Chapter 9

The features of Mercury are named in honor of famous people in which fields of endeavor?

Artists, writers, composers, and other contributors to the arts and humanities.

How did our exploration of the Moon differ from that of Mercury (and the other planets)?

Because of its proximity to Earth, we have been able to send manned missions to the Moon and return lunar material to Earth for study. Robotic missions have also been used to explore the lunar surface. All other solar system objects—planets, asteroids, and comets—have been studied only by telescope and/or robotic missions. No humans have visited them.

Frozen water exists on the lunar surface primarily in which location? Why?

Frozen water exists primarily in deep craters on the Moon's south pole. Parts of these craters are permanently in shadow and therefore do not receive enough energy from the Sun to evaporate the ice and escape into space.

What is the relationship between Mercury's rotational period and orbital period?

Mercury's rotation is 59 Earth-days long, while the year is 88 Earth-days long. That is a ratio of 3 to 2. Thus, three Mercury days equals two Mercury years.

Why does the Moon not have an atmosphere?

The Moon's mass, and therefore its gravitational force, is not large enough to retain gases and volatile compounds. Therefore, any gases released on the Moon quickly escape into space.

Explain how high-speed impacts form circular craters. How can this explanation account for the various characteristic features of impact craters?

A high-speed projectile will penetrate somewhat into the surface of the larger body before stopping. This abrupt loss of energy is transferred into a shock wave and heat that fractures and vaporizes some of the rock. It is this rapid heating and explosion that throws debris out of the impact site. The explosion tends to send energy in all directions, generating a circular crater. There is a rebound effect from the shock wave that will fill in the crater a little and flatten the floor, sometimes creating a central peak. Some of the debris ejected during the explosion will also fall back into the crater, the rest will be distributed outward from the impact site and make the ringed mountains that surround each crater. These are the characteristic features of impact craters.

Outline the main events in the Moon's geological history.

After the formation, more than four billion years ago, the low density silicates cooled first and made up the initial crust. This surface was exposed to heavy meteor cratering early in the Moon's history. About 3.8 to 3.3 billion years ago, extensive volcanism released large amounts of lava that flowed over the surface filling in the lowest parts of the large impact basins and forming the darker maria we see today. Both the highlands and the maria continue to be battered by additional impacts ever since.

With no wind or water erosion of rocks, what is the mechanism for the creation of the lunar "soil?"

Billions of years of impacts breaking up the rocks and scattering the debris over the surface have created lunar "soil." Enough impacts have occurred to cover much of the surface with sand- and dust-sized particles.

What differences did Grove K. Gilbert note between volcanic craters on Earth and lunar craters?

He found that Earth volcanoes have small, deep craters on mountaintops, whereas lunar craters are larger, have different shapes than volcanic craters and are mountain-rimmed and circular with floors generally below the level of the surrounding plains.

Describe the basic internal structure of Mercury.

Mercury is one of the densest of the planets, at 5.4 g/cm3. It has an enormous iron-nickel core nearly 3500 km in diameter, which is encased in a rocky/silicate crust about 700 km deep. The metallic core, representing nearly 60% of the planet's total mass, produces a weak magnetic field.

What is the main consequence of Mercury's orbit being so highly eccentric?

Mercury's distance from the Sun varies hugely, ranging from 46 million km at perihelion to 70 million km at aphelion.

What is the composition of the Moon, and how does it compare to the composition of Earth? Of Mercury?

The Moon is principally composed of silicate rocks, whereas Earth has more metals and volatile compounds. Earth has an iron core, but the Moon does not. Earth has liquid water in its surface layer, but the Moon does not. Mercury contains substantially more metals than the Moon, with a significant iron-nickel core.

What are the principal features of the Moon observable with the unaided eye?

The dark maria, the lighter highlands, and a few large craters such as Tycho are visible. The contrast between the light and dark of these features is sometimes called "the man in the Moon."

Summarize the four main hypotheses for the origin of the Moon.

The fission hypothesis suggests that the Moon was once part of Earth but separated early in their history. The sister hypothesis proposes that the Moon formed together with, but independent of, Earth. In the capture hypothesis, the Moon formed elsewhere in the solar system and was later captured by Earth. The newer giant impact hypothesis suggests that a Mars-sized object grazed Earth, ejecting material from both Earth and itself—material that condensed to form the Moon.

The mountains on the Moon were formed by what process?

The long, semi-circular mountain ranges that border the maria are debris ejected from the massive impacts that formed these basins, piled up at the edge of the bowl dug out by the explosion that resulted from the impact. The central peak mountains in large craters are due to rebound from the sudden removal of overlying rock.

What are the maria composed of? Is this material found elsewhere in the solar system?

The maria are composed chiefly of basalt. Basalts are also found on Earth, Venus, Mars—and to a much lesser extent—Mercury.

What do our current ideas about the origins of the Moon and Mercury have in common? How do they differ?

The similarities are mainly those of appearance: both suffered frequent and sometimes massive impacts early in their histories, with heavily cratered surfaces visible on each body. Mercury has much more iron than the Moon, so early on, Mercury must have lost most of its rocky mantle, probably due to impacts. At some point, probably due to internal cooling, Mercury shrank enough to create long "wrinkles" and scarps in the crust; such scarps are not seen on the Moon.

Explain the evidence for a period of heavy bombardment on the Moon about 4 billion years ago.

There are about 10 times more craters on the highlands than on a similar area of maria. The radioactive dating of highland samples shows that they are only slightly older than the maria, about 4.2 billion years versus 3.8 billion years. If the impact rate was constant over the Moon's history, the highlands would be least 10 times older than the maria, or about 38 billion years old. That's older than the age of the universe. Thus, the impact rate must not have been constant, and been at a much higher rate earlier than 3.8 billion years ago.

What are the difficulties with the capture hypothesis of the Moon's origin?

There does not seem to be any way that Earth could have captured a large satellite from elsewhere. One object approaching another cannot go into orbit around it without losing large amounts of energy. Also, the new satellite would go into a very eccentric orbit rather than the nearly circular orbit the Moon has now. Finally, there are too many compositional similarities between Earth and the Moon to warrant an independent origin.

How was the rotation rate of Mercury determined?

With the use of radar. Radar beams were directed to Mercury and the reflected beams showed the tell-tale signs of Doppler broadening. The measured degree of this frequency broadening provided an exact value for the rotation rate of Mercury.