Sky & Telescope January 1987 [antikvár]

In Search of Martian SeasAs SEEN through the television eyes . of Mariner and Viking spacecraft, Mars is truly a global desert. White polar caps, occasional clouds, and dustings of surface frost don't change that fact. But abundant evidence, like ancient river beds, demonstrates that such was not always the case (ST: July, 1986, page 17). In fact, planetary geologists now suspect that large lakes of water perhaps hundreds of meters deep once existed beneath a much denser atmosphere.If the seas did indeed exist, then they might have played a key role in the depletion of the planet's early atmosphere. Standing water may have slowly robbed the air of its carbon dioxide (COj). The thinning continued until enough of Mars' heat escaped into space, causing the lakes to freeze. The water then either evaporated or became trapped beneath the surface in permafrost.Although CO, molecules can become disassociated by solar radiation, or escape directly into space, these processes occur much too slowly to account for the elimination of Mars' thick, early atmosphere (estimated to be up to three times Earth's current surface pressure). More plausible, say geologists, is that CO, became locked in carbonate rocks, like limestone. Such a process occurred on Earth, sparing ourplanet from becoming the global greenhouse that Venus (whose CO! remained in its atmosphere) is today.To produce carbonates, large bodies of water are required to absorb CO2, which reacts to form insoluble sediments. One proof of the lake theory would be the detection of these carbonate rocks. Last June, University of Hawaii researcher Robert Singer thought he and his colleagues had found the weak spectral signature of such rocks in infrared observations from 3 to 5 microns. However, after completing the data processing, they possessed no conclusive evidence. Singer said that data from other wavelengths offered no positive results either.The carbonates may still exist, however. Infrared telescopes have relatively large fields of view, so much of Mars' disk must be examined as a whole. But Singer notes that the carbonates would exist as localized spots. Thus laboratory studies of soil mixtures are being made to find detectability limits, "to see how much [carbonate rock] you could hide." High-resolution data should become available from the spectrometer aboard NASA's Mars Observer, planned for 1992. The instrument will scan 1-kilometer-square areas from orbit to determine surface composition spectroscopically.Left: Jet Propulsion Labiflngerlike channels of Mars' Deuleronilus MensL This enlargement of the outlined area reveals shoreline of an ancient Martian lake 01 higher. Photographs courtesy JPL.6 Sky & Telescope, January, 1987tory researchers suggest that two to three billics ago themay have been filled with water. Rig/ii; dark vertical feature that could be the The land to the left of the 40-km-long feature isOther researchers believe the evidence for Martian lakes is already in hand. By examining photographs taken by the Viking orbiters, Timothy Parker and colleagues at the Jet Propulsion Laboratory have located features that suggest ancient shorelines. The photographs (two reproduced below) show topography resembling shorelines and flooded craters.Other Viking photographs reveal the existence of thick sedimentary deposits on the floor of the 4,000-km-long Valles Marineris canyon system, adding more support for the Martian lake hypothesis. From these images Susan Nedell (NASA-Ames Research Center), Steven Squyres (Cornell University), and David Andersen (San Jose State University) conclude that the layering in the canyon could have resulted from windblown dust that, after landing on the surface of ice-covered lakes, somehow migrated through the ice and settled to the lake bottom.The team first tested the hypotheses that the 5-km-high plateaus of deposits consisted of material from volcanic eruptions, landslides of canyon walls, or airborne dust. All three possibihties had shortcomings. They inferred that sedimentation through water was the most likely mechanism.Although Mars was probably warmer at some time in the past, surface temperatures may not have risen much above the freezing point of water. With this in mind, Nedell and colleagues attempted to determine how material could have collected underneath a canopy of ice. Each of the studied mechanisms down through the ice, up from the lake bottom, or in from the lake margins was possible, yet none proved conclusive.Ice-covered lakes in Antarctica may closely resemble the conditions on early Mars. So Nedell's team, along with F. Gordon Love (Radford University), traveled there to study how water on Mars may have existed. Although Antarctic lakes are constantly covered by some 3 meters of ice, the water underneath can reach temperatures of up to 77° F.Nedell and her colleagues also studied sedimentation methods in the Antarctic lakes. They have matched sediment trapped in the surface ice with that on the lake bottom, but the study was not conclusive. "We're not sure how [sediment] gets through," Nedell says, "but it does get through." Although the ice on Mars would have been thicker than that atop the Antarctic lakes, a similar process of sedimentation could have occurred.STUART J. GOLDMAN