Liquid
water runs down canyons and crater walls over the summer months on Mars,
according to researchers who say the discovery raises the chances of being home
to some form of life.
Images taken from the Mars orbit show
cliffs, and the steep walls of valleys and craters, streaked with summertime
flows that in the most active spots combine to form intricate fan-like
patterns. Scientists are unsure where the water comes from, but it may rise up
from underground ice or salty aquifers, or condense out of the thin Martian
atmosphere.
“There is liquid water today on the surface
of Mars,” Michael Meyer, the lead scientist on Nasa’s Mars exploration
programme, told the Guardian. “Because of this, we suspect that it is at least
possible to have a habitable environment today.”
The water flows could point Nasa and other
space agencies towards the most promising sites to find life on Mars, and to
landing spots for future human missions where water can be collected from a
natural supply. “Mars is not the dry, arid planet that we thought of in the
past,” said Nasa’s Jim Green. “Liquid water has been found on Mars.” Nasa
announce that there are watery flows on the surface of Mars during the red
planet’s summer months.
Some of the earliest missions to Mars
revealed a planet with a watery past. Pictures beamed back to Earth in the
1970s showed a surface crossed by dried-up rivers and plains once submerged
beneath vast ancient lakes. Earlier this year, Nasa unveiled evidence of an
ocean that might have covered half of the planet’s northern hemisphere in the
distant past.
Dark narrow
streaks called recurring slope lineae emanate out of the walls of Garni crater
on Mars.
But occasionally, Mars probes have found
hints that the planet might still be wet. Nearly a decade ago, Nasa’s Mars
Global Surveyor took pictures of what appeared to be water bursting through a
gully wall and flowing around boulders and other rocky debris. In 2011, the
high-resolution camera on Nasa’s Mars Reconnaissance Orbiter captured what
looked like little streams flowing down crater walls from late spring to early
autumn. Not wanting to assume too much, mission scientists named the flows
“recurring slope lineae” or RSL.
Researchers have now turned to another
instrument on board the Mars Reconnaissance Orbiter to analyse the chemistry of
the mysterious RSL flows. Lujendra Ojha, of Georgia Institute of Technology in
Atlanta, and his colleagues used a spectrometer on the MRO to look at infrared
light reflected off steep rocky walls when the dark streaks had just begun to
appear, and when they had grown to full length at the end of the Martian
summer.
Writing in the journal Nature Geosciences,
the team describes how it found infra-red signatures for hydrated salts when
the dark flows were present, but none before they had grown. The hydrated salts
- a mix of chlorates and perchlorates - are a smoking gun for the presence of
water at all four sites inspected: the Hale, Palikir and Horowitz craters, and
a large canyon called Coprates Chasma.
“These may be the best places to search for
extant life near the surface of Mars,” said Alfred McEwen, a planetary
geologist at the University of Arizona and senior author on the study. “While
it would be very important to find evidence of ancient life, it would be
difficult to understand the biology. Current life would be much more
informative.”
The flows only appear when the surface of
Mars rises above -23C. The water can run in such frigid conditions because the
salts lower the freezing point of water, keeping it liquid far below 0C.
“The mystery has been, what is permitting
this flow? Presumably water, but until now, there has been no spectral
signature,” Meyer said. “From this, we conclude that the RSL are generated by
water interacting with perchlorates, forming a brine that flows downhill.”
These channels, which are between 1 metre and 10 metres wide, are on a scarp in
the Hellas impact basin.
John Bridges, a professor of planetary
science at the University of Leicester, said the study was fascinating, but
might throw up some fresh concerns for space agencies. The flows could be used
to find water sources on Mars, making them prime spots to hunt for life, and to
land future human missions. But agencies were required to do their utmost to
avoid contaminating other planets with microbes from Earth, making wet areas
the most difficult to visit. “This will give them lots to think about,” he
said.
For now, researchers are focused on learning
where the water comes from. Porous rocks under the Martian surface might hold
frozen water that melts in the summer months and seeps up to the surface.
Another possibility is that highly concentrated saline aquifers are dotted
around beneath the surface, not as pools of water, but as saturated volumes of
gritty rock. These could cause flows in some areas, but cannot easily explain
water seeping down from the top of crater walls.
A third possibility, and one favoured by
McEwen, is that salts on the Martian surface absorb water from the atmosphere
until they have enough to run downhill. The process, known as deliquescence, is
seen in the Atacama desert, where the resulting damp patches are the only known
place for microbes to live. “It’s a fascinating piece of work,” Bridges said.
“Our view of Mars is changing, and we’ll be discussing this for a long time to
come.”
Πηγή: theguardian.com
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