Terrestrial Planets

Part I

Mercury

The surface is saturated in craters (a very old surface) and looks much like the Moon. Mercury has higher gravity making the features a little flatter than similar features on the Moon.

Two lines of evidence lead to the conclusion that Mercury rotated more rapidly in the past. There are long scarfs near the poles that could be the result of the shape of the planet adjusting to the slowing rotation rate. I have already mentioned that an explanation of the small magnetic field involves a more rapidly rotating planet also. Her the magnetic field observed is a small remnant of a larger field that existed when the planet rotated more rapidly.

A very large impact site called the Caloris Basin shows that Mercury (just like the Moon) suffered an early period of major impacts before settling down to the smaller impacts that cause the normal cratering.

The interior model includes a large iron core. Another of the explanations of the small magnetic field involves a permanent field due to the iron core. The iron may also be required to account for the rather large density of Mercury.

Venus

Venus has a very harsh environment: 50 km of sulfuric acid clouds, surface temperature of 900 °F and pressure of 90 atmospheres. We now have excellent radar imagery of the surface and we see many features that look like lava flows. The very high surface temperature limits what the surface composition can be and any increase in temperature can liquefy the rocks. Some of the volcanoes may even been active, since we have inferred lightning strikes around the volcanoes, a situation that exists on Earth.

The former Soviet Union set several probes to the surface to reveal lava fields.

The interior of Venus probably resembles that of the Earth.

Earth

Interior models are more accurate for Earth due to the study of seismic waves. An earthquake emits both transverse as well as longitudinal waves. Transverse waves can only penetrate through a solid, but the longitudinal wave can penetrate through all material. Observations of hundreds of earthquakes yield an interior model that consists of:

• Inner Core - solid, iron-nickel composition, high temperature
• Outer Core - fluid, iron-nickel composition - source of the magnetic field
• Mantel - plastic (i.e., moveable) composition
• Crust - surface skin - thicker under the continents

The surface of the Earth is made up of a system of interlocking plates that move around. Convective currents in the mantel cause fresh material to well up in oceanic ridges, spread out along the surface moving the continents as it goes, and finally dive back into the mantel at subduction zones or trenches. This idea is referred to as Plate Tectonics or Continental Drift. One of the consequences of plate tectonics is the formation of island chains. Volcanoes form over mantel hot spots. Because the crust is in motion, the volcano that forms on the surface will not remain over the hot spot long. As the crust moves along, new volcanoes form over the hot spot. The Hawaiian Island chain is formed in this way.

Likewise the Mid-Atlantic Ridge, running the length of the Atlantic Ocean, is bringing up new mantel material and causing North and South America on one side and Africa and Europe on the other to move farther apart.

Part II

Earthquakes and volcanoes tend to occur along the boundaries of the plates. As the plates move past one another vast amounts of friction are generated. The existence of plate tectonics means that the map of the world has been changing over time. About 250 million years ago all of the continents were together in one supercontinent called Pangea.

The Moon

Without a telescope we see light areas (Highlands) and dark areas (Maria). With a telescope we see that the maria are flat and have few craters, whereas the highlands are heavily cratered.

The maria represent the sites of major meteoric bombardment at the beginning of the solar system. So much energy was generated that the surface melted and lava flows filled in the impact crater. The lava flows solidified again about 3.3 billion years ago. Surface rocks returned from the maria are basalts, the result of volcanic activity.

The highlands have rocks that are much older (4.2 billion years old) and composed of breccias.

The origins of the Moon have been a source of controversy. Three ideas have been explored historically:

• Fission - the early Earth was spinning rapidly enough that a section was flung off to become the Moon. The surface density of the Earth and the average density of the Moon are very close. The volume of the Pacific Ocean basin is almost exactly the volume of the Moon. The composition of the two objects, however, argues against the Moon ever having been a part of the Earth.
• Capture - the Moon formed elsewhere in the solar system (relieving the composition problem noted above) and was captured by the Earth in a near encounter. The argument against this idea is the very low probability that the Moon could be captured in the nearly circular orbit we find it in.
• Binary Accretion - the Earth and Moon formed close together but separately. The differences in composition still plagued this idea and most theories are rather stretched here.
• Collisional Accretion - finally the advent of super-computers have allowed astronomers to test other ideas. Suppose a Mars sized object had a collision with the early Earth. The material combined in large part with the Earth, but a piece broke off at the impact site to eventually become the Moon. This theory has features of the fission and capture theories without their drawbacks. This theory is currently favored.

Mars

Modern Martian studies date to 1877 when Giovanni Schiaparelli recorded features on his hand-drawn map that he labeled "canali" (in Italian, trenches or grooves). The term was mistranslated canals implying an intelligent origin. The Boston businessman Percival Lowell was convinced that Mars possessed an intelligent species and built an observatory in Flagstaff, AZ to prove it.

From the early part of the 20^th century astronomers have known that, although Mars has an atmosphere, it is too thin to support liquid water on the surface. Still Mars has impressive polar caps that grow and shrink with the seasons and dark marking that show seasonal variations similar to vegetation on Earth. Perhaps primitive life does exist on Mars.

Several different terrain types exist on Mars:

• Cratered terrain - heavily cratered section of the planet. One notable difference between the craters of Mars and those of the Moon is that the Martian craters show the distinct markings of mud flows around the margins of the crater, indicating subsurface water. The Moon has no water.
• Sinuous channels - trenches formed by the combined action of water and wind. These are not the canals of Lowell, which have never been identified, but they do indicate that Mars has experienced a vast climatic change in its history. Perhaps two billion years ago, rivers, lakes, and even shallow seas existed on Mars.

Part III

A possible scenario that explains the climate change on Mars involves the small mass of the planet. Long ago Mars had huge, active volcanoes that supplied enough gas to the atmosphere to permit liquid water to exist on the surface. A mild Greenhouse Effect kept the temperature within the zone of liquid water. But the interior soon went cold since low mass planets cool rapidly. The volcanoes became extinct. The planet does not have sufficient gravity to retain a thick atmosphere, and the atmosphere left, leaving the very thin atmosphere we observe today.

Another terrain type on Mars is:

• Volcanoes - large shield volcanoes exist that are amoung the largest in the solar system. Nix Olympica is 2.5 times the size of Mt. Everest. The size of these volcanoes is probably evidence that Mars has no plate tectonics. The volcano that forms over hot spot stays there to grow larger and larger.

We have also been to the surface with the Viking probes of the 1970's and the Pathfinder in the 1990's. The Viking experiments were intended to look for life. Several different life experiments were conducted:

• Respiration - all life forms on Earth take in gases from the atmosphere and expel other gases.
• Metabolic changes - all life forms on Earth take in nutrients and give off waste.
• Organic molecules - all Earth life is based on the four elements C, H, O, and N. These are called organic.

The experiments gave mixed results. Most agree that the Viking found interesting chemistry but probably not life. Perhaps we looked in the wrong place. We know that there is a permanent water ice cap at each pole. The Mars Polar Lander was intended to test this idea. Perhaps we looked at the wrong time. Had space capable humans existed two billion years ago we may have found a very different Mars.

The surface is reddish due to iron oxide compounds in the soil. Periodic dust storms lift this dust into the atmosphere, making it take on a reddish hue.

The interior model is incomplete since one of the seismic stations on the Viking lander failed. It is driven by the relatively low density of Mars compared to the other terrestrial planets.

Mars also has two small moons (Phobos and Deimos) which are probably captured asteroids. These may act as launch sites for future Martian studies.