Issues relating to the selection of initial landing sites for manned Mars missions are discussed from a geological viewpoint. The two prime objectives for initial manned exploration should be the youngest unambiguous lava flows (to tie down the late end of the cratering history curve for Mars) and old highland crust, which is best sampled and studied through the use of large impact basins as natural, planetary drill-holes. Exploration of these two sites will provide data on Martian chronology, volcanism, impact processes and gross chemical structure that will enable a first-order global synthesis through integration of these results with the global remote-sensing data already in hand from Viking and that to be provided by the Mars Observer Mission

Spudis, Paul D.

English

NASA Technical Reports Server

N87-17769
SITE SELECTION FORMANNEDMARS LAI_II_:
A GEOLOGICAL PERSPECTIVE
Paul D. Spudls
U. S. Geological Survey
Flagstaff, Arizona 86001
ABSTRACT
Issues relating to the selection of initial landing sites for manned
Mars missions are discussed from a geological viewpoint. The two prime
objectives for initial manned exploration should be the youngest,
unambiguous lava flows (to tie down the late end of the craterlng history
curve for Mars) and old highland crust, which is best sampled and studied
through the use of large impact basins as natural, planetary drill-holes.
Exploration of these two sites will provide data on martian chronology,
volcanism, impact processes and gross chemical structure that will enable
a flrst-order global synthesis through integration of these results wlth
the global remote-senslng data already in hand from Viking and that to be
provided by the Mars Observer Mission.
INTRODUCTION
A system to deliver men to the surface of another planet implies
scientific capabilities many times greater than that of an automated,
unmanned exploration spacecraft {see Taylor, 1975). Although site selec-
tion for the initial manned landings on Mars will be guided by many
complex factors, the geological perspective is the purpose of this con-
tribution. Other scientific disciplines, such as geophysics, may be
interested in different sites. For the purpose of this discussion, I
will concentrate on potential landing sites that will fundamentally
contribute to deriving a detailed knowledge of martian geologic history.
This involves selecting landing sites that span the vast ranges of time
and processes that we observe on the surface of Mars.
A GEOLOGIC RATIONALE FOR MARTIAN LANDING SITE SELECTION
Although numerous studies are conducted during mannned missions,
from a geological point of view, we are interested primarily in: 1)
absolute ages of regional stratigraphic units; and 2) the composition,
lithology, and possible paleontology chemistry, of rocks that make up the
martian surface. Geological mapping based on returned photographs (e.g.
Scott and Carr, 1978) has shown that Mars is a complex, heterogeneous
planet, with regional geologic units that span the range from heavily-
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https://ntrs.nasa.gov/search.jsp?R=19870008336 2020-03-20T12:29:16+00:00Z
cratered terrain (representing the oldest units) to very sparsely-
cratered lava flows (representing some of the youngest units).
In selecting landing sites to address the global history of Mars, a
general strategy might be based on establishing two end points for
martian geologic history. First, we would like to know the absolute age
of the _ martian lava flows. This would answer the questions:
When did martian volcanism cease? By calibrating the lower end of the
planet-wide crater-frequency curve, the absolute age of most martian
geologic units could be derived. Horeover, sampling lava flows not only
gives us direct information concerning the composition of martian surface
units, it also indirectly provides data on the probable chemical and
petrologic nature of the martian mantle. Second, a landing site to
sample and investigate the oldest martian geologic units would provide
data at the opposite end of the age spectrum. This is best accomplished
on Hars, as it is on the Hoon, by sampling the rims of multl-ring basins,
which are large impact craters that have excavated many kilometers into
the crust of Hats. We therefore, have an opportunity not only to obtain
samples of the ancient martian crust for age dating, chemistry and petro-
logy, but also the potential to establish any vertical stratigraphy that
may exist within the crust by reconstructing the basin impact target.
Additionally, all martian basin landing sites appear to be partially
embayed by numerous geologic units of diverse ages. Thus, a manned
mission to one of these sites could not only provide data for early
martian history, but also fill in gaps by sampling and dating some
intermediate age units as well.
These two prime objectives, to investigate both the latest and
earilest martian geologic units, will enable global extrapolations that
should give us a fairly complete understanding of martian geologic
history. The intermediate phases of martian history could be
reconstructed by carefully integrating global photographic and remote-
sensing compositonal data (to be provided by the HGCO mission), with the
