Before an impact created Holden crater, large channels crossed and deposited sediments in Holden basin. Blocks as big as 50 meters across were blasted from Holden basin when Holden crater formed, then fell chaotically back to the surface and eventually formed "megabreccia," a conglomeration of large, broken boulders mixed with smaller particles. HiRISE images show megabreccia outcrops in Holden crater walls. This megabreccia may be some of the oldest deposits exposed on the surface of Mars.
HiRISE has also found that this "megabreccia" is topped by layers of fine sediments that formed in what apparently was a long-lived, calm lake that filled Holden crater on early Mars.
"Holden crater has some of the best-exposed lake deposits and ancient megabreccia known on Mars," said HiRISE's principal investigator, professor Alfred McEwen of the UA's Lunar and Planetary Laboratory.
"Both contain minerals that formed in the presence of water and mark potentially habitable environments. This would be an excellent place to send a rover or sample-return mission to make major advances in understanding if Mars supported life," he added. According to another instrument on the Mars Reconnaissance Orbiter - the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), at least 5 percent, by weight, of the fine sediments in the layer on top of the megabreccia consists of clay.
"The origin of the clays is uncertain, but clays in the probable lake sediments implies quiescent conditions that may preserve signatures of a past habitable environment," said HiRISE co-investigator John Grant of the Smithsonian National Air and Space Museum.
Topping the clay layers that formed in the placid Holden crater lake are layers of great boulder-filled debris unleashed later, when water breached Holden crater rim, creating a torrential flood that eroded the older lake sediments.
"The volume of water that poured through during this flood must have been spectacular," said Grant. "It ripped up finely bedded materials, including blocks 70 meters or 80 meters across -- blocks nearly the size of football fields," he added.
"If we were looking on Earth for an environment that preserves signatures related to habitability, this is one of the kinds of environments we would look at," said Grant.