Unit Three: Investigating Planets
Doing Science

3. Craters, Craters, and More Craters!

Introduction:
Impact craters have been found on (almost) every solid surface in our solar system, from Mercury to Neptune's moons, and even on comet nuclei from the Kuiper Belt and beyond. Consequently, planetary geologists recognize impact cratering as one most important geological processes operating in the solar system. Cratering seems like a pretty simple process: one object slams into another to form a hole in the ground while ejecting a whole bunch of stuff around. But are impact craters identical on all moons and planets? Some worlds have atmospheres while some do not. Some surfaces are made of rocks, while others are made of ice. Some surfaces may have liquids in or on them while others may be completely dry. Some surfaces have very low gravity, and some have fairly high gravity. Can astronomers see any differences in fresh craters on different planets and moons that may be the result of different conditions at the time of impact?

Directions:
Select six or eight planets and moons with different surface condition, like Venus (thick atmosphere, rocky surface), Mars (thin atmosphere, rocky surface), the moon (rocky surface, no atmosphere), Ganymede (ice surface, no atmosphere), Titan (icy surface, thick atmosphere), Phobos or Eros (rocky surface, no atmosphere, very small in size), Wild 2 nucleus (a comet - ice surface, no atmosphere, small size), etc. Make a table with all the characteristics of each object that might affect cratering on its surface.

Locate fresh craters in several images of each planet or moon. "Fresh" is somewhat subjective. Look for craters that look sharp and crisp, have recognizable ejecta blankets or white streaks called "rays," which are usually lighter or darker than the surrounding surface, like the craters Tycho or Copernicus on the moon. Select 2 or 3 images from each object showing representative fresh craters on each.

Good sources of planetary images are:

• National Space Science Data Center (good place for Mercury and Venus, the asteroids, and the moons of Uranus and Neptune): http://nssdc.gsfc.nasa.gov/image/. Click on Catalog of Spacebourne Imaging and select objects of interest.
• US Geological survey map-a-Planet: http://pdsmaps.wr.usgs.gov/. Select a planet or moon. Best for overall view and large features.
• Views of the Solar System (an individual's site): http://www.solarviews.com/. Click on http://saturn.jpl.nasa.gov/multimedia/images/raw/index.cfm.
• Mars: http://marsprogram.jpl.nasa.gov/index.html. Click on Multimedia, then Mars Images, and take your pick by subject area.
• More Mars: http://www.msss.com/. Click on Mars Images at Malin. Scroll down to Images by Subject, and take your pick.
• Jupiter Satellites: http://galileo.jpl.nasa.gov/gallery/index.cfm. Click on the satellite of choice.
• Saturn Satellites: http://saturn.jpl.nasa.gov/multimedia/images/raw/index.cfm. Select satellite of interest in "Raw Images" pull-down menu and click Go.

Your Report:

1. Create an "Impact Crater Gallery" from your selected images. Look for similarities and differences between the craters on each body (including size), and describe what you found. Try and correlate any differences in crater morphology (appearance) with differences in surface conditions (this may take a little searching for more information about each object). Make a list of different surface conditions that seem to affect the cratering process, and which do not. Make a poster or power point to display your images, observations, and conclusions.
2. Repeat this project looking at another common planetary feature, such as volcanoes or sinuous channels, or canyons.