results of manned sample return and geologic exploration. However,
detailed knowledge of martian geology will probably come only after many
generations of surface exploration. Such a long range plan is beyond the
scope of this paper; the following section will briefly describe some
selected landing sites that will maximize the geologic return of brief
579
series of manned missions and will give a broad knowledge of the geologic
history of Mars and the processes that have shaped its surface.
SOME RECOMMENDED MARTIAN LANDING SITES
Mars is such a geologically diverse and complex planet (e.g. Mutch
et. al., 1976; Cart, 1981), that to compile a list of geologically
interesting landing sites to inventory all the processes that have
operated during the planet's history would be an exercise in futility.
Instead, this discussion will be confined to the two prime objectives
listed above; some additional geologic "targets of opportunity" are
presented, in addition to the prime sites, in Table 1.
Prime Objective 1 - The youngest martian lava flows
The Tharsls province of Mars possesses some of the most spectacular
volcanoes observed in the solar system. It was recognized early in
martian exploration that vast regions of this area contain few superposed
impact craters, indicating a geologically-young age (Carr,1973; BVSP,
1981). Through detailed mapping and crater-counting of lava flows in the
Tharsls region (Schaber et. al., 1978; Plescia and Saunders, 1979;
Morris, in press), the youngest flows may be recognized (Fig. 1).
The smooth lava plains of the uppermost member of the Olympus Mons
Formation (Scott and Tanaka, 1985) have the lowest cumulative crater
density of all Tharsls flows (Number of craters > 1 km diameter = 78 /
106 km2; Morris, in press). Moreover, they are unambiguous lava flows,
displaying flow lobes and pressure ridges (Fig. 2). A mission to this
site would also have the opportunity to sample the basal scarp of Olympus
Mons, the youngest shield volcano of the Tharsls province. The elevation
of this site is between 2 and 3 km above the mean planetary level; if
this elevation is too high for a spacecraft to obtain the necessary
aerobraklng capability, an alternate site exists at about 20°N, 150 °
(Table 1; Fig.2). This site is near the 0 km contour on the global
topographic map. It consists of lava flows only slightly older than the
previously mentioned Olympus flows (N_ 1 km ~ 100-200/ 106 km2; Morris,
in press). In addition to these young lavas, a carefully selected site
at this locality could investigate both the distal margins of an eJecta-
flow impact crater and the enigmatic aureole deposits of Olympus Mons
(Fig. 2), for which diverse, and mostly unconvincing, origins have been
proposed (see review in Carr,1981).
580
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Either of these two sites would give us samples of the youngest
unambiguous martian lava flow. As such, they would calibrate the planet-
wide crater-frequency curve and enable us to extrapolate the results from
this site to volcanic plains across the martian surface, which cover over
60_ of the surface area of the planet (Greeley and Spudls, 1981).
Prime Objective 2 - The Ancient Narttan Crust
Experience with Apollo lunar surface exploration has shown that
investigations of multi-ring basins and their ejects provide good strate-
gies to reconstruct the composition and structure of planetary crusts.
The cratered terrain hemisphere of Mars displays numerous basins, in
preservation states ranging from near-pristine (e.g. Lowell; Wilhelms,
1973) to almost totally-obliterated (Schultz et. al., 1983). By Investi-
gating and sampling these basins, we can learn about the processes
involved in basin formation, the age and composition of the martian
highlands, and crusta] stratigraphy and structure.
The Argyre basin in one of the best preserved, large (800 km dia-
meter) martian multi-ring basins (Fig. 3; Table 1). A landing in this
location would have several objectives. The prime sampling objective
would be the basin massifs (Fig. 3). These mountains consist of both
uplifted and rotated crustal blocks and /or basin ejects, excavated from
many kilometers depth ( a model calculation suggests maximum depths of
excavation for an Argyre-size basin at 40- 50km, extrapolated from the
relation for lunar basins given in Spudis and Davis, 1985). Addi-
tionally, knobby-deposlts (FIg.3) may well conslst predominantly of pri-
mary basin ejecta, by analogy with similar deposits observed around the
lunar Orientale basin (e.g. Head, 1974). Old plains material partlally
embays Argyre basin terrain; these units may consist of old volcanic
flows that have resurfaced almost 50t of the martian cratered terrain
hemisphere (Greeley and Spudis, 1981). Finally, a variety of eolian
features, such as dune fields and etched terrain, occur within Argyre;
both the morphology and process of eolian activity could be investigated
at this site.
An alternate hlghland/basln site is the Isidls basin (Fig. 4; Table
i). This basin (1500 km diameter) may have been excavated to depths of
70 to 80 km into the martian crust. Objectives at this site consist of
basin masslfs as described above, basin-fllling lavas, and the dlstal
584
I 585

