November 17, 2008
An international team of scientists reports new evidence for the controversial idea that oceans once covered about a third of ancient Mars based on data from the Gamma Ray Spectrometer onboard NASA's Mars Odyssey.
The orbiter's Gamma Ray Spectrometer (GRS), controlled by William Boynton of University of Arizona's (UA) Lunar and Planetary Laboratory, can detect elements buried as much as 13 inches (1/3 meter), below the surface by the gamma rays they emit. That capability led to the instrument's 2002 discovery of water-ice near the surface throughout much of high-latitude Mars.
"We compared Gamma Ray Spectrometer data on potassium, thorium and iron above and below a shoreline believed to mark an ancient ocean that covered a third of Mars' surface, and an inner shoreline believed to mark a younger, smaller ocean," said James M. Dohm, University of Arizona planetary geologist and leader of the international investigation.
"Our investigation posed the question, 'Might we see a greater concentration of these elements within the ancient shorelines because water and rock containing the elements moved from the highlands to the lowlands, where they eventually ponded as large water bodies?'" Dohm said.
Results from Mars Odyssey and other spacecraft suggest that past watery conditions likely leached, transported, and concentrated such elements as potassium, thorium, and iron, Dohm said. "The regions below and above the two shoreline boundaries are like cookie cutouts that can be compared to the regions above the boundaries, as well as the total region."
The younger, inner shoreline is evidence that an ocean about 10 times the size of the Mediterranean Sea, or about the size of North America, existed on the northern plains of Mars a few billion years ago. The larger, more ancient shoreline that covered a third of Mars held an ocean about 20 times the size of the Mediterranean, the researchers estimate.
The potassium-thorium-iron enriched areas occur below the older and younger paleo-ocean boundaries with respect to the entire region, they said. The scientists used data from Mars Global Surveyor's laser altimeter for topographic maps of the regions in their study.
They report their findings in the article, "GRS Evidence and the Possibility of Paleo-oceans on Mars." The article will be published in a special edition of Planetary and Space Science, which stems from a June 2007 workshop on Mars and its Earth analogs held in Trento, Italy.
Scientific debate on the existence of ancient martian oceans marked by shorelines was sparked by several studies almost 20 years ago. One such study, by Baker and colleagues at the UA Lunar and Planetary Laboratory, proposed that a few billion years ago, erupting magma unleashed floods far greater in volume than Brazil's Amazon River. The floods ponded in the northern lowlands of Mars, forming seas and lakes that triggered relatively warmer and wetter conditions that lasted tens of thousands of years.
Spacecraft images going back to Mariner 9 in the early 1970s and the Viking orbiters and landers later in the 1970s show widespread evidence for a watery past for Mars. Images and other information from a flotilla of United States and European Mars orbiters have sharpened the details in the past decade. Results from Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter highlight a water-and-ice-sculpted martian landscape.
Scientists studying spacecraft images have a hard time confirming "shoreline" landforms, the researchers said, because Mars shorelines would look different from Earth's shorelines. Earth's coastal shorelines are largely a direct result of powerful tides caused by gravitational interaction between Earth and the Moon, but Mars lacks a sizable moon. Another difference is that lakes or seas on Mars could have formed largely from giant debris flows and liquefied sediments. Still another difference is that Mars oceans may have been ice-covered, which would prevent wave action.
"The GRS adds key information to the long-standing oceans-on-Mars controversy," Dohm said. "But the debate is likely to continue well into the future, perhaps even when scientists can finally walk the martian surface with instruments in hand, with a network of smarter spaceborne, airborne, and ground-based robotic systems in their midst."