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Planetary News: Cassini-Huygens (2005)Enceladus' South Polar Stripes Spew "Warm" WaterBy Emily LakdawallaJuly 30, 2005
Most of the solid-surfaced worlds in the Solar System are dormant or dead; the last chapters in their geologic histories were written long ago, and now they are only collecting craters. Until this week, only Earth and Jupiter's moon Io had ever been directly observed in the throes of geologic activity reshaping their surfaces. Now, Saturn's moon Enceladus has joined this select group. The great fissures seaming Enceladus' south pole are hot vents that actively spew water vapor into Enceladus' atmosphere. The discovery was made by nearly all of the Cassini orbiter's instruments acting in concert. Prior to Cassini's arrival, Enceladus had already been marked for particularly close attention during Cassini's orbital tour, thanks to the Voyager discovery that Enceladus had a geologically youthful surface, likely less than 100 million years in age. Also, Enceladus is theorized to be the origin of the particles that compose Saturn's most tenuous E ring. Enceladus was scheduled for three close flybys of less than 1000 kilometers (600 miles) altitude, more close encounters than for any other Saturn moon except Titan. In a statement issued yesterday, the Magnetometer team said: "Although no other instruments on the Cassini spacecraft had detected evidence of this atmosphere on the first two flybys, on the basis of the Magnetometer instrument observations alone a decision was made to modify the spacecraft trajectory for the 14th July encounter to fly much closer to the surface of Enceladus." Originally planned for 1000 kilometers (660 miles), the new flyby altitude was set at only 173 kilometers (107 miles). The new altitude would multiply the resolution of the measurements that Cassini's instruments could make at Enceladus by a factor of nearly 6. "It was really cool the way the story fell together," said Linda Spilker, the Deputy Project Scientist for the Cassini-Huygens mission. "We had the magnetometer data from the previous flyby showing an atmosphere around Enceladus. And once ISS [Imaging Science Subsystem] saw the south pole with no craters, immediately they knew this was one of the youngest regions on Enceladus. So that was intriguing, but by 'young' maybe they meant only as young as 10 or 100 million years."
Not only did ISS observe that the south polar area was young, but they also found it to be colorful. New color mosaics of the south pole showed it to be striped: long, parallel fissures are markedly blue in the ISS enhanced color images. The enhanced color images span a broader region of the electromagnetic spectrum than is visible to the human eye, from ultraviolet to near infrared wavelengths. Throughout the solar system, space weathering processes tend to make ancient surfaces grayer and redder over time. Spectrally bluer materials may also imply youth. In this case, Spilker said, "the blue color in those images was indicative of larger grain sizes. The analogy they are making is glacial blue ice, where you can see more deeply into the ice" because the ice grains are larger. "And then of course we have those pictures that showed a very rough surface at the south pole, which was surprising." So far, the new data was intriguing, but "no one at that point would have predicted or even guessed that we’d see thermal activity on Enceladus. Then the UVIS team had a stellar occultation, watching as the star [Bellatrix] went behind Enceladus, and they saw evidence of an atmosphere," something they had not achieved in the previous flyby. But there was something strange and exciting about the UVIS team's atmosphere detection. The geometry of the occultation was like this: as UVIS watched the star disappear behind Enceladus, the star traveled behind Enceladus' south polar region, from Cassini's point of view. After Cassini had traveled across Enceladus and watched Bellatrix reappear, Cassini was now looking at a latitude nearer Enceladus' equator. "They saw evidence of an atmosphere at ingress, at -80 degrees latitude, but none at egress, which was up near the equator," Spilker said. "So there was an indication that the atmosphere was not global, but was localized at the south pole. They also got spectra at the same time as they watched the star, and they saw water, H2O, signature in their data." So UVIS had detected an atmosphere at Enceladus, one that was made of water vapor -- but only at the south pole. That detection was confirmed by another instrument, the Ion Neutral Mass Spectrometer (INMS). INMS scoops up particles as Cassini travels. Although INMS was not pointed in its preferred "ram" direction to scoop up the most particles -- the pointing of the spacecraft was determined by UVIS -- "they still noticed that on the ingress part of the path they collected an order of magnitude more atmosphere than they did on egress, and they also saw water molecules in their data. And possibly something even heavier, around mass 28, maybe nitrogen or carbon monoxide coming out of Enceladus." A different in-situ instrument, the Cosmic Dust Analyzer (CDA), had measured Saturn's E Ring particles during previous trips through the ring plane. The vaporous atmosphere detected by UVIS and INMS does not match the particulate nature of the E ring, Spilker said. "The water vapor is very different from the E ring particles themselves. So we have this sort of cloud, patchy atmosphere over the south pole, and then the E ring particles seem to be coming uniformly off of Enceladus, probably through micrometeorite impact kicking up particles. So the vents are not the source of the E ring." In the meantime, the Composite Infrared Spectrometer (CIRS) instrument had also taken data. CIRS, UVIS, and ISS are collectively referred to as Cassini's Optical Remote Sensing instruments. They are all located on one pallet on Cassini and all point in the same direction, so they are able to capture concurrent measurements of the same target. CIRS measures thermal radiation, infrared wavelengths that are emitted by all objects as they radiate away their internal heat. Even very cold bodies like icy moons in the outer solar system emit heat, and CIRS can see that.
