Mars, Water on

Water and other easily vaporized molecules become more common as one moves outward from the Sun. Since Earth has a substantial amount of water, one can assume that Mars should as well.

Spectroscopic studies from Earth-based telescopes and Mars-orbiting spacecraft have detected water vapor in the Martian atmosphere, particularly in white clouds and fog. Temperature measurements made by the Viking Orbiter missions in the 1970s revealed that the north pole residual cap, which remains year-round, even at the height of Martian summer, was likely water ice, rather than carbon dioxide ice, which comprises the seasonal polar caps.

Although the Martian atmosphere and polar caps contain only a small amount of the water that scientists believe exists on Mars, geologic evidence from channels and craters suggests that much water (mainly ice) is buried below the surface of the planet, probably within the upper few kilometers of the surface. Recent results from Mars Odyssey seem to confirm that large amounts of water ice exist in the upper layers of Martian soil. Upcoming robotic Mars missions will directly measure the amounts and distribution of subsurface water by using ground-penetrating radar (Mars Express), seismic studies (Netlander), penetrators, and drilling.

Early Evidence: Cold and Dry

Early Mariner missions to Mars in the 1960s determined that its atmosphere is composed primarily of carbon dioxide and is extremely thin. At sea level on Earth, the atmosphere exerts a pressure of 1 atmosphere . At the surface of Mars, the atmosphere exerts a pressure of only 0.006 atmosphere. This very thin atmosphere has two important implications:

1. The temperature stays below freezing because the atmosphere is unable to retain heat; and
2. Liquid water cannot exist on the surface.

Consequently, the view of Mars in the 1960s was that it was a cold, dry world and that it had probably been that way throughout its history.

Later Evidence: Geologic and Fluvial Activity

The Mariner 9 spacecraft entered orbit around Mars in 1971 and revealed that 60 percent of the planet is very old, whereas the remaining 40 percent (located primarily in the northern hemisphere) has experienced more recent geologic activity. Among the features seen in the Mariner 9 images were huge volcanoes, a gigantic canyon system, and channels that had apparently been formed by a flowing liquid. After considering all other options, scientists concluded that the channels had been formed by flowing water.

Two main types of channels are seen on Mars: the small valley networks, and the large outflow channels.

The valley networks are similar in appearance to channels on Earth that form from rainfall runoff. Valley networks tend to be found on ancient terrain, which led scientists to speculate that Mars had a thicker atmosphere and warmer surface conditions during its early history than it does today. Alternately, the valley networks could form by groundwater sapping, a process whereby the surface collapses as underground water is removed. Sapping could occur under present climatic conditions.

The outflow channels appear to have formed by huge floods. Outflow channels typically begin in areas of collapse, called chaotic terrain, where water bursts out from the subsurface, quickly carving channel pathways. Calculations show that such channels could form in only a few days and under present atmospheric conditions. By the end of the Mariner 9 mission, scientists were beginning to see Mars as a place where limited amounts of water had affected its geology.

Other Geologic Evidence

Information obtained by the Viking Orbiters in the 1970s and 1980s led scientists to begin to look more closely at a number of mysterious geologic features and consider how water might be involved in their formation. Ejecta blankets surrounding fresh Martian impact craters display a fluidized appearance, quite different from the radial pattern seen around craters on dry bodies such as the Moon. Most scientists now believe that these ejecta patterns form by subsurface water and ice being vaporized during crater formation.

In the northern plains of Mars, features such as polygonal terrain (which look like large mud cracks) suggest that the area was once saturated with water and has since dried out. The rims of many craters in the ancient terrain are dissected by channels, and the floors of these craters appear to be covered by smooth sedimentary deposits.

A Martian Ocean?

Scientists proposed a radical idea in the mid-1980s: namely, that the lowlying northern plains of Mars may have contained an ocean within the past 2 billion years or so. They based this theory on a number of observations:

• Linear features along the edges of the plains that resemble shorelines;
• The concentration of polygonal terrain and fluidized ejecta morphologies in this region; and
• The large number of outflow channels that debouch (empty) into the northern plains.

Recent evidence from the Mars Global Surveyor (MGS) mission, in orbit around Mars since 1997, seems to support this view. MGS instruments have revealed that the northern plains are extremely flat, which is a characteristic of the deep ocean basins on Earth.

Evidence in Minerals and Sedimentary Features

The ancient terrain of Mars shows the effects of water. MGS has revealed large deposits of minerals, such as hematite, which on Earth commonly form by interaction with water. Gullies are found along crater and canyon walls and show evidence that they are recent features. Layered deposits have been identified in topographic depressions and are similar in appearance to sedimentary deposits on Earth.

Minerals found in Martian meteorites also show evidence of interaction with water. These rocks were formed and altered by geologic processes on Mars prior to being ejected off the planet by impact events (and which subsequently have fallen to Earth). Analysis of these meteorites reveals the environmental conditions under which they existed on Mars. Carbonates and other minerals formed by interactions with water, such as clays, are common in these meteorites.

Reconciling the Evidence

Scientists' perception of Mars has changed dramatically in 40 years, from that of a dry desert world to one where flowing water has played a major role in shaping its surface. How can this picture be reconciled with the current atmospheric conditions that prohibit liquid water on the surface?

Scientists now believe that Mars goes through episodes of climate change, caused by variations in the tilt of its rotational axis and orbital factors due to the gravitational effects of other planets. Right now, conditions on Mars are such that most water is tied up as ice in the polar caps or in the subsurface. However, when the Martian poles tip more than their current 25-degree tilt, increased solar insolation can cause the polar ices to evaporate, thickening the atmosphere and causing a greenhouse warming of the surface. Subsurface ice can then melt and appear on the surface in the form of rivers, lakes, and perhaps even oceans.

The episodic appearance of liquid water has important implications for the question of life (either present or past) on Mars and is of great interest as a resource for future explorers who may colonize Earth's neighboring world.

see also Astrobiology: Water and the Potential for Extraterrestrial Life; Comets and Meteorites, Water in; Life in Extreme Water Environments; Solar System, Water in the; Stream Erosion and Landscape Development.

Nadine G. Barlow

Bibliography

Carr, Michael H. "Mars." In The New Solar System, 4th ed. Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew L. Chaikin, eds. New York: Cambridge University Press, 1998.

Carr, Michael H. Water on Mars. New York: Oxford University Press, 1995.

CANALS ON MARS?

Although a few astronomers using Earth-based telescopes in the early twentieth century reported what they believed to be canals on Mars, spacecraft imagery in the 1960s and 1970s showed that the canals were optical illusions.