Image: Sand Pit On Mars. Where The Sarlacc At?

(Click image to enlarge. Via.)

Behold this righteous sand pit found on the surface of Mars. It leads to a cavern beneath the surface where undoubtedly Martians are planning their fatal assault on Mars-2, or what we call Earth. They terraformed it, they released us onto it, and now they’re coming home. Uh, right? Moving on.

Wondering how such a thing came to be?

Bad Astronomy:

Here’s how we think skylights like this form. In the distant past, Mars was geologically active. Rivers of lava ran across the surface. If the surface of the lava hardens it can form a roof, allowing the lava underneath to continue flowing; these are called lava tubes and there are bazillions of them in Hawaii, for example. Eventually, the source of the lava chokes off and the lava flows away, leaving the empty tube underground. If the roof is thin in one spot it can collapse. Sometimes that just leaves a hole, but apparently in this case it was under a sand field. Some of the sand must have fallen into the chamber below and eventually blown away, leaving the pit and the hole. The pit is located not too far from Pavonis Mons, a known (long-dead) Martian volcano.

The hole is about 35 meters (115 feet) across, so the pit is about 175 meters (nearly 600 feet) across the rim. I love how it sits in an otherwise nearly featureless sand field; the contrast is beautiful. In the high-res image you can see boulder perched on the rim, having rolled part of the way down as well. The inside of the pit has lines and furrows that are instantly recognizable to anyone who has tried to dig a hole at the beach and had sand continually flow down from the rim

Gorgeous.

(Phil Plait over at BA also made a sarlacc pit reference but it was it was the firs thing that popped into my head when I read the headline, so I had to go there as wel.)