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Mercury
Winter 2009 Table of Contents


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.
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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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