Cassini Finds “Heat” and More Geysers on Enceladus


Newly released images from last November’s close flyby over Saturn’s icy moon Enceladus the Cassini spacecraft reveal geyser jets spraying all along the prominent fractures, or “tiger stripes” that cross the moon’s south polar region. Additionally, a new detailed temperature map of one fracture reveals warmer temperatures than what was expected. “Enceladus continues to astound,” said Bob Pappalardo, Cassini project scientist at the Jet Propulsion Laboratory. “With each Cassini flyby, we learn more about its extreme activity and what makes this strange moon tick.”

The new images from the imaging science subsystem and the composite infrared spectrometer teams include the best 3-D image ever obtained of a tiger stripe fissure that sprays icy particles, water vapor and organic compounds. There are also views of regions not well-mapped previously on Enceladus, including a southern area with crudely circular tectonic patterns.

In this unique mosaic image combining high-resolution data from the imaging science subsystem and composite infrared spectrometer aboard NASA's Cassini spacecraft, pockets of heat appear along one of the mysterious fractures in the south polar region of Saturn's moon Enceladus. Image credit: NASA/JPL/GSFC/SWRI/SSI

For Cassini’s visible-light cameras, the Nov. 21, 2009 flyby provided the last look at Enceladus’ south polar surface before that region of the moon goes into 15 years of darkness, and includes the most detailed look yet at the jets.

Scientists planned to use this flyby to look for new or smaller jets not visible in previous images. In one mosaic, scientists count more than 30 individual geysers, including more than 20 that had not been seen before. At least one jet spouting prominently in previous images now appears less powerful.

“This last flyby confirms what we suspected,” said Carolyn Porco, imaging team lead based at the Space Science Institute in Boulder, Colo. “The vigor of individual jets can vary with time, and many jets, large and small, erupt all along the tiger stripes.”

A new map that combines heat data with visible-light images shows a 40-kilometer (25-mile) segment of the longest tiger stripe, known as Baghdad Sulcus. The map illustrates the correlation, at the highest resolution yet seen, between the geologically youthful surface fractures and the anomalously warm temperatures that have been recorded in the south polar region. The broad swaths of heat previously detected by the infrared spectrometer appear to be confined to a narrow, intense region no more than a kilometer (half a mile) wide along the fracture.

In these measurements, peak temperatures along Baghdad Sulcus exceed 180 Kelvin ( – 92 C, -135 F), and may be higher than 200 Kelvin (- 73 C, -100 F). These warm temperatures probably result from heating of the fracture flanks by the warm, upwelling water vapor that propels the ice-particle jets seen by Cassini’s cameras. Cassini scientists will be testing this idea by investigating how well the hot spots correspond with the jet sources.

“The fractures are chilly by Earth standards, but they’re a cozy oasis compared to the numbing 50 Kelvin (-223 C, -370 F) of their surroundings,” said John Spencer, a composite infrared spectrometer team member based at Southwest Research Institute in Boulder, Colo. “The huge amount of heat pouring out of the tiger stripe fractures may be enough to melt the ice underground. Results like this make Enceladus one of the most exciting places we’ve found in the solar system.”

Some of Cassini’s scientists infer that the warmer the temperatures are at the surface, the greater the likelihood that jets erupt from liquid. “And if true, this makes Enceladus’ organic-rich, liquid sub-surface environment the most accessible extraterrestrial watery zone known in the solar system,” Porco said.

The Nov. 21 flyby was the eighth targeted encounter with Enceladus. It took the spacecraft to within about 1,600 kilometers (1,000 miles) of the moon’s surface, at around 82 degrees south latitude.

Source: JPL

Are We Just ‘Lucky’ to See Activity on Enceladus?

