September 17, 2008

Water Flowed on Mars Longer than Thought

Parts of ancient
Mars may have been wet for a billion years longer than scientists previously
thought, a new study of images of the red planet's surface suggests.

Along with
Earth and the other inner planets of our solar system, Mars formed about 4.5 billion years ago. Scientists
have long known that flowing water formed many of the features seen on Mars
today, but previous studies suggested that water runoff from precipitation had
ceased after the first billion years of Mars' history, called the Noachian

But one
team of scientists thinks these rains and floods persisted into more recent --
geologically speaking -- periods in Mars' history.

Weitz, a senior scientist with Planetary Science Institute in Tucson, Ariz., and her colleagues examined close-up images of the plains surrounding the huge Valles
canyon system taken by the HiRISE instrument aboard NASA's Mars
Reconnaissance Orbiter (currently still circling the planet). HiRISE can
resolve features as small as 3 feet (1 meter) in diameter.

Weitz and
her team noticed that light-toned layered deposits in the plains around Valles
Marineris had different features from those inside the canyon; these features
suggested that water continued to flow on a large scale in these plans after
the Noachian, into the Hesperian epoch of Mars, until about 3.7 billion to 3
billion years ago. Phenomena associated with flowing water are called fluvial

was a big surprise because no one thought we'd be seeing these extensive
fluvial systems in the plains all around Valles Marineris that were formed
during the Hesperian Era," Weitz said. "Everyone thought that by then
the climate had pretty much dried out."

Another recent
suggested that periods of rain and flooding on Mars were longer in
duration, not just short bursts, as had previously been thought.

The deposits
that Weitz and her team observed outside the canyon showed "a lot of
variations in brightness, color and erosional properties that we don't see for
light-toned deposits inside Valles Marineris," Weitz said. "This
suggests that the processes that created the deposits outside Valles Marineris
were different from those operating inside."

locations near the canyon had inverted channels, which, on Earth, form when
sediment deposits in a streambed over time. After the stream dries up, the
softer terrain surrounding the streambed erodes away, leaving the harder,
cemented stream sediments forming a ridge above the surrounding terrain.

explanations for the deposits, such as explosive volcanism and wind deposition,
can't be ruled out, Weitz said, but the distinctiveness of the features
suggests a fluvial origin, she added.

we're seeing tells us that this light-toned layering on the plains was
associated with fluvial activity that wasn't occurring just in little pockets
over very brief episodes, but rather on a much larger scale for sustained time
periods," she added. "For some reason, there was precipitation around
Valles Marineris that allowed these systems to form out on the plains."

The details
of the study are posted online in the journal Geophysical Research Letters.