Images of planets’ surfaces sent back to earth often have abstract patterns that need to be interpreted.
Scientist use their knowledge of the formation of features on Earth and other background knowledge they have about a planetary body to identify geologic features of planets and hypothesize about how those features may form on these other worlds.
Two kinds of features are "aeolian": formed by wind, and "fluvial": formed by a liquid.
Scientists use their knowledge of what kinds of patterns liquid water and wind make to find out if the planet did have/has running water (or other liquid), and has wind (therefore an atmosphere).
Students make patterns with wind and water:
Aeolian (wind) patterns are made by spreading sand evenly in a tray, adding two rocks to one end of the tray, and blowing the sand along the bottom of the tray from the other end. A plexi sheet covering most of the tray prevents sand from blowing out of the tray. The sand is blown from around the rocks, but remains in a streak behind the rocks (see first two images).
Fluvial (liquid) patterns are made by swirling a tray of sand and water to mix them evenly, then tipping the tray so that the water runs through the sand, creating channels. (See third image).
Students compare the patterns they make to those seen on other planets, and deduce what formed each of the pictured planetary features. (See fourth image, and links below.)
Mars features are both fluvial and aeolian:
A drainage network of a liquid is clearly seen. There is no liquid on Mars now, only ice water. This fluvial feature is from extensive water flow in the past when Mars was warmer. Why was Mars warmer? Maybe it had an atmosphere to create a greenhouse effect. Or maybe asteroid and comet impacts which filled the atmosphere with vaporized rock and ice, resuled in several years of rainfall and flooding that formed the fluvial erosion features.
The wind streaks seen on Mars are from erosion of the surface rock. Erosion needs an atmosphere - a gas that can move past the surface and carry dust particles along. This is happening on Mars now - we have seen tornadoes on Mars.
We also see dunes on Mars, evidence of aeolian activity.
Wind streaks are seen. Winds are from carbon dioxide atmosphere blowing fine-grained particles around.
Titan (moon of Saturn) feature:
Drainage networks from a liquid are seen. They are formed by liquid methane! Venus has a methane cycle: it rains methane, which flows into methane rivers and lakes, from which it evaporates and rains again (just as we have a water cycle).
Images and information from the NASA ARES site resource. Specific web links:
Visual summary of how different planetary features are formed at https://ares.jsc.nasa.gov/interaction/eeab/pdfs/bmm/bmm_quickreferencesh...
Images of various planet features at https://ares.jsc.nasa.gov/interaction/eeab/pdfs/bmm/bmm_planetaryfeature...