Caption: Geysers on Enceladus. Credit: NASA, JPL, Space Science Institute

One of the most exciting but unexpected discoveries of the Cassini mission is seeing the activity taking place on Saturn’s small moon Enceladus. Between the active geysers, the unusual “tiger stripes” and the surprisingly young surface near the moon’s south pole, Enceladus has surprised scientists with almost all the images and data the gathered by the spacecraft. But is the moon always active, or are we just in the right place at the right time, lucky to be catching it during an active phase? A recent paper outlines a model in which the kind of geologic eruptions now visible on Enceladus only occur every billion years or so.

“Cassini appears to have caught Enceladus in the middle of a burp,” said Francis Nimmo, a planetary scientist at the University of California Santa Cruz. “These tumultuous periods are rare and Cassini happens to have been watching the moon during one of these special epochs.”

Nimmo and co-author Craig O’Neill of Macquarie University in Sydney, Australia propose that blobs of warm ice that periodically rise to the surface and churn the icy crust on Saturn’s moon Enceladus explain the quirky heat behavior and intriguing surface of the moon’s south polar region.

The most interesting features by far in the south polar region of Enceladus are the fissures known as “tiger stripes” that spray water vapor and other particles out from the moon. While Nimmo and O’Neill’s model doesn’t link the churning and resurfacing directly to the formation of fissures and jets, it does fill in some of the blanks in the region’s history.

Enceladus. Credit: NASA/JPL/Space Science Institute

“This episodic model helps to solve one of the most perplexing mysteries of Enceladus,” said Bob Pappalardo, Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., of the research done by his colleagues. “Why is the south polar surface so young? How could this amount of heat be pumped out at the moon’s south pole? This idea assembles the pieces of the puzzle.”

But not everyone is convinced this model answers all the questions about Enceladus. Carolyn Porco, who leads the imaging team for Cassini said via Twitter regarding this paper, “Beware! Several different models out there say different things.”

About four years ago, Cassini’s composite infrared spectrometer instrument detected a heat flow in the south polar region of at least 6 gigawatts, the equivalent of at least a dozen electric power plants. This is at least three times as much heat as an average region of Earth of similar area would produce, despite Enceladus’ small size. The region was also later found by Cassini’s ion and neutral mass spectrometer instrument to be swiftly expelling argon, which comes from rocks decaying radioactively and has a well-known rate of decay.

Calculations told scientists it would be impossible for Enceladus to have continually produced heat and gas at this rate. Tidal movement – the pull and push from Saturn as Enceladus moves around the planet – cannot explain the release of so much energy.

The surface ages of different regions of Enceladus also show great diversity. Heavily cratered plains in the northern part of the moon appear to be as old as 4.2 billion years, while a region near the equator known as Sarandib Planitia is between 170 million and 3.7 billion years old. The south polar area, however, appears to be less than 100 million years old, possibly as young as 500,000 years.

O’Neill had originally developed the model for the convection of Earth’s crust. For the model of Enceladus, which has a surface completely covered in cold ice that is fractured by the tug of Saturn’s gravitational pull, the scientists stiffened up the crust. They picked a strength somewhere between that of the malleable tectonic plates on Earth and the rigid plates of Venus, which are so strong, it appears they never get sucked down into the interior.

These drawings depict explanations for the source of intense heat that has been measured coming from Enceladus' south polar region. Credit: NASA/JPL

Their model showed that heat building up from the interior of Enceladus could be released in episodic bubbles of warm, light ice rising to the surface, akin to the rising blobs of heated wax in a lava lamp. The rise of the warm bubbles would send cold, heavier ice down into the interior. (Warm is, of course, relative. Nimmo said the bubbles are probably just below freezing, which is 273 degrees Kelvin or 32 degrees Fahrenheit, whereas the surface is a frigid 80 degrees Kelvin or -316 degrees Fahrenheit.)

The model fits the activity on Enceladus when the churning and resurfacing periods are assumed to last about 10 million years, and the quiet periods, when the surface ice is undisturbed, last about 100 million to two billion years. Their model suggests the active periods have occurred only 1 to 10 percent of the time that Enceladus has existed and have recycled 10 to 40 percent of the surface. The active area around Enceladus’s south pole is about 10 percent of its surface.

