Lecture of the Week: Part VI: Astrobiology

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

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

The talks center primarily around evolutionary biology, in all of its
aspects: cosmology, astronomy, planetology, geology, astrobiology,
ecology, ethology, biogeography, phylogenetics and evolutionary biology
itself, and are presented at a professional level, that of one
scientist talking to another. All of the talks were recorded live at
conferences.

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

The geological column on Earth has been divided and subdivided again
and again into increasingly finer segments, and ever increasingly more
accurately dated, so that we now know the age of the strata to three or
four decimal points.

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' life.

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.
The surface geology of Gusev observed by Spirit, as it traversed the
crater's floor, strongly argues for a dry and desiccating environment
since the Late Hesperian. The geological history of Mars essentially
died at that time. Erosion rates at Gusev are measured to be less than
0.1 billionth of a meter per year over the last 3.5 billion years,
approximately 1/10,000th of what they were during the Noachian.

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.

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