The milestone launches of NASA's latest Mars missions, called Spirit and Opportunity, provide the impetus to revisit the remarkable journey of the earliest martian missions. Excerpts from the lively debates that took place prior to the 1976 Viking missions give immediacy and perspective on both the rewards and challenges that the Red Planet offers. In this and forthcoming issues, Astrobiology Magazine is pleased to commemorate the descriptions offered in the words of then mission contemporaries. NASA historians have compiled these notes in their five-hundred page edition of: On Mars: Exploration of the Red Planet. 1958-1978 (NASA HQ SP-4212).
As anticipated, the information relayed to Earth by the Viking spacecraft has greatly affected man's perceptions and understanding of the planet Mars. The increase in basic, directly confirmed knowledge of the Red Planet began even before the landings. Once in orbit, the spacecraft began transmitting the first of tens of thousands of images of the planet and its satellites.
Heterogeneity was the most striking aspect of Mars as scientists identified a greater variety of terrains than known to exist on the moon or Mercury. Conway B. Leovy, a member of the meteorology team, noted: "Unlike the moon, whose story appears essentially to have ended one or two billion years ago, Mars is still evolving and changing. On Mars, as on the earth, the most pervasive agent of change is the planet's atmosphere, itself the product of the sorting of the planet's initial constituents that began soon after it condensed from the primordial cloud of dust and gas that gave rise to the solar system 4.6 billion years ago."
|Viking lander with extended surface sampler arm.
Some information about the nature of the Martian atmosphere had been derived from telescopic observations and from earlier Mariner missions, but those sources of data were "unverifiable and subject to misinterpretation." With the exception of its significantly different composition and its being "less than a hundredth as dense as that of the earth," the atmosphere of Mars behaves much like that of our own planet. "It transports water, generates clouds and exhibits daily and seasonal wind patterns." Responding to seasonal changes in the heat generated by solar radiation, localized dust storms occur and sometimes grow in strength until they cover the entire planet, a fact with which Mariner and Viking specialists were familiar. Global dust storms appear to be a phenomenon unique to Mars, which lacks large bodies of water that would prevent their buildup.
Atmospheric weathering of the primitive crystalline rocks on Mars has reduced them to fine particles that have oxidized and combined chemically with water to produce the reddish minerals so apparent in the color images returned from the Viking landers. Whereas on Earth the dominant weathering process has been from the movement of liquid water, on Mars the primary agent of change has been the wind. It erodes the landscape, transports the dust, and deposits it elsewhere on the planet. The Viking landing sites appear to have been "severely scoured by winds". In addition, pictures taken by the orbiter cameras reveal deep layers of wind-borne sediment in the polar regions, while dunefields of Martian dust and sand much larger than those on Earth were observed near the north pole.
The geologic history of Mars, according to orbiter imaging team leader Michael H. Carr, "shows evidence of floods and relatively recent volcanic eruptions, at least in the hundreds of millions of years that geology uses as a measure." There are also features that resemble terrestrial river systems. "Apparently tremendous floods occurred many times over Mars' history, indicating that the planet must have been drastically different in the past."
|Viking Mars image of laminated polar regions.
The large riverlike channels are one of the big Martian puzzles. Carr and his colleagues believe there are two major kinds of water features:
"There are the large flood features and then there are dendritic or branching drainage features that resemble terrestrial river systems. It appears from the crater counts that the fine terrestrial-like river channel systems are older than the flood features. It appears that the large flood features came in middle Mars history. There was a period of vast floods, then the flooding for some reason ceased or became less frequent because we don't have flood features with crater cutouts comparable to those we find on the Tharsis volcanoes. Very early in Mars' history, dendritic drainage patterns developed; in Mars' middle history it had a period of flooding, and then mostly after that the volcanics of Tharsis accumulated. This general picture has come out of the Viking data."
A lot of skeptics didn't believe there had been any period of surface drainage. Some said all those things could easily have been formed by faulting and soon. "The Viking pictures are full of examples of dendritic channels. I can't believe there are many skeptics left. I think we have really established that there was this early period of surface drainage. There can be very little doubt about that," said Carr.
The scientists are still left with explaining where all the water for the floods and rivers came from. More important, where did it go?
Where Did it Go?
