Artist's conception of gas giant planet HD 209458b in the constellation Pegasus, which has less water vapor in its atmosphere than expected. Credit: NASA, ESA, G. Bacon (STScI) and N. Madhusudhan (UC)
Surprise! Three planets believed to be good candidates for having water vapor in their atmosphere actually have much lower quantities than expected.
The planets (HD 189733b, HD 209458b, and WASP-12b) are “hot Jupiters” that are orbiting very close to their parent star, at a distance where it was expected the extreme temperatures would turn water into a vapor that could be seen from afar.
But observations of the planets with the Hubble Space Telescope, who have temperatures between 816 and 2,204 degrees Celsius (1,500 and 4,000 degrees Fahrenheit), show only a tenth to a thousandth of the water astronomers expected.
“Our water measurement in one of the planets, HD 209458b, is the highest-precision measurement of any chemical compound in a planet outside our solar system, and we can now say with much greater certainty than ever before that we’ve found water in an exoplanet,” stated Nikku Madhusudhan, an astrophysicist at the University of Cambridge, England who led the research. “However, the low water abundance we have found so far is quite astonishing.”
This finding, if confirmed by other observations, could force exoplanet formation theory to be revised and could even have implications for how much water is available in so-called “super-Earths”, rocky planets that are somewhat larger than our own, the astronomers said.
Kepler-62f, an exoplanet that is about 40% larger than Earth. It’s located about 1,200 light-years from our solar system in the constellation Lyra. Credit: NASA/Ames/JPL-Caltech
That theory states that planets form over time as small dust particles stick to each other and grow into larger bodies. As it becomes a planet and takes on an atmosphere from surrounding gas bits, it’s believed that those elements should be “enhanced” in proportion to its star, especially in the case of oxygen. That oxygen in turn should be filled with water.
“We should be prepared for much lower water abundances than predicted when looking at super-Earths (rocky planets that are several times the mass of Earth),” Madhusudhan stated.
The research will be published today (July 24) in the Astrophysical Journal.
Artist's impression (from 2002) of Rosetta orbiting Comet 67P/Churyumov-Gerasimenko. Credit: ESA, image by AOES Medialab
It’s no surprise that there is a lot of water in comets. The “dirty snowballs” (or dusty ice-balls, more accurately) are literally filled with the stuff, so much in fact it’s thought that comets played a major role in delivering water to Earth. But every comet is unique, and the more we learn about them the more we can understand the current state of our Solar System and piece together the history of our planet.
ESA’s Rosetta spacecraft is now entering the home stretch for its rendezvous with comet 67P/Churyumov-Gerasimenko in August. While it has already visually imaged the comet on a couple of occasions since waking from its hibernation, its instruments have now successfully identified water on 67P for the first time, from a distance of 360,000 km — about the distance between Earth and the Moon.
The detection comes via Rosetta’s Microwave Instrument for Rosetta Orbiter, or MIRO, instrument. The results were distributed this past weekend to users of the IAU’s Central Bureau of Astronomical Telegrams:
S. Gulkis, Jet Propulsion Laboratory, California Institute of Technology, on behalf of the Microwave Instrument on Rosetta Orbiter (MIRO) science team, reports that the (1_10)-(1_01) water line at 556.9 GHz was first detected in Comet 67P/Churyumov-Gerasimenko with the MIRO instrument aboard the Rosetta spacecraft on June 6.55, 2014 UT. The line area is 0.39 +/-0.06 K km/s with the line amplitude of 0.48 +/-0.06 K and the line width of 0.76 +/-0.12 km/s. At the time of the observations, the spacecraft to comet distance was ~360,000 km and the heliocentric distance of the comet was 3.93 AU. An initial estimate of the water production rate based on the measurements is that it lies between 0.5 x 10^25 molecules/s and 4 x 10^25 molecules/s.
Although recent images of 67P/C-G seem to show that the comet’s brightness has decreased over the past couple of months, it is still on its way toward the Sun and with that will come more warming and undoubtedly much more activity. These recent measurements by MIRO show that the comet’s water production rate is “within the range of models being used” by scientists to anticipate its behavior.
