Lecture 15 - Mercury and Venus

Mercury's orbit is highly elliptical, with an eccentricity of $e = 0.206$ .
The average Sun-Mercury distance is 0.39 AU.
Perihelion distance (closest to the Sun): 0.32 AU.
Aphelion distance (farthest from the Sun): 0.48 AU.
Elongation is the angle between Mercury and the Sun as viewed from Earth.
- Perihelion elongation: 18°
- Aphelion elongation: 28°
Mercury is never far from the Sun in the sky when viewed from Earth.
It is only visible just before sunrise or just after sunset.
The maximum elongation of Mercury is 28°.
It is visible for no more than about 2 hours.
Mercury shines by reflected sunlight, like the Moon.
Phases of Mercury are best seen at maximum elongation.
The semimajor axis of Mercury's orbit is $a = 0.39$ AU.
Kepler's Third Law allows calculation of Mercury's orbital period.
Orbital Period Formula: $P^2 \text{ (in Earth years)} = a^3 \text{ (in AU)}$
Orbital Period: $P = 0.24$ Earth years = 88 Earth days

Mercury's size is similar to the Moon, but its density is similar to Earth.
Radius: 2440 km
Mass: $3.3 \times 10^{23}$ kg
Density: 5400 kg/m3
Escape Speed: 4.2 km/s
Mercury's size is comparable to some moons in the solar system, such as Ganymede, Titan, Callisto, and Io.

Initially, Mercury was believed to be tidally locked to the Sun, with a synchronous orbit and a rotation period of 88 Earth days.
Radar measurements in 1965 determined Mercury's rotation period to be 59 Earth days, exactly two-thirds of a Mercury year.
Mercury's orbital and rotational motions combine to create a solar day that is 2 Mercury years long.
Mercury spins 3 times for every 2 orbits around the Sun, resulting in a 3:2 resonance.
The 3:2 resonance occurs because Mercury's orbit is highly eccentric.
Mercury's rotation speed (spin on its axis) is constant, but its orbital speed varies between aphelion and perihelion.
Spin and orbital rates are equal at perihelion but not at aphelion.
The Sun's tidal forces have created a tidal bulge on Mercury.
If spin and orbital rates are out of sync, the Sun will pull the bulge to re-establish resonance.
The same face of Mercury points toward the Sun every other time it is at perihelion, resulting in a 3:2 resonance.

Mercury has no significant atmosphere.
Being close to the Sun, any gas molecules near its surface have fast thermal motion.
The escape speed is only 4.2 km/s, so any gas molecules would have easily escaped.
Without an atmosphere to moderate the surface temperature, and with one Mercury day equaling 59 Earth days, the daily temperature variation is extreme.
$T_{\text{day}} = 700$ K (above the boiling point of water)
$T_{\text{night}} = 100$ K (below the freezing point of water)
Mercury is heavily cratered like the Moon due to the lack of an atmosphere providing protection from meteoroid impacts.
Most information about Mercury has come from the Mariner 10 and Messenger spacecraft.
Mariner 10:
- Launched in 1973 (stopped returning data in 1975)
- Passed by Mercury every six months
- Mapped 45% of the surface with 4000 photographs
Messenger:
- Orbited Mercury from 2011-2015
- Created detailed surface maps
- Studied composition
- Found water ice in craters

Mercury is less heavily cratered than the Moon.
Intercrater plains were formed by volcanic activity (3.9 to 3.7 billion years ago), covering older craters.
There is no evidence of plate tectonics.
Due to higher gravity than on the Moon:
- Crater walls are not as high.
- Ejecta material landed closer to the crater.
- Many large craters on Mercury have dark material around them, but the origin is not fully understood.

Scarps (cliffs):
- Not fault lines, as there are no tectonic plates
- Hundreds of km long and up to 3 km high
- Younger than most craters
- Occurred after a bombardment that liquefied the surface
- As the surface cooled, it cracked
Hollows:
- Lighter-colored, irregularly shaped depressions on a crater's rim and floor
- Formed when the Sun's heat vaporized loose material in the crater
- Not seen anywhere else in the solar system
Caloris Basin:
- Very large impact feature
- 1400 km diameter
- Ringed by mountain ranges that reach more than 3 km high
- On the opposite side of the planet from