Having realized that a new dark spot on Mars, seen in a red wide angle
camera image acquired on 6 January 2006, might be an indication of a
recent meteor impact site (see PIA09020 or MOC2-1611), the Mars Global
Surveyor (MGS) Mars Orbiter Camera (MOC) science operations team
considered it possible to find more such impact sites using the MOC red
wide angle camera. The most recent, freshest craters would be expected to
be quite small, ranging from a few meters across to maybe a few hundred
meters or so, at most, in diameter (100 meters is about 109 yards; compare
that with a 100 yard U.S.-style football field). Something less than 100
meters across would not show up easily in a 240 meters per pixel red wide
angle image. But the 6 January 2006 image showed that it could, because
these small impacts, if they occur in an area thickly mantled with dust,
will create a much larger "blast zone" around them.
Thus, the MOC science operations team set out to image a few of the
dustiest regions on Mars -- Tharsis, Amazonis, and Arabia -- with the red
wide angle camera. The same camera had, in May and early June 1999,
already imaged most of the planet at about 240 meters per pixel scale. By
repeating areas already imaged in May/June 1999 during the January/March
2006 timeframe, we would be able to identify more dark spots.
And, so, that is what we did. The Tharsis, Amazonis, and Arabia regions
were re-imaged using the MOC red wide angle camera during January through
March 2006. The data covered about 21,506,000 square kilometers (~8.3
million square miles; ~1/3 the surface area of Mars and more than twice
the area of the United States). As each picture was received on Earth, we
compared it with the images acquired during May/June 1999. Over the entire
area surveyed, we found 39 dark spots that were present in early 2006 but
not visible in May/June 1999.
The 39 dark spots, then, were the candidate impact sites. Each one of
these became a target for the MOC narrow angle camera, which would be used
to take an image of about 1.5 meters (4.9 feet) per pixel of each site.
The targets were entered into the MOC database. Then, as the predicted MGS
ground track came near each site, the MOC team targeted an image by
working with the spacecraft engineers at Lockheed Martin Astronautics
(Denver, Colorado) and the Caltech/Jet Propulsion Laboratory (JPL,
Pasadena, California) to point the spacecraft and camera at each site
using the Roll Only Targeted Observation (ROTO) maneuver.
Of the 39 dark spots, 20 turned out to be fresh impact sites, and 19 of
them were not. The other 19 included mistaken identifications (one was a
transient, large dust devil shadow; several were craters that had been
present in earlier images but had changed in brightness owing to dust
removal), new dark wind streaks, and new dark slope streaks created by
avalanching dust on steep slopes.
Some of the 20 new impact sites received further attention, as the
spacecraft and MOC were used to obtain cPROTO (compensated Pitch and Roll
Targeted Observations) views that have a spatial resolution of 0.5 meters
(1.6 feet) in the downtrack dimension and 1.5 meters (4.9 feet) in the
cross track direction. The cPROTO views, where obtained, have a higher
resolution and better signal-to-noise ratio than the original ROTO images.
Finally, while our approach of comparing MOC red wide angle camera images
obtained in May/June 1999 with those obtained in January/March 2006
constrains the 20 craters all to having formed during the May 1999 to
March 2006 time interval, we found in all cases that there were already
other images that had been received on Earth that helped constrain the
time of the impact more tightly. In some cases, the date of the impact
could be pinned down to within a month or two, in other cases the interval
covered several years. Data from the MGS MOC, Mars Odyssey
Thermal Emission Imaging System (THEMIS), and Mars Express
High Resolution Stereo Camera (HRSC) were all employed in the search.
Shown on this page (above) are pictures that illustrate our work to find
new impact craters:
Figure A: This picture shows one of the new impact sites identified by the
MOC team. Located in northern Arabia Terra near 29.3°N, 333.2°W, the
actual crater is quite small, only 11.2 ± 3.0 meters in diameter. This is
a sub-frame of MOC image S16-01105, acquired using a ROTO maneuver on 12
March 2006.
Figures B and C: These pictures are MOC red wide angle camera images,
obtained at a scale of about 240 meters per pixel, of portions of Arabia
Terra. Figure B is M01-01610 and was acquired during the MOC Geodesy
Campaign (see PIA02022 and PIA02023, or MOC2-127) on 14 May 1999. Figure
C, MOC S14-02741, was obtained on 26 January 2006 as part of the campaign
to find new impact craters. By comparing the two images, one from 1999 and
one from 2006, we were able to identify all new dark spots that formed
during that interval. In this case, the new dark spot seen in the 2006
image, S14-02741, is inside the white circle. The same location is also
indicated by a circle in the May 1999 image, but no dark spot is present
there. In both cases, the white circle is about 12 km (7.5 mi) across.
Figure D: This map of Mars, showing the location of all the MOC red wide
angle camera images acquired for the search for new craters during January
through March 2006. These images cover most of Amazonis, Tharsis, and
Arabia Terra. The base map is a product that combines the May/June 1999
MOC red wide angle data (plus later data for the south polar region) and
laser altimeter data from MGS.
Figure E: This picture shows portions of two red wide angle camera context
images that more tightly constrain when the new crater shown here (above,
top, left) formed. The first picture, R05-00427, was acquired on 5 May
2003 and shows no dark spot at the site of the impact. The second picture,
S05-01885, shows that the dark spot was present on 29 April 2005. Thus,
these two images tell us that the impact occurred sometime between those
dates: 5 May 2003 and 29 April 2005.
The Mars Global Surveyor mission is managed for NASA's Office of Space
Science, Washington, by the Jet Propulsion Laboratory, a division of the
California Institute of Technology, Pasadena. Lockheed Martin Space
Systems, Denver, developed and operates the spacecraft. Malin Space
Science Systems, San Diego, Calif., built and operates the Mars Orbiter
Camera.
For more information about images from the Mars Orbiter Camera, see
http://www.msss.com/mgs/moc/index.html.