rootless cones

Hieroglyphic-like features point to past subsurface water on Mars

Although these strange features on Mars look a bit like hieroglyphics or geoglyphs such as the mysterious Nazca lines on Earth, they are completely natural features, ones that are found on Earth too.

Called ‘rootless cones,’ they form on lava flows that interact with subsurface water or ice. Their formation comes from an explosive interaction of lava with ground ice or water contained within the regolith beneath the flow. Vaporization of the water or ice when the hot lava comes in contact causes an explosive expansion of the water vapor, causing the lava to shoot upward, creating a rootless cone.

The scene above is located in Amazonis Planitia on Mars, a vast region covered by flood lava. If this image were in color, we’d see the surface coated by a thin layer of reddish dust, which avalanches down steep slopes to make dark streaks.

Just how big are these strange features on Mars and how old are they? “The cones are on the order of a hundred meters across and ten meters high. The age of these specific cones isn’t known. They are on a mid- to late-Amazonian geologic unit, which means that they are young by Martian standards but could be as much as a few hundred million to over a billion years old.”

The water or ice that led to the formation of these cones was likely within a few meters (or less) of the surface, and so it’s probably not there anymore. At this low latitude (22 degrees north), shallow ground ice is currently unstable, and should sublimate on timescales much less than the likely age of the cones.

Image credit: NASA/JPL/University of Arizona