The Geography of Mars

Relative Age of Landforms and Sequencing of Processes

• to introduce you to Mars Odyssey THEMIS-derived imagery
• to give you practice in inferring relative ages of Martian surfaces through crater counting and assessment of crater condition
• to give you practice in applying superposition to infer the relative sequence of processes creating Martian landforms
• to alert you to imagery flaws that could be misinterpreted as surface features

The NASA Mars Odyssey orbiter (2001+) carries the Thermal Emission Imaging System, or THEMIS. This is a multispectral imager that offers five visible light bands (with 19 m pixels) and ten infrared bands (with 100 m pixels). In the IR, THEMIS picks up the emitted re-radiation of solar energy absorbed during the day. It creates images that resemble a lit landscape but it must be remembered that the bright areas are not reflecting sunlight but strongly emitting IR radiation based on absorption of sunlight).

For more information about THEMIS:

Christensen, P.R., N.S. Gorelick, G.L. Mehall, and K.C. Murray, THEMIS Public Data Releases, Planetary Data System node, Arizona State University, http://themis- data.asu.edu.

You can download your target image from https://home.csulb.edu/~rodrigue/mars/labs/gangeschasma.jpg. This depicts a section of Ganges Chasma on the eastern end of the Valles Marineris system, roughly -8° and 48° W.

Interpreting the image

Think about which slopes (north facing or south facing) would be the adret or ubac slopes (search those online if you don't remember what those are). On Earth, which slopes are generally warmer: west-facing or east-facing? Why? Use these ideas of warmer slope aspects to figure out the orientation of the image.

Examine the different parts of this image: the flat terrain dominating the top of the image, the scarp and large crater dominating the middle ground, and the wide variety of deposits dominating the lower portion of the image.

Note the varying density of craters found in each area and how fresh (sharp-edged and cup-shaped) or softened (dulled edges and flatter floors) they appear. The density and condition of the craters are related to the age of the various surfaces.

What is the lumpy, blocky material at the base of the scarp? What are the flatter, striated areas fanning out from those piles? You will probably need to look through your introductory physical geography or general geology textbooks to figure out what those features are and what the striations indicate.

Notice the superpositions of these fanned out deposits: They are a clue to the sequence of events here. You might want to identify each of the lobes of material by marking them with letters and then try to figure out which is younger than which other(s) on the basis of superposition relationships.

Keep an eye out for artificial patterns that indicate camera or data-transmission hiccups. Some are pretty obvious; one or two are really subtle.

Lab report

Write a brief lab report interpreting this landscape, responding to the following questions. You can deposit the report in the Dropbox for the course in BeachBoard (Lab: Ganges Chasma).