The Downselection Process and the Proposed Landing Sites
Downselection Process
A Mars Surveyor '01 Landing Site Workshop was held at the State University
of New York at Buffalo June 22-23, 1999. This workshop was open to the science
community and had sessions on the general project science and constraints,
new MGS results, general landing site considerations, and specific proposed
candidate landing sites. About 60 candidate landing sites were proposed
at the workshop [see URL http://cmex.arc.nasa.gov/MS_Landing_Sites/Workshop2/Wkshp2.html].
Most of the sites proposed met the engineering and remote sensing criteria
discussed earlier. These sites are located in the highlands (0-40°W),
Valles Marineris, Memnonia, Aeolis, Elysium, Terra Cimmeria, Isidis, and
Sinus Sabaeus. Unavailable prior to the workshop for most proposed sites
were the new MOC high-resolution images. Because of the importance of these
data in interpreting the meter-scale hazards at potential landing sites
and the inverse correlation between smoothness in Viking-scale (hundreds
of meters per pixel) and MOC-scale (meters per pixel) images, no attempt
was made at the workshop to downselect the number of sites under consideration.
Following the Buffalo workshop, there were two other significant meetings.
Immediately after the workshop, Steve Squyres met with the APEX team and
discussed the community input expressed at the workshop and summarized their
preferences based primarily on payload capabilities and science. Subsequently,
the Project Scientist sent out a Dear Colleague letter to the community
dated July 1, 1999 that outlined a strawman site selection policy. The Mars
Surveyor 2001 Project Science Group then endorsed the policy which is based
on the capabilities of the Mars '01 lander, its payload, and on the scientific
objectives of the Mars Surveyor Program. In summary:
1) The Mars 2001 Landing Site Workshop demonstrated that many scientifically
exciting landing sites can be found that are consistent with the mission's
engineering constraints.
2) The Mars '01 lander payload is excellent for studying soils. Soils
can be found virtually anywhere on Mars, and provide substantial new science.
Given this, safety is, therefore, the first priority (needless to say, safe
landing is required to do any new science).
3) After the discussions at the workshop, and further discussions with
the '01 investigator team, it was concluded that the best new science is
likely to come from landing within ancient highland crustal materials.
4) With the above scientific constraints (and within the engineering
constraints), the final site should be chosen so as to: a. - maximize total
mission duration b. - maximize rock abundance c. - maximize large-scale
topography in the visible distance, particularly if it exposes stratigraphy
d. - maximize the chances of finding aqueous minerals e. - consider potential
for future human/outpost base site
Given the present thermal and power constraints on the MS '01 lander,
the desire to maximize total mission duration (4a) places preferred landing
sites near the northern end of the latitude band (within a few degrees of
the equator). As the lander mission progresses, insolation at the southern
end of the latitude band decreases due to seasonal effects, which shortens
the length of daily operations and may end the mission prematurely. In addition,
early in the mission, warm conditions at the southern latitudes also have
the potential to shorten daily operations. Near the equator longer "sols"
(7 hrs) of operation are possible, and seasonal effects are minimized.
At the Mars 2001 workshop held in Houston Oct. 2-4, 1999, sessions were
held to discuss the downselection process and the science of the top candidate
landing sites. A description of the five sites that were evaluated during
these sessions and the pros and cons of each are listed below.
Isidis Rim:
This is a large zone with multiple possible landing ellipses (an image
of the zone with possible ellipses superposed was prepared by Tim Parker
and available during the discussion). Its great advantage is that it has
the potential to address a number of important science questions while,
at the same time, satisfying site constraints for the HEDS instruments.
Specific characteristics of scientific relevance include:
1. The zone is located on the rim of a Noachian (probably early Noachian)
impact basin (Isidis Basin), and thus there is an excellent chance that
Noachian rocks will be sampled.
2. There is clear morphologic evidence for the past presence of water in
the form of networks of small channels or valleys. This is a fundamental
requirement for exobiology objectives.
3. Because the Isidis impact would have excavated rocks from significant
depth, there is a chance that deep crustal or mantle rocks will be present.
4. This site is higher than all putative northern lowland shorelines, whereas
both Viking sites and the Pathfinder site are within the lowest shoreline.
If the northern ocean(s) existed, it is possible that the chemistry and
mineralogy of the soil will be different for sites that were and were not
flooded by the ocean(s).
5. This site is in the area where TES data seem to require that feldspar
be more common than pyroxene, a result in apparent conflict with liquid
descent models based on melts that could produce SNC meteorites and anorogenic
andesites (Rutherford). Thus the Isidis Rim may expose ancient rocks that
either have been altered in some unknown way to mask pyroxene, or else expose
ancient rocks derived from the cryptic high alumina layer predicted by Rutherford's
model.
