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... http://quest.arc.nasa.gov/mars/team/urquhart.html 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! http://quest.arc.nasa.gov/mars/ask/index.html ----- Jan Wee, discuss-lfm moderator Live From Mars http://quest.arc.nasa.gov/mars