Rosetta image of Comet 67P/C-G on June 4, 2014, from a distance of 430,000 km. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
This August Rosetta will become the first spacecraft to establish orbit around a comet and, in November, deploy its Philae lander onto its surface. Together these robotic explorers will observe first-hand the changes in the comet as it makes its closest approach to the Sun in August 2015. It’s going to be a very exciting year ahead, so stay tuned for more!
Artist's concept of a hot Jupiter exoplanet orbiting a star similar to tau Boötes (Image used with permission of David Aguilar, Harvard-Smithsonian Center for Astrophysics)
As more and more exoplanets are identified and confirmed by various observational methods, the still-elusive “holy grail” is the discovery of a truly Earthlike world… one of the hallmarks of which is the presence of liquid water. And while it’s true that water has been identified in the thick atmospheres of “hot Jupiter” exoplanets before, a new technique has now been used to spot its spectral signature in yet another giant world outside our solar system — potentially paving the way for even more such discoveries.
Researchers from Caltech, Penn State University, the Naval Research Laboratory, the University of Arizona, and the Harvard-Smithsonian Center for Astrophysics have teamed up in an NSF-funded project to develop a new way to identify the presence of water in exoplanet atmospheres.
Previous methods relied on specific instances such as when the exoplanets — at this point all “hot Jupiters,” gaseous planets that orbit closely to their host stars — were in the process of transiting their stars as viewed from Earth.
This, unfortunately, is not the case for many extrasolar planets… especially ones that were not (or will not be) discovered by the transiting method used by observatories like Kepler.
So the researchers turned to another method of detecting exoplanets: radial velocity, or RV. This technique uses visible light to watch the motion of a star for the ever-so-slight wobble created by the gravitational “tug” of an orbiting planet. Doppler shifts in the star’s light indicate motion one way or another, similar to how the Doppler effect raises and lowers the pitch of a car’s horn as it passes by.
The two Keck 10-meter domes atop Mauna Kea. (Rick Peterson/WMKO)
But instead of using visible wavelengths, the team dove into the infrared spectrum and, using the Near Infrared Echelle Spectrograph (NIRSPEC) at the W. M. Keck Observatory in Hawaii, determined the orbit of the relatively nearby hot Jupiter tau Boötis b… and in the process used its spectroscopy to identify water molecules in its sky.
“The information we get from the spectrograph is like listening to an orchestra performance; you hear all of the music together, but if you listen carefully, you can pick out a trumpet or a violin or a cello, and you know that those instruments are present,” said Alexandra Lockwood, graduate student at Caltech and first author of the study. “With the telescope, you see all of the light together, but the spectrograph allows you to pick out different pieces; like this wavelength of light means that there is sodium, or this one means that there’s water.”
Previous observations of tau Boötis b with the VLT in Chile had identified carbon monoxide as well as cooler high-altitude temperatures in its atmosphere.
Now, with this proven IR RV technique, the atmospheres of exoplanets that don’t happen to cross in front of their stars from our point of view can also be scrutinized for the presence of water, as well as other interesting compounds.
“We now are applying our effective new infrared technique to several other non-transiting planets orbiting stars near the Sun,” said Chad Bender, a research associate in the Penn State Department of Astronomy and Astrophysics and a co-author of the paper. “These planets are much closer to us than the nearest transiting planets, but largely have been ignored by astronomers because directly measuring their atmospheres with previously existing techniques was difficult or impossible.”
Once the next generation of high-powered telescopes are up and running — like the James Webb Space Telescope, slated to launch in 2018 — even smaller and more distant exoplanets can be observed with the IR method… perhaps helping to make the groundbreaking discovery of a planet like ours.