flows of the Syrtis Major shield volcano (Schaber, 1982). Additionally,
some drainage channels occur within the rugged basin terrain (Fig. 4);
another goal of this site would be to establish the nature of these
channels, which may be of fluvial origin (Carr, 1981). An advantage of
the Isidis site over the Argyre basin site described above is its near-
antipodal location to the young volcanic sites described earlier; the
placement of a geophysical station In both the Tharsts and Isidis regions
might enable a determination of the existence and properties of a martian
core.
Additional Sites of Geologic Interest
Six additional regions on Mars of geologic significance are listed
in Table 1. As mentioned previously, Mars is such a complex planet, that
a list hundreds of entries long could easily be given. In this tabula-
tion, I have attempted to rank other targets only in terms of how they
will help us address key issues in martian geologic history. After
satisfying the two prime objectives, perhaps the most Interesting site
from both a geological and resources viewpoint is the north polar region
(Table 1). Geologically, the polar layered deposits contain a record of
alternating deposition and quiescence that Is invaluable in terms of
recent martian history. In terms of resources, the permanent polar cap
Is composed of water ice (Kleffer et. al., 1977). This resource is
directly available at this site for life support at a permanent base and
for propulsion uses.
The list presented in Table 1 is not meant to be definitive in any
way. Thls is only an outline of site selection targets that will provide
answers to several key questions regarding Mars. If the lunar experience
is a guide, this initial exploration plan will probably raise many more
questions than it answers.
CONCLUSIONS
The site selection strategy proposed here will address two key
fundamental issues in martian geology: 1) the timing and composition of
martian volcanism; and 2) the nature of the martian highland crust.
Although detailed knowledge of martian geologic history will take decades
of manned surface exploration, these initial manned landings will, at the
very least, enable a formulation of the proper questions and provide a
587
framework within which the evolution of Mars as a terrestrial planet
be understood.
ACKNOWLEDGEMENTS
I thank E. C. Morris, D. H. Scott,
comments on this paper.
REFERENCES
1. Balsaltic Volcanism Study Project (BVSP)
(1981)Balsaltic volcanism on the terrestrial planets,
Pergamon Press, 1286 pp.
2. Carr, M. H. (1973) Volcanism on Mars. J. Geophys. Res.
78, 4049-4062.
3. Cart, M. H. (1981) The surface of Mar____s,Yale Univ.
Press, 232 pp.
4. Greeley, R. and Spudis, P. D. (1981) Volcanism on Mars,
Rev. Geophys. Space Phys. i_99,13-41.
5. Head, J. W. (1974) Orientale nulti-ring basin interior
and implications for the petrogenesis of lunar highland
samples. The Moon lj, 327-356.
6. Kieffer, H. H., Martin, T. Z., Peterfreund, A. R., and
Jakosky, B. M. (1977) Thermal and albedo mapping of
Mars during the Viking primary mission. J. Geophys.
Res. 822, 4249-4291.
7. Morris, E. C., in press, Geologic map of the Olympus
Mons region of Mars. U. S. Geol. Survey map.
8. Mutch, T. A., Arvidson, R. E., Head, J. W., Jones, K.
L., and Saunders, R. S., (1976) The Geology of Mars,
Princeton Univ. Press, 400 pp.
9. Plescia, J. B. and Saunders, R. S., (]979) The chrono-
logy of martian volcanoes. Proc. Lun. Planet. Sci.
Conf. lOt____h,2841-2859.
10. Schaber, O. O., (1982) Syrtis Major: A low-relief
volcanic shield. J. Geophys. Res. 8__7,9852-9866
11. Schaber, G. G., Horstman, K. C., and Dial, A. L.,
(1978) Lava flow materials of the Tharsis region of
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Schultz, P. H., Schultz, R. A., and Rogers, J., (1982)
The structure and evolution of ancient impact basins on
Mars. J. Geophys. Res. 8__?,9803-9820.
Scott, D. H., and Cart, M. H., (1978) Geologic map of
Nars. U. S. Geol. Survey Map I-I083.
Scott, D. H., and Tanaka, K., (1985) Geologic map of
the western region of Nars. U. S. Geol. Survey Nap (in
press).
Spudls, P. D., and Davis, P. A., (1985) How much anor-
thoslte In the lunar crust?: Implications for lunar
crustal origin. Lun. Planet. Sci. XV__!, 807-808.
Taylor, S. R. (1975) Lunar Science: A Post-Apollo
Vle___w,Pergamon Press, 372 pp.
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Site selection for manned Mars landings: A geological perspective

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