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Astronomy in the News

Mercury Winter 2009 Table of Contents


Lava Flow

A young lava flow (dark region) lies atop an older surface on Mars. The lava flow has visibly fewer impact craters than the background terrain, illustrating the general principle that crater numbers can reveal ages of surfaces.

Image courtesy of NASA / JPL / Malin Space Science Systems.

Dating Small Martian Features

Planetary Science Institute

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 features, such as riverbeds and lava flows, according to William K. Hartmann, a senior scientist at the Tucson-based Planetary Science Institute.

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 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 didn't arise until 1997, when the small craters first became visible in images returned by the Mars Global Surveyor high-resolution cameras. In recent years, many more high-resolution images have come from the HiRISE camera aboard Mars Reconnaissance Orbiter.

The crater-counting system, which Hartmann first proposed in the 1960s, was originally developed for counting large craters that are several miles wide.

"Using small craters to predict the age of landforms is complicated," Hartmann observed. 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.

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