• size

• surface composition

• atmosphere

• evidence of geologic activity

Possible answers:

Planet	Size	Composition	Atmosphere	Geologic Activity	Result: (cratering state)
Mercury	small	rocky	none	very little (some tectonics)	old, saturated surface
Moon	small	rocky	none	very little (some volcanism)	old surface, many areas saturated
Mars	mid-sized	rocky	thin	some (tectonics, large volcanoes, some wind, maybe water?)	some areas saturated, some craters removed by volcanism, wind, dust (no tiny craters)
Venus	almost Earth-sized	rocky	thick	some (volcanoes, maybe tectonics, little weathering)	few craters, (atmos. keeps out many), pristine condition
Earth	largest terrestrial planet	rocky + oceans	medium	lots! (tectonics, volcanoes, weathering - water, wind)	very few craters, most heavily modified by weathering

Scientific context: The most important factor in predicting how the surface of a planet will look is the degree of geologic activity, or how effective various agents of change are. The appearance of craters on the surfaces of the terrestrial planets is an indication of how geologically active the planets are. Since bolides impact all bodies in the solar system, a lack of craters must be explained by past or current geologic activity. There are many possible agents of change which could be responsible. Craters can be removed by having other craters form over them, as on a saturated surface. They can have lava flows bury them, or tectonic activity fracture them. They can be filled in by dust, blown away by wind, or washed away by water. They can even be obscured by vegetation. Using this indicator, we can rank the planets from old, inactive bodies to young, geologically active worlds.

Oldest on this scale are bodies like Mercury and the Moon. These are relatively small objects, with old, heavily cratered surfaces and little evidence of subsequent activity which could have covered or partially obscured their craters. Some of the large craters on the Moon, however, are filled in with lava flows, evidence that the Moon was once active. Mercury and the Moon are sometimes called "dead" bodies, because there is no evidence of current geologic activity.

The in-between cases are Mars and Venus. Mars has some craters on its surface, but also has other features like volcanoes and giant rift valleys, evidence that the planet was once much more active than it is today. Some craters on the surface seem to have been filled in with dust or eroded away, evidence that while Mars' thin atmosphere is not very efficient, it does affect surface features. There are relatively few craters on the surface of Venus, and most seem to be preserved in pristine condition. There is also evidence of volcanism and tectonic activity. Scientists have interpreted surface images as indicating that Venus underwent a period of great geologic activity about 500 million years ago, which removed all older craters. Since then, however, the planet has been relatively inactive, meaning that any craters that have accumulated in the last 500 million years have been preserved in relatively pristine form.

The other extreme is the Earth, which is a large, very active planet with very few craters preserved on its surface (Meteor crater, in Northern AZ, is one of the best-preserved.). Tectonics and volcanism are important processes on Earth, but even more important are erosional processes caused by wind and water. Earth is the only body with liquid surface water, which quickly washes away most craters. It is no accident that one of the most well-preserved craters on Earth is in a desert!

Much can be determined about the state of geologic activity on a planet merely by examining craters and other features on its surface. A heavily cratered surface (Mercury, Moon) indicates that the planet is not currently active, and has not been active for (perhaps) billions of years. A non-saturated surface is an indicator of past (Mars, Venus) or current (Earth) geologic or atmospheric activity of some sort. Whatever the crater removal process, it is important to understand that if the surface of a planet is not covered with craters, there must be a reason!

TEACHER FEATURE: Wrap-up Activity

Next Page

• Introduction

• I. Location of features on the surface of a planet: latitude and longitude

• II. Crater formation, modification, and removal
  • A. How are craters made?

  • B. Where have all the craters gone?

  • C. Focus on the Galilean satellites

• III. Crater Image Interpretation
  • A. Crater size

  • B. Crater depth

  • C. Crater size distribution and surface ages

• Conclusion