# The Moon and Mercury Scorched and Battered Worlds

## Similar Bodies

The Moon - Earth's only natural satellite
Mercury - Smallest terrestrial planet - closest planet to the Sun
These two bodies are similar in appearance in many respects:
• both heavily cratered, ancient surfaces
• no air, no water
• wild temperature swings, day to night

## The Moon - General Properties

• Mass = 7.4 x 1022 kilograms (about 1/80 of Earth)
• Average density = 3340 kg/m3
• => fewer heavy metals (like iron) than Earth
• Atmosphere (None)
• => very weak gravity
• => low escape speed (2.4 km/s, while earth is 11 km/s)
• Temperature
• 400K (daytime)
• 100K (nightime)
• Earth-moon distance = 384,000 km = 1.28 light-seconds

## The Moon's Orbit around the Earth

• Orbits Earth in 27.3 days
• Rotates synchronously with orbit (same side always faces Earth)
• From the Earth, we see the same side of the Moon all the time
• From the Moon, the Earth appears to hang fixed in one place in the sky, slowly rotating on its axis
• Elongated shape of the Moon results in the synchronous orbit
• Presumably, the Moon's elongated shape was created long ago by the Earth's tidal influence when the Moon was young, and the moon is now "tidally locked"

## Moon's Gravity on its Surface

Newton's Law:
F = G (M1)(M2) / R2, where G is a constant

So the force pulling on you (suppose you are M2), on the surface of the Moon, would be proportional to M1 / R2, or the mass of the Moon divided by the distance to the center of the Moon, squared.
Since the Moon has about 1/80 of the mass of the Earth, and it has about 1/4 of the radius of the Earth, gravity on the surface would appear about (1/80)/(1/4)2 = 1/5 as great as on the Earth. (a more accurate estimate comes out to 1/6.)

## The Moon's Surface Features and Character

• Maria - large flat areas resulting from lava flows
• Fourteen (14) named maria, the largest being Mare Imbrium (1100 km diameter)
• more iron than other parts of surface
• greater density (3300 kg/m3)
• material originated from mantle
• Highlands - lighter appearing, elevated areas
• rich in aluminum
• somewhat lower density (2900 kg/m3)
• material originated in crust
• Craters - very old
• Lunar Dust
• pulverized by meteoroids
• but erosion is slow -
• about 5 m per billion years, 1/10,000 of rate on Earth due to Earth's weather
• Barringer Crater in Arizona, only 25,000 years old, already shows erosion
• Barringer crater would likely survive on Moon for several billion years
• Lunar Ice
• Two 1990's lunar missions (Clementine and Prospector) found evidence for ice at the Moon's poles
• 2009 Lunar CRater Observation and Sensing Satellite (LCROSS) measured water
• 1 part in 100,000 - less than desert sand on Earth -but NOT ZERO!
• Volcanism
• explains some craters
• but volcanism ended early in the Moon's history
• Far side surprise
• Moon's surface is frozen in time: very little surface erosion
• no wind or running water, to cause erosion
• no plates on surface to cause volcanism or seismic activity
• some erosion from micrometeroids
• cumulative effect of micrometeroids => dust

## The Moon's Internal Features

• Moon's average density (3340 kg/m3) is similar to surface rock
• No magnetic field => iron deficiency
• Cooler (1500K) interior than Earth
• Crust is thicker on the lunar far side
• resulted from tidal forces
• and explains absence of maria on far side

## Origin of the Moon

Impact Theory
• Likely origin from collision of large object (Mars-sized) with young Earth
• Moon is similar to Earth's mantle

## Evolutionary History of the Moon

• Moon formed about 4.6 billion years ago
• Oldest rocks discovered on Moon about 4.4 billions years old
• During first 0.5 billion years, surface was heated and remelted by frequent meteoritic bombardment
• Then, volcanism filled basins with lava, which became the maria
• This phase lasted from 3.9 to 3.2 billion years ago
• Last 3 billion years has seen heavy erosion of the surface from constant meteoritic bombardment

## Lunar eclipses

Earth's shadow sweeps across the Moon, temporarily blocking the Sun's light and darkening the Moon in a lunar eclipse

When the Moon passes directly in front of the Sun, daylight is briefly turned into night in a solar eclipse

## Mercury

Mercury is the closest planet to the Sun

Named for the fleet-footed messenger of the gods by the Romans
because it seemed to move more quickly than any other planet.

