In 40 years of studying planetary surfaces, I've come across a lot of weird impact craters — oblongs, squares, and barely-theres — but nothing quite like this.
The team that operates the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter made this amazing image available a few days ago. Click here to open up a larger version in a new window, and then just take a moment to appreciate the view.
You're looking down on the Martian surface from an altitude of 190 miles (300 km), using an exquisite 20-inch (0.5-m) f/24 telescope that records details just a foot across. The surface textures inside and outside the crater's rim are incredible in their own right.
But which rim? This striking bull's-eye has at least three.
Now, multiple crater rims are not unusual, but they're exclusive to much larger impact basins (the Moon's Orientale comes to mind) often hundreds of miles across. This example, by comparison, is a much more modest 2,300 feet (700 m) across.
The HiRISE scientists are mulling two possible explanations for this striking bull's-eye pattern: unusual subsurface layering or a lucky second hit directly on top of an existing crater.
Located at 47° north, 165° west, the crater is set in a broad northern plain thought to be underlain by a thick layer of water ice. So it's not much of a stretch to imagine some warehouse-sized space intruder punching through the ice layer and into solid bedrock below, created the set of nested craters seen here. The layer-cake scenario is strengthened by the existence of a smaller crater, about a half mile to the southeast and pictured at right, that's also doubled.
But notice that the secondary crater within the big bull's eye isn't exactly symmetrical — it's just a bit off center. Also, it has a raised rim, which geologists are having trouble explaining if the root cause is a layered target.
Finally, note the two rounded lobes of debris at upper right on the crater floor. Could those be landslides triggered by a second strike inside a larger, preexisting crater?
It's fun to speculate about all this — especially when the picture itself is so arresting. For more information about this fascinating find, check out this posting.
HiRISE has been delivering thousands of incredible Martian vistas since its arrival in March 2006. There's a showcase of some of the best views in Sky & Telescope's September 2010 issue — and you can find a bonus selection, picked in consultation with the HiRISE team, right here.