Malin and Edgett identified three main outcrop types: layered units, massive units and thin mesa units.
Layered units consist of rock beds only a few yards (meters) or less thick, stacked on top of one another in distinct groups. Massive units appear as one bulky rock layer with no clearly defined horizontal bedding. In a few cases, these types appear together, with the massive unit always perched on top of the bedded unit.
Thin mesa units, with surfaces ranging from smooth to pitted, to ridged and grooved, are almost always found on top of eroded massive or layered sedimentary rock.
A paper on the images will appear in this Friday's issue of the journal Science. The findings were to be reported at a NASA news conference Thursday, after an embargo on the information lifted. That embargo was lifted today, after other news organizations published accounts of the study.
The sedimentary units show no telltale signs of wind deposition, and the researchers concluded that explosive volcanic eruptions and impact cratering probably could not have produced enough sediment to create the large-scale and geographically widespread outcrops seen on the Martian surface.
Although Malin and Edgett favor water as the sedimentary suspect, they also offer an alternative model that involves changes in atmospheric pressure on early Mars. They suggest that periods of relatively high atmospheric pressure -- caused by fluctuations in the amount of solid carbon dioxide on the planet’s surface --could have increased the atmosphere’s ability to carry dust produced by heavy cratering.
To confuse matters, the researchers don’t know where the original sediments came from, or how they were transported to their final resting places, since there are no traces of gullies or streams or other channels associated with the outcrops. They think that erosion may have wiped out both the source of the sediments and their travel routes.
In some cases, sedimentary rock has eroded out of the crater in which it formed, also vanishing without a geologic clue.
To Malin, the history of Martian geology looks like a jigsaw puzzle.
"In the center of the puzzle, we have these layered rocks, which are good evidence of an extremely dynamic environment. On either side of this well-developed puzzle piece, we have mysteries." In any case, Mars’ sedimentary rocks suggest a very active early history for the planet.
"This makes Mars more complicated and more exciting. This record is going to tell us a lot about what early Mars was like, and maybe the early Earth as well, since we don’t have a lot of rocks on our own planet from this time period," said Edgett.
Global Surveyor was launched in 1996 to make a detailed map of Mars from an orbital vantage point. It carries a suite of instruments designed to provide new details about the planet's surface.
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