Astronomy Ch 9

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.

One of the primary scientific objectives of the Apollo program was the return of lunar material. Why was this so important? What can be learned from samples? Are they still of value now?

Astronomy is usually only observing light, but bringing back material, allows scientists to experiment with it directly. Carbon dating and other experiments allow scientists to learn more about both moon and earth origins and chemistry

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.

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.

If, in the remote future, we establish a base on Mercury, keeping track of time will be a challenge. Discuss how to define a year on Mercury, and the two ways to define a day. Can you come up with ways that humans raised on Earth might deal with time cycles on Mercury?

Mercury days are 59 earth days and years are 88 earth days, so people would have to create astronomical landmarks to keep track of earth time. or some other way to recognize the day

Give several reasons Mercury would be a particularly unpleasant place to build an astronomical observatory.

Mercury is extremely hot/cold and the lack of an atmosphere makes it extremely dangerous for life (no protection of radiation from sun). Also wait time to observe sun then night (vice versa) would be a long time.

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 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.

The Moon has too little iron, Mercury too much. How can both of these anomalies be the result of giant impacts? Explain how the same process can yield such apparently contradictory results.

The Moon is made from ejecta from a giant impact between Earth and a mini-planet, and is thus mostly mantle material. Mercury is made of the core material that is left behind after a giant impact, and therefore is enriched in iron.

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.

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.

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."

ummarize 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.

Apollo astronaut David Scott dropped a hammer and a feather together on the Moon, and both reached the ground at the same time. What are the two distinct advantages that this experiment on the Moon had over the same kind of experiment as performed by Galileo on Earth?

The lower gravity allows more time to observe the drop, and the lack of air resistance (vacuum) increases the accuracy of the test.

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.

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.

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

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.