Figure 1 from the paper.
At the time of publication, I (Ken Edgett) was on vacation and could not be reached by news/media by telephone, internet, or other means. The vacation was scheduled many months ago, long before the publication schedule for the MARS Journal article was known.
As of 17 November 2005, I have returned to the office. The material that follows was prepared on 28 October 2005, to be used during my absence. What I have done here is to prepare a list of questions that are of the flavor that I think someone might ask about the work, and then provided my answers. I am hopeful that anyone interested in this reporting on this research will be able to mine this page for "quotes" from the author.
---Ken Edgett
Q: What is the main point of this article?
A: The goal of the research was to understand the context
of the sedimentary rocks being explored by the Mars Exploration
Rover, Opportunity. The main point is that the rover has only
been able to explore a very tiny fraction of the sedimentary
rock record known to exist--from orbiter data--in the region.
The rover has examined about 6 or 7 meters of the sedimentary
strata, which is less than 1% of the more than 800 meters
that are seen in the orbiter data.
Q: Was this work motivated by the Opportunity findings?
A: No, actually this paper is the product of research I began
way back in 1996, before Mars Global Surveyor even reached Mars.
The light-toned bedrock exposures in the Sinus Meridiani region
were obvious in images from Mariner 9 and Viking, but no one
really had a good concept of what it was we were looking at. During
that early phase of the research, Tim Parker (of JPL) and I
wrote a
speculative paper
about the possiblity that this region
harbored sediments deposited in an ocean that must have covered
much of the northern hemisphere of Mars. While it turns out
that we were basically wrong, it was clear to us then that the
region presented something odd and unusual. The study published
in this paper is something I've been picking away at for almost
10 years, but it really got underway once we had a wealth of
images from the Mars Global Surveyor Mars Orbiter Camera (MOC)
and Mars Odyssey Thermal Emission Imaging System (THEMIS).
Q: So, what is the context of the sedimenary rocks studied by the rover?
A: Well, there are really several contexts that I explore in
the paper:
As for how representative these rocks are--the vast exposures in Sinus Meridiani cover an area greater than the Colorado Plateau, which is where--in North America--we find a lot of vast, colorful, and famous exposures of sedimentary rock, like those in the Grand Canyon, Bruce Canyon, Zion National Park, the Canyonlands, the Painted Desert and Petrified Forest, Monument Valley-- all those cool places in Arizona and Utah. The images from MOC and THEMIS show that there is quite a diversity of sedimentary rocks exposed in the Meridiani region, more diverse than what the rover has been able to explore. We know of the diversity because different groups of layered sedimentary rock have different bedding style (thick layers, thin layers, etc), and different erosional expression-- some form cliffs, some for stair-stepped outcrops, what have you.
Finally, regarding the age, or temporal context of the rocks the rover has been examining-- we find similar light-toned, layered sedimentary rocks elsewhere on Mars. Some of them are in very ancient, heavily cratered terrain, such as an area near Mawrth Vallis in western Arabia Terra. Others are in very lightly-cratered terrain, as in the plains that are cut by the chasms of the Valles Marineris. These other occurrences suggest that conditions were right on Mars for sediments to be deposited, become cemented to form rock, and-- who knows--maybe subjected to groundwater like the rocks at the rover site--for some period of time beyond the most ancient epoch of Martian history. The period in which Mars became heavily cratered--when impactors were more common in the Solar System and most planets and satellites were subjected to more frequent impacts than they are today--is known as the Noachian Period for Mars. The period that followed is known as Hesperian. The point of all these observations about sedimentary rocks is that the rocks at the rover site need not be so ancient as to be called Noachian. They could be a bit younger, like Hesperian.
Q: What makes you think there were rocks above the plains
that Opportunity has been exploring, that were then eroded away?
A: First, there's all the hematite spherules on the plains.
These, we now know, formed as concretions inside of sedimentary
rock. To have an abundance of them on the plains means that
the rock in which these loose concretions--just sitting around
on the surface today--had to have been disintegrated and eroded
away, leaving behind the concretions as a lag. Second, there are
areas on Meridiani Planum where the sedimentary rocks occur
at a higher elevation than they do at the rover site-- if the
rocks are all flat-lying (horizonal layers), then there must
have been strata above the present rover site, which were eroded
away. Third, there is some evidence on the rim of Endurance Crater,
which the rover explored in 2004, that the crater was once
buried and is now exhumed. When the rover visited the rim
of the crater, it found no ejecta-- no boulders or cobbles
ejected from the impact when it occurred. Further, it found
flaps of sedimentary rock that had dropped down and covered the
upper walls of the crater. Where is the ejecta? I think it
might be buried beneath the layers of rock that the rover
was sitting on when it drove around the crater rim--the same
layers that form these dropped-down flaps.
Q: Is there a role for water, in all of this?
A: The Opportunity rover team has documented fantastic evidence
that the rocks the rover is exploring were subjected for a long time
to acidic groundwater. There is also some evidence that some of
the layers of rock may be been deposited in running water--streams,
perhaps--and that some of the sand grains in the sandstones might
have formed on muddy playas. To really understand the role of
water in the sedimentary rocks of this region--really requires
attention on the ground--rovers, people, etc-- but from orbit there
are a few things we can see. One is that there are, scattered
through some of the rocks, good evidence for ancient valleys
or streams through which water might have flowed. Some of these
valleys were inverted by erosion. Many were filled, buried,
and later exhumed.
Q: Buried and exhumed?
A: Oh, yes, this is a theme that repeats itself throughout
the Sinus Meridiani region. This, by the way, is one of a very few
places on Mars--and this is by far the largest--where we have really
good exposures of the martian bedrock. What we see here is that
impact craters and valleys have been incorporated into the rock.
A crater would form on a surface, become filled, then buried, then
later it would be exhumed by erosion. Some of the filled and buried
craters are more than 50 kilometers in diameter. This idea of layers
and layers of rock, with filled and buried and exhumed valleys
and craters all interbedded and exposed in various places by erosion,
is a theme that repeats itself all over the planet. It's really one
of the keys to understanding the planet's geology and history.
Q: How about life on Mars?
A: Who knows? I don't spend much time worrying about this
subject. The rover team and their work does far more to address
this question than I can do from orbiter results. The region
had groundwater. The groundwater was acidic. It probably played
a key role in shaping the region's present geomorphology, by
contributing to the retreat of the scarps that form cliffs in
the sedimentary rocks. Certainly no signs of life can be
seen from orbit--except the evidence that humans sent a rover
there.
Q: In Figure 1 of the paper, you show the rover site in
comparision to another place in Sinus Meridiani, with lots of exposed
rock. What's the story there?
A: This is my favorite figure, because it shows the rover
site and one very small portion of a vast sedimentary rock
exposure in northern Sinus Meridiani. If you look at the rover
site, even at the highest resolution (about 1/2 a meter per pixel)
from MOC, you really can't see the rock outcrops the rover has
been studying--they are very small, even in the highest resolution
images we have from orbit. And then you look elsewhere in Sinus
Meridiani, and you find vast areas of exposed sedimentary rock.
It would have been wonderful if the rover could have been sent to
one of these areas-- more diversity of rocks to explore, and they
wouldn't be all covered-up with sand and hematite spherules--but
the landing ellipse for MER was too large to accommodate these
vast outcrop areas because the terrain they create is too
rugged-- for the rover, they needed a flat plain. One can
imagine that, in the future, a rover might have a much, much
smaller landing ellipse, and then we could really get in there
and explore there vast outcrops of sedimentary rock. But the
ellipses would have to be something like a 1-2 kilometers in
diameter--pretty tough to do.