What became of the water on Mars? 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

Natural History, Dec, 2003 by Michael H. Carr

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 Mars 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 so-called solar wind, made up of highly energetic particles 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 Planitia [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. Their presence will dramatically pick up the tempo of the research begun by the Viking missions and, more recently, by the 1997 Pathfinder rover. Those missions failed to find any evidence of life on the martian surface. Yet of all the extraterrestrial bodies in the solar system, Mars is still the most likely place where conditions might have been hospitable for life. If Spirit and Opportunity successfully carry out their missions, planetary scientists will have a much better idea of whether some form of life evolved on Mars in the past, and of where we might best go to look for it, or for its remains.

The modern roots of people's fascination with Mars extend at least as deep as the late eighteenth century. By that time observations had already revealed that Mars has some remarkably Earth-like qualities: polar caps, seasons, clouds, a day that lasts roughly twenty-four hours, and even, it seemed, oceans. On the basis of those observations, the contemporary English astronomer William Herschel speculated that life existed on Mars.

It wasn't until the late nineteenth century, however, that the public became caught up in what quickly grew to be a frenzied discussion about the ways of martians. The dynamo behind the popular hysteria was a nineteenth-century American named Percival Lowell. Lowell, a scion of a prominent Boston family, was a devotee of Asian culture and an accomplished amateur astronomer. The main "evidence" Lowell offered for his speculations about life was an elaborate network of "canals" that had been observed and mapped by the Italian astronomer Giovanni Schiaparelli. Lowell suggested that intelligent martians had built the "canals" to transport water from the polar caps to the equatorial deserts. Other observers failed to see the waterworks, but the possibility of civilizations populated by martian little green men led to a torrent of writings about martian invasions, the potential colonization of Mars, and the threat of interplanetary wars.