The Moon and Mercury:Airless Worlds

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The moon’s orbit around the Earth is tidally locked. This means that …

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1. the moon is rotating is rotating about its axis once per tidal period on Earth.

2. the moon is orbiting around the Earth once per tidal period on Earth.

3. the moon is rotating about its axis once per synodic orbital period.

4. the moon is rotating about its axis once per siderial orbital period.

5. the moon is not rotating at all.

The Moon:The View from Earth

From Earth, we always see the same

side of the moon.

Moon rotates around its axis in the same time that it takes to orbit around Earth:

Tidal coupling:

Earth’s gravitation has produced tidal bulges on the Moon;

Tidal forces have slowed rotation down to same period as orbital period

Lunar Surface FeaturesTwo dramatically different

kinds of terrain:

• Highlands: Mountainous terrain, scarred by craters

• Lowlands: ~ 3 km lower than highlands; smooth

surfaces:

Maria (pl. of mare):

Basins flooded by lava flows

Highlands and Lowlands

Sinous rilles = remains of ancient

lava flows.

May have been lava tubes which later collapsed due to

meteoride bombardment.

Apollo 15 landing site

Which are the older surface features on the moon?

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1. The highlands (cratered areas).

2. The lowlands (maria).

The HighlandsSaturated with craters

Older craters partially obliterated by more

recent impacts

… or flooded by lava flows

Impact CrateringImpact craters on the moon

can be seen easily even with small telescopes.

Ejecta from the impact can be seen as bright rays originating

from young craters

How long after the origin of the solar system did it take to clear away the remaining debris

from the inner solar system?

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1. 100,000 years

2. 5 million years

3. 100 million years

4. ½ billion years

5. 2 billion years

History of Impact Cratering

Rate of impacts due to interplanetary bombardment

decreased rapidly after the formation

of the solar system.

Most craters seen on the Moon’s (and Mercury’s) surface were formed within the first ~ ½ billion

years.

Formation of Maria

Impacts of heavy

meteorites broke the crust and produced large basins

that were flooded with

lava

Formation of Maria (II)Major impacts forming maria might have

ejected material over large distances.

Large rock probably ejected during the formation of Mare Imbrium (beyond the horizon!)

Apollo 14

The Apollo Missions

Apollo Landing SitesFirst Apollo missions landed on safe, smooth terrain.

Apollo 11: Mare Tranquilitatis; lunar lowlands

Later missions explored more varied terrains.

Apollo 17: Taurus-Littrow; lunar highlands

Apollo Landing Sites (II)

Selected to sample as

wide a variety as possible of

different lowland and

highland terrains.

Lowlands (maria)

Highlands

Modern Theory of Formation of the Moon

The Large-Impact Hypothesis

• Impact heated material enough to melt it

→ consistent with “sea of magma”

• Collision after differentiation of Earth’s interior

→ Different chemical compositions of Earth and moon

(in particular: The moon does not have a large iron core.)

The History of the Moon

Alan Shepard (Apollo 14) analyzing a moon rock, probably

ejected from a distant crater.

Moon is small; low mass → rapidly cooling off; small

escape velocity → no atmosphere → unprotected against meteorite impacts.

Moon must have formed in a molten state (“sea of lava”);

Heavy rocks sink to bottom; lighter rocks at the surface

No magnetic field → small core with little metallic iron.

Surface solidified ~ 4.6 – 4.1 billion years ago.

Heavy meteorite bombardment for the next

~ ½ billion years.

Where is this terrain?1. On the lunar highlands.

2. On the lunar lowlands.

3. On Mercury.

4. On Venus.

5. On Mars.

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Mercury

Very similar to Earth’s moon in several ways:

• Small; no atmosphere

• lowlands flooded by ancient lava flows

• heavily cratered surfaces

Most of our knowledge based on

measurements by Mariner 10 spacecraft

(1974 - 1975)

View from Earth

Rotation and Revolution

Like Earth’s moon (tidally locked to revolution around Earth), Mercury’s rotation has been

altered by the sun’s tidal forces,

but not completely tidally locked:

Revolution period = 3/2 times rotation period

Revolution: ≈ 88 days

Rotation: ≈ 59 days

The Orbit and Rotation of Mercury

How long is the Mercurian day?

(Rotation period: 58 days; orbital period: 88 days)

1. 44 Earth-days

2. 58 Earth-days

3. 88 Earth-days

4. 120 Earth-days

5. 170 Earth-days

Rotation and Revolution

Like Earth’s moon (tidally locked to revolution around Earth), Mercury’s rotation has been

altered by the sun’s tidal forces,

but not completely tidally locked:

Revolution period = 3/2 times rotation period

Revolution: ≈ 88 daysRotation: ≈ 59 days

→ Extreme day-night temperature contrast: 100 K (-173 oC) – 600 K (330 oC)

The Surface of Mercury

Very similar to Earth’s moon:

Heavily battered with craters, including some large basins.

Largest basin: Caloris BasinTerrain on the opposite side jumbled by seismic waves

from the impact.

Lobate Scarps

Curved cliffs, probably formed when Mercury shrunk while cooling down

The Interior of MercuryLarge, metallic core.

Over 60 % denser than Earth’s moon

Mercury has a very large, iron-rich metallic core. Thus, would you expect

that it has a strong magnetic field?

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1. Yes.

2. No.

The Interior of MercuryLarge, metallic core.

Over 60 % denser than Earth’s moon

Magnetic field only ~ 0.5 % of Earth’s

magnetic field.

Difficult to explain at present:

Liquid metallic core should produce

larger magnetic field.

Solid core should produce weaker field.