Exploring Mars: Old, Relativity
Craters and landslides at the wall of Valles Marineris. Viewed at an angle, scene is 60 kilometers across.
For exploring Mars, it is important to know which events happened in which order, and which areas are older than others. A simple way of figuring out the sequence of events is superposition: most of the time, younger things are on top of older things, and younger (more recent) events affect older things.
1. Superposition in your life. Is there a pile of stuff on your desk? On your teacher's? On a table or the floor at home? Where in the pile is the thing you used most recently? The thing next most recently? Where in the pile would you look for something you put down 10 minutes ago? When was the last time you (your teacher or your parent) used the things at the bottom of the pile?
2. Superposition on Mars. Using superposition, we can sort out many of the complicated events in the history of Mars. For example, you can sort out all the events that affected the area shown in the figure, which shows a small part of the wall of the great canyon system of Valles Marineris. Toward the top of the picture is a high plateau (labeled "P" on the picture), with a large circular impact crater ("C"). The crater formed when a large meteorite hit Mars' surface. Below the plateau is the wall of Valles Marineris. Here, the wall has been cut away by huge landslides ("L"), which leave bumpy rough land at the base of the wall and a thin, broad fan of dirt spreading out into the canyon floor. In the canyon wall, almost at its top, alternating layers of light and dark rock are exposed.
To discover the history of this part of the Valles Marineris, start by listing all the landscape features you can see, and the events that caused them (don't bother listing every small crater by itself). Now list the events in order from oldest to youngest. [Hints: How many separate landslides are there? Is the large crater ("C") younger than the landslides? Are the landslides younger than the rock layers at the top of the walls? Are the small craters older or younger than the landslides?] Sometimes, you cannot tell which of two events was younger. What additional information would help you tell? To learn more about this image, visit Exploring Gangis Chasma.