Because of low atmospheric pressure at the surface, there are no contemporary large pools, rivers, or collection basins filled with water, and because of low temperatures the atmosphere cannot contain much water. However, there is probably a great quantity in the permanent polar caps and within the surface. The low pressure permits water to be present only in the solid (ice) or gaseous (water vapor) state. One possible explanation for the apparently contradictory vision of rushing rivers on Mars was presented by Gerald A. Soffen: "Broad channels formed when subsurface water-ice (permafrost) was melted by geothermal activity from deep volcanic centers. When the melting of the permafrost reached a slope the interstitial water suddenly released great flows, sometimes a hundred kilometers wide that modified the channels."
Seasonal heating of the permafrost may have occasionally released large flows of water, as well-a possible explanation for the channels that originate in box canyons and spill onto the plains. The easiest method of accounting for the dendritic channels is to conjure up a Martian rainstorm, but that suggestion raises many problems, all of which hinge on the basic question: "How is it possible that these ancient rivers could [have] existed and there be none today?" Obviously, atmospheric pressure would have to have been different during such a period. This hypothesis seems to be supported by studies of the Martian atmosphere encountered by Viking.
If the atmospheric pressure once was sufficient to permit the formation of liquid water, how long ago was that? This is still a subject of some debate. Harold Masursky and his colleagues estimated the relative age of the channels by counting the number and judging the age of the craters in and near the channels. The different kinds of channels appear to have been created in different epochs, or episodes, and all of them at least 50 million years ago and perhaps as long ago as several billion years.
|Viking image of Gusev Crater, an ancient proposed lakebed that will be targeted in forthcoming Mars Exploration Rover mission.
Permafrost: Permanent or Not?
Shifting of the permafrost also is believed to have influenced the texture of the planet's surface. Investigators assume the existence of permafrost, sometimes to the depth of several kilometers and generally thought to have been present for billions of years. Carr stated:
"To me one of the more exciting things we've observed is the abundant evidence of permafrost. The most striking features indicative of permafrost occur along the edge of old crater terrain. They form by mass movement of surface material probably aided by the freezing and thawing of ground ice. Another possible indicator of ground ice is the unique character of material ejected from impact craters that is quite different from the pattern on the Moon and on Mercury. We interpret the difference as due to ground ice on Mars. The impact melts the ground ice and lubricates the [ejecta] that is thrown out of the crater so when it lands on the ground it flows away from the crater in a debris flow and forms the characteristic features we have observed."
Slow movement and a freeze-thaw cycle could account for the chaotic, jumbled terrain seen over vast stretches of the Martian surface. Irregular depressions caused by localized collapsing of the crust when permafrost thawed could have formed the flat-floored valleys in Siberia and the table-lands of Mars. Large polygonal patterned regions on Mars resemble the ice wedges in terrestrial glacial areas.
One of the major questions posed by the Mariner 9 data was the composition of the residual polar cap left when the winter polar cap, made of frozen carbon dioxide, retreated in midsummer. A major controversy existed over whether this summer cap seas also frozen carbon dioxide or was frozen water.
According to Viking data, the temperatures of the residual cap are near -68° to -63°C, making a case for water frost. Also. the brightness of the frost "indicates it has a lot of dirt mixed in with it. The dirty nature of the ice had also been seen now by the orbital imaging system." Apparently there is no permanent reservoir of carbon dioxide in the polar regions of Mars, a finding that tends to rule out the theory of a rapid climate change induced by the instability of the carbon dioxide on the planet. "This means we still don't have an adequate explanation of how the atmosphere could have been of sufficient density to sustain the liquid water that appears to have flowed at one time in streams and rivers on the surface of Mars,'' said Kieffer.
Just as the evolution of Earth's atmosphere helped determine the nature of its environment, the evolution of Mars is linked with the development of its atmosphere. As Jerry Soffen concluded: "It appears that there was a considerably denser atmosphere in the past, somewhere between 10 and 50 times the present value of 7.5 millibars at the surface. This denser atmosphere would account for the possibility of the ancient river [beds] seen from the orbiter."
Whatever explanation the scientific community comes to accept, Viking has made two points very clear-the Red Planet's environment has not been static, and in the past was very dynamic.