http://www.planetary.org/blog/article/00002324/ wrote:
Opportunity's thousand-year-old crater
By Emily Lakdawalla | Jan. 29, 2010
Please donate to support our blog, website, and podcast.
<<Since leaving Marquette Island on sol 2,122, Opportunity has been barreling southward on her journey toward Endeavour crater. On her horizon for the last several sols has been a very small but very fresh looking crater named Concepción.
Concepcion crater (Opportunity sol 2138)
Credit: NASA / JPL / mosaic by Emily Lakdawalla
How do I know the crater is "fresh?" The ejecta is very very blocky and is mostly resting on top of the surrounding soil. There's certainly no evidence that the sand ripples have marched over the blocks, so the crater formed since the time that the Meridiani dunes have last moved. However, it didn't form yesterday, or its center wouldn't be filled with dust.
So how do we know this thing is 1,000 years old? Matt Golombek was kind enough to explain things to me. In an abstract that he and several coauthors will be presenting at this year's Lunar and Planetary Science Conference, he explains the process by which they arrived at the age of a different set of craters visited by Opportunity, the so-called "Resolution" cluster. This included craters referred to as Resolution, Adventure, Discovery, and Granbee, which Opportunity visited from sols 1,818 to 1,854. They used three different methods to estimate when the ripples in Meridiani last moved to be approximately 100,000 years ago; and
the Resolution cluster is superposed on the ripples with no evidence of being deformed by ripple motion, so it's younger than 100,000 years old.
Concepcion crater from HiRISE___The Resolution cluster of craters
Matt told me that the Resolution cluster "has no rays, which have been found for the very youngest craters that have impacted in Meridiani (and elsewhere).
The Concepción crater has what look like degraded dark rays in HiRISE images, so we would say it is younger than the Resolution cluster.... but not as young as other very fresh impacts in Meridiani that have both bright and dark rays." These very fresh, bright-and-dark-rayed impacts have not been visited by Opportunity. You can perform statistical analyses to arrive at an age estimate for them of some decades old. Concepción's morphology -- having degraded rays -- places it intermediate in time between the Resolution cluster and these other very fresh craters, or between several decades and about 100,000 years old. Thus the age estimate for Concepción of about 1,000 years old. It's not exactly a definite age -- Matt told me "all of these ages should be considered as nothing more than educated guesses, but they do provide some constraints.">>