A1.  The only spacecraft that has visited Mercury is Mariner 10, which made three fly-by passes of the planet in the mid-1970's. Unfortunately, each fly-by observed the same side of the planet. In addition, these observations are limited by the relatively crude technology available at that time.

A2.  While both Mercury and the Moon are made of rocky surfaces and iron cores, the proportions of the two parts are entirely different. Mercury has a very large iron core, while the Moon has a very small one.

A3.  The presence of an atmosphere depends on the balance between gravity (which holds the atmosphere) and temperature (rapid motion of atoms allows them to escape). Mercury is close to the sun, so the temperature is quite high. Titan is far from the sun and has a very cold atmosphere. This Titan is able to hold onto its atmosphere while Mercury cannot.

A4.  Because Mercury does not have an atmosphere, the heat gained from the Sun during the daytime is quickly lost at night. Since Mercury is very close to the Sun, the day side is heated to a high temperature. At night the surface cools to very low temperatures.

A5.  The age of a rock can be determined by measuring the amount of a radioactive substance in the rock compared to the amount of its decay product in the rock. This ratio tells us how many half lives (the length of time required for half of the radioactive substance to decay) have occurred since the rock formed. Laboratory measurements of the half life then allow us to find the age of the rock in years.

A6.  Mercury's weak magnetic field indicates that some part of its core remains molten, since magnetism is generated by currents in a fluid iron core. That is surprising since we would have expected a small planet like Mercury to have lost its internal heat and solidified throughout by now.

A7.  Scarps formed on Mercury's surface when it cooled very quickly soon after formation. As it cooled, the planet shrank slightly and the stress was relieved when the surface cracked in various places. While the same process has not occurred elsewhere in the solar system, similar looking cracks are often associated with earthquake faults on Earth. Valles Marineris on Mars is also a (much larger) crack caused by expansion or up-thrusting of the surface.

A8.  A resonance occurs when a small force acted repeated at the same point in the cycle of motion of an object. Even though the force is very small, its effect accumulates over time to produce a noticeable change in the motion of the object. The force of Earth acting on the "heavy" side of the Moon has slowly pulled that side to always face Earth as the Moon orbits Earth. The action of the sun on Mercury has produced a similar result there, except that Mercury rotates three times for every two orbits around the sun.

A9.  Mercury has a much larger core, in proportion to its size, than does Earth. This conclusion is reached by comparing the average density of Mercury to that of Earth, after compensating for the compression caused by Earth’s greater gravity. The large core of Mercury occurred because it formed closer to the sun, where the temperature was higher. Fewer rocky materials could condense to help form a planet there, although iron could still easily condense under those conditions.

A10.  Mercury is so close to the sun that any atmosphere it had would be very hot, which means that atoms in its atmosphere would be moving very rapidly. However, Mercury is also a rather small planet, which means that its gravity is not very strong. It does not have an atmosphere because its gravity is too weak to hold onto a high temperature atmosphere.