since it landed in the Jezero Crater on Feb. 18th, 2021, the Perseverance rover has been prepping its scientific instruments to begin searching for signs of past life on the Red Planet. These include spectrometers that will scan Martian rocks for organics and minerals that form in the presence of water and a caching system that will store samples of Martian soil and rock for retrieval by a future mission.

These telltale indicators could be signs of past life, which would most likely take the form of fossilized microbes. In the near future, a similar instrument could be used to search for present-day extraterrestrial life. It’s known as the Wireline Analysis Tool for the Subsurface Observation of Northern ice sheets (WATSON), and could be used to find evidence of life inside “ocean worlds” like Europa, Enceladus, and Titan.

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Beyond “Fermi’s Paradox” XIII: What is the “Ocean Worlds” Hypothesis?

Welcome back to our Fermi Paradox series, where we take a look at possible resolutions to Enrico Fermi’s famous question, “Where Is Everybody?” Today, we examine the possibility that the reason for the Great Silence is that most life out there exists in warm water oceans under sheets of ice!

In 1950, Italian-American physicist Enrico Fermi sat down to lunch with some of his colleagues at the Los Alamos National Laboratory, where he had worked five years prior as part of the Manhattan Project. According to various accounts, the conversation turned to aliens and the recent spate of UFOs. Into this, Fermi issued a statement that would go down in the annals of history: “Where is everybody?

This became the basis of the Fermi Paradox, which refers to the disparity between high probability estimates for the existence of extraterrestrial intelligence (ETI) and the apparent lack of evidence. Since Fermi’s time, there have been several proposed resolutions to his question, which include the possibility that Oceans Worlds (and not rocky planets) might be the best candidates for finding life.

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Planets With Large Oceans are Probably Common in the Milky Way

Within our Solar Systems, there are several moons where astronomers believe life could be found. This includes Ceres, Callisto, Europa, Ganymede, Enceladus, Titan, and maybe Dione, Mimas, Triton, and the dwarf planet Pluto. These “ocean worlds” are believed to have abundant liquid water in their interiors, as well as organic molecules and tidal heating – the basic ingredients for life.

Which raises the all-important question: are similar moons to be found in other star systems? This is the question NASA planetary scientist Dr. Lynnae C. Quick and her team from NASA’s Goddard Space Flight Center sought to address. In a recent study, Quick and her colleagues examined a sample of exoplanet systems and found that ocean worlds are likely to be very common in our galaxy.

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An Insulating Layer of Gas Could Keep a Liquid Ocean Inside Pluto

In July of 2015, NASA’s New Horizons mission made history by becoming the first spacecraft to ever conduct a flyby with Pluto. In addition to providing the world with the first up-close images of this distant world, New Horizons‘ suite of scientific instruments also provided scientists with a wealth of information about Pluto – including its surface features, composition, and atmosphere.

The images the spacecraft took of the surface also revealed unexpected features like the basin named Sputnik Planitia – which scientists saw as an indication of a subsurface ocean. In a new study led by researchers from the University of Hokkaido, the presence of a thin layer of clathrate hydrates at the base of Pluto’s ice shell would ensure that this world could support an ocean.

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NASA Wants to Send a Low-Cost Mission to Explore Neptune’s Moon Triton

In the coming years, NASA has some bold plans to build on the success of the New Horizons mission. Not only did this spacecraft make history by conducting the first-ever flyby of Pluto in 2015, it has since followed up on that by making the first encounter in history with a Kuiper Belt Object (KBO) – 2014 MU69 (aka. Ultima Thule).

Given the wealth of data and stunning images that resulted from these events (which NASA scientists are still processing), other similarly-ambitious missions to explore the outer Solar System are being considered. For example, there is the proposal for the Trident spacecraft, a Discovery-class mission that would reveal things about Neptune’s largest moon, Triton.

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Are There Enough Chemicals on Icy Worlds to Support Life?

For decades, scientists have believed that there could be life beneath the icy surface of Jupiter’s moon Europa. Since that time, multiple lines of evidence have emerged that suggest that it is not alone. Indeed, within the Solar System, there are many “ocean worlds” that could potentially host life, including Ceres, Ganymede, Enceladus, Titan, Dione, Triton, and maybe even Pluto.