Source: JPL

Mystery Solved? New Clues Point to a Liquid Ocean on Enceladus.


A liquid plume is spewing from Saturn’s icy moon Enceladus — but is it coming from heated ice on the surface, or a liquid ocean underneath?

Analysis of the plume’s chemistry, detailed in the Cassini (CICLOPS) image above and reported in Nature this week, may put the debate to rest.

Enceladus. Credit: CICLOPS
Enceladus. Credit: CICLOPS

Lead author Jack Hunter (J.H.) Waite, of the Southwest Research Institute in San Antonio, Texas and his colleagues say ammonia detected in the jets from Enceladus’ south pole provides the strongest evidence yet for the existence of liquid water beneath the surface.

A previous paper led by Frank Postberg of the University of Heidelberg in Germany, published in Nature just last month, reported the discovery of salts in E-ring particles derived from the plume, also suggestive of a liquid reservoir.

But Susan Kieffer of the University of Illinois at Urbana–Champaign and her colleagues proposed in a 2006 Science paper that warm ice is heated near the surface, causing dissociation of clathrate hydrates. And Nicholas Schneider, of the University of Colorado at Boulder, and his colleagues published a paper in the same Nature issue as Postberg’s team (June 24) — reporting that there’s not enough sodium in the plume to support a liquid ocean.

The ammonia may tip the scales, say the authors of the new paper.

“The presence of ammonia provides strong evidence for the existence of at least some liquid water, given that temperatures in excess of 180K have been measured near the fractures from which the jets emanate,” the authors write. “We conclude, from the overall composition of the material, that the plume derives from both a liquid reservoir (or from ice that in recent geological time has been in contact with such a reservoir) as well as from degassing, volatile-charged ice.”

Besides ammonia, the authors detected various organic compounds and deuterium — ‘heavy’ hydrogen abundant in the oceans of Earth. Ammonia, together with methanol and salts, acts as an antifreeze, allowing liquid water to exist at below-freezing temperatures. The authors suggest that preserving even a residual oceanic layer during cooling episodes would maintain conditions necessary for tidal heating and geologic activity.

Enceladus is one of only three moons in the Solar System known to be volcanically active. The plume of gas and particles is thought to make up Saturn’s outermost ‘E’ ring.

UT ran a story last month, when Nature ran two papers with different ideas about whether Enceladus harbors a liquid ocean. See that story here.

Source for text: Nature. Source for images: Cassini Imaging Central Laboratory for Operations (CICLOPS), with thanks to study co-author William Lewis for the tip.

Does Enceladus Harbor a Liquid Ocean? Reasonable Minds Disagree


Two papers in the journal Nature this week come down on opposite sides of the question about whether Saturn’s moon Enceladus contains a salty, liquid ocean.

One research team, from Europe, says an enormous plume of water spurting in giant jets from the moon’s south pole is fed by a salty ocean. The other group, led out of the University of Colorado at Boulder, contends that the supposed geysers don’t have enough sodium to come from an ocean.  The truth could have implications for the search for extraterrestrial life, as well as our understanding of how planetary moons are formed.

The Cassini spacecraft first spotted the plume on its exploration of the giant ringed planet in 2005. Enceladus ejects water vapor, gas and tiny grains of
ice into space hundreds of kilometers above the moon’s surface.

The moon, which orbits in Saturn’s outermost “E” ring, is one of only
three outer solar system bodies that produce active eruptions of dust
and vapor. Moreover, aside from the Earth, Mars, and Jupiter’s moon
Europa, it is one of the only places in the solar system for which
astronomers have direct evidence of the presence of water.

The European researchers, led by Frank Postberg of the University of Heidelberg in Germany, are reporting  the detection of sodium salts among the dust ejected in the Enceladus plume. Postberg and colleagues have studied data from the Cosmic Dust Analyzer (CDA) onboard the Cassini
spacecraft and have combined the data with laboratory experiments.

