As we have seen in our tour through the solar system, the surfaces of the planets look very different. Geologists and geomorphologists have played an important role in interpreting the surface features of the planets. Those features can provide information not only about the current conditions on other planets but also provide a basis for interpreting what has taken place on those planets in the past.
Geologists have long been interested in unraveling the history of changing environments on Earth based on evidence that is preserved in rocks. To do this we apply the axiom that states:
“The present is the key to the past”.
This means that we can interpret how rocks formed in the past by studying how they form today. If we see ancient rocks with the same characteristics as the rocks that are forming today we can infer that they formed by the same processes that form them today.
A similar axiom helps us interpret bodies in space for which much of our information is based largely on photographic images of the surface of those bodies.
This axiom is that:
“The Earth is the key to interpreting the planets”
This is based on the assumption that features we see on the surfaces of planets and other bodies formed by the same processes that produce the same features on Earth.
For example, river channels form on Earth when water flows over the land surface and erodes the substrate. These channels form specific patterns of erosion on the Earth's surface. When we see similar linear patterns of erosion on the surface of other planets we can infer that they formed by fluids flowing over the surface…..even if there is no water (or other fluids) there today. So as an example, compare the orbital image of the surface of Mars at right with the title image above. Similarities between the channel morphologies in the two images hints similar processes shaping the surfaces. There are big differences, however, including an abundance of circular craters on the surface of Mars that are rare on Earth. Also, the spatial scale is really different between the images; the Sharonov Crater at the center of the Mars image is 100 km across whereas the total width of the title image is only about 25 km. But the similarities provide a starting point to interpret the history of the Mars surface.
Of course, there are processes that act on other solar system bodies that cannot act on Earth. Flowing glaciers of methane, carbon monoxide and nitrogen are found on Pluto where the average surface temperature is -229ºC but such glaciers could never form on Earth. In cases like this we can make our best interpretations based on what we know about flowing water ice on Earth.
In sections 2 through 6 of this module we will look at the processes that act on Earth and the landscapes that they produce so that we have a basis for interpreting those landscapes when we find them on other solar system bodies. The remaining sections of the module will focus on the surface features of other planets and smaller bodies.