INTRODUCTION.
The Mars Pathfinder Mission is a
Discovery Class mission that will place a small lander and rover
on the surface of Mars in July of 1997. It is primarily a technology
demonstration to test the feasability of a direct entry-delivery
system, but carries a nominal scientific payload that includes
rover-lander and instrumentation for limited mineralogical analysis
(1). The nominal landing site was selected by the Pathfinder
Team under the leadership of Dr. Matthew Golombek (JPL) based
input from 60 participants at a Landing Site Workshop held last
Spring at the Lunar Planetary Institute in Houston (1). The mission
constraints for the landing site were 0-30°
N latitude, and below the 0.0 elevation datum. Over 20 landing
sites were proposed and a nominal site was selected on southern
Chryse Planitia near the terminae of the Ares and Tui outflow
channels. In part, the decision to land at this location was
based on the opportunity to sample a potentially large number
lithologies in a small area (the rover will have a range of a
few tens of meters from the lander). The purpose here is to review
the general geological context of the landing site and the rationale
for Exobiology's recommendation of the Ares site given at the
workshop last spring (1). Because Ares and Tui Valles are sourced
within terranes that may have originated by thermokarst processes,
hydrothermal processes could have operated there for some time.
Hydrothermal systems are presently regarded as important sites
for a fossil record on Mars (2,3). Models for the formation of
the outflow channels suggest that thermal spring sinters and associated
aqueous mineral deposits, high priority targets for Mars Exopaleontology
(2,3), could have formed in association with thermokarst processes
and subsequently been delivered to the landing site in large quantities
during the periodic cataclysmic outflows that created the channels.
GEOLOGICAL SETTING OF THE PATHFINDER
LANDING SITE.
The Ares and Tui
outflow channels originate from highly fractured, elongate to
circular, collapsed zones called chaotic terranes (4,5). These
terranes consist of irregularly broken and jumbled blocks probably
formed by the withdrawal of subsurface water (5,6). On a regional
scale, the channels are broadly anastomosing networks that include
a variety of macro- and mesoscale flow features (7). Catastrophic
flooding by water has been invoked to explain many of the features
of the large outflow channels on Mars (4), although sources of
the water and mechanisms of release are more controversial (8,
9). Features of the martian outflow channels compare favorably
with terranes on Earth formed by catastrophic flooding. Analogs
for martian terranes include the channeled scablands of eastern
Washington, U.S.A. (4). The immense erosive capability of such
outflows significantly modifies older features of the landscape,
depositing vast quantities of sedimentary material downstream.
For example, Komar (10) compared the hydraulic capacity of the
outflows that created the Channeled Scablands to turbid flows
capable of transporting boulders up to a meter in suspension.
Masursky et al. (8) suggested a thermokarst origin for chaos source
regions of many martian outflow channels. Outfloods of water would
have occurred by the melting of water ice stored in the martian
regolith. The favored mechanism is heating by shallow intrusives.
It is likely, under this scenario, that epithermal and subaerial
hydrothermal systems would have developed and persisted for some
time prior to and following major outflow events. Some small channels
on the flanks of circular chaos terranes, such as Aram and Hydaspis,
possess amphitheater-shaped head reaches with patchy albedos.
Such areas are logical sites for ancient subaerial thermal spring
systems and their associated deposits.
STRATEGY FOR MARS EXOPALEONTOLOGY.
On Earth, microbial fossils are
preserved only where contemporaneous mineralization entombs organisms,
organic matter and biomolecules before they can be destroyed by
oxidation (3). Organic materials carried to the
martian surface from subsurface oases
(see 11) would almost certainly be destroyed by oxidation, if
not accompanied by early mineralization. On Earth, high priority
geological environments (and associated mineral deposits), where
rapid mineralization frequently occurs in the presence of microorganisms,
include the following (3): springs (subaerial and subaqueous,
thermal and non-thermal) playa lakes (terminal lake basin evaporites),
and mineralized paleosols (shallow subsoil hard-pans, including
calcretes, ferracretes, silcretes). During crystallization, aqueous
minerals also incorporate liquid and volatile phases present in
the depositional environment. For stable minerals such as silica
or phosphate, primary volatiles may be retained for billions of
years and are not only prime targets for fossils and biomolecular
compounds, but also for pre-biotic organic compounds, and volatiles
of fundamental importance to Exobiology and Mars Science in general.
APPLICATION TO THE PATHFINDER
LANDING SITE.
As mentioned earlier,
the outflows that created the Ares and Tui outflow channels originate
from chaotic terranes that may have formed by thermokarst processes
(8). Aram Chaos and other semi-circular areas within these terranes
may have formed above focused subsurface heat sources, possibly
shallow igneous intrusives which interacted with ground ice. Near
surface heat sources associated with such features are likely
to have sustained hydrothermal activity for some time, depositing
thermal spring sinters around vents at the surface, or in the
shallow subsurface in association with epithermal systems. Such
deposits are likely to have been carried down outflow channels
and deposited at the landing site. As noted previously, such aqueous
mineral deposits have a high priority for Mars Exopaleontology
(3). The high rates of mineralization typically observed in such
settings, driven by outgassing and declining tempertaure, provide
an important mechanism for entrapping and preserving organisms
and their byproducts. If Mars harbors a subsurface microbiota
(11), it is likely that organisms were delivered to the surface
by such outflows. Where associated mineralization occurred, organisms
are likely to have been entrapped and fossilized. In the absence
of life, the same aqueous mineral deposits of interest for Exopaleontology
are also potential repositories for pre-biotic organic molecules.
This is especially important for Exobiology, because the prebiotic
chemical record, which has been lost on the Earth by crustal recycling,
may still be preserved on Mars.
A key question is whether aqueous mineral
deposits are present on the martian surface. Despite the elemental
analyses obtained from Viking, mineralogy of the martian surface
materials is still debated (12). There is a basic need for future
missions to go beyond simple elemental analysis, and include information
on mineral structures. This is central to the strategy for Mars
Exopaleontology (3). The Mars Global Surveyor aims to map the
surface composition of Mars by using infrared and gamma ray spectroscopy
(13). Such information is crucial for site selection for future
missions with the goal of searching for evidence of ancient martian
life. Future landed missions should carry instrumentation capable
of the in situ identification of a wide variety of aqueous
minerals as the basis for evaluating exopaleontological potential,
as well as for ground truthing orbital mapping data. Although
the Pathfinder camera will have sensitivities for only iron-oxides
and iron-bearing silicates (1), it will provide our first opportunity
to assess mineralogy and the past activity of mineralizing aqueous
systems on Mars. This is of broad interest for Mars Science, and
is also a fundamental objective of the Mars Surveyor Program as
a whole (13).
REFERENCES.
NASA Space Science Solar System Exploration Exobiology Program Exobiology Branch at ARC