Lecture 14 - The Moon(PHYS1901)

The Moon

Overview

• The Moon is Earth's only natural satellite.

• It lacks atmosphere, water, and sound, making it a desolate environment.

• The Moon's surface temperature varies drastically due to the absence of an atmosphere:

  • T_{noon} = 400 K (above water's boiling point)

  • T_{night} = 100 K (below water's freezing point)

Why Air Sticks Around

• Gas molecules move at high speeds due to thermal motion.

• If the average molecular speed is significantly below a planet's escape speed, few molecules escape.

• Earth's escape speed is 11.2 km/s, and few air molecules exceed this, allowing Earth to retain its atmosphere.

• Escape probability increases with:

  • Higher temperatures

  • Lighter molecules (higher speed at a given temperature)

  • Smaller planets (weaker gravity, lower escape speed)

• The Moon's escape speed is only 2.4 km/s compared to Earth's 11.2 km/s.

• Light elements escaped the Moon's gravity long ago, resulting in no atmosphere, save for trace amounts of Argon.

Surface Alterations

• The lunar surface has been shaped by numerous meteoroid and asteroid impacts.

• The Moon's lack of atmosphere means there is no wind or water erosion, preserving crater evidence.

Orbital Properties of the Moon

• The Moon orbits Earth with a period of about one month.

  • Sidereal Month: 27.3 days (time for one complete orbit of 360 degrees around Earth)

  • Synodic Month: 29.5 days (time for the Moon to go through a full cycle of phases, longer than the sidereal month because of Earth’s orbit around the Sun).

  • The distance between the Earth and Moon is measured with lasers with accuracy of a few centimeters. Apollo missions placed Corner Cube Retroreflectors on the moon.

• The lunar orbit is not perfectly circular.

  • Eccentricity: e = 0.055

  • The average Earth-Moon distance is approximately 384,000 km.

Physical Properties of the Moon

The Tides

• Tides are caused by the gravitational force of the Moon on Earth.

• The force on the near side of Earth is greater than on the far side.

• Water flows freely in response, creating a tidal bulge on Earth.

• Earth's rotation results in 2 high tides and 2 low tides per day.

• The difference in gravitational forces on either side of Earth is called differential force or tidal force.

• The Sun also influences tides but to a lesser extent due to its greater distance.

• Spring Tides: Occur when the Moon is full or new, aligning Earth, Moon, and Sun, resulting in the largest tides.

• Neap Tides: Occur when the Moon is in the first or third quarter phase, partially canceling the tidal effects of the Moon and Sun, leading to the smallest tides.

Tidal Friction

• As Earth rotates, it pulls the tidal bulge ahead of the Moon.

• The Moon's gravity pulls on the tidal bulge, trying to align it, causing tidal friction that slows Earth's rotation.

• Earth's rotation slows by approximately 1.5 milliseconds per century.

• About 500 million years ago, a day was only 22 hours long, and there were 397 days in a year.

• The energy lost by Earth through tidal friction is gained by the Moon, causing it to speed up in its orbit and spiral away from Earth by 4 cm per year.

Rotation Rate of the Moon

• Earth's tidal forces on the Moon cause a tidal bulge that always points toward Earth.

• The Moon is tidally locked to Earth, resulting in a synchronous orbit.

• Tidal locking makes the Moon's rotation rate the same as its orbital period (sidereal month: 27.3 days).

• This is why the same side of the Moon always faces Earth.

• In the past, when the Moon spun relative to Earth, the tidal bulge caused strain on the rocks, leading to tidal heating of the lunar crust.

• The Moon's rotational kinetic energy converted to heat, slowing its rotation until it tidally locked.

Tidal Locking of Earth and Moon

• Earth's rotation is slowing, and the Moon is spiraling away at 4 cm per year.

• Eventually, Earth will rotate on its axis in the same time the Moon orbits (47 days), with the Moon at 550,000 km, taking billions of years.

• Eventually, Earth will also become tidally locked to the Moon, with the same side of Earth always facing the Moon.

Surface Features of the Moon

• Galileo was the first to observe the Moon with a telescope and recognize its terrain.

• Maria: Large, dark, flat areas on the Moon's near side, thought to be oceans by early observers (maria = "seas" in Latin), but are actually solidified lava (basalt) from early volcanism.

  • There are 14 maria forming the "Man in the Moon" appearance.

  • Apollo 11 landed in the Mare Tranquilitatis (the Sea of Tranquility).

• Terrae: Lighter areas, elevated above the Maria, called the lunar highlands, elevated several km above the maria, and older than the maria.

• Craters: Numerous on the Moon's surface due to meteorite impacts, most visible near the terminator (day/night boundary) due to long shadows.

The Far Side of the Moon

• The far side of the Moon was not seen until explored by spacecraft in the early 1960s due to the Moon's synchronous orbit.

• The far side is more heavily cratered but has no maria.

• The absence of maria indicates a thicker crust on the far side than the near side.

• Tidal forces from Earth may have caused this difference when the Moon was more molten.

• Molten material could more easily come up through the thinner crust on the near side, forming the maria.

• The maria are younger than the highlands because they have fewer craters.