They say the icy grains in the Enceladus plume contain
substantial quantities of sodium salts, hinting at the salty ocean
deep below.

The results of their study imply that the concentration of sodium chloride in the ocean can be as high as that of Earth’s oceans and is about 0.1-0.3 moles of salt per kilogram of water.

But the Colorado study suggests a different interpretation.

Nicholas Schneider, of CU-Boulder’s Laboratory for Atmospheric and Space Physics, and his colleagues say high amounts of sodium in the plume should give off the same yellow light that comes off street lights, and that the world’s best telescopes can detect even a small number of sodium atoms orbiting Saturn.

Schneider’s team usied the 10-meter Keck 1 telescope and the 4-meter Anglo-Australian telescope, and demonstrated that few if any sodium atoms existed in the water vapor. “It would have been very exciting to support the geyser hypothesis. But it is not what Mother Nature is telling us,” said Schneider.

One suggested explanation for the contrasting results, said Schneider, is that deep caverns may exist where water evaporates slowly. When the evaporation process is slow the vapor contains little sodium, just like water evaporating from the ocean. The vapor turns into a jet because it leaks out of small cracks in the crust into the vacuum of space.

“Only if the evaporation is more explosive would it contain more salt,” he said. “This idea of slow evaporation from a deep cavernous ocean is not the dramatic idea that we imagined before, but it is possible given both our results so far.”

But Schneider also cautions that several other explanations for the jets are equally plausible. “It could still be warm ice vaporizing away into space. It could even be places where the crust rubs against itself from tidal motions and the friction creates liquid water that would then evaporate into space,” he said.

“These are all hypotheses but we can’t verify any one with the results so far,” said Schneider. “We have to take them all with, well, a grain of salt.”

Lead photo caption: Image of Enceladus from Cassini. Credit: NASA/JPL/Space Science Institute

Sources: Press releases from CU Boulder and the University of Leicester, via Nature and Eurekalert (a news service through the American Association for the Advancement of Science).

Book Your Tours of Titan and Enceladus Today!

Looking to go somewhere far-flung and exotic? Now is the time to book your excursion, and the Cassini spacecraft has several flyby tours of the moons Titan and Enceladus scheduled for the next few months. Major tour operators say the cost of long-haul flights and summer holidays prices are at an all time low. But with Cassini, you can travel for FREE! just by following the along with Universe Today and the Cassini website. Thrill with some of the closest flybys ever of the mystery moon Titan, and delight in explorations of the geyser plumes of Enceladus. As a special bonus, if you book today, you can experience Saturn’s solar equinox, as in August the sun crosses from the southern hemisphere to the north. Wonderful worlds are beckoning – come away starting June 6 with Cassini! It’s a worry free vacation. See below for available tour dates! Destination fees do not apply.

Seriously now, here are the upcoming dates scheduled for Cassini flybys of Titan and Enceladus:

Dates listed in Spacecraft Event Time (SCET) — the time the event happens at the spacecraft based on Coordinated Universal Time (UTC).
Click here for details about time conversions.

June 6 — Titan flyby (965 kilometers) — T-56: This is the only dusk side observation at mid Southern latitudes, and the Ion and Neutral Mass Spectrometer (INMS) will be taking advantage of that, riding along with Cassini’s RADAR at closest approach. It’s the only time in the mission the spacecraft will get simultaneous coverage of the dusk side while in the wake magnetospheric interaction region. The Visual and Infrared Mapping Spectrometer (VIMS) and the Imaging Science Subsystem ISS will observe eastern Tsegihi, a bright region in Titan’s southern mid-latitudes.

