Water is
present on Mars today, but it is entirely bound up in ice because the surface
is too cold for liquid water.
But
evidence has been mounting that shows water once flowed across the Martian
surface, potentially supporting life. While water does not mean there was life,
it's a key prerequisite.
A new study
of a system of gullies worn into the surface of Mars suggests the most recent
period of water flow on the red planet was only 1.25 million years ago.
But
throughout the more than 4 billion-year history of our neighbor, its climate
has cycled
back and forth between warmer and cooler periods as the planet wobbled on
its axis. Like Earth, Mars' axis is tilted with respect to its orbital plane
and the degree of tilt changes over thousands of years. But Mars' tilt changes
more over time, alternately heating up or cooling down parts of the planet as
the amount of sunlight falling on them changes.
Over the past
decade, scientists have found numerous geological features, such as gullies and
possible lakebeds, that indicate water was once present on the surface. They
have also found
water-bearing minerals, such as opals and carbonates that show that water
has reacted with the Martian regolith, or dirt.
Gullies
are known to be young surface features on Mars, but pinning a precise date to
them has been difficult.
The new
study, detailed in the March issue of the journal Geology, did just that
with a gully system located on the inside of a crater in Promethei Terra,
showing that water flowed on Mars more recently than previously thought.
"We
think there was recent water on Mars," said study team member Jim Head of Brown University in Providence, R.I. "This is a big step in the direction to proving
that."
Crater
counting
The gully
system the team examined shows that water-borne sediments were carried down
steep slopes of nearby alcoves and deposited in a fan-like shape during four
different intervals.
"You
never end up with a pond that you can put goldfish in," said study team
member Samuel Schon, a graduate student at Brown. "You had ice that
typically sublimates. But in these instances, it melted, transported, and
deposited sediment in the fan. It didn't last long, but it happened."
Viewed from
far away, the fan looks like one continuous feature, but close-up images taken
by the NASA Mars
Reconnaissance Orbiter HiRISE camera show four distinct lobes in the
alluvial fan.
Schon was
able to determine that the lobes were created at different times and could tell
which was the oldest because it was pockmarked with craters, while the younger
lobes were left relatively unblemished. (The longer a surface has been exposed,
the more meteorites have had a chance to leave their mark.)
Schon
linked the craters in the oldest lobe of the fan to a rayed crater more than 50
miles (80 kilometers) to the southwest. He dated the crater to about 1.25
million years, which meant the younger lobes of the fan could not be any older
than that.
The team
also determined that ice and snow deposits formed in the alcoves when Mars was
tilted so that it plunged into an ice age and ice could form in the
mid-latitude areas, instead of being confined to the poles, as it is today.
About half a million years ago, the planet's tilt change and the ice began to
melt or sublimate.
Schon said
that other explanations for the water's presence in the gully were ruled out:
Groundwater bubbling up seemed unlikely to have occurred multiple times, and
dry mass wasting (for example, a rockslide) also didn't seem to fit the
pattern.
The
findings add more evidence that Mars underwent a recent, geologically-speaking,
ice age that moved water closer to the equator of the planet.
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