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A very dark spot on Mars could be an entrance to a deep hole or cavern, according to scientists studying imagery taken by NASA's Mars Reconnaissance Orbiter.

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The geological oddity measures some 330 feet (100 meters) across and is located on an otherwise bright dusty lava plain to the northeast of Arsia Mons, one of the four giant Tharsis volcanoes on the red planet.

The hole might be the sort of place that could support life or serve as a habitat for future astronauts, researchers speculated.

Must be deep

The Mars Reconnaissance Orbiter (MRO) used its High Resolution Imaging Science Experiment (HiRISE) instrument to draw a bead on the apparent deep hole - a feature that may cause more scientists to ponder about potential subsurface biology on Mars.

Because the spot lacks a raised rim or tossed out material called ejecta, researchers have ruled out the pit being an impact crater. No walls or other details can be seen inside the hole, and so any possible walls might be perfectly vertical and extremely dark or - more likely - overhanging.

HiRISE image specialists said the pit must be very deep to prevent detection of the floor from natural daylight, which is quite bright on Mars.

In April, it was announced that the NASA Mars Odyssey and its Thermal Emission Imaging System (THEMIS) found near the equator seven dark spots that scientists think could be entrances to underground caves.

Meanwhile, MRO is ready to target the dark spots on Mars over the coming months as opportunities arise, explained HiRISE principal investigator, Alfred McEwen, of the University of Arizona in Tucson.

"We especially want oblique images from the west, to see illuminated walls. These are deep holes with overhanging walls, but perhaps not long caves," McEwen told SPACE.com.

Cave