Evidence found for recent shallow ground ice on Mars
Posted: June 14, 2001

New high-resolution images from Mars Global Surveyor's Mars Orbiter Camera (MOC) show evidence of ground ice on Mars as recently as 10 million years ago.

More striking is that the signs of geologically recent ground ice deposits are near Mars' equator, where ice was probably no deeper than 5 meters (15 feet) below the surface, University of Arizona scientists say.

Cluster of cones north of the Cerberus plains on Mars, as seen in an image from Mars Global Surveyor. Photo: NASA/JPL/Malin Space Science
"If ground ice was present within 5 meters of the surface only a few million years ago, it is very likely to persist today within about the upper 10 meters," said UA planetary sciences Professor Alfred S. McEwen. "This is especially interesting because it is an equatorial region of Mars, more accessible to exploration."

Peter D. Lanagan, McEwen and Laszlo P. Keszthelyi of the UA Lunar and Planetary Laboratory, and Thorvaldur Thordarson of the University of Hawaii have discovered clusters of tens to many hundreds of small "rootless" cones in MOC images of the Cerberus plains, Marte Valles, and Amazonis Planitia region near Mars' equator.

The martian cones are similar both in morphology and size to rootless cones in Iceland, features which form when surface lava interacts explosively with near-surface groundwater.

"The martian cones sit on pristine lava surfaces, and the cones are generally close to fluvial (water-carved) channels. The lavas do not appear to have been modified since they were emplaced, and some of the channels appear to be similarly pristine," Lanagan said.

Using crater counts and other geologic evidence seen in the detailed new MOC images, William K. Hartmann of the Planetary Science Institute in Tucson and others recently determined these lava flows to be as young as 10 million years.

"We consider recent fluvial recharge to be the most likely origin for the shallow ground ice," the UA/Hawaii team conclude this week in Geophysical Research Letters. "If shallow ground ice in these regions was present less than 10 million years ago, deposits of shallow ground ice probably persist in the vicinity of the cone fields to the present day."

Rootless cones, or pseudocraters, do not form over volcanic vents.

Thordarson concludes from years of fieldwork that in Iceland, rootless cones form where molten lava flows over marshy terrain. A crust forms over the lava flow, while molten lava continues to pump through tubes or pathways beneath the crust. As lava is shoved through the tubes, it mixes with some of the underlying water-rich sediment, and in the process of mixing, the water is heated by lava until it flashes to steam. When the steam pressure exceeds the pressure of the lava above it, there's a "phreatomagmatic" -- or groundwater and magma -- explosion. The result of several such sustained explosions is a cluster of cones associated not with any deep fault or fissure but with a network of lava tubes over the marshy area.

"We see many hundreds of similar cones in the Mars scape, and they appear to be associated either with low plains areas or with recent outflow channels," Lanagan said. Water would flow to low areas, pond and percolate in low plains during the floods, recharging ground ice.

Air photo of rootless cone field in Laki lava flow north of Innryi Eyrar, Iceland. Photo: Courtesy University of Arizona
"If the terrestrial rootless cone analogy is extended to Mars," he added, "lava flows erupted over surfaces with ground ice -- probably at a depth of less than 5 meters -- where they melted the ice to form a water-rich slurry which mixed with the tube-fed lavas. The process likely would have resulted in a series of phreatomagmatic explosions, which formed cones on the top of the chilled lava crust.

"The martian cones are close to outflow channels, so the cones formed in regions that were probably water- or ice-rich. Also, the martian cones generally are seen to sit on platy-ridged lavas similar to Icelandic 'rubbly pahoehoe' lava flows, where lavas delivered through tube networks breaks the hardened, chilled crust of the flow and move the resulting pieces around like a pulled-apart jigsaw puzzle. This suggests that the martian cones formed over lava flows fed by lava tubes, similar to rootless cones in Iceland."

Researchers debate whether the shallow ground ice that exploded to create the cones is relic ice leftover from the planet's formation, recondensed water vapor from the soil-atmosphere water vapor exchange, or recharge from surface flooding events.

"It is unlikely that relic ground ice has survived for 4 billion years in equatorial regions of Mars," the UA/Hawaii team concludes.

The argument that equatorial ground ice could be recharged by an exchange of water vapor between the ground and the atmosphere -- as Arizona State University scientists have modeled -- is perhaps more plausible, the team wrote.

"However, because these cones appear to be near outflow channels, we think that the water the formed the cones is probably recharge from floods," Lanagan said.

Scientists studying Viking imagery in the late 1970s and 1980s noted structures they interpreted to be rootless cones. Most of these were twice the size of the largest terrestrial cones, however, and it was unclear if some of these rested on actual volcanic surfaces.

The UA researchers looked at some of these areas again, this time using high-resolution MOC images, but still could not tell if the cones sit on volcanic surface because the terrain is either heavily mantled by dust or significantly eroded, Lanagan said.

"The structures observed by MOC are the first clearly identified martian cones having dimensions, morphologies, and geologic settings similar to terrestrial rootless cones," the team wrote in GRL.

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