6. The scenery should be spectacular because the landing ellipses are all
in plains areas that lie between the massifs that make up the Isidis basin
rim. This is of obvious PR value, but the side view of the rocks in these
massifs may well provide data of scientific importance as well.
The only significant weakness is that MOC images indicate that many of the
landing ellipses selected on the basis of apparent smoothness on Viking
images are in areas with rather ominous appearing m-scale roughness.
Hematite:
As pointed out by Steve Squyres, the instrument payload is ideally suited
for characterizing this site, in large part because of the Mössbauer
spectrometer. The site is of great potential interest because of the expectation
that the high content of crystalline hematite implies the presence of water
in general, and possibly hydrothermal water in particular. The high hematite
values correspond with a mappable geomorphic/geologic unit visible on Viking
Orbiter images. This correspondence between geology and mineralogy is uncommon.
This, in turn, raised the question of why this correspondence exists here
but apparently nowhere else (so far, anyway). In particular, does it imply
that the surface is completely stripped of dust (an undesirable characteristic
for the HEDS objectives)? In general it was believed that a total absence
of dust is highly unlikely. However, it is not clear if the high hematite
content is associated with a young layer superposed on surrounding older
highland crust, or if the high hematite content is associated with an older
layer that has been exposed by localized erosion. Thus possible scientific
strengths include:
1. Probable involvement of water in the formation of the hematite.
2. The possibility of preservation of microfossils, by analogy with banded
iron formations (BIF) on Earth.
3. Site uniqueness: If we do not go here in 01, we probably will not do
so in the foreseeable future (a fundamental rule of field geology is that
most collected and returned samples should be representative of the most
abundant rocks and soils, not of oddities; thus a sample-return mission
is not likely to be targeted to the hematite site).
4. MOC images of the western part of the hematite site show a surface that
looks very safe. Thus this site could be considered as a safe backup if
one is needed.
Some weaknesses pointed out by participants included:
1. Site uniqueness: Not representative.
2. Other than the hematite itself, we do not yet have a clear set of scientific
justifications for picking this site (this could change with time, however).
A good story needs to be assembled, comparable to what we have for the Isidis
Rim.
3. The smooth, safe surface may mean few rocks, and it almost certainly
implies no scenery.
NE Meridiani:
The site has surfaces that are generally smooth on MOC images. The bright
terrain is interesting and not really understood; it is smooth, whereas
in most places on Mars bright terrain is rough. This site provides an opportunity
to visit an area with low albedo (past landing sites have been in areas
with higher albedo). The rocks may well be different. The general feeling
was that this site would satisfy the need for a safe backup if safety becomes
the only issue of importance. However, the group felt that the hematite
site would be a more interesting safe backup than this one.
Amenthes Highlands:
Although in an area likely to contain Noachian rocks, this site does
not provide the variety of the Isidis rim. In addition, it is marginally
too dusty.
Hesperia Paleolake:
This is a very interesting site where it appears as if a lake was once
present within an old impact crater. Scientifically interesting characteristics
include:
1. There are low, presumably erosional, scarps within the crater lake basin
that can be reasonably interpreted as eroded lake beds, assuming that the
lake hypothesis is valid.
2. There is at least one post-lake impact crater with fresh-looking ejecta,
and thus samples of the underlying putative lake beds could be available.
3. There is an impact crater just outside of the crater rim that has emplaced
ejecta from the surrounding Noachian terrain into the crater (not clear,
however, if this occurred before or after the lake deposits were emplaced).
4. There would be interesting scenery (crater walls, ejecta blanket edge,
intra-basin scarps).
Problems include:
1. The relatively abundant MOC coverage shows a very rough surface.
2. Because of the presence of low scarps, it is possible to fit only one
landing ellipse within the crater, and this is in an area with no MOC coverage.
3. 2001 roving capability will not permit visiting any of the scarps or
the apparently fresh crater ejecta because these features cannot be within
the landing ellipse. This constraint may well eliminate this site from consideration
in 03 and 05 as well unless landing ellipses are shrunk sufficiently to
allow roving beyond their limits.
Workshop Results
At the conclusion of these workshop discussions, it was mutually agreed
to carry two areas forward for further evaluation and study: Isidis Rim
and Hematite. The NE Meridiani site was recommended as a distant third,
to be used only if a super safe site is required and if the Hematite site
does not satisfy this need. The coordinates of the Isidis Rim area are 3N-1S,
270-280W. The coordinates of the Hematitie site are not certain yet because
existing MOC images show the eastern part of this deposit to be too rough.
The landing zone will need to be confined to the western portion of the
deposit where available MOC images indicate the region is smoother (0-3S,
2-7W).
The MDIM of each region can be seen here:
Hematite
Isidis Rim
Malin Space Science Systems has created a website that shows all available
MOC images of the two sites. These can be seen at MOC
Images of Landing Sites
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