The Evolutionary Biology Lecture of the Week for June 26, 2006 is now available
at:

http://aics-research.com/lotw/

This is the sixth lecture in a summer-long series on the new science of
astrobiology.

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June 26, 2006

Part VI: Astrobiology

Surficial Geology of the Sprit Rover Traverse
in Gusev Crater: Dry and Desiccating
Since the Hesperian

Matt Golombek, Jet Propulsion Laboratory, Caltech

20 min.

"To set foot on the soil of the asteroids, to lift by hand a rock from the
Moon, to observe Mars from a distance of several tens of kilometers, to land on
its satellite or even on its surface, what can be more fantastic? From the
moment of using rocket devices a new great era will begin in astronomy: the
epoch of the more intensive study of the firmament."

— Konstantin E. Tsiolkovsky, 1896

Less than 200 years ago, we only had the crudest estimates of the age of the
Earth. We identified those ages by the kinds of fossils found in particular
rocks, giving the strata the names of the places in which they were found:
Devon, Cambria, Siluria, Ordovicia and the like, only later sorting out which
were older.

Mars is a new world to us geologically, and our current knowledge of Mars is as
rudimentary as Earth once was to us. Armed with the first topographic maps of
Mars derived from orbiting spacecraft, three crude epochs in the history of
Mars were outlined based on cratering intensities at three locations: Amazonis
Planitia, Hesperia Planum and Noachis Terra. These names mean nothing; they
were assigned to regions of Mars by earth-bound astronomers early in the 20th
Century, long before we had the capacity to see the regions in detail.

The Noachis region is ancient cratered highlands, Moon-like, a region left
essentially undisturbed from the time of heavy bombardment during the formation
of the solar system, ending ca. 3.9 billion years ago. Hesperia is a lower,
much less cratered region, while Amazonis lies in the lowland half of Mars in
which the craters have been erased.

Originally, in the absence of any other information, the three epochs were
roughly assigned equal ages, but as recent evidence has accumulated, it has
become increasingly more clear that the first two epochs were reasonably short,
and that the current dry, dessicating epoch, the Amazonian, has dominated for
three-quarters of Mars' existence.

In this week's lecture, Matt Golombek, one of the principals responsible for
choosing the landing sites for the Spirit and Opportunity rovers, speaks
wistfully about the traverse of Gusev Crater by the Spirit rover. Layered
sediments are the gold a geologist seeks. They tell a sequential story, but
none were found on the floor of Gusev. Rather, the plains of Gusev were
discovered to be a heavily impacted basaltic (lava) regolith, perhaps 10 meters
or more in depth.

Although no clear evidence of rain-driven or lake activity was identified in
the cratered plain, its surface geology still tells an important story. Gusev's
geology strongly limits any warmer and/or wetter period of Mars history to be
only pre-Late Hesperian at best.

As we'll see in next week's lecture, we now have very strong evidence that Mars
once had oceans, but the measurements at Gusev greatly constrain how long those
oceans could have persisted, and by consequence the likelihood that Mars had
sufficient time to evolve life.