Mars has hidden depths hinted at by intriguing pits scattered around the planet. Some of these tantalizing windows may be entrances to underground caverns. If humans ever make it to Mars, they may want to consider hunkering down underground to protect themselves from space radiation.
This image from NASA's Mars Reconnaissance Orbiter (MRO) in 2011 shows a hole on the slopes of the Pavonis Mons volcano. "Why there is a circular crater surrounding this hole remains a topic of speculation, as is the full extent of the underlying cavern," NASA said in an image feature in 2020.
No sarlacc here
This MRO image from 2015 shows a pit with a rocky-looking bottom. "No Sarlacc here, we think," the MRO HiRise camera team tweeted. This was the perfect image for a Star Wars joke. No aliens were found.
This wonderfully circular hole and others like it first showed up in images from NASA's Mars Odyssey spacecraft,which reached Mars in 2001. MRO later went in for a better look and produced this image in 2007. NASA estimated the hole to be about the size of a football field.
"Such holes and underground caves might be prime targets for future spacecraft, robots, and even the next generation of human interplanetary explorers," NASA said in an image release.
Pit with channels
There's more than just a pit here. Notice the channels leading off from the edge. "The pit, which formed after the channels, exposes a bouldery layer below the dusty surface mantle and is underlain by sediments," NASA said in a release in 2017.
According to NASA, this pit and others like it may have subsurface water ice. Future human visitors will be looking for accessible water as a resource.
What's better than a dark, weird pit on Mars? Two pits.
The HiRise camera on NASA's MRO snapped these "dark rimless pits" in late 2010. "The wispy deposit may consist of dark material that has been either blown out of the pits or from some other source and scattered about by the local winds," said the HiRise team.
NASA/JPL-Caltech/Univ. of Arizona
Pit or impact crater?
NASA gazed at this fascinating indentation found in the south polar region of Mars in 2017 and contemplated whether it might actually be an impact crater. If it's not a crater, then it's likely a collapsed area of ground.
The MRO HiRise camera team tweeted a short and sweet description to go with this wild Mars view from 2008: "Mind. Blown. A pit within an unnamed crater to the north of Rabe Crater." It's a good reminder of just how crazy the Martian landscape can get.
This side-by-side view shows the pit spotted by NASA's MRO in January 2020 on the left and the brightness-enhanced version on the right. It doesn't look quite as intimidating with the brightness turned up.
NASA's Mars Global Surveyor was really feeling the love from the red planet when it captured this picture of a heart-shaped pit in 1999. The space agency said this is "actually a pit formed by collapse within a straight-walled trough known in geological terms as a graben."
NASA/JPL/Malin Space Science Systems
Chain of pits
These pits seen on the Ascraeus Mons volcano on Mars are related to each other. "Pit chains such as this are the result of collapse along fault lines. In this case, before the collapses occurred, the fault was a conduit for molten rock -- magma -- which erupted to form a suite of lava flows," NASA said in an image release in 2005.
NASA's Mars Global Surveyor captured this sculptural view.
This collapse pit was shadowed by the angle of the sun when NASA's MRO snapped the picture in late 2008. "Pits like this form by collapse into underground voids, such as those left by propagating magma-filled dikes," said the HiRise camera team. Essentially, the ground underneath gives way, creating the haunting formation visible by NASA's orbiter.
Making Mars pits
These pits formed along the flank of the Elysium Mons volcano on Mars. NASA's MRO HiRise camera team called them an important clue to the origin of some nearby valleys. "These pits form as the ground is pulled apart by Marsquakes," the team said in a 2009 image release.
This isn't a natural pit on Mars. It a small drill hole made by NASA's Curiosity rover in 2013. The hole's diameter is about 0.6 inch (1.6 cm), and its depth is about 2.6 inches (6.6 cm). The rover used a close-up camera to get a good look at its handiwork.
Humans are already making a mark on Mars, just on a very small scale.