The Moon and Mercury
Scorched and Battered Worlds

April 7, 2011, jeb

Outline THE MOON General Properties The Moon's Orbit around the Earth Moon's Gravity on its Surface The Moon's Surface Features and Character The Moon's Internal Features Origin of the Moon Evolutionary History of the Moon Phases of the Moon Lunar eclipses Other Sources on the Moon MERCURY Observing Mercury from Earth Character of the Mercury's Surface Mercury - General Properties Mercury's Spin-orbit Resonance Mercury's Internal Features Mercury's Basic Properties Other Sources on Mercury Messenger Mission

go to the Moon
go to Mercury

Similar Bodies

• both heavily cratered, ancient surfaces
• no air, no water
• wild temperature swings, day to night

The Moon - General Properties

• Radius = 1738 kilometers (about 1/4 of Earth)
• 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

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
  • Image from Apollo 8 orbiter shows craters as small as 500 m across
    • Craters range in size from 50 km to 500 m
    • diameter about 10 times the size of the incoming meteoroid and depth twice its size.
  • mostly from meteoritic impact - very old
  • Orientale Basin
    • large lunar crater with concentric rings of cliffs
  • Copernicus and secondary cratering of Eddington and Reinhold
  • Steady "rain" of micrometeoroids falls on Moon's surface
    • Beads formed during explosive impacts show subsequent miniature craters
• Lunar Dust
  • about 20 meters deep
  • 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
    • some lava found in maria
      • Hadely Rille
    • Crater-chain probably due to cracking of surface
  • but volcanism ended early in the Moon's history
• Far side surprise
  • no major maria => due to asymmetric crust
  • almost entirely highlands
• 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 dense core
• 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

• 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

Phases of the Moon

The Moon's cycle of phases takes a little over 29 days to complete
although, one sidereal month is 27.3 days

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

Other Sources on the Moon

StarDate Guide to the Moon
Views of the Solar System - the Moon
The Nine Planets - the Moon

Mercury

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 28^o, 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
  • Images of Mercury from the Earth are not nearly as revealing as images from passing spacecraft
    Best images from Earth at maximum elongation, when only "half Mercury" is shown

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

• Radius = 2440 kilometers (about 2/5 of Earth)
• 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
    • perihelion = 0.32 A.U
    • aphelion = 0.48 A.U.
  • Orbital plane is inclined 7^o 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
  • Reflected radar beam yields information about both the planets line-of-sight motion and its rotation rate
• 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 0^o 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

• NASA Messenger Web Site

Planet order from Sun Orbital period Semi-major axis Mass Radius Density Surface Gravity Escape speed Rotational Period Inclination Surface Magnetic Field Surface Temperature

1 0.24 years (88 days) 0.39 AU 0.055 x Earth 0.38 x Earth 5430 kg/m3 0.38 x Earth 4.2 km/s 58.6 days 0.0° 0.011 x Earth 100-700 K

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Golden Globe Awards for Mercury

Closest planet to the Sun Lightest planet Surface temperature swings are the most extreme Orbit has largest deviation from the plane of the ecliptic Orbit is most eccentric of the planets Spin-orbit resonance locks 3 rotations every 2 orbits

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Other Sources on Mercury

StarDate Guide to Mercury
Welcome to the Planets - Mercury
Views of the Solar System - Mercury
The Nine Planets - Mercury