But what if the elements for life as we know it are not abundant enough on these worlds? In a new study, two researchers from the Harvard Smithsonian Center of Astrophysics (CfA) sought to determine if there could in fact be a scarcity of bioessential elements on ocean worlds. Their conclusions could have wide-ranging implications for the existence of life in the Solar System and beyond, not to mention our ability to study it.

The study, titled “Is extraterrestrial life suppressed on subsurface ocean worlds due to the paucity of bioessential elements?” recently appeared online. The study was led by Manasvi Lingam, a postdoctoral fellow at the Institute for Theory and Computation (ITC) at Harvard University and the CfA, with the support of Abraham Loeb – the director of the ITC and the Frank B. Baird, Jr. Professor of Science at Harvard.

Artist’s depiction of a watery exoplanet orbiting a distant red dwarf star. Credit: CfA

In previous studies, questions on the habitability of moons and other planets have tended to focus on the existence of water. This has been true when it comes to the study of planets and moons within the Solar System, and especially true when it comes the study of extra-solar planets. When they have found new exoplanets, astronomers have paid close attention to whether or not the planet in question orbits within its star’s habitable zone.

This is key to determining whether or not the planet can support liquid water on its surface. In addition, astronomers have attempted to obtain spectra from around rocky exoplanets to determine if water loss is taking place from its atmosphere, as evidenced by the presence of hydrogen gas. Meanwhile, other studies have attempted to determine the presence of energy sources, since this is also essential to life as we know it.

In contrast, Dr. Lingam and Prof. Loeb considered how the existence of life on ocean planets could be dependent on the availability of limiting nutrients (LN). For some time, there has been considerable debate as to which nutrients would be essential to extra-terrestrial life, since these elements could vary from place to place and over timescales. As Lingam told Universe Today via email:

“The mostly commonly accepted list of elements necessary for life as we know it comprises of hydrogen, oxygen, carbon, nitrogen and sulphur. In addition, certain trace metals (e.g. iron and molybdenum) may also be valuable for life as we know it, but the list of bioessential trace metals is subject to a higher degree of uncertainty and variability.”

Artist rendering showing an interior cross-section of the crust of Enceladus, which shows how hydrothermal activity may be causing the plumes of water at the moon’s surface. Credits: NASA-GSFC/SVS, NASA/JPL-Caltech/Southwest Research Institute

For their purposes, Dr. Lingam and Prof. Loeb created a model using Earth’s oceans to determine how the sources and sinks – i.e. the factors that add or deplete LN elements into oceans, respectively – could be similar to those on ocean worlds. On Earth, the sources of these nutrients include fluvial (from rivers), atmospheric and glacial sources, with energy being provided by sunlight.

Of these nutrients, they determined that the most important would be phosphorus, and examined how abundant this and other elements could be on ocean worlds, where conditions as vastly different. As Dr. Lingam explained, it is reasonable to assume that on these worlds, the potential existence of life would also come down to a balance between the net inflow (sources) and net outflow (sinks).

“If the sinks are much more dominant than the sources, it could indicate that the elements would be depleted relatively quickly. In other to estimate the magnitudes of the sources and sinks, we drew upon our knowledge of the Earth and coupled it with other basic parameters of these ocean worlds such as the pH of the ocean, the size of the world, etc. known from observations/theoretical models.”

While atmospheric sources would not be available to interior oceans, Dr. Lingam and Prof. Loeb considered the contribution played by hydrothermal vents. Already, there is abundant evidence that these exist on Europa, Enceladus, and other ocean worlds. They also considered abiotic sources, which consist of minerals leached from rocks by rain on Earth, but would consist of the weathering of rocks by these moons’ interior oceans.

Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Credit: NASA/JPL

Ultimately, what they found was that, unlike water and energy, limiting nutrients might be in limited supply when it comes to ocean worlds in our Solar System:

“We found that, as per the assumptions in our model, phosphorus, which is one of the bioessential elements, is depleted over fast timescales (by geological standards) on ocean worlds whose oceans are neutral or alkaline in nature, and which possess hydrothermal activity (i.e. hydrothermal vent systems at the ocean floor). Hence, our work suggests that life may exist in low concentrations globally in these ocean worlds (or be present only in local patches), and may therefore not be easily detectable.”

This naturally has implications for missions destined for Europa and other moons in the outer Solar System. These include the NASA Europa Clipper mission, which is currently scheduled to launch between 2022 and 2025. Through a series of flybys of Europa, this probe will attempt to measure biomarkers in the plume activity coming from the moon’s surface.

Similar missions have been proposed for Enceladus, and NASA is also considering a “Dragonfly” mission to explore Titan’s atmosphere, surface and methane lakes. However, if Dr. Lingam and Prof. Loeb’s study is correct, then the chances of these missions finding any signs of life on an ocean world in the Solar System are rather slim. Nevertheless, as Lingam indicated, they still believe that such missions should be mounted.

Artist’s concept of a Europa Clipper mission. Credit: NASA/JPL

“Although our model predicts that future space missions to these worlds might have low chances of success in terms of detecting extraterrestrial life, we believe that such missions are still worthy of being pursued,” he said. “This is because they will offer an excellent opportunity to: (i) test and/or falsify the key predictions of our model, and (ii) collect more data and improve our understanding of ocean worlds and their biogeochemical cycles.”

In addition, as Prof. Loeb indicated via email, this study was focused on “life as we know it”. If a mission to these worlds did find sources of extra-terrestrial life, then it would indicate that life can arise from conditions and elements that we are not familiar with. As such, the exploration of Europa and other ocean worlds is not only advisable, but necessary.

“Our paper shows that elements that are essential for the ‘chemistry-of-life-as-we-know-it’, such as phosphorous, are depleted in subsurface oceans,” he said. “As a result, life would be challenging in the oceans suspected to exist under the surface ice of Europa or Enceladus. If future missions confirm the depleted level of phosphorous but nevertheless find life in these oceans, then we would know of a new chemical path for life other than the one on Earth.”

In the end, scientists are forced to take the “low-hanging fruit” approach when it comes to searching for life in the Universe . Until such time that we find life beyond Earth, all of our educated guesses will be based on life as it exists here. I can’t imagine a better reason to get out there and explore the Universe than this!

Further Reading: arXiv

Could There be Alien Life Right Beneath the Surface of Icy Worlds Like Enceladus and Europa?

For decades, scientists have been speculating that life could exist in beneath the icy surface of Jupiter’s moon Europa. Thanks to more recent missions (like the Cassini spacecraft), other moons and bodies have been added to this list as well – including Titan, Enceladus, Dione, Triton, Ceres and Pluto. In all cases, it is believed that this life would exist in interior oceans, most likely around hydrorthermal vents located at the core-mantle boundary.

One problem with this theory is that in such undersea environments, life might have a hard time getting some of the key ingredients it would need to thrive. However, in a recent study – which was supported by the NASA Astrobiology Institute (NAI) – a team of researchers ventured that in the outer Solar System, the combination of high-radiation environments, interior oceans and hydrothermal activity could be a recipe for life.

The study, titled “The Possible Emergence of Life and Differentiation of a Shallow Biosphere on Irradiated Icy Worlds: The Example of Europa“, recently appeared in the scientific journal Astrobiology. The study was led by Dr. Michael Russell with the support of Alison Murray of the Desert Research Institute and Kevin Hand – also a researcher with NASA JPL.

Vestimentiferan tubeworms (Riftia pachyptila) found near the Galapagos islands. Credit: NOAA Okeanos Explorer Program, Galapagos Rift Expedition 2011.

For the sake of their study, Dr. Russell and his colleagues considered how the interaction between alkaline hydrothermal springs and sea water is often considered to be how the key building blocks for life emerged here on Earth. However, they emphasize that this process was also dependent on energy provided by our Sun. The same process could have happened on moon’s like Europa, but in a different way. As they state in their paper:

“[T]he significance of the proton and electron flux must also be appreciated, since those processes are at the root of life’s role in free energy transfer and transformation. Here, we suggest that life may have emerged on irradiated icy worlds such as Europa, in part as a result of the chemistry available within the ice shell, and that it may be sustained still, immediately beneath that shell.”

