PASS Planetarium Activities for Student Success Please send new material for this page to Alan Gould <agould @uclink.berkeley.edu> [space before "@" is antispam device; please remove it when sending e-mail] Martian Gully Mystery Solved? by DAVID TYTELL March 2002, Sky & Telescope magazine, p. 19. PLANETARY SCIENTISTS MAY BE (JOSE TO EXPLAINING HOW THE enigmatic little Martian gullies formed relatively recently. According to research published in Science Express by François Costard and François Forget (University of Paris) and others, changes in Mars's obliquity (polar tilt) can easily lead to the melting of shallow subsurface ice in just the right places. Forget describes how in Jameson Land, Greenland, the tops of thick permafrost layers can thaw in the summer sun. Gullies exactly like those seen on Mars form on steep hillsides as a result. "These gullies don't involve water runoff but are instead debris floss's the melt of subsurface water mixed with rock," says Forget. Mars's obliquity is known to cycle radically between 00 and 60° on a time scale of just hundreds of thousands to a few million years. Currently the planet's axis is tilted 25.2° to its orbit. When the obliquity is greater than 30°, the poles can get enough sunlight to warm the topmost ground layers above freezing. In fact, at times of high obliquity a planet's poles are actually warmer than the equator. This neatly explains why the Martian gullies are found at high latitudes and on poleward-facing slopes, just the opposite of where the Sun shines most today. Costard and Forget's theory argues strongly against several other proposed scenarios. One widely debated idea involves subsurface aquifers that are capped by ice plugs. When a plug on a cliff face cracks, water gushes out, forming a gully. But no one can explain how the aquifers could stay liquid so near the surface, given Mars's very weak geothermal activity. Outflows of thawing carbon dioxide ice have also been proposed. Martian hydrologist Stephen M. Clifford (Lunar and Planetary Institute) has argued for the importance of obliquity changes ever since Michael Malin and Kenneth Fdgctt (Mali Space Science Systems) first announced finding the gullies (S&T: September 2000, page 56). "I think all the other proposed methods [to make gullies] require too many ad hoc assumptions," Clifford says. "[They rely on] things that don't occur in nature)' The obliquity theory isn't flawless, though. Some argue that given Mars's current climate, it would be difficult if not impossible to keep liquid water on the surface for long; it would rapidly boil away in the low atmospheric pressure (barely over 0.65 percent of Earth's sea level atmospheric pressure). But Clifford disagrees." Liquid water is metastable at current Martian conditions for an hour or two," he explains. "It could [flow] hundreds of meters easily." In addition, during a high-obliquity era Mars's atmosphere should be thicker due to the sublimation of polar carbon dioxide ice (see the story below). A thicker atmosphere combined with continuous high polar sunlight could keep water above freezing for days or even months clearly long enough to make lasting erosive marks on the Martian surface. How did the Martian gully features form? Changes in Mars's polar tilt may hold the answer. Prolonged Sun exposure near the poles could have melted the top of a Martian permafrost layer and led to outflows of rubble and mud (left), just as happens in Greenland (right).

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