Seven very dark holes on the north slope of a Martian volcano are seen in this undated handout photo. An orbiting spacecraft has found evidence of what look like seven caves on the slopes of a Martian volcano, the space agency NASA said on Friday. REUTERS/ Handout/NASA

WASHINGTON (Reuters) - An orbiting spacecraft has found evidence of what look like seven caves on the slopes of a Martian volcano, the space agency NASA said on Friday.

The Mars Odyssey spacecraft has sent back images of very dark, nearly circular features that appear to be openings to underground spaces.

“They are cooler than the surrounding surface in the day and warmer at night,” said Glen Cushing of the U.S. Geological Survey’s Astrogeology Team and Northern Arizona University.

“Their thermal behavior is not as steady as large caves on Earth that often maintain a fairly constant temperature, but it is consistent with these being deep holes in the ground.”

The holes, which the researchers have nicknamed the “Seven Sisters,” are at some of the highest altitudes on the planet, on a volcano named Arsia Mons near Mars’ tallest mountain, the researchers report in the journal Geophysical Research Letters.

“Whether these are just deep vertical shafts or openings into spacious caverns, they are entries to the subsurface of Mars,” said USGS researcher Tim Titus.

“Somewhere on Mars, caves might provide a protected niche for past or current life, or shelter for humans in the future.”

But not these caves.

“These are at such extreme altitude, they are poor candidates either for use as human habitation or for having microbial life,” Cushing said. “Even if life has ever existed on Mars, it may not have migrated to this height.”