A1.    Volcanoes on Mars built mountains just as they did on Earth, as opposed to volcanism on the Moon which erupted from many cracks and fissures to fill low lying areas without building mountains. Volcanoes on Mars are much bigger than Earth’s volcanoes, because on Earth a single hot spot makes many volcanoes as the crust moves past the hot spot. We know that all of Mars volcanoes are extinct because all of them have impact craters on their flanks.

A2.    The relative age of a surface is determined from the density of craters in it. The greater the number of craters per square mile, the older the surface. Mercury has a very old, cratered surface. Venus has a very young surface with few craters. Mars is split into two regions, one very old and one relatively young.

A3.    Venus has always been too hot to have liquid water, due to its proximity to the sun. When carbon dioxide is released into its atmosphere from volcanoes, it merely builds up in the atmosphere. On Earth carbon dioxide dissolves in the oceans and does not accumulate in the atmosphere. The extra greenhouse effect on Venus’ atmosphere has run away to produce a very hot environment.

Mars is a smaller planet than Earth, and its weak gravity is not able to hold its atmosphere permanently. Now that Mars’ volcanoes are all extinct (also because it is a small planet and has lost its internal heat), the atmosphere is slowly dissipating. Hence, it is very thin today.

A4.    The four processes which shape the surfaces of solid planets are: cratering (impacts not only create the craters but they also pulverize the rocks and spread them around the new craters), volcanism (the outflow from volcanoes covers large areas of the surface with new rocky material), tectonism (horizontal plate motion creates new surface material at spreading centers and recycles old surface material where plates collide), and erosion (the action of wind and water grinds away at the surface features and generally smooths them out).

A5.    The atmospheres of both Venus and Mars are mostly CO₂ while Earth's is N₂ (78%) and O₂ (22%). Surface temperatures range from 860 ^oF at Venus to about 40 ^oF at Earth and about 0 ^oF at Mars. Surface pressures on Venus are 90 times those on Earth, while the pressure at Mars' surface is only 0.8% of Earth's surface pressure.

A6.    Assuming that equal numbers of meteorites land on all parts of a planet, the lightly cratered surface must be much younger than the heavily cratered surface. Without additional evidence we cannot say what may have formed the young surface.

A7.    Compared to its size, Mercury's core is the biggest in the solar system. The Moon's is the smallest and Mars' is also relatively small. Earth's and Venus' cores are in between these extremes in relative size.

A8.    Examples: Valles Marineris is a large rift canyon on Mars longer than the entire US is wide. Olympus Mons is a huge volcano on Mars, much larger than any mountain on Earth. The Caloris Basin is a large impact crater on Mercury which is almost 1,000 miles across. Maxwell Montes is a large mountain range on Venus that may be volcanic or may be related to plate motion.

A10.    The competing factors which determine whether or not a planet can retain an atmosphere are its gravity and the temperature of the atmosphere. For higher temperatures, the atoms in the atmosphere move faster and can more easily escape the bonds of gravity.

A11.    On the Moon and Mercury, volcanism occurred only early in the history of the planet as general oozing from cracks in the crust. The lava released this way filled low lying basins and did not form volcanic mountains. On Mars there was some of this general volcanism, but there are also very large volcanic mountains. These mountains built up to huge sizes because the crust remained stationary over hot spots. Volcanism on Earth is concentrated at the convergent boundaries between plates where subduction returns material to the mantle and hot spots develop. Isolated hot spots in the middle of plates do not build volcanoes to the same degree as on Mars because of the motion of the plates over the hot spot. A few volcanic plains exist on Earth as well. Volcanism on Venus has been global, recovering the entire surface within the last few hundred million years. Volcanic mountains of various sizes and types as well as volcanic plains exist across the planet.

A12.    Small planets cooled quickly and formed a single very thick plate for the crust. This crust was dominated by vertical motion and heat loss by conduction, and had no horizontal motion. Larger planets cooled more slowly and their crust separated into multiple plates which moved horizontally. Heat was lost primarily through this tectonic motion.

A13.    The most significant geologic process on each of the planets is: Mercury — cratering; Venus — volcanism; Earth — plate tectonics; and the Moon — cratering.

A14.    The Moon and Mercury (and part of Mars) have been strongly affected by cratering. Their surfaces are covered by a large number of impact sites. Volcanism has been important on the Moon, Venus, and Mars (and possibly Mercury). The outpouring of molten material from the interior has covered large parts of the surface of these bodies with fresh material. The Earth has been strongly affected by tectonics. Where plates meet, mountain ranges and trenches are formed; at spreading centers, ridges of new crustal material are seen. Erosion has also been important on Earth and Mars, gradually wearing away the sharp surface features from other processes.

A15.    A planet forms from whatever material can condense into solid particles at its position in the solar cloud. Mercury formed close to the sun where the temperature was high. Many of the common rocky materials could not condense as solid particles at those temperatures. Thus, iron (which could condense there) became a more prevalent part of Mercury. Mars, on the other hand, formed farther from the sun where it was cooler. That allowed a greater variety of rocky material to condense to participate in its formation. Thus, iron was a smaller fraction of all the material going into Mars.

A16.    Mercury has a large (for its size) iron core. Earth has a much smaller core which is partly solid and partly liquid. Mars core is even smaller -- the planet is not completely differentiated. For each planet, the lighter silicate rocks float in the mantle above the core.

A17.    Venus atmosphere is mostly CO₂, has a surface temperature of 860^oF, and 90 times Earth pressure. Mars is also mostly CO₂, but has a surface temperature of only -20^oF and a surface pressure of only 0.8% Earth's. Earth's atmosphere is composed mostly of N₂ and O₂, and has an average temperature of about 60^oF.

A18.    Venus has had such extensive volcanism that the entire surface has been recoated within the last few hundred million years. Mars had extensive volcanism in the past, but predominantly in the northern hemisphere. Volcanism on Earth occurs primarily at plate boundaries and at isolated hot spots.

A19.    New atmospheric gases are released on terrestrial planets by volcanic activity and, in their very early history of the solar system, by numerous comet impacts.

A1.    Planet formation begins as small solid particles stick together to form progressively larger and larger particles. Farther from the sun where the temperature was cooler, more kinds of material could exist in solid form. As a result, the outer planets had more raw material to grow from and became much larger than the inner planets.

A2.    All the gas giant planets have banded atmospheres, with wind patterns parallel to their equators. Those on Jupiter are the most obvious, but faint impressions of bands can be seen on all the gas giants. Oval cyclonic storms also are ubiquitous to the gas giant planets. Some, such as the Great Red Spot on Jupiter, seem very stable while others come and go rather quickly. Uranus and Neptune have a characteristic blue color from methane clouds high in their atmospheres.

A3.    All the gas giant planets have cores of rock and ice that are about the same size. Jupiter and Saturn have extensive layers of metallic atomic hydrogen outside the cores (Jupiter’s is much larger than Saturn’s). All of the gas giant planets have molecular hydrogen layers on the outside, although the layers for Uranus and Neptune are rather small compared to those on Jupiter and Saturn.

A4.    Pioneer 10 & 11 flew past Jupiter and Saturn. Voyager I and II also flew past Jupiter and Saturn. Voyager II went on to Uranus and Neptune. Galileo is on the way to Jupiter now.

A5.    The solar system consists of the sun, and all objects whose motion is controlled by the sun. That includes the nine major planets, about 50 satellites of the planets, asteroids or minor planets, and many, many comets. The solar system is a very tiny part of the overall universe. In a scale model where the solar system is about a mile across, the nearest star is about 2000 miles away. Our galaxy alone contains more than 100 billion stars.