Origin of Sand on Mars

From: Jan Wee <jwee@mail.arc.nasa.gov>
Subject: Origin of Sand on Mars
Date: Mon, 13 Oct 1997 17:42:06 -0500

Dear discuss-lfm members,

You may recall that Pedro Escudero shared a question about
the source of sand on Mars via this forum.  Pedro followed
Sandy Dueck's and my suggestion to send the question to 
the Mars Team experts via the question-lfm@quest.arc.nasa.gov
email address.   He would like to share the answer with you
via this posting....

QUESTION from Pedro to Mars Team:
If there is sand in Mars and it atmosphere is extremely thin, which 
is the origin of that sand?  The barometric preasure on Mars is 
about 7 milibars on Earth we have 1.O12 milibars as normal preasure. 
That means that the air in Mars is not dense enough to produce 
erosion on the Planet rocks. Could it be that Mars had a heavy 
atmosphere millions of years ago ? Or that the sand comes from 
the erosion of ancient waters on that Planet ? 

We are told that the Martian surface we see in the Pathfinder images is
several billion years old. We are also told that there are dust storms, and
wind. I would have thought that these latter processes would have completely
transformed the surface features. Billions of years is a long time even
considering that erosion must be slow due to the atmospheric density being
1/100th of Earth's. I would have thought that exposed rocks would have been
weathered down to nothing and all we would see would be vast expanses of
dust like some Earth deserts. What is wrong with my thinking?

Looking forward to your ansers,
Pedro Escudero

ANSWER from Mary Urquhart 
The area where Pathfinder landed is between and a few and about 1 billion
years old based on the impact cratering record.  The thin atmosphere is just
one factor in the slow erosion rate we observe on Mars.  On the Earth, water
is by far a more effective eroding agent than wind.  In desert areas on the
Earth, such as our own desert southwest, features that have been wind eroded
have also been greatly affected by water erosion.  Much of the sand we see
in desert areas is brought in by flowing water or created during the
occational heavy rainfalls and associated flooding events.

Water is a very effective mechanical erosion agent. In addition, water
chemically erodes rocks as well as mechanically eroding them. Without
rainfall or flowing surface water, even with an atmosphere as dense as our
own, the erosion rate will be *very* slow compared to the typical erosion
rates we see here on the Earth. Slow erosion rates on Mars as indicated by
the age of surfaces are one of the major arguments that climate on Mars
hasn't been able to support rainfall for most of the history of the planet.
Wind velocity, the ability of the wind to carry abrasive agents such as sand
(a function of velocity and density), the availability of abrasive agents
such as sand, and topography each also play a role.  Typical wind speeds on
Mars are not very high (despite the occasional large scale dust storm).

Mary Urquhart*                                    
Laboratory for Atmospheric and Space Physics  
University of Colorado at Boulder    

*To learn more about Mary, visit the LFM the Mars Team journals...

Thanks, Pedro, for sharing.  By the way... all the Question and
Answer Pairs are archived at the LFM web site via this
link... it's intriguing just to see the variety of questions
and to read the Mars Team answers!


Jan Wee, discuss-lfm moderator
Live From Mars