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Study
of Mars
Landing Sites |
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Study
of Mars
Landing Sites |
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Project Description:We are currently working on a NASA/JPL project to characterize potential landing sites for Mars missions projected for 2001, 2003, and beyond. Several candidate regions have been identified, particularly low-lying areas along Mars' equator. Our approach so far has been to try to characterize these sites in terms of their crater populations, which can be a tool for estimating age and erosion history of the surface units (see our MARS GLOBAL SURVEYOR page).Mars Global Surveyor has taken many high-resolution images of two of the three previous landing sites: Viking 1, and Pathfinder. MGS is also acquiring new images of suggested 2001 sites and the Viking 2 site. These will allow us to characterize these sites from orbit with new information about the small crater populations, and their implications for age and geologic history. |
The crater count diagrams posted here have background isochron lines derived by assuming that Mars has twice the crater production rate as the moon, for a specified size of craters. This is a current estimate of the Martian crater production rate. Further information about the derivations of these isochrons is given is given on our MGS ISOCHRON web page.
At meetings at ISSI (International Space Science Institute, Bern, Switzerland) in July 1998, Gerhard Neukum and Bill Hartmann compared the Neukum estimate of the production function with the shape of the Hartmann isochrons used here (which are intended to have the production function shape) and determined that the two curves have virtually the same shape. Neukum's production function curve is a polynomial fit to his data and has a slightly more S-shaped curvature than the curves used here. However, our position is that the Neukum production function curves, as adapted to Mars by Neukum and Ivanov, can equally well be used to represent the input size distribution on Mars, prior to any erosive losses.
In this phase of the work, three independent data sets are generated. First, Dr. Eileen Ryan has made new counts from Viking and MGS digitized images. Craters are counted, binned, and classified using the free-ware image processing package NIH-Image. Second, Hartmann has combined new counts from Viking and MGS with his older Viking era counts. These are generally made from paper prints of the images. Third, with Hartmann, a group of student volunteers from Madrid, Spain have collaborated in producing MGS crater count data. These are also done on paper copies; the in the diagrams here, the Spanish counts have all been combined into a single curve by averaging the number of craters counted by different students in each bin on each photo, one photo at a time.
| THE VIKING 1 LANDING
SITE:
Figure 1 shows the combination of the three independent data sets for the Viking 1 site. At smaller sizes (D<~3km), this figure includes craters that appear to be superposed on the last flow unit(s), and thus allows an attempt to estimate the crater population formed since the end of formation of Chryse Planitia near the Viking 1 site. The data at these sizes appear to fit the production function shape quite well. The model age assigned to this general surface would be about 1-2 Gy, with the usual factor of perhaps 2 or 3 Gy uncertainty in abolute age. However, note that the largest craters
(D>3km)
have more scatter and appear
to cross older isochrons on the diagram, suggesting that we may
see some
older features, formed on underlying flow surfaces, but showing
through
the surface flows (See Figure 2). These older craters
may date back to 2-4 Gy.
The bent, solid line is a theoretical steady state assuming
steady crater
infill and obliteration; it fits old upland surfaces on Mars, but
the
Viking 1 data fall below it at D=250m to 32km, and fit better to
the
isochrons. The interpretation is that the Viking 1 site is young
enough to
preserve the production function at D>250m. |
 b) MGS images of Viking 1 landing site. |
a) Viking 1 area
b) Pathfinder area |
Figure 2. At the Viking 1 site, a number of large, older craters appear to be formed on one or more basement units, and to protrude through what appear to be Chyrse Planitia lava units. Evidence of this lava-flooding of craters is also visible at the Pathfinder site. Examples of some of these are shown in Figure 2. These support the idea that we see older craters added to the population at sizes large enought to show through the last surface flows. If the basement unit were much older than the upper units, these bigger craters would be much more numerous than the well preserved ones at at the same sizes in the size range D > 2 km. |
 b) MGS image of Pathfinder landing site. |
THE PATHFINDER LANDING
SITE:
Figure 3 shows the combination of
the
three
independent data sets for the Pathfinder site. This figure
includes
craters that appear to be superposed on the last flow
unit(s).
Similar to the Viking 1 site, this site suggests a model age
for the surface layers of about 0.5 to 2 Gy, based on craters
from D=~200m to ~8km. Also as at Viking 1, there is evidence
of larger size craters showing through the surface units from
layers that are more like 2 to 3.5 Gy in age. |
We have done some preliminary studies on the hematite region, one of the sites that had been considered for the Mars 2001 mission, which is now canceled.
Figure 2: Large
craters
in-filled by lava on older regions of the Viking 1 (a) and
Patfinder
(b) landing sites.