In the case of moon’s like Europa, hydrothermal springs would be responsible for churning up all the necessary energy and ingredients for organic chemistry to take place. Ionic gradients, such as oxyhydroxides and sulfides, could drive the key chemical processes – where carbon dioxide and methane are hydrogenated and oxidized, respectively – which could lead to the creation of early microbial life and nutrients.

At the same time, the heat from hydrothermal vents would push these microbes and nutrients upwards towards the icy crust. This crust is regularly bombarded by high-energy electrons created by Jupiter’s powerful magnetic field, a process which creates oxidants. As scientists have known for some time from surveying Europa’s crust, there is a process of exchange between the moon’s interior ocean and its surface.

Artist’s concept of plume activity on the surface of Europa. Credit: NASA/JPL-Caltech

As Dr. Russell and his colleagues indicate, this action would most likely involve the plume activity that has been observed on Europa’s surface, and could lead to a network of ecosystems on the underside of Europa’s icy crust:

“Models for transport of material within Europa’s ocean indicate that hydrothermal plumes could be well constrained within the ocean (primarily by the Coriolis force and thermal gradients), leading to effective delivery through the ocean to the ice-water interface. Organisms fortuitously transported from hydrothermal systems to the ice-water interface along with unspent fuels could potentially access a larger abundance of oxidants directly from the ice. Importantly, oxidants might only be available where the ice surface has been driven to the base of the ice shell.”

As Dr. Russel indicated in an interview with Astrobiology Magazine, microbes on Europa could reach densities similar to what has been observed around hydrothermal vents here on Earth, and may bolster the theory that life on Earth also emerged around such vents. “All the ingredients and free energy required for  life are all focused in one place,” he said. “If we were to find life on Europa, then that would strongly support the submarine alkaline vent theory.”

This study is also significant when it comes to mounting future missions to Europa. If microbial ecosystems exist on the undersides of Europa’s icy crust, then they could be explored by robots that are able to penetrate the surface, ideally by traveling down a plume tunnel. Alternately, a lander could simply position itself near an active plume and search for signs of oxidants and microbes coming up from the interior.

Artist’s impression of a hypothetical ocean cryobot (a robot capable of penetrating water ice) in Europa. Credit: NASA

Similar missions could also be mounted to Enceladus, where the presence of hydrothermal vents has already been confirmed thanks to the extensive plume activity observed around its southern polar region. Here too, a robotic tunneler could enter surface fissures and explore the interior to see if ecosystems exist on the underside of the moon’s icy crust. Or a lander could position itself near the plumes and examine what is being ejected.

Such missions would be simpler and less likely to cause contamination than robotic submarines designed to explore Europa’s deep ocean environment. But regardless of what form a future mission to Europa, Enceladus, or other such bodies takes, it is encouraging to know that any life that may exist there could be accessible. And if these missions can sniff it out, we will finally know that life in the Solar System evolved in places other than Earth!

Further Reading: Astrobiology Magazine, Astrobiology

Scientists Find that Earth Bacteria Could Thrive on Enceladus

For decades, ever since the Pioneer and Voyager missions passed through the outer Solar System, scientists have speculated that life might exist within icy bodies like Jupiter’s moon Europa. However, thanks the Cassini mission, scientists now believe that other moons in the outer Solar System – such as Saturn’s moon Enceladus – could possibly harbor life as well.

For instance, Cassini observed plume activity coming from Enceladus’ southern polar region that indicated the presence of hydrothermal activity inside. What’s more, these plumes contained organic molecules and hydrated minerals, which are potential indications of life. To see if life could thrive inside this moon, a team of scientists conducted a test where strains of Earth bacteria were subjected to conditions similar to what is found inside Enceladus.

The study which details their findings recently appeared in the journal Nature Communications under the title “Biological methane production under putative Enceladus-like conditions“. The study was led by Ruth-Sophie Taubner from the University of Vienna, and included members from the Johannes Kepler University Linz, Ecotechnology Austria, the University of Bremen, and the University of Hamburg.

Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Credit: NASA/JPL

For the sake of their study, the team chose to work with three strains of methanogenic archaea known as methanothermococcus okinawensis. This type of microorganism thrives in low-oxygen environments and consumes chemical products known to exist on Enceladus – such as methane (CH4), carbon dioxide (CO2 ) and molecular hydrogen (H2) – and emit methane as a metabolic byproduct. As they state:

“To investigate growth of methanogens under Enceladus-like conditions, three thermophilic and methanogenic strains, Methanothermococcus okinawensis (65 °C), Methanothermobacter marburgensis (65 °C), and Methanococcus villosus (80 °C), all able to fix carbon and gain energy through the reduction of CO2 with H2 to form CH4, were investigated regarding growth and biological CH4 production under different headspace gas compositions…”

These strains were selected because of their ability to grow in a temperature range that is characteristic of the vicinity around hydrothermal vents, in a chemically defined medium, and at low partial pressures of molecular hydrogen. This is consistent with what has been observed in Enceladus’ plumes and what is believed to exist within the moon’s interior.

These types of archaea can still be found on Earth today, lingering in deep-see fissures and around hydrothermal vents. In particular, the strain of M. okinawensis has been determined to exist in only one location around the deep-sea hydrothermal vent field at Iheya Ridge in the Okinawa Trough near Japan. Since this vent is located at a depth of 972 m (3189 ft) below sea level, this suggests that this strain has a tolerance toward high pressure.

Hydrothermal vents on Earth’s ocean floor. Credit: NOAA

For many years, scientists have suspected that Earth’s hydrothermal vents played a vital role in the emergence of life, and that similar vents could exist within the interior of moons like Europa, Ganymede, Titan, Enceladus, and other bodies in the outer Solar System. As a result, the research team believed that methanogenic archaea could also exist within these bodies.

After subjecting the strains to Enceladus-like temperature, pressure and chemical conditions in a laboratory environment, they found that one of the three strains was able to flourish and produce methane. The strain even managed to survive after the team introduced harsh chemicals that are present on Enceladus, and which are known to inhibit the growth of microbes. As they conclude in their study:

“In this study, we show that the methanogenic strain M. okinawensis is able to propagate and/or to produce CH4 under putative Enceladus-like conditions. M. okinawensis was cultivated under high-pressure (up to 50 bar) conditions in defined growth medium and gas phase, including several potential inhibitors that were detected in Enceladus’ plume.”

From this, they determined that some of the methane found in Enceladus’ plumes were likely produced by the presence of methanogenic microbes. As Simon Rittmann, a microbiologist at the University of Vienna and lead author of the study, explained in an interview with The Verge. “It’s likely this organism could be living on other planetary bodies,” he said. “And it could be really interesting to investigate in future missions.”

Artist impression of an interior cross-section of the crust of Enceladus, which shows how hydrothermal activity may be causing the plumes of water at the moon’s surface. Credits: NASA-GSFC/SVS, NASA/JPL-Caltech/Southwest Research Institute

In the coming decades, NASA and other space agencies plan to send multiple mission to the Jupiter and Saturn systems to investigate their “ocean worlds” for potential signs of life. In the case of Enceladus, this will most likely involve a lander that will set down around the southern polar region and collect samples from the surface to determine the presence of biosignatures.

Alternately, an orbiter mission may be developed that will fly through Enceladus’ plumes and collect bioreadings directly from the moon’s ejecta, thus picking up where Cassini left off. Whatever form the mission takes, the discoveries are expected to be a major breakthrough. At long last, we may finally have proof that Earth is not the only place in the Solar System where live can exist.

Be sure to check out John Michael Godier’s video titled “Encedalus and the Conditions for Life” as well:

Further Reading: The Verge, Nature

Icy Worlds Like Europa and Enceladus Might Actually be too Soft to Land On

Some truly interesting and ambitious missions have been proposed by NASA and other space agencies for the coming decades. Of these, perhaps the most ambitious include missions to explore the “Ocean Worlds” of the Solar System. Within these bodies, which include Jupiter’s moon Europa and Saturn’s moon Enceladus, scientists have theorized that life could exist in warm-water interior oceans.