What CIRS saw at Enceladus was a stunner. The team expected to see a relatively homogeneous temperature map, warmest where the Sun's rays strike the moon most directly, and coolest in the night side and at the poles. But CIRS found a whopping hot spot at the south pole of Enceladus. "It was a complete surprise," Spilker said. But an even greater surprise was in store from data captured closer in. "The hottest features in the CIRS data appear to correspond with the tiger stripes. There is clear evidence that the hottest footprint straddles one of those tiger stripes." Each CIRS footprint is relatively large, spanning a 6 kilometer (4 mile) square area. But the tiger stripes are much narrower, only 1 to 2 kilometers wide, and their central fissures are narrower than that. If the CIRS temperature measurements represent an average of the temperature within the footprint, the temperature at the point of the fissure could be much higher. The official NASA statement said the temperature could be as high as 110 Kelvin, 30 Kelvin warmer than the area outside the fissures. "I think they were conservative in the press release," Spilker stated. "Once UVIS saw the CIRS data, they confirmed they saw warmer water coming out. From the UVIS data they could be seeing something 140 or even greater. And it’s probably cooling as it comes up the crack, so we’re probably not seeing the hottest temperature." 140 Kelvin is still very, very cold by human standards, but it is very close to a critical temperature for icy satellites. "170 Kelvin is what they call the water-ammonia eutectic. It means that water can exist as a liquid if it has ammonia mixed in. So anything from 170 up to 273 you can have a source of liquid water." In other words, if there is a significant quantity of ammonia mixed in to Enceladus' composition, the presence of that ammonia could lower the melting temperature of water to 170 Kelvin. If Enceladus' fissures are spewing water vapor at 140 Kelvin, could it be 170 Kelvin below the surface? The high temperatures observed by CIRS in the tiger stripes open up the exciting possibility that there is a reservoir of liquid, or at least slushy, water-ammonia below Enceladus' south pole. The hot source is forcing water vapor out of vents, forming a tenuous atmosphere at Enceladus' south pole. The discovery is thrilling. Recent activity on Enceladus has been considered a possibility since Voyager, but there is no known explanation for how a body as small as Enceladus can generate enough internal heat to drive active geology. Spilker grappled with the problem: "Radiogenic heating is impossible for a body the size of Enceladus, there might not be much heat left over -- and tidal heating, from what we understand about how the tidal heating works, that’s not enough." But there's hope for people who want to explain Enceladus, she said. "I think the key here is that it’s not a global heat source, it’s really concentrated at the south pole. And that might make the problem a little easier in that you only need a localized heat source. How it got there, why it’s still there, -- people are busy scratching their heads!" Cassini has years of work ahead; the next and final close encounter with Enceladus does not take place until 2008. But Spilker is elated with the way that the science teams worked together to make this discovery, and thinks it bodes well for the future of the mission. "It was neat to see the in-situ instruments and the optical remote sensing instruments working together to build this picture. CIRS saw the warm pole, but it took UVIS and INMS to see the atmosphere to reveal it was water and at the south pole, and the magnetometer also saw the atmosphere. It's fun to see everyone working together." |
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