[ image of Mars ]       

The Geography of Mars


Profiling the Crustal Dichotomy

Christine M. Rodrigue, Ph.D.

Department of Geography
California State University
Long Beach, CA 90840-1101
1 (562) 985-4895

Visualizing the Great Crustal Dichotomy
Using IDL Virtual Machine and Gridview

This lab has the following objectives:
  • to familiarize you with MOLA data
  • to introduce you to IDL Virtual Machine software and the Gridview application for viewing the MOLA derived digital elevation model for Mars
  • to use Gridview to construct longitudinal profiles
  • to use Gridview to measure the length of your profiles and figure out the rise and run of your profile

Gridview is a NASA tool for visualizing various aspects of the martian surface. It does this by projecting the martian digital elevation model based on millions of points collected by the Mars Global Surveyor Mars Orbiter Laser Altimeter (MOLA). It has loads of tools for moving around Mars, zooming in, playing around with various color schemes for hypsometric tinting of the MOLA elevations, and an ability to project these color schemes on a shaded relief map or a contour map. You can use Gridview to measure distances between points on Mars and to construct highly vertically exaggerated longitudinal profiles between points. You can even design fly through trips around Mars (my flights have generally ended badly, along the lines of riding a bucking bronco back to Earth!).

In this lab, you'll use Gridview to construct several longitudinal profiles in order to see the contrast in elevations between the Southern Highlands and the Northern Lowlands and how sharp that contrast is in many places. You will often encounter the dramatic local changes in elevation found around craters, volcanoes, and the great Valles Marineris rift zone.

For more information:

Your data

Your data consist of elevation readings collected by the Mars Orbiter Laser Altimeter or MOLA. MOLA was one of the instruments carried on the Mars Global Surveyor orbiter, which operated from 1997 to 2006. It worked by sending laser beams to the surface of Mars and recording the time of return of the reflected beam (kind of like a satellite-borne total station). These laser returns eventually generated millions of discrete elevation estimates, and these are the basis of the Mars digital elevation model.

The MOLA data are saved as a "grid" in a .sav file for use in IDL and the Gridview application. To get at them, you will need to be able to fire up the IDL Virtual Machine FIRST and Gridview and then use Gridview to open this grid.

IDL and Gridview

To get to your data, go to the Desktop and double-click on the Mars folder icon. In there, double-click the IDL Virtual Machine. When the IDL VM dialogue comes up, pick Click to Continue

Once IDL opens up Windows Explorer, click the Desktop and then the Mars folder and then the Gridview subfolder. In there, open the gridview.sav file.

This will open a white geographic grid on a black background as a separate software environment. In this Gridview environment, click on File and then on Load (.sav) Grid and then open the MEG04 folder and then double-click on the Mola_04.sav, and up will come Mars hypsometrically tinted in a red-blue color ramp.

Whatever you do, do NOT just use Windows Explorer to navigate to Mars, Gridview, and the gridview.sav (or the MOLA .sav files). That file extension is used by SPSS, and our lab defaults to it, so the statistics program will open and try to open the gridview .sav files, which will not be a happy experience!

To move around Mars, you need to type in various latitude and longitude coördinates and then hit Reset. Do NOT type in coördinates and then hit Enter: It'll scold you. This is the only way to move around in Gridview: There is no panning function. The geographic grid used here represents latitude by plusses and minuses, not N and S. North is positive latitude, while south is negative latitude. Longitude here is in eastings: Longitude goes eastbound from 0°: full circle to 360°. On Earth, our custom is to have longitude values increase in both directions from Greenwich, differentiating the hemispheres with W and E Here, all longitudes are eastings, the values increasing going east. So, 30° W would be shown as 330°. This may take a little discomfort getting used to. Later in the class, we'll discuss the cartographic quirks of the Mars community!

First, type in 0.000 for Center lon and hit Reset. That will center Mars at the beginning of its geographic grid.

In Tools, select Profile. Then, go back to the globe and carefully move your cursor until it exactly aligns on the 0° meridian at the -30° parallel and click to establish your first point. Then, move it exactly along the 0° meridian until it aligns perfectly with the 60° parallel and click to establish the end point.

A window will pop up, showing a longitudinal profile along the path you selected. You can move the cursor, a vertical and horizontal crosshair line, around and it will give you elevation readings (in meters) for various points along the profile. Save it as a .bmp file. Also, note the elevation at the beginning of your profile and the end. These are given in meters above and below the martian geoid or "areoid," or the datum used to establish an analogue for "sea level" on Mars (equipotential gravitational/rotational surface).

Go back to Tools and this time select Calculate Distance. Using the line from the profile, again align your first point with the -30° parallel and the end point with the 60° parallel. Note the distance, which is in kilometers.

Once you have safely stowed your profile and noted its distance and the start and end elevations, go back and reposition the globe at 45° E longitude. Then, use Tools -- Profile and then Tools -- Distance to do another profile, saving the profile as a .bmp file. Repeat this process every 45° of longitude, giving you eight such profiles and distance readings.


For each profile, subtract the elevation in meters at -30° latitude (generally a positive number) from the elevation at 60° (a negative number) to get the total elevation change along your profile: its rise and run (like the Opposite and the Adjacent sides of a triangle). Divide that total elevation change (rise) by the length of the profile (run). Make sure both the rise and the run are in the same unit of measure: both in kilometers or both in meters. You need to convert one of them into the other unit of measure (m to km or km to m).

Now that you have the rise/run (tangent in trig), you can multiply that answer by 100 to get the percent slope.

You can calculate the slope angle by taking the arctangent (inverse tangent) of the rise/run (inv tan or Tan-1 on a calculator or =degrees(atan(#)) in OpenOffice/LibreOffice Calc, where # is your rise/run above).

Look at all your profiles and find the area on most of them where the floor of the Northern Lowlands transitions to the Southern Highlands. If you look closely, you'll find that most profiles have an abrupt rise, usually on the order of a kilometer or more, in less than 500 km. We'll be talking a lot about this feature: the marked difference in elevation between the two hemispheres of the planet and the often very abrupt boundary between them. This feature is called the crustal dichotomy.

Lab report

The deliverable from this lab is pretty basic: Copy and paste your eight profiles into an Open/Libre Office Calc or Writer document, resizing them to get two or four of them on one page. For each profile, also write down:

  • the beginning elevation
  • the ending elevation
  • the length of the profile (either kilometers or meters)
  • the percent slope (make sure rise and run are both in the same kinds of units!)
  • the slope angle.

"Autograph" the document and turn it in. I am just looking for evidence that you did the profiles and slope calculations and became familiar with the way Gridview works. Print the file and turn it in (with your name on it ...).

You can also deposit the report in the Dropbox for the course in BeachBoard (Lab 2).

Dr. Rodrigue's Home |   GEOG 441/541 Home |   Geography Home |   ES&P Home |   EMER Home |
BeachBoard |   CSULB |   Library |   Bookstore
This document is maintained by Dr. Rodrigue
First placed on the web: 01/11/12
Last updated: 08/30/22