Posted: 19 November, 2008
An international team of scientists working with data gleaned from Mars Odyssey’s Gamma Ray Spectrometer report new evidence for the controversial idea that oceans once covered as much as one-third of ancient Mars.
Today, on the Earth, oceans occupy around two-thirds of our planet, but all of the Martian oceans have long since dried up. Although the fleet of spacecraft and landers visiting Mars since the 1970s have demonstrated widespread evidence for a watery past, planetary scientists find it considerably challenging trying to identify ancient ‘shorelines’ on Mars, since they look a lot different to those we are more familiar with on Earth. For example, Earth's well-defined 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 that could have exerted such a force on the planet. In addition, that lakes or seas on Mars could have formed from giant debris flows and liquefied sediments, and Martian oceans may have been ice-covered, which would prevent any wave action and subsequent erosion of the coastline.
This 3D map superimposes gamma ray data from Mars Odyssey's GRS onto topographic data from the laser altimeter onboard the Mars Global Surveyor. The red arrow indicates the shield volcanoes of Elysium rise in northern Mars. Blue-to-violet colours mark areas poor in potassium and red-to-yellow colours mark potassium-rich sedimentary deposits in the lowlands below the Mars Pathfinder landing site (PF) and Viking 1 landing site (V1).
In the new study, lead by University of Arizona planetary geologist James Dohm, the shorelines are based on past investigations using topographic measurements. The team used data from NASA’s Mars Odyssey spacecraft, in particular the Gamma Ray Spectrometer (GRS), which is capable of detecting elements buried as much as 33 centimetres below the surface of Mars by the gamma rays they emit, to delineate the ancient shorelines on Mars. The capability of the GRS was previously demonstrated in the ground-breaking discovery of water-ice near the surface throughout much of high-latitude Mars in 2002.
"We compared GRS 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," says Dohm. "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?"
Results from Mars Odyssey and other spacecraft suggest that past watery conditions likely leached, transported and concentrated such elements as potassium, thorium and iron into the lowlands. The team found that the potassium-thorium-iron enriched areas occur below the older and younger ocean boundaries with respect to the entire region. "In other words, GRS elemental information is consistent with the ancient aqueous activity documented in the literature, such as the transferral of volatiles and rock materials to the northern plains and the formation of lakes and oceans in the northern plains, which includes marine deposits that either remained unburied and/or are exposed by erosion and deformation," Dohm tells Astronomy Now. "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."
An illustration showing the location of the Tharsis volcanic region and Valles Marineris in the context of the hypothesized larger, ancient ocean and smaller, more recent ocean in Mars' northern lowland planes. It is argued that Tharsis volcanism unleashed great floods that carved large outflow channels and deposited sediment carried from the southern cratered highlands to the northern lowland plains, where water formed lakes and oceans and changed climate for thousands of years.
The research team liken the younger inner shoreline to an ocean ten times the size of our Mediterranean Sea that existed on the northern plains of Mars a few billion years ago, and think that the larger, more ancient shoreline that covered a third of Mars held an ocean about 20 times the size of the Mediterranean.
Understanding how and when water existed on Mars is crucial in determining the habitability of the red planet, since water is a vital ingredient for life as we know it. The debate as to the possible existence of ancient Martian oceans marked by shorelines has been a colourful area of discussion for over twenty years. A trigger for global oceans, according to a report by Professor Victor Baker and colleagues at the University of Arizona Lunar and Planetary Laboratory, is that erupting magma caused extreme heating, and therefore resulted in vast areas of ice melting and unleashing floods that ponded in the northern lowlands of Mars, forming seas and lakes.
“Several investigators hypothesize that before the magmatic-driven release, the ice begins to melt and carbon dioxide gases begin to build below impermeable ice somewhat like a lid - think of shaking a pop can,” explains Dohm. “Eventually, the carbon dixoide-charged water is released catastrophically in floods of enormous magnitudes.”
The latest analysis of GRS data adds key information to the long-standing oceans on Mars controversy. "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,” says Dohm.