How do planetary geologists learn about the geology of other planets? Since much of the available information is from spacecraft flying high above the planet, these scientists compare images taken of the planet with images taken on Earth. Look at the images above and see if you can find the similarities and differences. Which is from Earth and which from another planet?

It’s easy to figure out that the top image is Earth since there are plants. It is Devil’s Tower in Wyoming and the bottom image is Marte Vallis on Mars. The features that are common to the images are columnar basalts showing columnar jointing and talus slopes. Columnar basalts form from basalt lavas that flow in a thick layer along a cool surface. The vertical joints speed cooling. Weathering causes the columns to break and form talus slopes.

On Mars the columnar basalts are exposed on the rim of a 160 km diameter crater. Evidence of the lava flows covers more than 200 square km (77 square miles), similar to terrestrial flood basalts. A meteorite impact appears to have created a crater to expose the columnar basalts.

Devil’s Tower is a magma that cooled below the surface in sedimentary rock layers and formed columnar joints. The sedimentary rocks have since been eroded, exposing the tower. At the base are slopes of broken columns creating talus slopes.

^{Mars image courtesy High Resolution Imaging Science Experiment, Arizona State University. Devil’s Tower image courtesy Wyoming Geographic Information Advisory Council. earthobservatory.nasa.gov/IOTD/view.php?id=38904. Public Domain.}