June 22 –– Titan flyby (955.5) kilometers — T-57: RADAR and INMS again share prime opportunities near closest approach. The RADAR synthetic aperture radar (SAR) imager observation runs parallel to observations in the T-55 and T-56 flybys in the southern hemisphere mapping sequence. Earlier, RSS observes an occultation on the inbound leg. T-57 is another flank-out, post-dusk flyby, with a minimum altitude of about 1000 kilometers. As in T-55 and T-56 flybys, magnetometer measurements will provide a description of the draping and the pileup of the external magnetic field around Titan on the nightside hemisphere. The flyby will also be a good complement to previous flybys in order to characterize the background field for a similar local time with respect to Saturn and different SKR (Saturn kilometric radiation) longitudes.

July 8 — Titan flyby (965 kilometers) — T-58: The Ultraviolet Imaging Spectrograph UVIS observes a solar occultation while inbound towards Titan, and then a stellar occultation on the spacecraft’s outbound trajectory. RADAR’s SAR swath runs along the western edge of Xanadu to study its boundary with Shangri-La, a large equatorial dark region. The swath runs parallel to the T-55/56/57 mapping sequence and covers Ontario Lacus, a methane-ethane lake near the south pole of Titan.

July 24 — Titan flyby (955 kilometers) — T-59: The spacecraft’s instruments sample Titan’s southern mid-latitudes, with the Cassini Plasma Spectrometer (CAPS) controlling pointing at closest approach.

Aug. 9 –– Titan flyby (970) kilometers — T-60: RADAR takes a South pole pass. The resulting swath links up with the T-13 flyby swath at the edge of Xanadu, an Australia-sized, bright region on Titan. ISS will acquire high-resolution, low-phase-angle imaging of western Senkyo, a wide dark region near the equator.

Aug. 11 –– Saturn will go through the solar Equinox as the Sun crosses from the southern hemisphere to the north. For about two months on either side of that date rings scientists will be running an Equinox campaign to observe the rings in this season change.

The Cassini team will be watching for topographic features in the rings that can only be seen in this special geometry. Any features in the rings that are not exactly in the ‘ring plane’ will be seen to cast shadows.

Cassini scientists will also be looking at the thermal properties of the rings in this season change. Most rings have seen heating on the north side over the past 14.5 years, but the B ring’s densest portions have remained cold with no solar heat penetrating that ring for the past 14.5 years.

This Rings Equinox campaign is a unique opportunity provided by the long duration of the Cassini Mission.

Aug. 25 — Titan flyby (970 kilometers) — T-61: RADAR gathers a SAR swath over the Huygens landing site. The swath is near-equatorial, covering Dilmun, Adiri and Belet. As the SAR parallels and overlaps the T-8 flyby, this should provide a good stereo opportunity over the Belet sand dunes. T-61 is the only southern equatorial wake observation in the mission, so the Magnetosphere and Plasma Science (MAPS) instruments take advantage of the opportunity.

Oct. 12 — Titan flyby (1,300 kilometers) — T-62: This flyby offers excellent VIMS and UVIS observing opportunities, including a UVIS solar occultation that reaches down to Titan’s surface. CIRS takes observations while Titan is in eclipse, measuring the temperature, aerosol density and composition near 75 South. This is the only low altitude in nose side magnetospheric interaction pass in the extended mission.

Nov. 2 — Enceladus flyby (99 kilometers) — 120EN: This is the seventh targeted Enceladus flyby of the Cassini mission and will take the spacecraft to the lowest altitude above the active south pole region. This will also be the deepest plume passage of the tour, allowing for sensitive measurements of the geyser-like plume composition and density.

Nov. 21 — Enceladus flyby (1,603 kilometers) — 121EN: The eighth targeted Enceladus flyby, this is an approximate 1600 kilometer pass over the south pole enabling imaging of the warm, active tiger stripes.

Dec. 12 — Titan flyby (4,850 kilometers) — T-63: CAPS takes advantage of the T-63 flyby being the best wake passage in the extended mission to direct pointing at closest approach.