“While the current state of the technique cannot detect earthlike planets around stars like the Sun, with Keck it should soon be possible to study the atmospheres of the so-called ‘super-Earth’ planets being discovered around nearby low-mass stars, many of which do not transit,” said Caltech professor of cosmochemistry and planetary sciences Geoffrey Blake. “Future telescopes such as the James Webb Space Telescope and the Thirty Meter Telescope (TMT) will enable us to examine much cooler planets that are more distant from their host stars and where liquid water is more likely to exist.”
The findings are described in a paper published in the February 24, 2014 online version of The Astrophysical Journal Letters.
Anyone who’s ever seen a map or a globe easily knows that the surface of our planet is mostly covered by liquid water — about 71%, by most estimates* — and so it’s not surprising that all Earthly life as we know it depends, in some form or another, on water. (Our own bodies are composed of about 55-60% of the stuff.) But how did it get here in the first place? Based on current understanding of how the Solar System formed, primordial Earth couldn’t have developed with its own water supply; this close to the Sun there just wouldn’t have been enough water knocking about. Left to its own devices Earth should be a dry world, yet it’s not (thankfully for us and pretty much everything else living here.) So where did all the wet stuff come from?
As it turns out, Earth’s water probably wasn’t made, it was delivered. Check out the video above from MinuteEarth to learn more.
*71% of Earth’s surface, yes, but actually less total than you might think. Read more.
MinuteEarth (and MinutePhysics) is created by Henry Reich, with Alex Reich, Peter Reich, Emily Elert, and Ever Salazar. Music by Nathaniel Schroeder.
UPDATE March 2, 2014: recent studies support an “alien” origin of Earth’s water from meteorites, but perhaps much earlier in its formation rather than later. Read more from the Harvard Gazette here.
UV observations from Hubble show the size of water vapor plumes coming from Europa's south pole (NASA, ESA, and M. Kornmesser)
It’s been known since 2005 that Saturn’s 300-mile-wide moon Enceladus has geysers spewing ice and dust out into orbit from deep troughs that rake across its south pole. Now, thanks to the Hubble Space Telescope (after 23 years still going strong) we know of another moon with similar jets: Europa, the ever-enigmatic ice-shelled moon of Jupiter. This makes two places in our Solar System where subsurface oceans could be getting sprayed directly into space — and within easy reach of any passing spacecraft.
(Psst, NASA… hint hint.)
The findings were announced today during the meeting of the American Geophysical Union in San Francisco.
“The discovery that water vapor is ejected near the south pole strengthens Europa’s position as the top candidate for potential habitability,” said lead author Lorenz Roth of the Southwest Research Institute (SwRI) in San Antonio, Texas. “However, we do not know yet if these plumes are connected to subsurface liquid water or not.”
The 125-mile (200-km) -high plumes were discovered with Hubble observations made in December 2012. Hubble’s Space Telescope Imaging Spectrograph (STIS) detected faint ultraviolet light from an aurora at the Europa’s south pole. Europa’s aurora is created as it plows through Jupiter’s intense magnetic field, which causes particles to reach such high speeds that they can split the water molecules in the plume when they hit them. The resulting oxygen and hydrogen ions revealed themselves to Hubble with their specific colors.
Unlike the jets on Enceladus, which contain ice and dust particles, only water has so far been identified in Europa’s plumes. (Source)
Rendering showing the location and size of water vapor plumes coming from Europa’s south pole.
The team suspects that the source of the water is Europa’s long-hypothesized subsurface ocean, which could contain even more water than is found across the entire surface of our planet.
“If those plumes are connected with the subsurface water ocean we are confident exists under Europa’s crust, then this means that future investigations can directly investigate the chemical makeup of Europa’s potentially habitable environment without drilling through layers of ice,” Roth said. “And that is tremendously exciting.”
One other possible source of the water vapor could be surface ice, heated through friction.
Cassini image of ice geysers on Enceladus (NASA/JPL/SSI)
In addition the Hubble team found that the intensity of Europa’s plumes, like those of Enceladus, varies with the moon’s orbital position around Jupiter. Active jets have been seen only when Europa is farthest from Jupiter. But the researchers could not detect any sign of venting when Europa is closer.
