The Snows of Mars

The Snows ofMarsNASA scans thepolar wastelands. около 5000 знаков. Mars holds a special place inthe human imagination as the planet most like the Earth. It has an atmosphere,seasons, and distinctive polar ice caps. The ice caps, first observed byGiovanni Cassini in 1666, immediately raised tantalizing questions. Are theymade of water ice like the giant glaciers that smother

Antarctica? Are they thefrozen remains of long-vanished oceans? If they melted, could Mars become ahabitable place? NASA s Mars Global Surveyor, currently in orbit about the RedPlanet, is finally providing some solid answers. The Surveyor hasalready revealed unexpected details about the size and structure of

Mars snorthern cap. By the end of February, the spacecraft will begin mapping, forthe first time, the topography and composition of the even more poorlyunderstood southern polar ice cap. The new information along with upcomingdata from the Mars Polar Lander, which will arrive in December will strip awaymany of the lingering mysteries of the Martian poles. On Mars, thepresence of water essential for life, past or present is always an issue

ofgreat interest. Some people have proposed that there were oceans early inMartian history others have said there were not. But for all of thosetheories, one needs to understand the water cycle how much water there was,where it went to, and where it s at now. If scientists find substantialreserves of frozen water, it would bolster the view that Mars was once a balmy,moist world where life could have started.

Until about twomonths ago, planetary astronomers believed that the southern cap containednothing but frozen carbon dioxide, also known as dry ice. New research suggestsotherwise a thick sheet of carbon dioxide ice would be too soft to staystable. The thought now is that carbon dioxide ice is so weak that itwould flow away, like a glacier, even at very low temperatures, Zuberexplains. So to maintain the topography of the south polar cap, there hasto be water ice in there

stiffening it up. Zuber and hercolleagues also analyzed Mars s much larger northern polar cap. The ice cap iscut by deep troughs and chasms some of these depressions extend down over amile to the base of the planet s crust. Many researchers off guard. There are no troughs of that kind in any of the ice caps on Earth, said Global. We don t know how this formedZuber s resultsconfirmed that the northern cap is composed

entirely of water ice, in someareas interspersed with layers of wind-blown dust and sediment. That piece ofgood news came as no surprise, because summer temperatures at the cap whichhas an elevation several miles lower than the southern cap are high enough tovaporize frozen carbon dioxide. But the Global Surveyor also produced the firstaccurate measurement of the size of the northern cap and that was a surprise. Seven hundred andfifty miles across, and up to two miles thick, the northern

cap has a volumejust half that of the Greenland ice sheet. It may sound large, but doesn tcontain nearly enough water to account for the flood channels and other erosionfeatures that appear all over the place on Mars. It s not even close towhat is generally believed to have once been on the surface, says Zuber.Scientists like Michael Carr at the U.S. Geological

Survey who believe oceansonce covered much of Mars face a serious challenge from the Global Surveyorstudies. The northern cap contains no more than one tenth the amount of waterneeded to fill an ancient ocean. On the other hand, the fissures and ring ofresidual ice around the perimeter of the cap suggest it has lost a great dealof water over the millennia. The Global Surveyor has alsoprovided some clues about the way water circulated about on

Mars in the distantpast. The northern ice cap sits nestled within a deep depression that coversessentially the entire northern hemisphere of Mars and drops in elevation as itnears the pole. The cap looks something like a hockey puck in thatdepression, David Smith of NASA s Goddard Space Flight Center reported atthe AGU press conference. Researchers are not sure how the giant lowland formed perhaps through a large

impact , but they do know that it has been there sincevery early in Martian history, and so has clearly played an important role inthe planet s water cycle. Before wemade these measurements of the northern hemisphere, it used to be thought thatthe only way you could get water to the north pole of Mars was through theatmosphere, Zuber says. But because the northern cap lies at a lower elevationthan the rest of the planet, water

than you put down almost anywhere inthe northern hemisphere is going to flow toward the pole. It is quite probable,then, that you once had standing bodies of water at high northern latitudes.They might not have persisted for very long, because we don t know how warm itwas and things may have frozen over quickly. But you clearly could get thewater up close to the pole. Clearly, Mars wasnot always the frozen wasteland it is today.

What happened? Some of the ancientwater could have been lost to the atmosphere and then, over countlessmillennia, ejected into space through complicated interactions with the Martianmagnetic field. Some might still be locked in aquifers and other formationsbeneath the surface. And some may exist in the southern polar cap but notmuch. The southern cap is significantly smaller than the northern one.

Even ifthe Mars Global Surveyor finds water ice in the south, it won t come close toeliminating the water shortage, according to Zuber. We haven t either improved ordiminished the possibility of life on Mars, she says. Essentially,what we have done is exacerbate the problem of there being too little water onMars today compared to where there was earlier. Now those people who haveproposed oceans have a bigger task in explaining where the water went.

Kathy SvitilPosted 2 19 99