Costa del Mars


By Jeff Hecht in Boston TRACES of an ancient Martian seashore show up on images from the Mars Global Surveyor spacecraft, researchers told last week’s American Geophysical Union meeting in Boston. The spacecraft has also found evidence that hot hydrothermal vents once bubbled on the surface of Mars, and that its atmosphere has curious bulges which have so far defied explanation. The Mars Global Surveyor has been observing the Red Planet since September. At last week’s meeting, Maria Zuber of the Massachusetts Institute of Technology said the images show that the edge of the northern polar icecap climbs steeply to a height of about a kilometre, then rises more gently to a peak of more than 2.5 kilometres near the pole. The images show that dark arcs which appeared in pictures taken by the Viking spacecraft in the 1970s are chasms up to a kilometre deep, often with terraced sides. Zuber says that by comparing the slopes of the icecap with models of ice flow it has been possible to work out whether the icecap is changing. “The northern polar cap is not growing today,” Zuber told the meeting. It may be shrinking, she says, but at most by only a few centimetres a year. Data from the spacecraft have provided more evidence that an ocean once covered the northern Martian lowlands. Points along one clear contour vary in height by less than half a kilometre, according to Jim Head of Brown University in Providence, Rhode Island. This suggests the contour marks the rim of an ancient ocean. The Mars Global Surveyor has also found the first clear signs that water collected in ponds on the Martian surface. Mike Malin of Malin Space Science Systems in San Diego says the images show a crater 50 kilometres wide in the southern hemisphere which could have been “a pond fed by seepage from the walls of the crater”. Channels in the crater wall suggest fluid seepage, while the outlines of deposits on the crater are shaped like bays and peninsulas, possibly formed when water flooded the crater. An infrared spectrometer on the spacecraft has revealed an equatorial region several hundred kilometres across that appears rich in coarse-grained haematite. These large crystals typically grow in warm, iron-rich fluids, for example where water seeps into volcanoes or at hydrothermal vents on ocean floors. The location was a surprise, because it had not been recognised as a volcanic region or an old seabed. “We haven’t found anything like this anywhere else,” says Phil Christensen of Arizona State University in Tucson. NASA scientists believe hydrothermal systems could have incubated Martian life, and Christensen suggests the newly discovered zone would be a good target for a future lander. The data have turned up yet another surprise—twin bulges on opposite sides of the Martian atmosphere. Atmospheric density at an altitude of 125 kilometres in each bulge can be twice that at low points, says Gerald Keating of George Washington University in Washington DC. He thinks the bulges are density waves linked to peaks in the Martian highlands,
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