Astronomy_Mod09_chunk3.pdf - Chapter 10 Earthlike Planets:...

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Figure 10.23 Dust Devil Tracks and Sand Dunes.(a) This high-resolution photo from theMars Global Surveyorshows the dark tracks of severaldust devils that have stripped away a thin coating of light-colored dust. This view is of an area about 3 kilometers across. Dust devils are one ofthe most important ways that dust gets redistributed by the martian winds. They may also help keep the solar panels of our rovers free of dust.(b) These windblown sand dunes on Mars overlay a lighter sandy surface. Each dune in this high-resolution view is about 1 kilometer across.(credit a: modification of work by NASA/JPL/University of Arizona; credit b: modification of work by NASA/JPL-Caltech/University of Arizona)10.5WATER AND LIFE ON MARSLearning ObjectivesBy the end of this section, you will be able to:Describe the general composition of the atmosphere on MarsExplain what we know about the polar ice caps on Mars and how we know itDescribe the evidence for the presence of water in the past history of MarsSummarize the evidence for and against the possibility of life on MarsOf all the planets and moons in the solar system, Mars seems to be the most promising place to look for life,both fossil microbes and (we hope) some forms of life deeper underground that still survive today. But where(and how) should we look for life? We know that the one requirement shared by all life on Earth is liquid water.Therefore, the guiding principle in assessing habitability on Mars and elsewhere has been to “follow the water.”That is the perspective we take in this section, to follow the water on the red planet and hope it will lead us tolife.Atmosphere and Clouds on MarsThe atmosphere of Mars today has an average surface pressure of only 0.007 bar, less than 1% that of Earth.(This is how thin the air is about 30 kilometers above Earth’s surface.) Martian air is composed primarily ofcarbon dioxide (95%), with about 3% nitrogen and 2% argon. The proportions of different gases are similar tothose in the atmosphere of Venus (seeTable 10.2), but a lot less of each gas is found in the thin air on Mars.While winds on Mars can reach high speeds, they exert much less force than wind of the same velocity wouldon Earth because the atmosphere is so thin. The wind is able, however, to loft very fine dust particles, which cansometimes develop planet-wide dust storms. It is this fine dust that coats almost all the surface, giving Mars itsdistinctive red color. In the absence of surface water, wind erosion plays a major role in sculpting the martiansurface (Figure 10.24).Chapter 10 Earthlike Planets: Venus and Mars359
Figure 10.24 Wind Erosion on Mars.These long straight ridges, called yardangs, are aligned with the dominant wind direction. This is a high-resolution image from theMars Reconnaissance Orbiterand is about 1 kilometer wide. (credit: NASA/JPL-Caltech/University of Arizona)Although the atmosphere contains small amounts of water vapor and occasional clouds of water ice, liquidwater is not stable under present conditions on Mars. Part of the problem is the low temperatures on the planet.

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