Back in 1954, Jerome Bixby wrote a fantastic story called "The Holes Around Mars". It was all about the discovery of series of weird holes discovered in the surface of Mars, holes seemingly punched through rocks and hills and mountains. We wouldn't dream of spoiling the ending (you can read it here.) But it's become weirdly topical, now that astronomers have been faced with the fact that Mars really is as full of holes as an M. Night Shyamalan plot.
One of the first was discovered by NASA's Mars Reconnaissance Orbiter (MRO) in 2007. It is more than 490 feet across. Like most similar features, it's probably a collapsed lava tube. Holes such as these are officially known as "skylights". Skylights on Earth often form in volcanic regions where ancient lava flows have created underground lava tubes. When the flow of lava ceases and the tubes become empty, caverns may form. If the roof of one these caverns collapses, a skylight forms. This theory gains support from the fact that the Martian hole lies in the midst of the Arsias Mons volcano system. Most similar Martian skylights are also found in volcanic areas. Skylights have also been observed on the Moon.
A photo released just last week of a Mars hole is really spectacular. Originally spotted on the flank of the Pavonis Mons volcano by the MRO's Context Camera earlier this year, mission managers decided to zoom in on the crater with the orbiter's High Resolution Imaging Science Experiment (HiRISE) camera. At the bottom of an incredibly symmetric crater is a 115-foot-wide hole. Just barely visible is the floor of a cavern about 65 feet below. One of the big question on everyone's mind is: Just how large are the caverns under these holes?
The Pavonis Mons skylight has raised a great many other questions. Is the crater an ancient impact feature that breached the roof of an underlying cavern? Or is it simply an erosion feature where surface material has slumped and drained into the skylight? It is clear from the photo that material has been sliding down the crater wall into the pit.
NASA scientists are excited about the discovery of the skylights. "Holes such as this," the agency explains, "are of particular interest because their interior caves are relatively protected from the harsh surface of Mars, making them relatively good candidates to contain Martian life. These pits are therefore prime targets for possible future spacecraft, robots, and even human interplanetary explorers."
Caves and caverns would also be ready-made shelters for future Mars explorers and colonists. This idea has inspired the Cave of Mars Project, which was established to search for just such features.