Planetary Surfaces

Things to notice on a planetary surface Number and size of craters

The density or number of craters on the surface and whether or not they overlap provides insight to the inner workings and age of the surface. The term crater saturation means that the surface density of craters is so high that the next event (impact or volcanic) basically just puts a new crater on top of an old crater so the crater density has reached its physical limit.

Let's Examine A Cratering Simulation

The relative size distribution of craters on a planetary surface is also very important because taken together, the crater density and its size distribution provides insight on the relative age of a planetary surface. Saturated surfaces are significantly older than surfaces with low crater density. If a surface of a rocky planet is relatively smooth, there must be some kind of resurfacing mechanism obliterating the old craters. As we explore the planets,we will find out what these mechanisms are for each object and learn why there should be craters in the first place.

Thus the surface on the top is significantly older than the surface on the bottom in the two images below.

Another important thing to look for is the presence of a central peak in the crater. If the cratered surface is primarily rock, then there will be a small rebound effect of the imploded surface. If the surface is mostly ice, there usually will not be a central peak.

Impact craters are essentially explosive events and proceed through several stages as shown in the animation below. Often times the ejecta blanket (which comes from material at depth) is lighter than the surface material leading to the apparance of rays:

Another subtle but important feature of craters is the thickness/thinness of the crater walls.

The Earth has impact craters on its surface as well - old ones have eroded away and are hard to find. Here are some examples:

38 Million Years Old in Canada

200 Million Years Old Near Quebec

290 Million years old, Clearwater Lakes, Canada

5 Million years ago, Namibia Desert

The Earth Impact Database (Vredfort example) 2 billion year old Earth crater!

Finally there is the issue of "flow" features on surfaces that could be fractures, ice flow, lava flows, rivers, etc.