Photographs of the Martian surface find no sign of a sea cliff along a possible ancient shoreline.
To test the hypothesis that oceans once covered much of the northern hemisphere of Mars,
scientists at Malin Space Science Systems (MSSS) of San Diego, CA, have used high resolution images of Mars taken with the Mars Orbiter Camera (MOC)
on Mars Global Surveyor.
"The ocean hypothesis is very important, because the existence of large bodies of liquid water in the Martian past would have had a tremendous impact on ancient Martian climate and implications for the search for evidence of past life on the planet," said Dr. Kenneth Edgett, a staff scientist at MSSS.
Features in earlier Mars probes, in particular the startling Viking images, led a number of researchers to look for remnants of ancient coastlines and further raised the possibility that such a body of water once existed.
"So things now with respect to the oceans," said Dr. Michael Carr, of the US Geological Survey, 'they're kind of in limbo right now. I have looked at a lot of the MOC images in places where the shorelines are supposed to be and I can't find any evidence. You can see features there, but whether these are shorelines or not is kind of difficult."
Beginning in 1998, MSSS scientists Michael Malin and Kenneth Edgett set out to answer this question with higher resolution cameras - five to ten times better than Viking. Initially the team targeted about 2% of the MOC images in places that would test shorelines proposed by others in the scientific literature.
With the researchers' visual identification at higher resolution, none of these features appeared to have been formed by the action of water in a coastal environment. Their analysis first appeared in Geophysical Research Letters, in a paper entitled "Oceans or Seas in the Martian Northern Lowlands: High Resolution Imaging Tests of Proposed Coastlines."
Fourteen images were analyzed of areas that had been indicated, from Viking images, to be candidates for shorelines. Whether a larger image sample or confirming data will bear out the visual interpretation of what expectations of a Martian shoreline should look like, remains a scientific conclusion ripe for debate. The Mars Global Surveyor carries onboard the Mars Orbiter Laser Altimeter instrument (MOLA), which uses infrared laser pulses to measure the surface below.
"The MOC images we took in the late '90s do not show any coastal landforms in areas where previous researchers - working with lower resolution Viking images - proposed there were shorelines."
As presented in the Geophysical Research Letters paper, the analysis focused on four different areas that had been proposed as coastlines. One of these areas is northwest of the great volcanoOlympus Mons (Figure 3 ). Viking images of the linear feature separating the western margin of the Lycus Sulci from the lower, smoother Amazonis plains (upper left in Figure 4 ) led some researchers to conclude that the two surfaces were in contact along a cliff.
Previously, since the proposed cliff faces toward the smooth plains, it was suggested that this feature might be the signature of a cliff that forms from erosion by waves in a body of water as they break against a coastline. Three MOC images were acquired along this proposed shoreline, covering the areas indicated by the white boxes in Figure 4. Each image was targeted to straddle the feature, a rise that runs diagonally across the scene from near the lower left toward the upper right. The middle section of the central image, shown in Figure 5, was taken in July 1998. The Lycus Sulci uplands (lower half) here are roughly-textured while the flat Amazonis plains (upper half) appear more smooth.
This image in particular shows that the contact between Amazonis and Lycus Sulci is clearly not a wave-cut cliff, and that there are no features that can be unambiguously identified as coastal landforms. "But what bothers me," said Carr, "is that throughout this latitude band where the ocean shorelines are being mapped, the surficial geology is very complicated. There's a lot going on. You get all kinds of very complicated morphology."
"Even on Earth, looking for ancient shorelines from the air or space is a challenge," said Dr. Malin. "But, despite the difficulties in identifying ancient coastlines remotely, we believe these MOC images of the proposed shorelines are of a high enough resolution that they would have shown features indicative of a coastal environment had there been an ancient ocean on Mars."
Martian seas would have been influenced by only one third the gravity of Earth's seas and would not have been subject to strong tidal forces, like that arising from the Earth's Moon. Because of the Earth's active erosion, there are fewer chances to compare an ancient Mars coast with a present eroding Earth coast as such a rift would appear seen from space.
While the suggestion that Mars at one time had oceans cannot be ruled out, the foundation for the "ocean hypothesis" developed in the 1980s on the basis of suspected shorelines appears now to require a broader scan of any apparent beachfront real estate on Mars.
However, it should be understood that there is significant other evidence of water on Mars in the past, both from Mars Global Surveyor and from previous missions. To search for clues to the very important question of the role of water in the evolution of Mars, the MOC continues to acquire new high resolution pictures.