Smaller and more recent geological features on Mars can now be dated

October 13th, 2008 - 2:09 pm ICT by ANI  

Washington, Oct 13 (ANI): The crater-counting system that scientists have used since the 1970s to determine the age of large geologic features on Mars will also allow them to date small and more recent features, such as riverbeds and lava flows.

Crater counting relies on the density, or crowding, of craters to determine the age of planetary surfaces. It works on the assumption that older landforms have been exposed for a longer periods and have been hit by more meteorites than younger surfaces.

While the method is widely recognized as valid for large, miles-wide craters, some scientists had questioned whether the rate at which small craters form is well enough understood and constant enough to be trusted in predicting the age of a landform.

The issue didnt arise until 1997, when the small craters first became visible in images returned by the Mars Global Surveyor high-resolution cameras.

The crater-counting system, which William K. Hartmann, a senior scientist at the Tucson-based Planetary Science Institute, first proposed in the 1960s, was originally developed for counting large craters that are several miles wide.

While the large craters are formed by a single event, many small craters can be formed simultaneously when a large meteorite slams into the planet and throws debris into the air, which then falls as secondary meteorites, he explained.

Meteor showers also can produce many small craters in a short time.

These phenomena caused some scientists to question the lower limits of crater-counting accuracy.

To test whether small craters can reliably determine the age of planetary features, Alfred McEwen, principal investigator for the HiRISE camera, proposed that researchers add up the number of small craters that form inside some of the youngest large craters on Mars.

The idea was that, if the system works, these small craters should give roughly the age expected for the youngest crater.

Young craters were needed because theres general agreement on the rate at which miles-wide craters form on the Red Planet. If the crater is new, they can assume it formed sometime between now and a single interval of crater formation.

McEwen and his colleagues identified some of the youngest large craters in HiRISE images.

These were craters in the range of about two to 10 miles in diameter that have perfectly sharp rims that show no signs of erosion, indicating that they formed in recent geologic time.

Weve tested this theory on about eight of those large craters and in every case, the count of small craters has given the expected approximate age, Hartmann noted.

According to Hartmann, counting the number of small craters around other Martian formations, such as a dry river channel or lava flow, will yield an accurate age for that feature. (ANI)

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