Small view (315 KBytes)
Medium view (1 MByte)
Large view (3.6 MBytes)
The Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) Extended
Mission has included dozens of opportunities to point the spacecraft
directly at features of interest so that pictures of things not seen
during the earlier Mapping Mission can be obtained. The example shown
here is a small meteorite impact crater in northern Tharsis near
17.2°N, 113.8°W. Viking Orbiter images from the late 1970’s
showed at this location what appeared to be a dark patch with dark
rays emanating from a brighter center. The MOC team surmised that the
dark rays may be indicating the location of a fresh crater formed by
impact sometime in the past few centuries (since dark ray are quickly
covered by dust falling out of the martian atmosphere). All through
MOC’s Mapping Mission in 1999 and 2000, attempts were made to image
the crater as predictions indicated that the spacecraft would pass
over the site, but the crater was never seen. Finally, in June 2001,
Extended Mission operations allowed the MOC team to point the
spacecraft (and hence the camera, which is fixed to the spacecraft)
directly at the center of the dark rays, where we expected to find the
crater.
The picture on the left (above, A) is a mosaic of three MOC high
resolution images and one much lower-resolution Viking image. From
left to right, the images used in the mosaic are: Viking 1 516A55,
MOC E05-01904, MOC M21-00272, and MOC M08-03697. Image E05-01904 is
the one taken in June 2001 by pointing the spacecraft. It captured
the impact crater responsible for the rays. A close-up of the crater,
which is only 130 meters (427 ft) across, is shown on the right
(above, B). This crater is only one-tenth the size of the famous
Meteor Crater in northern Arizona.
The June 2001 MOC image reveals many surprises about this feature.
For one, the crater is not located at the center of the bright area
from which the dark rays radiate. The rays point to the center of
this bright area, not the crater. Further, the dark material ejected
from the crater–immediately adjacent to the crater rim in the picture
on the right (above, B)–is not continuously connected to the larger
pattern of rays. Asymmetries in crater form and ejecta patterns are
generally believed to occur when the impact is oblique to the surface.
The offset of the crater from the center of the rays suggests that the
meteor struck at an angle, most likely from the bottom/lower right
(south/southeast). The strange geometry of the rays is quite different
from that seen for rays associated with impact craters on the Moon
and other airless bodies; one possible explanation is that they
resulted from disruption of dust on the martian surface by winds
generated by the shock wave as the meteor plunged through the martian
atmosphere before it struck the ground.
