Head's team set out to test the hypotheses of scientists who suggested the possibility of oceans on Mars in 1989 and 1991. The team used data from the Mars Orbiter Laser Altimeter, which beamed a pulsing laser to Mars' surface.

In this topographic portrayal of Mars, above, the northern lowlands are occupied by an ocean (blue) whose shoreline is placed at the position of Contact 2, the line that Parker and co-workers interpreted as an ancient shoreline. Thus, this view shows Mars as it might have looked mid-way through its history according to the oceans hypothesis. The Tharsis region, with numerous very large shield volcanoes is seen in the central part of the globe. In the upper right, many channels flow into the northern lowlands at Chryse Planitia. Credit: MOLA Team. Rendering by Peter Neivert, Brown University.

Scientists measured the time it took for the laser to return to the satellite; the laser traveled a shorter length of time from mountain peaks and longer from craters. MOLA is the first instrument to provide scientists the information required to construct a topographic map of the entire surface of the planet.

For years, scientists have known about channels in which water once flowed into the northern lowlands on the surface of Mars. "The question is whether it collected in large standing bodies," Head said. "This is the first time we could get instruments to comprehensively test these ideas."

According to Head, the team has found four types of quantitative evidence that points to the possible ancient ocean:

• The elevation of a particular contact (the border between two geological units, such as where one type of surface meets another) is nearly a level surface, which might indicate an ancient shoreline.

• The topography is smoother below this possible ancient shoreline than above it, consistent with smoothing by sedimentation.

• The volume of the area below this possible shoreline is within the range of previous estimates of water on Mars.

• A series of terraces exists parallel to the possible shoreline, consistent with the possibility of receding shorelines.

Head's team concludes that further tests are necessary, including analysis of meteorites from Mars and of landing sites, checking for the presence of salts that may be related to former oceans.

The importance of determining whether there were ancient oceans -- and life -- on Mars is that scientists may be able to learn more about long-term climate change and why climate changed on Mars, which has relevance to the future of the Earth, Head said.

Science