Astronomy Ch. 8 – The Moon and Mercury

The best way to find the exact distance to the Moon is to:

bounce lasers off the retroreflectors left on the surface by the Apollo landings.

What is the reason that it is so difficult to view Mercury from Earth?

Mercury is always very close to the Sun.

Mercury is very hard to observe from Earth because:

it never gets more than 28 degrees from the Sun’s glare.

From Earth, due to their motions and the fact that the Sun lights only a portion of each surface, both Mercury and the Moon:

appear to go through phases.

Mercury experiences extreme high and low temperatures between night and day because:

it has no atmosphere to moderate temperatures over the globe.

In size, Mercury is intermediate between:

the Moon and Mars.

How do the atmospheres of the Moon and Mercury compare?

Neither body has a permanent atmosphere.

If the Earth’s surface temperature were increased to that of Mercury’s day side, then:

we would lose most of our water vapor into space.

Which of the following is NOT a factor in determining whether a body in the solar system retains an atmosphere?

rotation rate

Comparing the densities of the Moon and Mercury, we find:

the Moon’s is similar to Earth’s crust, while Mercury’s is similar to the entire Earth.

One of the effects of Mercury’s very slow spin is

extreme variations in its surface temperature.

The lunar mare are found:

almost entirely on Earth side, where the crust was thinner.

The lunar highlands are:

more rugged, heavily cratered , and older than the lunar mare.

The youngest features visible with telescopes on the Moon are:

the craters sitting atop the mare.

What is true of the lunar highlands?

They are the oldest part of the lunar surface.

The lunar mare are radioactively dated at:

3.9-3.2 billion years old, forming after most of the bombardment was over.

To measure how Mercury spins, astronomers sent ________ to Mercury and used the Doppler shift to determine how fast it was rotating.

radar beams

What did radar astronomers find in the polar regions of Mercury?

water ice that never melts in the deep craters

Which statement about the rotations of the Moon and Mercury is FALSE?

Like our Moon, Mercury does not rotate at all, keeping the same side facing the Sun.

How does Mercury’s rotation relate to the Sun?

Its rotation rate is 2/3 as long as its year, due to tidal resonances.

What causes Mercury’s 3:2 spin-orbit resonance?

the planet’s very eccentric orbit the planet’s closeness to the Sun the planet’s high density tidal torques operating on the planet All of the above are factors.

Mercury presents the same side to the Sun

every other orbit.

What is true of the Moon’s orbital and rotational periods?

They are equal.

The chief erosive agent now on the Moon is:

the rain of micrometeorites chewing up the regolith.

The rate of cratering:

shows that most interplanetary debris was swept up soon after the formation of the solar system.

The rate of cratering in the lunar highlands shows us that

they range from 4.6 – 4.4 billion years old, on average.

The average rate of erosion on the Moon is far less than on Earth because

the Moon lacks wind, water and an atmosphere.

Which type of feature is the best evidence of lunar volcanism?

rilles associated with lava flows accompanying the mare formation

The spacecraft which reveal the possibility of lunar ice are:

Clementine and Lunar Prospector.

Mercury’s surface most resembles that of which other body?

Moon

Which of these features is attributed to the shrinking of Mercury’s core?

scarps

Almost all we know about Mercury has come from:

the three flybys of Mariner 10.

Mercury and the Moon appear similar, but we note that:

Mercury has "weird terrain" opposite its huge Caloris basin. Mercury does not always keep the same face toward the Sun, while the Moon does have the Earthside always facing us. the lunar mare are darker than Mercury’s intercrater plains. Mercury has striking lobate scarps due to the shrinking of its core. All of the above are correct.

The scarps on Mercury were probably caused by

the interior cooling and shrinking.

Mercury’s surface most resembles which of these?

the lunar far side

Mercury’s Caloris basin is aptly named, since:

it is the hottest region, turning to face the Sun when Mercury is at perihelion.

The Moon’s huge Mare Orientale basin has a twin on Mercury named:

Caloris.

Moonquakes on the Moon were detected by:

the seismographs left there by the Apollo astronauts.

What do moonquakes reveal about the Moon?

Its small, partially molten core has been pulled toward us by tidal forces.

How does Mercury’s magnetic field compare to our own?

It is 1/100th as strong as ours, but does deflect the solar wind to some degree.

What two properties of Mercury imply that it is differentiated?

its large average density and its magnetic field

The presence of a Mercurian magnetic field surprised the planetary scientists on the Mariner 10 team because

the dynamo theory predicted that Mercury was spinning too slowly for one.

Which of these theories seems to best explain the Moon’s origin?

Impact Theory

What are the major factors that rule out the co-formation theory for the Moon-Earth system?

Each body has a different density and a different chemical composition.

Which of these would support the capture theory of the Moon’s origin?

the retrograde orbit and large orbital inclination of Neptune’s moon Triton

The cratering of the lunar highlands shows us:

they are older than the smoother maria.

How are the polar regions of Mercury and the Moon similar?

Both seem to have ice pockets in the deepest, darkest crater floors.

Mercury’s evolution was different from the Moon’s because:

dense Mercury had an iron core that shrank, creating the lobate scarps.

Both the Moon and Mercury are geologically inactive and have been that way for most of the history of the solar system. However, about 4 billion years ago, it is thought that

Mercury had more common volcanic activity than the Moon.

Astronomers believe that the Moon did not differentiate to the same degree as Earth because:

the less dense and smaller moon did not have as much radioactivity as the larger Earth in its core.

(SA) Would an observer on Mercury see the Sun rise in the east or the west?

Most of the time, the Sun would appear to move east to west from Mercury’s surface. Near perihelion, however, the Sun goes into retrograde for a few days and moves west to east.

(SA) How is it possible for Mercury and the Moon to have water ice at their poles?

Shadows at the bottom of craters keep sunlight from hitting the ground, so the temperature has always been low enough that ice there has never vaporized and escaped, or that came in with impacting comets and asteroids.

(SA) Why was the discovery of a substantial magnetic field around Mercury a surprise? How was it detected?

In its close passes by Mercury, Mariner 10 found a field that is much stronger than Mercury’s very slow rotation would have led us to expect with the dynamo theory.

(SA) What is the primary source of erosion on the Moon? Why does change there take so long?

A constant fall of meteoroids from space pelts the moon, pulverizing the surface with tiny craters. But really big impacts are rare, and these microscopic changes take a long time to show up as seen from Earth. Our erosive agents like wind, water, and ice can make much more dramatic changes in short periods of time, such as floods, sandstorms, glaciers, etc.

(SA) Relate the formation of Mercury’s scarps to its differentiation.

As Mercury was molten, the dense iron and nickel sank to the core. But smaller Mercury cooled much faster than larger Earth and Venus, so as its core cooled, it also contracted as it solidified. The crust above wrinkled and formed the scarps due to this shrinkage.

(SA) Why has Mercury traditionally been such a hard planet to observe and study?

There are two reasons. First, Mercury is a very small planet, so it never appears very large in telescopes. Second, because it orbits so close to the Sun, it can never be viewed under favorable conditions. It is always seen near the horizon, through a lot of atmosphere which distorts its image. Whenever it is in a favorable position for viewing, so is the Sun!