Fluvial Mapping of Mars
It took fifteen years of imaging and nearly three years of stitching the
pieces together to create the largest image ever made, the 8-trillion-pixel
mosaic of Mars’ surface. Now, the first study to utilize the image in its
entirety provides unprecedented insight into the ancient river systems that
once covered the expansive plains in the planet’s southern hemisphere.
These three billion-year-old sedimentary rocks, like those in Earth’s
geologic record, could prove valuable targets for future exploration of
past climates and tectonics on Mars.
The work, published this month in Geology, complements existing
research into Mars’ hydrologic history by mapping ancient fluvial (river)
ridges, which are essentially the inverse of a riverbed. “If you have a
river channel, that’s the erosion part of a river. So, by definition, there
aren't any deposits there for you to study,” Jay Dickson, lead author on
the paper, explains. “You have rivers eroding rocks, so where did those
rocks go? These ridges are the other half of the puzzle.” Using the mosaic,
as opposed to more localized imagery, let the researchers solve that puzzle
on a global scale.
Mars used to be a wet world, as evidenced by rock records of lakes, rivers,
and glaciers. The river ridges were formed between 4 and 3 billion years
ago, when large, flat-lying rivers deposited sediments in their channels
(rather than only having the water cut away at the surface). Similar
systems today can be found in places like southern Utah and Death Valley in
the U.S., and the Atacama Desert in Chile. Over time, sediment built up in
the channels; once the water dried up, those ridges were all that was left
of some rivers.
The ridges are present only in the southern hemisphere, where some of Mars’
oldest and most rugged terrain is, but this pattern is likely a
preservation artifact. “These ridges probably used to be all over the
entire planet, but subsequent processes have buried them or eroded them
away,” Dickson says. “The northern hemisphere is very smooth because it’s
been resurfaced, primarily by lava flows.” Additionally, the southern
highlands are “some of the flattest surfaces in the solar system,” says
Woodward Fischer, who was involved in this work. That exceptional flatness
made for good sedimentary deposition, allowing the creation of the records
being studied today.
Whether or not a region has fluvial ridges is a basic observation that
wasn’t possible until this high-resolution image of the planet’s surface
was assembled. Each of the 8 trillion pixels represents 5 to 6 square
meters, and coverage is nearly 100 percent, thanks to the “spectacular
engineering” of NASA’s context camera that has allowed it to operate
continuously for well over a decade. An earlier attempt to map these ridges
was published in 2007 by Rebecca Williams, a co-author on the new study,
but that work was limited by imagery coverage and quality.
“The first inventory of fluvial ridges using meter-scale images was
conducted on data acquired between 1997 and 2006,” Williams says. “These
image strips sampled the planet and provided tantalizing snapshots of the
surface, but there was lingering uncertainty about missing fluvial ridges
in the data gaps.”
The resolution and coverage of Mars’ surface in the mosaic has eliminated
much of the team’s uncertainty, filling in gaps and providing context for
the features. The mosaic allows researchers to explore questions at global
scales, rather than being limited to patchier, localized studies and
extrapolating results to the whole hemisphere. Much previous research on
Mars hydrology has been limited to craters or single systems, where both
the sediment source and destination are known. That’s useful, but more
context is better in order to really understand a planet’s environmental
history and to be more certain in how an individual feature formed.
In addition to identifying 18 new fluvial ridges, using the mosaic image
allowed the team to re-examine features that had previously been identified
as fluvial ridges. Upon closer inspection, some weren’t formed by rivers
after all, but rather lava flows or glaciers. “If you only see a small part
of [a ridge], you might have an idea of how it formed,” Dickson says. “But
then you see it in a larger context—like, oh, it’s the flank of a volcano,
it’s a lava flow. So now we can more confidently determine which are
fluvial ridges, versus ridges formed by other processes.”
Now that we have a global understanding of the distribution of ancient
rivers on Mars, future explorations—whether by rover or by astronauts—could
use these rock records to investigate what past climates and tectonics were
like. “One of the biggest breakthroughs in the last twenty years is the
recognition that Mars has a sedimentary record, which means we’re not
limited to studying the planet today,” Fischer says. “We can ask questions
about its history.” And in doing so, he says, we learn not only about a
single planet’s past, but also find “truths about how planets evolved… and
why the Earth is habitable.”
As this study is only the first to use the full mosaic, Dickson looks
forward to seeing how it gets put to use next. “We expect to see more and
more studies, similar in scale to what we're doing here, by other
researchers around the world,” he says. “We hope that this ‘maiden voyage’
scientific study sets an example for the scale of science that can be done
with a product this big.”
FEATURED ARTICLE
The global distribution of depositional rivers on early Mars
J.L. Dickson; M.P. Lamb; R.M.E. Williams; A.T. Hayden; W.W. Fischer
Contact:
jdickson@caltech.edu
GEOLOGY articles are online at
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