The Inner Planets
Sean C. Solomon
Department of Terrestrial Magnetism, Carnegie Institution of Washington
The President Mr. Ohlmacher called the 2044th meeting to order at 8:16 p.m. on April 21, 1995. The Recording Secretary read the minutes of the 2043rd meeting and they were approved. The President then read a portion of the minutes of the 436th meeting April 13, 1895.
The President introduced Mr. Sean C. Solomon of the Department of Terrestrial Magnetism, Carnegie Institute of Washington to discuss “Worlds Apart: the Inner Planets”.
As earth scientists we can try to understand how the earth has evolved using other “laboratories”, the terrestrial planets in the solar system that arose at about the same time and out of the same material in the collapsing solar nebula. Although they were born from the same common material, yet they are all different, and our challenge is to explain why this is so. George Weatherall has proposed a computer model of terrestrial planet formation which manages to predict a small number of inner planets roughly similar in size and composition. In order for the model to produce the variations in planetary structure observed in our solar system, it is necessary to invoke historical accidents like very large body collisions. In one model something like the earth-moon system can be produced as the result of a body the size of Mars colliding with a proto-earth and throwing off matter that coalesces into a body the size of the moon beyond the Roche limit.
One thing that has influenced these considerations is the theory of plate tectonics which was advanced in its modern form about 25 years ago and rapidly gained acceptance. This theory accounts for the fact that the ages of rocks under the oceans are generally less than 200 million years old while the ages of rocks under the continents are generally more than 4 billion years old. This latter age coincides with another event that can be dated in the earth- moon system. The moon is covered by impact craters of all sizes, and there are other features that can be attributed to volcanic action and lava flows. The last major volcanic activity was the filling of the mares by lava that has been dated to 4 billion years ago, about the same time as continental crust formation on earth. Measurement of the cratering density of these lunar surfaces can be used to date other planetary surfaces since then. Some of these measurements were made by the Clementine mission last year, the first probe to return to the moon since the last Apollo mission 20 years ago.
Mercury is 4/3 the size of and superficially similar to the moon but is as dense as the earth. Most of Mercury looks like the lunar highlands. Some scarps are observed 10's of kilometers long, suggestive of faulting.
Mars is more varied; the southern hemisphere is heavily cratered while the northern hemisphere is covered with lava flows and faults. The uplands of Mars are the same age as the lunar highlands but the lava flows are younger. The largest volcano is the 25 km high Olympic Mons.
Venus has been a challenge to explore because of the perpetual cloud cover. This has now effectively been penetrated by synthetic arpeture radar imaging. This method has a resolution of 100 meters which is better than some sea floor images. 10% of the surface is so heavily deformed that the surface cannot be resolved even at that resolution. The crater density age seems to be about the same as the continents on earth, about 500 million years. The largest craters on earth and Venus are about the same size, 300 km. 85% of the craters on Venus are preserved and all the surface is about the same age.
Venus and Mars have essentially no water, while the earth is covered with it. Despite its solar proximity there do appear to be polar caps on Mercury revealed as radar bright regions with the reflective properties of water ice. On Mars there are seasonal polar caps of carbon dioxide ice at the south pole and carbon dioxide ice with some water ice at the north pole. Some of the impact craters on Mars look as though they produced fluid ejecta and flow channels. The current thinking is that Mars retains a great deal of water locked in permafrost.
The greatest differences among the inner planets are found in their internal structures and dynamics. The earth has a large molten iron core, the origin of its magnetic field, a thick rocky mantle, and a thin crust. The moon is 10% crust, and most of the rest is mantle with possibly a small iron core that solidified 3 billion years ago. Mercury has relatively high density and a relatively high magnetic field, so it may also have a large molten core. It seems to be lacking light crustal material; it may have been lost in a collision event. There is little information available on the internal structure of Mars; it is still an open question whether it has a magnetic field. The hemispheric dichotomy of Mars may have arisen either because the northern plains are the vestige of a very large impact event, or the southern highlands are the vestige of a period of plate tectonics. The volcanoes on Mars and Venus have large gravity anomalies; the most similar feature on earth is Hawaii. On Mars and Venus these volcanoes overlie elastic plates 30 to 50 km thick.
As Carl Sagan has said, “we are fortunate to have lived in the era when extraterrestrial exploration began, but it will be many generations before the questions about the origins of these planets are satisfactorily resolved.”
Mr. Solomon kindly answered questions from the audience. The President thanked the speaker on behalf of the Society, announced the date for the next meeting, restated the parking policy, and adjourned the 2044th meeting at 9:37 p.m.
Weather: partly cloudy
John S. Garavelli