Watering Mars with volcanism.

Watering Mars with volcanism 

The role of water may not be the ultimate question in the study of the planet Mars, but it is an essential factor in understanding the answer(s). Was Mars once wetter? Warmer? More earth-like? It is a vastly complicated problem, from the amount of water originally present within the planet to the quantities that remain today in such diverse reservoirs as permafrost, hydrated minerals and perhaps even subterranean aquifers of liquid.

Various estimates of the total volume of water on Mars have been based on such factors as the distribution of elements throughout the solar system, studies of certain meteorites believed to have come from Mars, examination of what appear to be water-carved "outflow channels" on the Martian surface and more. Few of these studies, however, says Ronald Greeley of Arizona State University in Tempe, have dealt with the amounts and timing of water released from the planet's interior in the course of its evolution.

Some of the water would have been present since the planet's formation, but much would have formed later on in association with volcanism, the evidence of which is visible in thousands of the photos taken by the two Mars-orbiting Viking spacecraft. Greeley estimates in the June 26 SCIENCE that volcanism could have acconted for the equivalent of a ater layer 46 meters deep over the entire planet. (Previous water estimates, he says, not confined to the role of volcanism, have ranged from 1 meter to 1 kilometer.)

In 1979, he and a colleague concluded from the Viking photos that volcanism has "resurfaced" more than 41 percent of the Martian surface, by their best estimate, and that the total could be as high as 64 percent (SN:11/10/79, p.329). Recently prepared global geologic maps based on the Viking data, says Greeley, still indicate that materials apparently of volcanic origin cover more than half the surface.

Constructing a panorama of Martian water history from such a finding, however, is a formidable task. It is one of the key issues that have been addressed in recent years as part of a NASA-funded multidisciplinary research project called MECA, or Mars: Evolution of its Climate and Atmosphere. A major problem is the lack of knowledge about the amount of volatile materials, or gases, in Martian magmas. Even for earth, Greeley says, such calculations have large uncertainties, and extrapolating them to Mars just makes them more uncertain. Another difficulty, long familiar to planetary geologists, is assigning ages to surface features, since almost the only available tool (in the absence of actual rock samples that can be dated) is to count visible meteorite craters and attempt to relate their number to how long a given surface has been exposed to impacts.

Acknowledging these problems, Greeley nonetheless cites a general history of Martian volcanism in terms of "age scales" estimated for major Martian geologic epochs by another researcher (Kenneth L. Tanaka of the U.S. Geological Survey in Flagstaff, Ariz.). As for the amount of water associated with those upheavals, he uses a figure of 1 percent by weight, based on comparisons with earth and the assumption that Martian volcanism is dominated by mafic materials.

"Most of this water," according to Greeley, "was released in the first 2 billion years of Martian history." However, he notes, "volcanism has occurred from at least the close of the period of heavy impact cratering (about 3.9 billion years ago) to the age of the youngest rocks visible on the planet." And since the volcanism probably did not only form water from magmatic volatiles but also released substantial quantities that had already existed beneath the surface, he says, "the results substantiate the growing perception of Mars as a 'wet' planet in the first third of its history."