National Geographic : 2004 Jan
MARS'S ICE DANCE i Ice-free Microns mm cm surface SOLID WHITEAREAS INDICATEWATERICETHAT LASTS THROUGHTHEMARTIANSUMMER. RANG ING FROMCENTIMETERSTO MANYMETERSDEEP. TRANSLUCENTWHITEAREASINDICATETHIN ICE THAT MELTSIN THE SUMMER. CARBONDIOXIDE ICE,OR DRY ICE,EXISTSON MARS BUT IS NOT DEPICTED.DATA:MARKI. RICHARDSONAND MICHAELA. MISCHNA.CALIFORNIAINSTITUTEOF TECHNOLOGY;R. JOHNWILSON, NOAA/GEOPHYS ICALFLUIDDYNAMICS LABORATORY and night in visible and infrared light. Other sensors on Odyssey detect gamma rays and neutrons radiated from the minerals below, revealing to those versed in nuclear physics the abundance of different elements, such as hydrogen and iron. When Mars Express reaches the planet, it will add yet more instruments to those already aloft, including a multispectral imaging spectrome ter that can identify minerals using visible and infrared wavelengths, and a radar that may detect ice and liquid water below the surface. Also due to arrive in December is Japan's Nozomi orbiter, designed to study Mars's atmosphere. As they beam a continuous stream of fresh data to Earth, the orbiters evoke a thrilling new picture of Mars-and a genuinely puzzling one: While searching for answers to old mysteries, the orbiters are bombarding us with new ones. Hugh Kieffer, a Mars veteran at the United States Geological Survey (USGS) who has been study ing the planet for almost four decades, tells his younger colleagues they are in a "period of maximum confu sion." New data whole new types of data-are accu mulating faster than researchers can make sense of them. The result is something like an optical illusion. Contradic tory images of Mars seem to flicker in and out of focus in the mind's eye. "Although Mars is supposed to be the god of war, the planet is much more like a prima don na" says Nathalie Cabrol, a planetary geologist at NASA's Ames Research Center. "When you think you have it right, Mars always has a surprise." he man whose imagination first took him to the crater rim on the edge of the but terscotch plain is Philip Christensen, a geology professor at Arizona State Univer sity in Tempe. Christensen leads the team respon sible for designing (Continuedon page 14) MARS 9 How can there be water ice at the equator inone era but not inanother? Scientists point to Mars's wildly fluctuating tilt as a key to climate change. Several factors, such as the positions of other planets, cause Mars to tilt back and forth inrelation to the sun. The planet has swung between 150 and 35 ° some 50 times in five million years, scientists believe, and between 0° and 60 ° inthe more distant past. When Mars isheavily tilted (two globes at left)-high obliquity the poles get more sun exposure, causing water ice to vaporize and build up at lower, colder latitudes. At a milder tiltof 35 ° (third globe), the process begins to reverse, and ice leaves lower latitudes and condenses at the poles. At the current 25 ° (lower right), visible water ice is concentrated at the north pole. (The diagrams show Martian winter in the northern hemisphere.) Obliq uity variations as small as one degree can contribute to ice ages on Earth.