One explanation for the variability is Europa undergoes more tidal flexing as gravitational forces push and pull on the moon, opening vents at larger distances from Jupiter. The vents get narrowed or even seal off entirely when the moon is closest to Jupiter.
Still, the observation of these plumes — as well as their varying intensity — only serves to further support the existence of Europa’s ocean.
“The apparent plume variability supports a key prediction that Europa should tidally flex by a significant amount if it has a subsurface ocean,” said Kurt Retherford, also of SwRI.
(Science buzzkill alert: although exciting, further observations will be needed to confirm these findings. “This is a 4 sigma detection, so a small uncertainly that the signal is just noise in the instruments,” noted Roth.)
“If confirmed, this new observation once again shows the power of the Hubble Space Telescope to explore and opens a new chapter in our search for potentially habitable environments in our solar system.”
– John Grunsfeld, NASA’s Associate Administrator for Science
So. Who’s up for a mission to Europa now?(And unfortunately in this case, Juno doesn’t count.)
“Juno is a spinning spacecraft that will fly close to Jupiter, and won’t be studying Europa,” Kurt Retherford told Universe Today. “The team is looking hard how we can optimize, maybe looking for gases coming off Europa and look at how the plasma interacts with environment, so we really need a dedicated Europa mission.”
We couldn’t agree more.
The findings were published in the Dec. 12 online issue of Science Express.
Image credits: Graphic Credit: NASA, ESA, and L. Roth (Southwest Research Institute and University of Cologne, Germany) Science Credit: NASA, ESA, L. Roth (Southwest Research Institute and University of Cologne, Germany), J. Saur (University of Cologne, Germany), K. Retherford (Southwest Research Institute), D. Strobel and P. Feldman (Johns Hopkins University), M. McGrath (Marshall Space Flight Center), and F. Nimmo (University of California, Santa Cruz)
Curiosity is only eight miles (five kilometers) from a hematite-rich ridge at the bottom of Mount Sharp (Aeolis Mons). Credit: NASA/JPL-Caltech/MSSS
So Curiosity has been on Mars for an Earth year and is now, slowly, making its way over to that ginormous mountain — Mount Sharp, or Aeolis Mons — in the distance. The trek is expected to take at least until mid-2014, if not longer, because the rover will make pit stops at interesting science sites along the way. But far-thinking scientists are already thinking about what areas they would like to examine when it gets there.
One of those is an area that appears to have formed in water. There’s a low ridge on the bottom of the mountain that likely includes hematite, a mineral that other Mars rovers have found. (Remember the “blueberries” spotted a few years ago?) Hematite is an iron mineral that comes to be “in association with water”, a new study reports, and could point the way to the habitable conditions Curiosity is seeking.
The rub is scientists can’t say for sure how the hematite formed until the rover is practically right next to the ridge. There are plenty of pictures from orbit, but not high-resolution enough for the team to make definitive answers.
Colors map percentages of hematite in the surface materials in Meridiani Planum on Mars from 5 percent (aqua) to 25 percent (red). Opportunity landed within the black oval. MER scientists say the rocks there had once been drenched in water. Credit: NASA
“Two alternatives are likely: chemical precipitation within the rocks by underground water that became exposed to an oxidizing environment — or weathering by neutral to slightly acidic water,” wrote Arizona State University’s Red Planet Report. Either way, it shows the ridge likely hosted iron oxidation. Earth’s experience with this type of oxidation shows that it happens “almost exclusively” with microorganisms, but that’s not a guarantee on Mars.
Mars Reconnaissance Orbiter images show that the ridge is about 660 feet (200 meters) wide and four miles (6.5 kilometers) long, with strata or layers in the ridge appearing to be similar to those of layers in Mount Sharp.
While Curiosity is not designed to seek life, it can ferret out details of the environment. Just a few weeks ago, for example, it uncovered pebbles that likely formed in the presence of water. Other Mars missions have also found evidence of that liquid, with perhaps some of it once arising from the subsurface. Where the water came from, and why the environment of Mars changed so much in the last few billion years, are ongoing scientific questions.
