The comparative planetology of these bodies is a key to the understanding of the formation of the Earth, its atmosphere and oceans, and the physical and chemical conditions that led to the origin and evolution of life.
-- Committee on Planetary and Lunar Exploration of the National Academy of Sciences, 1978
It’s fairly hard just to get a robot in orbit around other solar system bodies. Landing on them is even harder and much more expensive. Additionally, landers tend to have a limited range, assuming they can move at all: they’ll tell you a lot about a small area, but not much about the object as a whole. Because of that, most of what we know of non-terrestrial geology comes from orbital craft.
One of the primary pieces of information on the geology of other worlds comes from pictures taken from space, similar to aerial photos taken on Earth. However, these images are usually monographic (as opposed to the 3-D stereographic images), which can make it difficult to interpret the landforms. In order to correctly interpret images from other planets, we need to start with something known: the Earth.
An important word of warning: you need two images to correctly interpret depth. Your brain will try to add depth to the monographic images, but it will sometimes get it wrong because it only has one image to work with. So be wary of interpretations of an image that rely solely on your interpretation of whether or not something is a bump or a depression!
There are four major geologic processes that reshape the surface of the terrestrial planets and larger moons in our solar system. They are volcanism, tectonics, gradation and impact cratering.
Volcanism occurs when magma from the mantel wells up to the planet’s crust. Lava flows form when the magma flows out onto the surface, usually from a volcano. Shield volcanoes form over hot spots in the mantle. The magma wells up due to convection and eventually pushes its way through the crust.
Tectonics is a movement of the planet’s crust. Plate tectonics dominates much of Earth’s geologic activity, however it is possible to have tectonic activity without having separate plates. During the formation of a shield volcano, the uplift may crack the crust around the volcano forming a scarp. A graben forms when two nearby fault lines separate, allowing the strip of crust between them to drop down, creating a wide flat bottomed valley bounded by two scarps. Grabens may be only a few feet wide or a few miles. They often come in groups, forming near parallel (but not necessarily straight) lines with a raised horst between each graben.
Gradation occurs when material is transported from a high region to a lower one. The overall result is a gradual leveling of the surface. There are three primary agents of gradation: wind; water; and gravity. Gravitational slumping results in events such as landslides or rockslides. Gradation by water can be very complex. For example rainwater may erode the top rim of a scarp carrying water and sediment into a graben, creating tributaries. The water then follows the valley created by the graben, turning it into a deep river valley. Eventually, the river empties into a larger body like an ocean. It drops the sediments along the shoreline, creating a delta.
Cratering occurs when objects with a lot of energy, usually from space, strike the ground. Craters happen to all solar system objects with roughly the same rate, but they are erased by the other processes. Therefore, we can use the craters to help determine the relative age of a surface region.
You'll begin with some images of Earth. Since we can freely access most areas of this planet most of the time, we know what these pictures actually are. You'll use the information from this section to help you with the images of Mars in the second part.
Look at the images on this page and answer the following questions.
Download this section ot the NASA produced planetary geology activity book, in pdf format.
Last modified: 3/7/06