Lunar Cratering and Surface Composition

• Meteoroids are the primary agents of change on the lunar surface.

• Most meteoroids burn up in Earth's atmosphere, creating meteors (shooting stars), but the Moon lacks an atmosphere, so meteoroids constantly collide with the surface.

• When a meteoroid strikes, it releases a large amount of energy.

  • A 1 kg object colliding at 10 km/s releases the energy equivalent to 10 tons of TNT.

• The explosion ejects material from the impact site and sends shock waves through the surface, creating a crater surrounded by ejected material.

• Craters are typically 10 times as wide and twice as deep as the meteoroid creating them.

  • A 1 kg meteoroid with a 10 cm diameter forms a crater 1 m wide and 20 cm deep.

• Rock is pulverized to a greater depth due to the shock wave.

• Craters range in size from very large (e.g., Orientale Basin, Aitken view) to very small micrometeoroid craters (0.01 mm across) in glassy beads retrieved by Apollo astronauts.

• Most lunar craters are at least 3.9 billion years old.

• The current cratering rate is:

  • 10 km crater every 10 million years

  • 1 m crater every month

  • 1 cm crater every few minutes

Regolith

• A thick layer of dust (about 20 m thick) from pulverized ejecta covers the lunar surface, called the regolith.

• The Moon is still bombarded, especially by micrometeoroids, softening surface features.

• The erosion rate is very slow (5 m per billion years).

• There are few craters 20 m deep on the lunar surface because they have been covered by the regolith in the 3.9 billion years since formation.

Craters on Earth

• Meteorites also hit Earth, such as the Barringer Crater in Arizona, which is 1.2 km in diameter and 0.2 km deep, formed by a 50 m across and about 200,000 tons meteoroid about 25,000 years ago.

• Erosion is much faster on Earth, resulting in:

  • Very few small craters

  • Craters that are much younger than those on the Moon

• Manicouagan Reservoir in Quebec is a 100 km diameter crater, 200 million years old, created by a 5 km diameter asteroid.

Spacecraft Exploration of the Moon

• 12 humans have walked on the Moon's surface.

• Apollo 11 (1969) was the first manned mission.

• Apollo 17 (1972) was the most recent.

• Apollo missions brought back crucial rock samples for understanding the Moon's formation.

• China's Chang'e 5 mission delivered more lunar rocks and dust to Earth in December 2020.

• Radioactive dating of Apollo rocks gives an age of 4.6 billion years for the Moon.

Ice on the Moon

• There is no liquid water on the Moon, but ice is believed to exist.

• Crater interiors near the poles are always shadowed with the temperature not exceeding 100K (below freezing point of water)

• Missions such as Clementine (1996), Lunar Prospector (1998), LCROSS (2009), and LRO (2012) have provided evidence for ice on the Moon.

• Ice on the Moon may have been caused by meteoroid and comet impacts, the same way water arrived on Earth.

• Ice on the Moon may be crucial for accessing water for future manned missions.

Volcanism on the Moon

• More than 3 billion years ago, the Moon was volcanically active.

• Maria flooded with lava and solidified.

• Volcanic rilles (ditches where lava flowed) were also formed.

• Volcanic activity on the Moon ended about 3 billion years ago, making the maria younger than the lunar highlands.

The Moon's Interior

• The Moon's density is relatively low: 3300 kg/m3 compared to Earth's 5500 kg/m3.

• The Moon has no magnetic field, suggesting it lacks a sizeable iron/nickel core.

• Core:

  • Radius of 330 km (much smaller than Earth's core)

  • 240 km solid Iron inner core, with fluid outer core

• Inner Mantle:

  • 400 km thick

  • Semisolid rock similar to Earth's asthenosphere

• Outer Mantle:

  • 900 km thick

  • Solid rock

• Crust:

  • 30 km average thickness (up to 60 km on the far side)

  • No plate tectonics

Origin of the Moon

• Several theories have attempted to explain the Moon's formation.

  • Sister (or Co-Formation) Theory: The Moon formed near Earth from the same material in the Solar Nebula, but this doesn't explain the density differences.

  • Capture Theory: The Moon formed far from Earth and was captured by Earth's gravity, which could account for density differences but makes it difficult for Earth to capture such a large moon.

  • Daughter (or Fission) Theory: Matter forming the Moon was torn from Earth due to rapid spin of a young molten Earth, potentially explaining density differences if it happened after iron sank to the core, but with no explanation for how Earth could spin so fast, and now ruled out by computer simulations.

Impact Theory

• The currently accepted theory involves a glancing impact of a Mars-sized body on the young, molten, partially differentiated Earth.

• The impact caused enough material, mostly from the mantle, to be ejected to form the Moon.

• Most of the impactor's metallic core became part of Earth, leaving the Moon composed mainly of rocky material.

• This theory is supported by computer simulations.

Evolutionary History of the Moon

• 4.6 billion years ago: Formation of the Moon; Heavy bombardment liquefies the surface.

• 3.9 billion years ago: Bombardment less intense; Lunar volcanism fills maria.

• 3.2 billion years ago: Volcanic activity ceases.

Next Lecture

• Chapter 8: Mercury

• Chapter 9: Venus