An immense 130-foot-thick slab of frozen water has been discovered beneath the surface of Mars. While astronomers have already found ice at the planet’s poles and traces of water in the soil, the sheer size of this sheet and its location at midlatitudes is unprecedented.
The discovery resulted from a group of peculiar craters spotted by Ali Bramson, a graduate student at the University of Arizona. She observed a cluster of the geological formations that, unlike most craters, have terraced walls rather than a smooth bowl shape. The uneven walls suggest that the shockwave that formed each crater encountered layers of different materials as it moved through the surface. At each interface between a weaker and stronger material, a terrace would have formed. Since all of these terraced craters were located in the same area, Bramson realized that there was more going on than meets the eye.
To tease out the origins of the terraces, the research team used a high resolution camera on board NASA’s Mars Reconnaissance Orbiter to model the craters and measure the depths of the terraces. They also used the MRO’s Shallow Radar to beam signals to Mars and time how long it took for the signals to permeate the surface and rebound. The speed at which the radar beams traveled through the surface indicated that they were moving through a thick sheet of ice just beneath the planet’s dusty surface.
"It extends down to latitudes of 38 degrees. This would be like someone in Kansas digging in their backyard and finding ice as thick as a 13-story building that covers an area the size of Texas and California combined," Bramson told Space.com.
The scientists theorize that an ice field of this magnitude must be tens of millions of years old, having formed during one of the planet’s past ice ages. Mars has a much less stable climate than Earth because of its wobbly orbit, which occurs because its moons are too small to stabilize its axis of rotation. As a result, the planet has carried highly variable amounts of water over its geologic history. Learning more about where water used to exist and how it disappeared can reveal whether Mars was ever truly habitable, and could even lead to the discovery of Martian lifeforms.
This particular ice sheet likely formed from heavy, continuous snowfall that was buried and preserved. But its current stability doesn’t match up with the planet’s exceptionally dry atmosphere. The researchers plan to further investigate why the ice has persisted this long, and whether it harbored alien life at some point in the planet’s past.