If an ocean once lapped at the rusty shores and cliffs of Mars, it left no clear traces of an ancient coastline on the planet's surface. That's the story told by new images from the Mars Global Surveyor spacecraft, currently orbiting the Red Planet. However, a lack of obvious shoreline patterns doesn't rule out the possibility of vast torrents scouring Mars billions of years ago, researchers write in the 1 October Geophysical Research Letters.
Visions of a watery Mars date to fanciful perceptions of canals and oases by astronomer Percival Lowell near the turn of the century. NASA's two Viking orbiters dashed those dreams in the 1970s, but their pictures revealed tantalizing hints of a wet Mars long ago. The evidence included sinuous gullies, branching networks of valleys, and teardrop-shaped mounds that looked as if raging water had swept past. Planetary geologist Timothy Parker of the Jet Propulsion Laboratory in Pasadena, California, and his colleagues also speculated that what they saw were faint coastal imprints from a vanished ocean in the northern hemisphere of Mars. These included embayments, cliffs, and other features that often appear along seashores on Earth.
Planetary scientists tested that scenario with high-resolution photos from the Mars Orbiter Camera (MOC) aboard Surveyor. Michael Malin and Kenneth Edgett of Malin Space Science Systems in San Diego scrutinized 14 images taken in 1997 and 1998. The images, up to 10 times more detailed than those from the Viking missions, covered areas designated by Parker and others as possible shorelines. But in each case, Malin and Edgett saw only gradual slopes lacking abrupt transitions that wave-cutting action should have produced. Also absent were any telltale linear deposits of eroded rocks. However, they acknowledge, a true test requires on-site studies, because it's often difficult to detect ancient shorelines even in satellite images of Earth. "The possibility that the planet once had oceans and coastal landforms cannot be dismissed," Malin says.
The big-picture Viking images may reveal sweeping coastal patterns more effectively, says Parker--like standing farther away from an impressionist painting. "These are subtle features," he says. "It's not easy to make a definitive statement on their origins." It's proper to interpret the MOC images carefully, agrees Mars Global Surveyor project scientist Arden Albee of the California Institute of Technology in Pasadena. However, he notes, the images do "call into question whether there was a huge body of water" that once filled much of the Northern Hemisphere.