CRATER REMOVAL SERVICE. Opportunity's long drive has taken it past craters of many different ages. These range from ones less than 10 years old (a) to those that are roughly 10 milllion years old (g). (Image is Figure 1 from the paper.)
The smooth, flat plain where Mars rover Opportunity landed in January 2004 looks spookily empty. Only a few rocks and meteorites, plus foot-high sand dunes and ripples, break the endless vista under a clear tawny sky.
How long has the scene looked like this? Or to put the question another way, how old is this surface?
About 10 million years, says Matt Golombek (Jet Propulsion Laboratory), reporting (PDF) at the 43rd Lunar and Planetary Science Conference in The Woodlands, Texas. “That’s the crater retention age.”
He points out that’s not the whole picture, however. “Crater counts of Meridiani Planum show a surface with two ages. Craters larger than about 2 kilometers [6,600 feet] in diameter are highly degraded with light-toned rims. These are Noachian in age, or more than 3.7 billion years old.”
That’s the approximate age of the light-colored sulfate-rich sandstones that Opportunity drove across between its landing and its current operations at Cape York. The latter is a heavily eroded rim segment of Endeavour Crater, and its rocks are even older than the Meridiani sandstones.
But Golombek says craters smaller than about 100 m (330 ft) in diameter are much, much younger: only about 10 million years old. As he describes it, “Fresh craters have sharp, blocky rims with clearly defined blocky ejecta blankets and rays. With time, these soft sulfate rocks are planed off parallel to the surface by saltating sand grains, and crater interiors are filled with sand.”
What’s left in the end, he explains, “is a flat crater rim surrounding a subtle, broad topographic depression.”
Golombek explains that erosion operates relatively quickly at first because the crater’s features stand out above the plain. But the removal rate then becomes more and more gradual as crater features erode, its profile lowers, and sand fills it in.
Opportunity has driven 34 kilometers (21 miles) across the Meridiani plains since landing. “Craters in all stages of degradation have been visited by the Opportunity rover and imaged by HiRISE from orbit,” Golombek says.
As a result, he notes, “We understand how large and deep a crater of a given diameter is when fresh. And when we compare that with what Opportunity sees, we can estimate how old the crater is.”