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] Mars's Active Snow and Ice March 2002, Sky & Telescope, p. 18. IMAGES RETURNED BY THE MARS GLOBAL SURVEYOR (MGS) orbiter suggest that the Martian polar caps and perhaps Mars's entire atmosphere and climate may be surprisingly unstable on a time scale of just a few centuries or thousands of years. Astronomers analyzed images of the south polar region and found ice cliffs retreating surprisingly fast. The speed of their shrinkage, given the temperatures and amounts of sunlight in the region, confirms that the material is frozen carbon dioxide (CO2). Water ice could not sublimate (evaporate directly from solid to gas) nearly so fast. Moreover, the observations confirm something astronomers have long suspected: the south cap contains so much frozen CO, that it serves as a reservoir regulating the pressure of Mars's entire atmosphere year round. Nothing of the sort exists on Earth. "The fact that we see the reservoir means that not all the carbon dioxide that can be in the atmosphere is in the atmosphere today," says Michael C. Mali (Mali Space Science Systems). "This means that the climate is really dynamic and changing with time?" Pits in Mars's south polar cap widened noticeably from October 1999 (top panels) to August 2001 (bottom panels). Their walls, several meters high, retreated by up to 3 meters during one Martian year, causing small ridges and outcrops to narrow or disappear. So much change from one year to the next suggests that the polar cap is out of equilibrium with the atmosphere, at least for now. A quarter to half of the ice escarpments in the south cap, Mali and his colleagues found, retreated by 1 to 3 meters during a Martian year. The eroding areas failed to recover during the winter, a sign of year-to-year climate variability and possibly longer-term climate change. In fact, if the loss of the south polar cap were to continue at the same annual rate, the whole cap would he gone in a few thousand years and the Martian atmosphere would become twice as dense and more able to trap solar heat. Such instability suggests that even small changes in temperatures or seasonal patterns, such as would he caused by changes in Mars's obliquity (the tilt of its axis), may he enough to set off runaway climate changes large enough to allow liquid water to carve gullies. Malin's team published their work in the December 7th Science. Meanwhile, another group publishing in the same issue of Science has tracked the seasonal cycle of ice buildup and loss all across the Martian high latitudes. David F. Smith (NASA/Goddard Space Flight Center) and Gregory A. Neumann and Maria T. Zuher (MIT) analyzed more than 400 million extremely precise elevation measurements made by the Mars Orbiter Laser Altimeter (MOLA) aboard MCS. In the polar regions they found the height of the Martian surface to he fluctuating by 1.5 to 2 meters from winter to summer, the result of carbon dioxide freezing onto the ground and then sublimating away. Local variations in this pattern may have been caused by winter storms, windblown drifting, and unseasonal warm weather caused by dust storms elsewhere on the planet. Team members also used orbital tracking data from MGS to map the planet's gravitational field in extremely fine detail so fine that they could estimate the density of the seasonally deposited layer even though it adds less than one part in a million to the planet's polar radius. The average density of the winter-deposited carbon dioxide turned out to be about 0.9 gram per cubic centimeter, much denser than Earthly snow and suggesting compacted slabs of dry ice. A. M.

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