MERCURY
Mercury's Diameter: 4878 km, 0.382 Earth diameters
Mass: 3.3x1023 kg, 0.055 Earth masses
Average Distance from Sun: 58 million km, 0.387 AU (astronomical units)
Length of Day: 58.6 Earth days
Length of Year: 88 Earth days (Mercury = the fleet-footed messenger of the gods)
Number of natural satellites: 0
Planetary ring system: No
Average temperature: 500 degrees Celsius (Day) and 150 degrees Celsius (Night)
Atmospheric composition:
42% Oxygen (O2), 29% Sodium (Na), 22% Hydrogen (H2), 6% Helium (He), 0.5% Potassium (K)
Atmospheric Pressure is 10-12 that of Earth's atmosphere (not enough gravity to hold an atmosphere like Earth's)
Axis of rotation not tilted.
Magnetic Field 1/100 x Earth's (not rotating fast enough? Rapid rotation of liquid core in
Earth generates Magnetic field by Dyanamo)
Day/Year
resonance: Mercury's day
(rotation rate) is 59 Earth days, and is exactly 2/3 of its 88 day year.
Mercury rotates three
times in two of its years.
Mercury is the only body in the solar system known to have an
orbital/rotational resonance
with a ratio other than 1:1.
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).
Results from tidal forces of the Sun's gravity -- c.f. Moon's
resonance.(recall the resonance of the
Moon's orbital & rotational periods)
Images from Mariner Fly-by in 1974-1975
Montage
of surface 125,000
miles from surface
Close up of
crater 100 km in diameter.
Size of
Mercury compared to other terrestrial planets
Smooth,
volcanic area on Mercury
IMPACT CRATERS
Impact
Crater Animation
Animation of impact and the structure of a typical
impact crater.
After the impact, the material
immediately below the impactor
is vaporized or melted.
Material from about 1/3 the crater depth (or about 1/10 the crater
diameter) is ejected
onto the surface.
The material in the lower 2/3 of the
crater is crushed and displaced downward forming an ejecta
blanket.
By
counting craters, and studying the superposition of craters on each
other, you can get a history of the rate of impacts during the life of
the solar system.
Early on: very heavy cratering, rate of impacts high.
Later, and Today: Impacts still occuring, but rate much lower
More pictures:
Impact
Craters and smooth volcanic plain
Bright
rays --> ejecta from impact
Bright Rays from
craters
Double Ringed crater
Degas
Crater, with rays
Large Faults
Caloris Basin Ridges
Possible Ice at poles (water or sulfur compounds)