Artist's impression of a rocky and water-rich asteroid being torn apart by the strong gravity of the white dwarf star GD 61. Credit: Mark A. Garlick, space-art.co.uk, University of Warwick and University of Cambridge
Remains of a water-filled asteroid are circling a dying white dwarf star, right now, about 150 light-years from us. The new find is the first demonstration of water and a rocky surface in a spot beyond the solar system, researchers say.
The discovery is exciting to the astronomical team because, according to them, it’s likely that water on Earth came from asteroids, comets and other small bodies in the solar system. Finding a watery rocky body demonstrates that this theory has legs, they said. (There are, however, multiple explanations for water on Earth.)
“The finding of water in a large asteroid means the building blocks of habitable planets existed – and maybe still exist – in the GD 61 system, and likely also around substantial number of similar parent stars,” stated lead author Jay Farihi, from Cambridge’s Institute of Astronomy.
Earth’s oxygen and water as detected by Venus Express (ESA)
“These water-rich building blocks, and the terrestrial planets they build, may in fact be common – a system cannot create things as big as asteroids and avoid building planets, and GD 61 had the ingredients to deliver lots of water to their surfaces. Our results demonstrate that there was definitely potential for habitable planets in this exoplanetary system.”
More intriguing, however, is researchers found this evidence in a star system that is near the end of its life. So the team is framing this as a “look into our future”, when the Sun evolves into a white dwarf .
The water likely came from a “minor planet” that was at least 56 miles (90 kilometers) in diameter. Its debris was pulled into the atmosphere of the star, which was then examined by spectroscopy. This study revealed the ingredients of rocks inside the star, including magnesium, silicon and iron. Researchers then compared these elements to how abundant oxygen was, and found that there was in fact more oxygen than expected.
White Dwarf Star
“This oxygen excess can be carried by either water or carbon, and in this star there is virtually no carbon – indicating there must have been substantial water,” stated co-author Boris Gänsicke, from the University of Warwick.
“This also rules out comets, which are rich in both water and carbon compounds, so we knew we were looking at a rocky asteroid with substantial water content – perhaps in the form of subsurface ice – like the asteroids we know in our solar system such as Ceres.”
The measurements were obtained in ultraviolet with the Hubble Space Telescope’s cosmic origins spectrograph. What’s more, the researchers suspect there are giant exoplanets in the area because it would take a huge push to move this object from the asteroid belt — a push that most likely came from big planet.
“This supports the idea that the star originally had a full complement of terrestrial planets, and probably gas giant planets, orbiting it – a complex system similar to our own,” Farihi added.
This question comes from Andrew Bumford and Steven Stormont.
In a previous episode I’ve talked about how the entire Solar System collapsed down from a cloud of hydrogen and helium left over from the Big Bang. And yet, we stand here on planet Earth, with all its water. So, how did that H20 get to our planet? The hydrogen came from the solar nebula, but where did the oxygen come from?
Here’s the amazing part.
The oxygen came from stars that lived and died before our Sun was even born. When those stars puffed out their final breaths of oxygen, carbon and other “metals”, they seeded new nebulae with the raw material for new worlds. We owe our very existence to the dead stars that came before.
When our Sun dies, it’ll give up some of its heavier elements to the next generation of stars. So, mix hydrogen together with this donated oxygen, and you’ll get H20. It doesn’t take any special process or encouragement, when those two elements come together, water is the result.
But how did it get from being spread across the early Solar System to concentrating here on Earth, and filling up our oceans, lakes and rivers? The exact mechanism is a mystery. Astronomers don’t know for sure, but there are a few theories:
Idea #1: impacts. Take a look at the craters on the Moon and you’ll see that the Solar System was a busy place, long ago. Approximately 3.8 to 4.1 billion years ago was the Late Heavy Bombardment period, when the entire inner Solar System was pummeled by asteroids. The surfaces of the planets and their moons were heated to molten slag because of the non-stop impacts. These impactors could have been comets or asteroids.
