What Became of the Water on Mars?

By Carr, Michael H. | Natural History, December/January 2003 | Go to article overview

What Became of the Water on Mars?


Carr, Michael H., Natural History


This January, a cluster of spacecraft will converge on the Red Planet, probing for clues to the mysterious but unmistakable role of water in its past.

As this issue of Natural History went to press, at least six spacecraft were already orbiting-or speeding toward a rendezvous with-the planet Mars. In the vanguard of this wave of martian exploration are two NASA orbiters, the MJB Global Surveyor, in orbit since 1997, and the Mars Odyssey, in orbit since 2001, which have by now collectively observed the planet for eight years. The two have already returned an enormous amount of data about Mars: its topography, which reflects a surprisingly complex geological history, incorporating thick stacks of layered sediments and seemingly recently waterworn gullies; its ancient magnetic field, now vanished because its core has cooled, but still traceable in the magnetization of ancient rocks; its surface chemistry and its primarily basaltic mineralogy; and its fine-scale surface structures, sculpted by wind and ice. The data from the two orbiters have also been crucial for planning the other missions now approaching Mars, particularly in helping planetary geologists pick exploration sites that are both scientifically interesting and relatively free of hazards to landing.

First among the approaching missions is another orbiter, Nozomi, launched by Japan's Institute of Space and Aeronautical Science in 1998. It is due to arrive in January. Nozomi will examine the interaction of the planet's upper atmosphere with the socalled solar wind, made up of highly energetic partides from the Sun. Since Mars has no magnetic field, it is constantly bombarded by the solar wind. The particles carry enough energy to break molecules in the upper atmosphere into their atomic constituents. Some of the lighter resultant elements get carried away in the solar wind, and so the planet is gradually losing its atmosphere. Knowing how fast that is happening today will enable scientists to estimate how thick the atmosphere was in the past, and . so-because of the greenhouse effect of an atmosphere-how warm the planet may once have been.

This past june the European Space Agency launched the Mars Express, made up of an orbiter, the eponymous Mars Express, and a lander known as the Beagle 2. Mars Express will go into orbit this Christmas Day, minutes after Beagle 2 is scheduled to land on Isidis Planifia [see map on page 35]. The lander is to measure surface and atmospheric properties, and will probe as deep as five feet into the martian soil. Its onboard instruments will seek bulk organic matter, as well as the isotopic signature of the biologically important element carbon. Most elements occur in nature as a mix of isotopes of slightly differing atomic weights. On Earth, some biological processes preferentially utilize certain isotopes of some elements, so that the carbon isotopes that occur in organic molecules, for instance, have different weights than the ones that occur in inorganic compounds. Measuring the isotopic ratios on Mars will provide clues about possible biological activity.

The orbiter Mars Express has numerous instruments for analyzing the surface and atmosphere, including a high-resolution stereo camera and instruments for measuring surface composition that complement the ones on Mars Global Surveyor. Mars Express also has a radar device for detecting water more than a mile below the surface.

Finally, this past summer NASA launched two Mars rovers, which will join the two U.S. spacecraft already examining the planet. Spirit, the first rover, is scheduled to land on the surface on January 4, 2004; Opportunity, the second, will land on January 25. The two rovers will land on opposite sides of the planet and investigate the geology of regions where liquid water might once have been present. The targets of their searches will be water-bearing minerals and sediments laid down by water.

The two rover missions, along with the other four, constitute by far the greatest assemblage of spacecraft people have ever sent to Mars. …

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