Did an ocean flatten Mars' northern half?

It's flatter than the Sahara Desert and about five times as large. The northern hemisphere of Mars at latitudes above 50 [degrees] is virtually devoid of hills and valleys, according to an experiment aboard the Mars Global Surveyor spacecraft, which has begun to map the planet's surface.

"It's one of the main surprises" about Mars, says David E. Smith of NASA's Goddard Space Flight Center in Greenbelt, Md. His team describes the study in the March 13 Science.

The only places on Earth that are as flat lie at the bottom of the oceans, notes Smith. Indeed, planetary scientists argue that the northern face of Mars is a basin, now bone-dry, that once held an ocean. On Earth, plate tectonics--the movement of huge sheets of the planet's crust--created deep basins. Some researchers believe a similar process may have shaped the northern half of Mars. Alternatively, a giant comet or asteroid may have flattened the region, which is markedly smoother and younger than the southern highlands.

If an ocean did sculpt the northern lowlands, says Smith, further mapping with Surveyor may reveal a shoreline or terracing--the irregular layering of material that occurs as water recedes. Surface deposits, like the silt on the floor of Earth's oceans, would offer another clue.

To measure topography, Surveyor bounces laser light off the Martian surface 10 times a second and records the amount of time it takes signals to return to the craft. The new findings are based on measurements made last October. The laser experiment was then turned off while the craft moved closer to the optimum orbit for mapping.

The experiment resumed at the end of March and continue until September, after which Surveyor will refine its orbit before the main mapping mission begins early next year (SN: 12/6/97, p. 360).