Dec. 28 — Titan flyby (955 kilometers) — T-64: RADAR captures HiSAR SAR over the North polar lakes to perform stereo and/or seasonal change detection. This is the only north polar SAR in the extended mission. Due to the location of the point of closest approach, this is a potentially important flyby in the effort to detect an intrinsic magnetic field within Titan. This is also an opportunity to sample the high northern atmosphere.

Source: Cassini website

Salt in Enceladus’ Geysers Hints at Subsurface Liquid Ocean

Planetary scientists say the geysers shooting from Saturn’s moon Enceladus are likely to come from a subsurface sea of liquid water. During the Cassini spacecraft’s fly-through of the geyser’s plume on October 9, 2008, the instruments on board were able to measure the molecular weight of the chemicals in the ice. Detected were traces of sodium in the form of salt and sodium bicarbonate. The chemicals would have originated in the rocky core of Enceladus, so to reach a plume they must have leached from the core via liquid water.

Frank Postberg of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, and colleagues, are presenting their findings at the European Geophysical Union meeting in Vienna this week.

Although the salt could have been leached out by an ancient ocean which since been frozen solid, that freezing process would concentrate most of the salt very far from the surface of the moon’s ice, says Julie Castillo of NASA’s Jet Propulsion Laboratory in Pasadena, California, quoted in New Scientist. “It is easier to imagine that the salts are present in a liquid ocean below the surface,” she says. “That’s why this detection, if confirmed, is very important.”

Observations of the plumes from Earth in 2007 detected no sign of sodium, casting doubt on such a subsurface sea. But the new in-situ detection may change our understanding of Enceladus’ geysers.

Source: New Scientist

Cassini “Skeet Shoot” of Enceladus a Success

Scientists for the Cassini mission called their flyby of Saturn’s small moon Enceladus on August 11 a “skeet shoot,” partially in honor of the current Olympic games underway, but mostly because the spacecraft would be trying to shoot rapidly at the moon with its array of cameras and scientific instruments. As the images begin to stream back, the scientists are definitely excited about what they’re seeing.

“What a dazzling success!” said Carolyn Porco, the Cassini Imaging Team Leader. “There doesn’t even appear to be any smear.” Scientists compared Cassini’s fast flyby of Enceladus to trying to capture a sharp, unsmeared picture of a roadside billboard about a mile away with a 2,000 mm telephoto lens held out the window of a car moving at 50 mph. The imaging team is still poring over the pictures to see if they were successful in “shooting” their target: the active vent regions on the tiger stripe-like features on the moon’s south pole that create the geysers on Enceladus. But the amazingly clear images show a fractured surface littered with boulders and what Porco said could possibly be ice blocks.

Cassini flew over the surface of Enceladus at tremendous speed; about 18 km/sec (about 40,000 mph), which makes taking clear images very difficult. The imaging team devised a technique of turning the spacecraft while taking pictures in rapid succession, shooting at seven, very high priority surface targets. The suite of images ranged in resolution from 8 to 28 meters/pixel, using exposure times that were long enough to see the surface in the twilight near the terminator yet short enough to avoid smear.

Overview of Cassini's flyby.  Credit: NASA/JPL
Overview of Cassini's flyby. Credit: NASA/JPL

The tiger stripes, officially called sulci, have been identified by the imaging cameras on earlier flybys of Enceladus as the sources of the jets, and also as the “hot spots” or warmer areas on the moon identified by the Cassini’s Composite Infrared Spectrograph.

Region "7" of the Cassini's skeet shoot of Enceladus.  Credit:  NASA/JPL

Porco said the team still has much work to do to decipher all the information in the images and data from the other instruments. “In this painstaking work, we proceed, step by step, to lay bare those things which hold the greatest promise of comprehension, the greatest significance for piecing together the story of the origins of the bodies in our solar system, our Earth, and indeed ourselves,” she wrote in her blog.

We’ll provide further updates on the flyby images as information becomes available.

Sources: Cassini’s website, NASA blogs, CICLOPS flyby preview