The Medusae Fossae Formation (MFF) is a thick deposit of soft materials that erodes easily by wind. The formation spreads in several large patches between the volcanic provinces of Elysium and Tharsis. In addition, outliers extend farther afield, with some reaching close to Gale Crater, the landing site for Curiosity, NASA’s Mars Science Laboratory rover, en route to an August 2012 landing.
The origin of the Medusae Fossae Formation is thought by many scientists to be volcanic ash, possibly from Apollinaris Patera as one group of scientists has argued. But the question of its age is unanswered. For a long time, scientists estimated the entire formation as Amazonian in age, or younger than 3.5 billion years old.
James Zimbelman and Stephen Scheidt (National Air and Space Museum) argue in a brief report in Science Express that most of the Medusae Fossae formation is Hesperian in age, dating to 3.6 to 3.7 billion years old. The upper surface of the lowest MFF member appears Amazonian in age — about 3.4 billion years old — but that’s because erosion has erased the craters that would reveal its true age, which is Hesperian also.
The team arrived at these ages by counting craters in daytime infrared images from the THEMIS camera on Mars Odyssey. “We found the dust-mantled MFF materials actually show more subtle features better in daytime IR rather than with a high-resolution imager such as HiRISE. Daytime IR revealed very subtle slopes and more craters,” Zimbelman says.
When they examined the surfaces of the formation near Gale Crater, they found the lowest (oldest) layer shows enough craters that it must have formed before the late Hesperian. They also found that it’s difficult to see much difference between these lowest sediments in the Medusae Fossae Formation and those making up the top layers of Mt. Sharp, the 5-kilometer (3 mile) high mound in Gale.
“A Hesperian age for the western Medusae Fossae Formation has implications for materials at the Mars Science Laboratory landing site,” Zimbelman and Scheidt explain. “Our results are consistent with a recent cratering study for the entire Gale mound, which indicates a late Hesperian to early Amazonian exposure age.”
They add that there may not be a major time gap between the upper and lower parts of the Gale mound, despite an apparent erosion interval between the mound units. “The hypothesized ignimbrite origin for the Medusae Fossae Formation may thus apply to the regularly layered upper units of the Gale mound.”
And they note, “Curiosity may test this interpretation while exploring the Gale mound.”