Iapetus His dark materials
SATURN’S mottled moon Iapetus can blame the sun for its blemished complexion and split personality. Iapetus is a yin-yang satellite, with a black belly and a bright back. The two sides of its character were already evident to 17th-century astronomers, but not the source – which has been a mystery ever since.
Now new clues come from images showing that the dark spots appear mainly on the sunward-facing slopes of craters and mountains. This suggests that they are the sun’s handiwork, says the team analysing the images from Cassini. The team released photos of the 1470-kilometre wide moon on Monday at a planetary sciences meeting in Orlando, Florida.
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Dark organic-rich gunk, probably from another moon, spatters the side that faces forward as Iapetus orbits Saturn. So the moon seems to pick up gunk much as dead insects accumulate on your car’s windshield. Since dark stuff reflects less sunlight, Iapetus’s leading hemisphere gets slightly warmer than its trailing hemisphere. Over time, thin layers of ice evaporate, exposing even more of the dark material. The net result, so astronomers figure, is a self-enforcing thermal segregation process that creates and maintains the satellite’s strong bright/dark dichotomy. Now the latest Cassini images, taken during the spacecraft’s close flyby of Iapetus on 10 September back up the idea.
Team leader Carolyn Porco, of the Space Science Institute in Boulder, Colorado, who has been studying the Saturn system since the 1970s, says the team feel sure they are seeing the effects of thermal segregation.
“We got a very much better look at the relation between the black material and the ice. That’s the key on Iapetus, obviously,” Porco says.
It turns out that there are sharp-edged dark spots all over the surface, even on the bright trailing hemisphere of the dinky moon. The Cassini imagery reveals that these spots are preferentially located on the sunward-facing slopes of craters. These slopes get slightly warmer; ice starts to evaporate, and the dark stuff is exposed, ready to retain even more solar heat. “It’s a runaway process,” says Porco.
Unfortunately, knowing that thermal segregation is at work doesn’t tell you anything about the exact nature of the dark stuff. Part of it has apparently been mixed in with the ice for a very long time, while another part has spattered the surface of Iapetus more recently. According to Porco, the presence of dark material on some crater floors suggests that there might be down-slope motion at play, too. “We certainly don’t know all the details yet.”
Enceladus There she blows
IT IS a freezing lump of ice, a mere snowball next to its giant parent. Yet it is geologically active. By rights it is too small at only 500 kilometres across and – with an average surface temperature of -200 °C – way too cold for such activity.
In their quest to solve the mystery, planetary scientists have pinpointed the source of the geyser plumes near the moon’s south pole. They are emanating from the warmest regions of four giant fractures, known as Enceladus’s “tiger stripes”. Originally labelled A, B, C and D, they go by the nicknames Alexandria, Baghdad, Cairo and Damascus. The plumes of water vapour were discovered by Cassini in 2005 and seemed to be related to the tiger stripes, but their precise source was unknown until now.
Joseph Spitale and Carolyn Porco of the Space Science Institute have triangulated the most prominent jets as seen on Cassini images taken from different angles over the past two years. The plumes do indeed come from the four tiger stripes, with the most violent activity occurring in Baghdad and Damascus (Nature, ).
What’s more, the sources of the plumes coincide with the “hottest” areas in the tiger stripes, where Cassini’s spectrometer registered temperatures between -116 and -156 °C.
The finding supports the shear heating model for the satellite’s geological activity: heat is generated by periodic lateral motion along the fractures, resulting from tidal forces as the satellite revolves around Saturn. However, one mystery remains: according to model calculations, Baghdad should have the lowest temperatures, and thus few powerful geysers, but it turns out to be the most active of the four stripes. It seems that Enceladus still has some tricks up its sleeve.
Titan They call it paradise
AT A chilly -179 °C, with a permanent haze blotting out the light of the sun, it’s hardly your typical tropical paradise. Yet that’s how Titan has been described.
The biggest of Saturn’s 60-odd companions, Titan is the only moon in the solar system large enough to have its own atmosphere. Although it is much too cold for liquid water, Raymond Pierrehumbert, a geophysical scientist at the University of Chicago, nevertheless labels Titan’s climate “tropical”, by analogy to the weather system of Earth’s equatorial regions (Proceedings of the National Academy of Sciences, vol 103, p 18,421).
Using observations from Cassini and the Hubble Space Telescope, as well as computer modelling, Pierrehumbert and colleagues have worked out the dynamics of the methane clouds in Titan’s atmosphere. Methane on Titan plays the role of water vapour on Earth, and as a result, the atmospheric processes on both worlds are pretty similar. The latest Cassini images reveal that the circulation processes on Titan mimic the tropical weather systems of our home planet.
On Earth, surface winds from the north and the south meet at the equator, and warm, moist air is pushed upwards, where most of the water vapour condenses into clouds. On Titan the same process occurs with methane, but partly because of the lack of extended oceans, the “tropical” conditions, with methane rain, can form almost anywhere on the surface, depending on Titan’s seasons.
Since Titan only rotates once every 16 days, the circulation pattern can extend all the way to the north and south poles. Earth’s fast rotation confines tropical systems to latitudes between 30 degrees north and south.
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