Comets are 80% water, and would deliver vast amounts of water to Earth, but they’re also volatile, and would have a difficult time surviving the harsh radiation of the young Sun. Asteroids have a lower ratio of water, but they could protect that water a little better, delivering less with each catastrophic impact.
A false-color, visible-light image of Comet ISON taken with Hubble’s Wide Field Camera 3. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Astronomers have also found many hybrid objects which contain large amounts of both rock and water. It’s hard to classify them either way.
Idea #2 is that large amounts of water just came directly from the solar nebula. As we orbited around the young Sun, it passed through the water-rich material in the nebula and scooped it up. Gravitational interactions between the planets would have transferred material around the Solar System, and it would have added to the Earth’s volume of water over hundreds of millions of years.
Of course, it’s entirely possible that the answer is “all of the above”. Asteroids and comets and the early solar nebula all delivered water to the Earth. Where did the Earth’s water come from? Astronomers don’t know for sure. But I’m sure glad the water is here; life here wouldn’t exist without it.
Spitzer image of an asteroid's surprise coma and tail (NASA/JPL-Caltech/DLR/NAU)
It’s a case of mistaken identity: a near-Earth asteroid with a peculiar orbit turns out not to be an asteroid at all, but a comet… and not some Sun-dried burnt-out briquette either but an actual active comet containing rock and dust as well as CO2 and water ice. The discovery not only realizes the true nature of one particular NEO but could also shed new light on the origins of water here on Earth.
JPL Near-Earth Object database map of 3552 Don Quixote’s orbit
Designated 3552 Don Quixote, the 19-km-wide object is the third largest near-Earth object — mostly rocky asteroids that orbit the Sun in the vicinity of Earth.
According to the IAU, an asteroid is coined a near-Earth object (NEO) when its trajectory brings it within 1.3 AU from the Sun and within 0.3 AU of Earth’s orbit.
About 5 percent of near-Earth asteroids are thought to actually be dead comets. Today an international team including Joshua Emery, assistant professor of earth and planetary sciences at the University of Tennessee, have announced that Don Quixote is neither.
An asteroid is coined a near-Earth object (NEO) when its trajectory brings it within 1.3 AU from the Sun and within 0.3 AU of Earth’s orbit. (IAU)
“Don Quixote has always been recognized as an oddball,” said Emery. “Its orbit brings it close to Earth, but also takes it way out past Jupiter. Such a vast orbit is similar to a comet’s, not an asteroid’s, which tend to be more circular — so people thought it was one that had shed all its ice deposits.”
Using the NASA/JPL Spitzer Space Telescope, the team — led by Michael Mommert of Northern Arizona University — reexamined images of Don Quixote from 2009 when it was at perihelion and found it had a coma and a faint tail.
Emery also reexamined images from 2004, when Quixote was at its farthest distance from the Sun, and determined that the surface is composed of silicate dust, which is similar to comet dust. He also determined that Don Quixote did not have a coma or tail at this distance, which is common for comets because they need the sun’s radiation to form the coma and the sun’s charged particles to form the tail.
The researchers also confirmed Don Quixote’s size and the low, comet-like reflectivity of its surface.
“The power of the Spitzer telescope allowed us to spot the coma and tail, which was not possible using optical telescopes on the ground,” said Emery. “We now think this body contains a lot of ice, including carbon dioxide and/or carbon monoxide ice, rather than just being rocky.”
This discovery implies that carbon dioxide and water ice might be present within other near-Earth asteroids and may also have implications for the origins of water on Earth, as comets are thought to be the source of at least some of it.
The amount of water on Don Quixote is estimated to be about 100 billion tons — roughly the same amount in Lake Tahoe.
“Our observations clearly show the presence of a coma and a tail which we identify as molecular line emission from CO2 and thermal emission from dust. Our discovery indicates that more NEOs may harbor volatiles than previously expected.”
– Mommert et al., “Cometary Activity in Near–Earth Asteroid (3552) Don Quixote “
The findings were presented Sept. 10 at the European Planetary Science Congress 2013 in London.
