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Figure 1: Small crater with dark, rayed ejecta was not present in 1976; rate at which ejecta faded from 1999 to 2005 suggests mid-1980s age. | Figure 2: Comparison of MOC wide angle views with Viking 2 view
> | Figure 3: Context view, Ulysses Patera volcano in U.S. Geological Survey mosaic of Viking Orbiter images
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Scientists using the Mars Orbiter Camera on NASA's Mars Global Surveyor
spacecraft have discovered a crater that appears to have formed on Mars
in the past 20 or so Earth years, and have used it and several other
similar craters to estimate the present cratering rate on Mars.
One of the basic tenets of planetary geology is that impact craters have
accumulated on planetary surfaces at roughly a constant rate since the
early history of the solar system. This appears to have been the case for
small craters on the surface of the Moon, as shown by measurements of the
length of time that lunar rocks created by small impacts have been exposed
to cosmic rays, as determined by laboratory measurements of samples
returned to Earth by the Apollo astronauts. This principle should permit
the number of craters found on a planetary surface to be used to determine
the age of that surface, if the rate at which new craters form is known.
Scientists have previously estimated the cratering rate of Mars by scaling
the lunar cratering rate based on the proximity of Mars to the asteroid
belt, and by performing calculations based on orbital mechanics. Another
way to establish the cratering rate of Mars would be to use long-term
observations, say, from orbiting spacecraft, to actually locate new
craters.
The new crater is located on the southern rim of the summit crater, or
caldera, of the intermediate-sized martian volcano, Ulysses Patera. The
site was imaged by the Viking 2 orbiter in 1976 (left, an enlarged portion
of the image) and in narrow-angle views by the Mars Orbiter Camera in
1999 (center) and 2005 (right). The new crater, about 25 meters (82 feet)
across, is marked by a distinct dark, rayed pattern of ejected material,
or ejecta, which is seen to have faded somewhat between 1999 and 2005.
Ulysses Patera, a volcanic shield about 100 kilometers (62 miles) in
diameter volcanic shield, located near 2.5 degrees north latitude, 121.3
degrees west longitude, is one of the Tharsis volcanoes and is partly
buried by younger lava flows. The summit caldera is about 55 kilometers
(34 miles) in diameter.
The amount that the crater's rays faded between 1999 and 2005 can be used
to help estimate how many years ago the crater formed. The actual contrast
between the ejecta and the undisturbed volcano summit materials is
actually much less than it appears to be in these processed images, and
the amount of fading is also much less. Images of disturbed surfaces from
various parts of Mars, such as dust devil tracks, dark slope streaks and
rover tracks, indicate that disturbed surfaces on Mars are dark and that
they lighten with time. Using these other examples to estimate how dark
the ejecta from the Ulysses crater was originally, and how much it has
faded in six years, suggests the crater formed in the early to mid 1980s.
The rate at which dark surfaces lighten on Mars is not uniform over the
whole planet, but scientists using the Mars Orbiter Camera have found a
number of other craters with dark ejecta that have faded during the Mars
Global Surveyor mission. The scientists estimate that these craters
probably formed within the past 100 years. Although the sample is very
small (the Mars Orbiter Camera narrow angle camera has imaged barely 4
percent of Mars), it appears that the recent cratering rate for craters
on Mars 25 to 100 meters (82 to 328 feet) in diameter is about 0.000000003
to 0.000000006 craters per square kilometer (0.39 square mile) per Earth
year, which is about five times lower than previous estimates.
The site of the new crater is shown in wider context in a comparison of
the 1976 Viking image with wide-angle views taken by the Mars Orbiter
Camera in 1999 and 2005 (figure 2), and in even wider context in a
regional mosaic of Viking images (figure 3).
The Mars Orbiter Camera was built and is operated by Malin Space Science
Systems, San Diego, Calif. Mars Global Surveyor left Earth on Nov. 7,
1996, and began orbiting Mars on Sept. 12, 1997. JPL, a division of the
California Institute of Technology, Pasadena, manages Mars Global Surveyor
for NASA's Science Mission Directorate, Washington.