## Observing Mercury from Earth

• Greatest elongation
• Since Mercury is on an orbit close to the Sun, it never separates from the Sun by more than 28o, as viewed from the Earth
• Consequently, Mercury is visible to the naked eye only when the Sun's light is blotted out, just before sunrise, or just after sunset, or occasionally during a total solar eclipse
• in this image the Moon is illuminated by earthshine

• Viewing Mercury

## Character of Mercury's Surface

• Craters from meteoritic bombardment
• many of the large craters tend to have unexplained dark halos
• less densely packed than Moon (covered at some time by lava?)
• lower crater walls than on Moon (stronger gravity)
• little evidence of Maria-like features (not much lava flow)
• Conclusion: some lava flow (not all craters remain) but not much (no maria)
• Scarps (or cliffs or ridges) are evidence for collapse of surface when interior cooled
• Caloris Basin - 1400 kilometer diameter impact feature ringed by concentric mountain ranges of more than 3 km in height
• Opposite the Caloris Basin is a region of oddly rippled and wavy surface features - "Weird" terrain

## Mercury - General Properties

• Mass = 3.3 x 1023 kilograms (about 5.5% of Earth)
• Average density = 5430 kg/m3
• => pretty similar to Earth's 5500 kg/m3
• Atmosphere (Negligibly small, about 10-14 compared to Earth)
• => very weak gravity
• => low escape speed (4.2 km/s, while earth is 11 km/s)
• Temperature
• 700K (daytime)
• 100K (nightime)
• typical Sun-Mercury distance (semi-major axis) = 58,000,000 km = 0.39 A.U.
• Mercury has most eccentric ("out of round") orbit of the planets
• Orbital plane is inclined 7o relative to the plane of the Solar System (the ecliptic), largest deviation of the planets from this plane
• Weak magnetic field (1/100 x Earth)

## Mercury's Spin-orbit Resonance

• The rotation of Mercury was measured by the Arecibo radio dish, which received radar pulses reflected from the surface of Mercury
• Mercury rotates on its axis once every 59 days, exactly 2/3 of the planet's orbital period of 88 days
• due to the a 3:2 spin-orbit resonance
• 3 rotations every 2 orbits
• large eccentricity of orbit prohibits 1:1 resonance (as the Earth's moon)
• The net effect is that noon (with the Sun directly overhead) occurs once for every two times Mercury travels around the Sun (or once every 176 days).
• Expressing this differently, the Sun stays "up" in the black Mercury sky for almost 3 Earth months, which is followed by nearly 3 months of darkness (and it gets cold)
• The spin-orbit resonance is similar to the tidal lock seen in the Moon's rotation
• Here the large eccentricity of the orbit, moves the dominate tidal influence to the perihelion point, resulting in the 3:2 resonance
• Also explains 0o inclination (no tilt of rotation axis)
• rotation axis exactly perpendicular to orbital plane

## Mercury's Internal Features

• Mercury's average density (5430 kg/m3)
• much larger than Moon's 3340, but about the same as Earth
• Small magnetic field => about 1/100 x Earth
• This is surprising:
• Dynamo Theory of planetary magnetic field generation requires rapid rotation of conducting liquid core
• Mercury rotates slowly
• There is thought to be little, if any molten iron in Mercury (there must be some)
• But the field is small, so perhaps the slow rotation does it
• or the core may have solidified, but still carries magnetic imprint
• Differentiated - variation of the density and composition
• explains large average density and weak magnetic field

## Messenger Mission

• The second NASA mission to Mercury
• MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging)
• launched on August 3, 2004
• three Mercury flybys in 2009-10
• entered elliptical orbit around Mercury on March 18, 2011
• Key issues under investigation
• Mercury's high density
• geological history
• the nature of the magnetic field
• the structure of the core
• whether there is ice at the poles
• where the tenuous atmosphere comes from