Mars once had surprisingly frigid primeval oceans complete with their own icebergs, new evidence suggests.
There are currently two leading ideas for what the climate of ancient Mars might have been like.
One is that it was cold and dry, contending that valley networks and other geological features suggestive of liquid water in Mars' past were essentially results of bursts of heat confined in space and time, suggesting that Mars could not have sustained oceans. The other is that Mars was once warm and wet, implying that it could once have supported lakes, seas and rainfall for long periods.
Now researchers have found evidence of icebergs on Mars, supporting a third idea of the Red Planet's ancient climate — that of a cold and wet Mars, governed by oceans or seas covered partly in ice, as well as glaciers and massive polar caps.
Boulder and craters
To peer into Mars' climatic past, scientists focused on the flat, smooth, featureless Martian lowlands, which some have equated to an ancient ocean basin.
However, images captured by the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter revealed the presence of boulders about 1.5-6.5 feet (0.5-2 meters) across, as well as chains of roughly one or two dozen craters measuring 330-1,300 feet (100-400 meters) wide scattered throughout the northern plains. Both these details are hard to reconcile with the notion of fine-grained sediments deposited on a deep ocean basin, and had been used to cast doubts on the concept of an ocean on Mars.
Now astrobiologist Alberto Fairen at the SETI Institute and NASA Ames Research Center and his colleagues suggest the presence and distribution of these boulders and chains of craters could have been caused by rock fragments carried by icebergs, a common process on Earth.
They suggest glaciers in the highlands could have eroded the terrain, transporting rock within them and on their surfaces. Armadas of icebergs would have formed at the edges of glaciers as they melted and broke apart, which could then float thousands of miles on the ocean before they disappeared, depositing rock downward.
Also, on Earth, when icebergs scrape against the ocean floor, they can rain boulders down in clumps, which could explain boulder clusters up to about a mile (1.6 km) wide that scientists have seen on Mars. In addition, when icebergs roll along the sea floor on Earth, they can generate strings of dents, perhaps explaining the chains of craters seen on the Martian lowlands.
Seas or oceans
If there were icebergs, then there were open and sizable bodies of stable liquid water on the surface of Mars, Fairen said.
"The size of the water bodies may have ranged from several local seas to a single hemispheric ocean, and they may have been continuous in time or episodic," he told SPACE.com.
Some might suggest that the scattered boulders were deposited by so-called periglacial processes, Fairen said — that is, processes that take place at the edges of glaciers. However, such processes cannot give a satisfactory explanation for the boulder clusters that HiRISE also saw, he noted.
Others have also suggested that the crater chains were formed by volcanic processes.
"But our analyses can discard this hypothesis, especially because all the craters within one chain are almost identical in shape and dimensions, and that's neither expected nor usual in a volcanic process, but is expected if all the craters in the chain are carved by the same iceberg," Fairen explained.
Fairen added that scour marks some 0.6 to 3 miles (1 to 5 km) long seen in the northern plains and Hellas Basin of Mars could be evidence of icebergs as well. These could have been carved by the keels of icebergs scraping against the ocean floor.
"The scours are the most clear evidence for icebergs that we are finding," he said.
Fairen and his colleagues detailed their findings at the 2010 Astrobiology Science Conference in April.