3552 Quixote isn’t the only asteroid found to exhibit comet-like behavior either — check out Elizabeth Howell’s recent article, “Asteroid vs. Comet: What the Heck is 3200 Phaethon?” for a look at another NEA with cometary aspirations.
This set of images from NASA's Cassini mission shows the turbulent power of a monster Saturn storm. The visible-light image in the back, obtained on Feb. 25, 2011, by Cassini's imaging camera, shows the turbulent clouds churning across the face of Saturn. The inset infrared image, obtained a day earlier, by Cassini's visual and infrared mapping spectrometer, shows the dredging up of water and ammonia ices from deep in Saturn's atmosphere. This was the first time water ice was detected in Saturn's atmosphere. Credit: NASA/JPL-Caltech/SSI/Univ. of Arizona/Univ. of Wisconsin
Remember the huge storm that erupted on Saturn in late 2010? It was one of the largest storms ever observed on the ringed planet, and it was even visible from Earth in amateur-sized telescopes. The latest research has revealed the tempestuous storm churned up something surprising deep within Saturn’s atmosphere: water ice. This is the first detection of water ice on Saturn, observed by the near-infrared instruments on the Cassini spacecraft.
“The new finding from Cassini shows that Saturn can dredge up material from more than 100 miles [160 kilometers],” said Kevin Baines, a co-author of the paper who works at the University of Wisconsin-Madison and NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “It demonstrates in a very real sense that typically demure-looking Saturn can be just as explosive or even more so than typically stormy Jupiter.”
While Saturn’s moons have lots of water ice, Saturn is almost entirely hydrogen and helium, but it does have trace amounts of other chemicals, including water. When we look at Saturn, we’re actually seeing the upper cloud tops of Saturn’s atmosphere, which are made mostly of frozen crystals of ammonia.
Beneath this upper cloud layer, astronomers think there’s a lower cloud deck made of ammonium hydrosulfide and water. Astronomers thought there was water there, but not very much, and certainly not ice.
This series of images from NASA’s Cassini spacecraft shows the development of the largest storm seen on the planet since 1990. These true-color and composite near-true-color views chronicle the storm from its start in late 2010 through mid-2011, showing how the distinct head of the storm quickly grew large but eventually became engulfed by the storm’s tail. Credit: NASA/JPL-Caltech/Space Science Institute
But the storm in 2010-2011 appears to have disrupted the various layers, lofting up water vapor from a lower layer that condensed and froze as it rose. The water ice crystals then appeared to become coated with more volatile materials like ammonium hydrosulfide and ammonia as the temperature decreased with their ascent, the authors said.
“The water could only have risen from below, driven upward by powerful convection originating deep in the atmosphere,” said Lawrence Sromovsky, also of the University of Wisconsin, who lead the research team. “The water vapor condenses and freezes as it rises. It then likely becomes coated with more volatile materials like ammonium hydrosulfide and ammonia as the temperature decreases with their ascent.
Big storms appear in the northern hemisphere of Saturn once every 30 years or so, or roughly once per Saturn year. The first hint of the most recent storm first appeared in data from Cassini’s radio and plasma wave subsystem on Dec. 5, 2010. Soon after that, it could be seen in images from amateur astronomers and from Cassini’s imaging science subsystem. The storm quickly grew to superstorm proportions, encircling the planet at about 30 degrees north latitude for an expanse of nearly 300,000 km (190,000 miles).
The researchers studied the dynamics of this storm, and realized that it worked like the much smaller convective storms on Earth, where air and water vapor are pushed high into the atmosphere, resulting in the towering, billowing clouds of a thunderstorm. The towering clouds in Saturn storms of this type, however, were 10 to 20 times taller and covered a much bigger area. They are also far more violent than an Earth storm, with models predicting vertical winds of more than about 300 mph (500 kilometers per hour) for these rare giant storms.
The storm’s ability to churn up water ice from great depths is evidence of the storm’s explosive power, the team said.
Their research will be published in the Sept. 9 edition of the journal Icarus.
