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Date: 15 May 2007 18:33:09
From: Sam Wormley
Subject: Martian sands shift slowly but surely
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Martian sands shift slowly but surely
http://physicsweb.org/articles/news/11/5/14/1
15 May 2007
Physicists may have solved a long-standing mystery about sand dunes
on Mars. The dunes looks as if they should have been created by the
action of wind, but the Martian atmosphere is so still and thin that
it was thought impossible for wind to have played a part. New
computer simulations suggest that the wind can indeed drive the sand
dunes on Mars -- it's just that the dunes are formed far more slowly
than back on Earth (Phys. Rev. Lett. 98 198001).
Astronomers have long been puzzled by the sand dunes on Mars, which
were first discovered in 1971. The dunes look very much like those on
Earth, which suggests they were created by the action of wind. The
problem is that the Martian atmosphere is so thin and still -- so how
could the wind have played a part?
Even more curious is the fact that successive missions to Mars have
not detected any change in the positions of the dunes, whereas the
dunes on Earth are shifting constantly. Some scientists have
therefore suggested that the dunes were created long ago, when the
Martian atmosphere could have been much denser than it is today.
Now, however, Eric Parteli at the University of Stuttgart and Hans
Hermann at the Swiss Federal Institute of Technology in Zürich reckon
the Martian dunes could indeed form and shift under the planet's
current atmospheric conditions. They have carried out a series of
computer simulations of the formation of two distinct types of dunes
seen on Mars -- arrowhead-shaped "barchan" dunes, which are formed
when the wind blows mainly in one direction, and elongated
"exotic" dunes that are formed when the wind alternates
between two directions (see figures).
Assuming that the sand was subject to just one 40-second gust of wind
every five years -- conditions experienced by Mars probes -- the
simulations suggested that the exotic dunes could have formed from
barchan dunes over a period of about 10,000 to 50,000 years. Parteli
told Physics Web that under the same conditions, it would take about
4000 years for a dune to shift by 1 m, which could explain why the
dunes have not appeared to move since they were first discovered.
According to Parteli, the key to modelling the dunes is a process
called "saltation", whereby a grain of sand is lifted by the wind,
and driven along above the sand until it falls back, creating a
splash of ejected grains. The ejected grains could then also be
picked up by the wind leading to a rapid multiplication of the amount
of sand that is being blown along.
Saltation has been successfully modelled for dunes on Earth using
parameters determined experimentally. But when Parteli and Herrmann
used the same parameters under Martian conditions, they found that
dunes would not form. Undeterred, they turned to research done nearly
30 years ago that established that the airborne grains should travel
10 times faster on Mars than on Earth thanks to the planet\u2019s
thinner atmosphere and lower gravity. As a result, each Martian grain
should eject about ten times more sand upon impact than does a grain
on Earth. Using these saltation parameters, Parteli and Herrmann were
able to reproduce both the barchan and exotic dunes.
According to Parteli, the simulations suggest that atmospheric
conditions on Mars have been relatively stable over the past 50,000
years and that the dunes are not necessarily evidence that the
Martian atmosphere had been more Earth-like in the past.
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Date: 16 May 2007 07:07:07
From: AustinMN
Subject: Re: Martian sands shift slowly but surely
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On May 15, 6:26 pm, "MTA" <M...@nospan.com > wrote:
> Well, you make a valid point. Sam makes no point at all. I surmise that
> high and low pressure systems exist
> on mars..why wouldn't they?
Empty-A strikes again. Another one for the killfile.
Austin
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Date: 16 May 2007 07:05:57
From: AustinMN
Subject: Re: Martian sands shift slowly but surely
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On May 15, 4:12 pm, "S. Caro" <s...@muxnet.com > wrote:
> Sam Wormley wrote:
> > Martian sands shift slowly but surely>
> >http://physicsweb.org/articles/news/11/5/14/1
> > 15 May 2007
> > (snip)
> > Astronomers have long been puzzled by the sand dunes on Mars, which
> > were first discovered in 1971. The dunes look very much like those on
> > Earth, which suggests they were created by the action of wind. The
> > problem is that the Martian atmosphere is so thin and still -- so how
> > could the wind have played a part?
>
> Strange.. It seems there's plenty of wind on Mars despite the thin
> atmosphere. I can understand dunes moving much more slowly than on
> earth due to the thinner atmosphere, but don't know why the scientists
> are puzzled.
>
> The mars rover Spirit filmed 'dust devils' moving across the surface of the
> planet. These winds have enough force to leave marks on the desert surface.
>
> http://marsrover.nasa.gov/gallery/press/spirit/20050819a.html
>
> Also.. Major dust storms that whip dust high into the atmosphere aren't
> uncommon occurrences either.
>
> http://antwrp.gsfc.nasa.gov/apod/ap990809.html
>
> I'd imagine a wind that can carry tonnes of dust aloft could push the
> odd grain of sand along the surface.
Sorry, but not quite. The dust particles carried by the dust devils
and by Martian winds are thousands of times smaller than grains of
sand.
Austin
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Date: 15 May 2007 17:12:07
From: S. Caro
Subject: Re: Martian sands shift slowly but surely
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Sam Wormley wrote:
> Martian sands shift slowly but surely>
> http://physicsweb.org/articles/news/11/5/14/1
> 15 May 2007
> (snip)
> Astronomers have long been puzzled by the sand dunes on Mars, which
> were first discovered in 1971. The dunes look very much like those on
> Earth, which suggests they were created by the action of wind. The
> problem is that the Martian atmosphere is so thin and still -- so how
> could the wind have played a part?
Strange.. It seems there's plenty of wind on Mars despite the thin
atmosphere. I can understand dunes moving much more slowly than on
earth due to the thinner atmosphere, but don't know why the scientists
are puzzled.
The mars rover Spirit filmed 'dust devils' moving across the surface of the
planet. These winds have enough force to leave marks on the desert surface.
http://marsrover.nasa.gov/gallery/press/spirit/20050819a.html
Also.. Major dust storms that whip dust high into the atmosphere aren't
uncommon occurrences either.
http://antwrp.gsfc.nasa.gov/apod/ap990809.html
I'd imagine a wind that can carry tonnes of dust aloft could push the
odd grain of sand along the surface.
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Date: 15 May 2007 23:26:11
From: MTA
Subject: Re: Martian sands shift slowly but surely
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Well, you make a valid point. Sam makes no point at all. I surmise that
high and low pressure systems exist
on mars..why wouldn't they?
> Strange.. It seems there's plenty of wind on Mars despite the thin
> atmosphere. I can understand dunes moving much more slowly than on
> earth due to the thinner atmosphere, but don't know why the scientists
> are puzzled.
>
> The mars rover Spirit filmed 'dust devils' moving across the surface of
> the
> planet. These winds have enough force to leave marks on the desert
> surface.
>
> http://marsrover.nasa.gov/gallery/press/spirit/20050819a.html
>
> Also.. Major dust storms that whip dust high into the atmosphere aren't
> uncommon occurrences either.
>
> http://antwrp.gsfc.nasa.gov/apod/ap990809.html
>
> I'd imagine a wind that can carry tonnes of dust aloft could push the
> odd grain of sand along the surface.
>
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Date: 16 May 2007 21:53:02
From: Trane Francks
Subject: Re: Martian sands shift slowly but surely
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On 2007-05-16 08:26 +0900, MTA wrote:
> Well, you make a valid point. Sam makes no point at all.
It's well worth noting that Sam was quoting an article and wasn't going
about attempting to make points at all.
trane
--
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// Trane Francks trane@gol.com Tokyo, Japan
// Practice random kindness and senseless acts of beauty.
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