Evidence Mars Had Vast Seas And Big Rivers Mounting Quickly

By David L. Chandler
Boston Globe Staff

The latest news from Mars shows that the red planet - which today is drier than the Sahara - may once have had vast expanses of blue.

Data sent back by the Mars Global Surveyor spacecraft over the last few weeks have provided several lines of evidence to bolster the idea that an ancient ocean existed on Mars, more than a mile deep and thousands of miles across. If these early indications are confirmed, it would greatly raise the odds that there could have been life on Mars, and even that some primitive, single-celled creatures may be living there now.

Today, the vast northern lowland area of Mars is the smoothest, most featureless expanse of land of its size ever seen on any planet, according to the latest information sent back by the spacecraft. And, it turns out, the rim of this smooth basin is virtually level - just as would be expected if it was, in fact, the floor of an ancient sea.

And that's not all. Six different river channels, which were photographed by the Viking mission to Mars in 1976, all empty into this northern lowland, and all signs of their channels disappear at just the level of the supposed ocean shoreline. Unless they reached a body of standing water at that point, why would the channels suddenly stop, scientists wonder, since the downward slope continues for hundreds of miles further.

All of this information ''is consistent with a standing body of water,'' said James Head, a planetary scientist at Brown University, last week at a meeting in Boston of the American Geophysical Union. Head added that the new data ''do not necessarily prove'' that such an ocean existed, although it is very difficult to explain the features any other way.

These findings are among the most dramatic new discoveries so far from Mars Global Surveyor, which after nearly two years of adjusting its orbit has been mapping the red planet in detail since early March.

The craft's high-precision laser altimeter, a device that can pinpoint details of surface topography to within one foot of elevation, has also shown clear signs of a relatively flat shelf of land bordering this ancient sea, somewhat like a continental shelf on Earth.

Altogether, Head and his colleagues say, the amount of water contained in the ancient Martian ocean would have been enough to cover the entire planet to a depth of more than 300 feet, if it were spread out uniformly over the whole planet. (The surface area of Mars is approximately equal to the dry- land surface of Earth). This ocean, Head estimates, would have been about one-third the size of the Atlantic.

Most startling of all is the new data showing the remarkable smoothness of this region of Mars - perhaps buried under millions of years worth of sediments deposited by this ancient sea.

''We don't know of anywhere smoother,'' said David Smith, one of Global Surveyor's principal scientists. ''There's nothing on Earth, nothing on Venus that smooth, on that kind of scale. It's like the Bonneville Flats,'' a perfectly flat, dried lake bed in the Nevada desert, but much more extensive. ''You can go for probably 1,000 miles with only variations [in height] of 5 to 10 meters,'' or 15 to 30 feet, he said (not counting the continual downward slope of this whole expanse).

Not that any of the new data can actually prove that there was an ocean on the surface of Mars long ago. For clear proof, humans may need to travel to Mars and dig deeply into the basins in search of evidence of an accumulation of water-borne sediments, or of a high water table or extensive permafrost in the region. ''I'm guessing we'll have to visit,'' Smith said.

But the likelihood of a past ocean has certainly increased dramatically as a result of the new findings. The data make it ''much more likely'' that such an ocean existed, Smith said. Still, he added, there is no ''smoking gun.''

Head has carried out ''a really careful analysis,'' said planetary scientist Maria Zuber of the Massachusetts Institute of Technology, and ''everything he says is consistent with large standing bodies of water.'' While nothing found so far proves it, she said, new images from Global Surveyor could eventually make the case by, for example, showing clear, distinct shoreline features.

The suggestion that Mars may once have had an ocean was made before, based on evidence from the Viking mission of 1976, but the signs at that time were far more ambiguous. The more detailed Global Surveyor data greatly strengthened the case, providing several lines of additional evidence, including these:

Planetary scientist Timothy Parker published a paper in 1989 describing martian features that looked like an ancient shoreline, but he had no information at that time about the actual elevations of the features he saw in Viking pictures. Now that the true elevations of points on the surface are known, it turns out that these shoreline features - identified solely on the basis of their appearance - really are all at about the same level. ''The fact that it is that close - that doesn't happen by luck,'' said Smith. (Small changes in elevation may have occurred as a result of millions of years of volcanic activity, wind erosion and other processes since the oceans dried up).

The volume of water that would have been contained in these ancient oceans matches very well with previous estimates of how much water must have flowed out of all the apparent river channels seen on the surface. The estimates reached by Head and his co-workers fall about midway between the maximum and minimum estimates others have made of the total amount of water that may have existed on Mars.

Research by a separate team also found a remarkable dichotomy that corresponds to this same boundary line. An analysis of the shapes of 1,600 small craters, carried out by James Garvin of NASA's Goddard Space Flight Center, found that virtually all of the craters inside the boundary are distinctly different from those outside it. ''When we saw that, we were stunned,'' Garvin said. ''We saw a striking difference.'' He thinks this might reflect a layer of ice beneath the soil of the ''ocean'' area, adding some independent support - though still not proof - to Head's analysis.

All of this is good news for those who hope to find signs of life on Mars. If there was indeed an ocean there, most scientists think it would only have existed for a brief period just after the planets were formed - the time when life is believed to have originated very rapidly on Earth.

If the same kind of conditions existed on Mars at that time, some biologists think it would be surprising if life had not begun there.

And if there was life, Head said in an interview, it is unlikely that it could have been completely eliminated. An abundance of evidence that has accumulated in recent years makes it ever more clear that living organisms are incredibly difficult to eliminate. They can survive in a much wider range of seemingly destructive conditions than any biologist would have dared to claim even a few decades ago.

''There's no way to autoclave a planet,'' Head said, referring to the equipment used to kill bacteria and viruses on surgical instruments. In other words, once life begins, it is almost impossible to wipe out.

All in all, the signs of a possible ocean are the latest - but doubtless not the last - of the dramatic changes in scientists' understanding of the red planet being delivered by the Global Surveyor data. ''It's a different planet,'' said Zuber in an interview last week.

The detailed information being sent back now, she said, make this a time of great excitement for scientists who have been studying the geology of Mars. ''It's like Balboa seeing the Pacific ocean for the first time,'' she said.