Caves are nothing new to the Red Planet, but a recent photo taken by NASA’s Mars Reconnaissance Orbiter (MRO) reveals a particularly large example on the flanks of the shield volcano Pavonis Mons.
The walls of the pit are very steep – if they were any steeper, debris would crumble off the walls and roll down through the hole at the crater’s base. Material that once filled the pit drained down the walls to form a pile of debris in a subterranean chamber below. The top of this debris pile can be seen through the opening about 92 feet (28 meters) farther down, although only a hint of it appears in these photographs.
Based on a digital model of the ancient volcano’s terrain, scientists can estimate how much material was once in the pit and how big the pile below must be. The results are amazing – a huge hill of soil and rocks some 203 feet (62 meters) tall stands below the opening in the crater’s floor. Given that the top of this pile is 92 feet below the rim of the central hole, this tells us that the empty cavity was once 295 feet deep (90 meters) deep, prior to collapse and infilling.
Only a few caves on Earth are this deep. Most of those are created when water dissolves limestone. Limestone remains elusive on Mars, so planetary astronomers look to lava tubes as the most likely source of the subterranean cavern beneath the pit.
Most Martian volcanoes are built up from multiple lava flows pouring down their flanks eruption after eruption. Sometimes the surface lavas cool and solidify to form a roof over lavas that continue to flow in underground lava tubes.
As the tubes drain, they can leave empty caverns – caves as it were. Sections of the roof can later collapse forming openings into an underground network of skylights.
Perhaps that’s what were seeing here – a window into the past when lava coursed across the thickening slopes of Pavonis Mons. One wonders whether geothermal springs might still bubble and trickle within the cave’s recesses. Could thermophilic (heat-loving) bacteria have evolved on Mars as they did on Earth and gained a foothold there? And might their descendants still be holed up as the rest of the planet became a desert sterilized by ultraviolet light from the sun? In my crystal ball I see future mini-drone missions to Martian caves followed by visits from astronauts.
Maybe someday we’ll see what’s up down that hole.