By the 2020s and 2030s, robotic missions are expected to reach these worlds and set down on them, sampling ice and exploring their plumes for signs of biomarkers. But according to a new study by an international team of scientists, the surfaces of these moons may have extremely low-density surfaces. In other words, the surface ice of Europa and Enceladus could be too soft to land on.

The study, titled “Laboratory simulations of planetary surfaces: Understanding regolith physical properties from remote photopolarimetric observations“, was recently published in the scientific journal Icarus. The study was led by Robert M.Nelson, the Senior Scientist at the Planetary Science Institute (PSI) and included members from NASA’s Jet Propulsion Laboratory, the California Polytechnic State University at Pomona, and multiple universities.

Artist’s rendering of a possible Europa Lander mission, which would explore the surface of the icy moon in the coming decades. Credit: NASA/JPL-Caltech

For the sake of their study, the team sought to explain the unusual negative polarization behavior at low phase angles that has been observed for decades when studying atmosphereless bodies. This  polarization behavior is thought to be the result of extremely fine-grained bright particles. To simulate these surfaces, the team used thirteen samples of aluminum oxide powder (Al²O³).

Aluminum oxide is considered to be an excellent analog for regolith found on high aldebo Airless Solar System Bodies (ASSB), which include Europa and Encedalus as well as eucritic asteroids like 44 Nysa and 64 Angelina.  The team then subjected these samples to photopolarimetric examinations using the goniometric photopolarimeter at Mt. San Antonio College.

What they found was that the bright grains that make up the surfaces of Europa and Enceladus would measure about a fraction of a micron and have a void space of about 95%. This corresponds to material that is less dense than freshly-fallen snow, which would seem to indicate that these moon’s have very soft surfaces.  Naturally, this does not bode well for any missions that would attempt to set down on Europa or Enceladus’ surface.

But as Nelson explained in PSI press release, this is not necessarily bad news, and such fears have been raised before:

“Of course, before the landing of the Luna 2 robotic spacecraft in 1959, there was concern that the Moon might be covered in low density dust into which any future astronauts might sink. However, we must keep in mind that remote visible-wavelength observations of objects like Europa are only probing the outermost microns of the surface.”

Enceladus in all its glory. NASA has announced that Enceladus, Saturn’s icy moon, has hydrogen in its oceans. Image: NASA/JPL/Space Science Institute

So while Europa and Enceladus may have surfaces with a layer of low-density ice particles, it does not rule out that their outer shells are solid. In the end, landers may be forced to contend with nothing more than a thin sheet of snow when setting down on these worlds. What’s more, if these particles are the result of plume activity or action between the interior and the surface, they could hold the very biomarkers the probes are looking for.

Of course, further studies are needed before any robotic landers are sent to bodies like Europa and Enceladus. In the coming years, the James Webb Space Telescope will be conducting studies of these and other moons during its first five months in service. This will include producing maps of the Galilean Moons, revealing things about their thermal and atmospheric structure, and searching their surfaces for signs of plumes.

The data the JWST obtains with its advanced suite of spectroscopic and near-infrared instruments will also provide additional constraints on their surface conditions. And with other missions like the ESA’s proposed Europa Clipper conducting flybys of these moons, there’s no shortage to what we can learn from them.

Beyond being significant to any future missions to ASSBs, the results of this study are also likely to be of value when it comes to the field of terrestrial geo-engineering. Essentially, scientists have suggested that anthropogenic climate change could be mitigated by introducing aluminum oxide into the atmosphere, thus offsetting the radiation absorbed by greenhouse gas emissions in the upper atmosphere. By examining the properties of these grains, this study could help inform future attempts to mitigate climate change.

This study was made possible thanks in part to a contract provided by NASA’s Jet Propulsion Laboratory to the PSI. This contract was issued in support of the NASA Cassini Saturn Orbiter Visual and Infrared Mapping Spectrometer instrument team.

Further Reading: Planetary Science Institute, Icarus