Mercury

General Characteristics

• Diameter is 4878 km (note: our Moon size = 3476 km).

• Density is 5.4 g/cm3 (similar to Earth at 5.52 g/cm3; Moon is 3.34 g/cm3) .
  Density is anomalously high for its size
  Small size & mass, and so lower gravity than Earth. Interior must be uncompressed.
  Suggests high density, Fe rich interior (large iron core);
  Core may be 80% by volume (larger than our Moon

• Interior is differentiated

• Crust is probably made of silicate minerals like the moon (similar appearance)

• Magnetic field - it has one, but it is weak (100 times weaker than Earth's field)
  This is significant because Mercury rotates so slowly; inner metallic core would rotate slowly
  Mercury was not expected to have a magnetic field.
  It may have a liquid core with convection to generate magnetic field, or the core may be solid due to cooling of the small planet.

• Surface Temperature
  max. = 700 K (430 C or 800 F)
  min. = 100 K (-170 C or -280 F)

• Hard to observe from Earth
  No more than 280 from Sun; normally 150 - 200 from Sun
  therefore can never be seen in dark sky from Earth,
  very small planet; hard to see.

• Most information from Mariner 10 (3 flybys, 1974 - 1975).
  The surface is highly cracked & moon-like, ).

• Orbit is eccentric; e = 0.206 (Earth e = 0.017).
  It has the most highly eccentric orbit, except for Pluto (e = 0.248).
  • perihelion = 46 million km from Sun.
  • aphelion = 70 million km from Sun.

  • Sunlight approximately 2 times as intense at perihelion.

  • Sun also changes size by approximately 2 times during year.

  • Orbital speed increases perihelion;
    Sun moves E in sky at that time (very odd).

• Orbit is also inclined with respect to ecliptic @ 7 degrees
  (inclination of orbits of other planets < 3.5 degrees, but Pluto's orbital inclination is 17 degrees)

• In general, 1 day (orbit on its axis) = 58.646 days long
  (1965 radar observations from Aricebo)

  1 year = 87.97 days

  period of daylight differs from 58 days , however;
  orbits Sun 2 times for every 3 rotations on axis
  2-to-3 spin orbit coupling

  - used to think its orbit was synchronous like Moon's.
  - at inferior conjunction (when it is between us and Sun; at its closest to Earth), we see dark side of Mercury.

  - temperature measurements of dark side show it is not cold,
  began search for an atmosphere to distribute heat.

  - very thin atmosphere of Na + K (volatiles from rocks)
  atmospheric pressure is 6 millibars.

Surface

Craters

• wide range of shapes

• some differ from lunar craters, due to higher gravity of Mercury

• ejecta travels shorter distances due to higher gravity

• more slumping of small craters

• Caloris Basin 1200 - 1400 km in diameter = hottest place on planet
  (faces Sun at perihelion.

  - large double or multi-ringed basin.

• many overlapping craters.

• large craters have no central peaks, smooth rims (no terraces);
  these craters may have formed when surface was more fluid.

• contrast between light and dark areas is much less than on Moon (no dark maria).

• there are bright-rayed craters (Kuiper)

Inter-Crater Terrain

• large craters are not closely packed.

• plains between big craters are very heavily cratered.

• high density of small craters in many places, few craters in other plains.

  Why? Buried under ejecta? Melted?

  - some sort of smoothing process.
  - smooth plains similar to maria; cover approx. 15% of surface.
  - not dark in color.
  - also more heavily cratered than maria.
  

Scarps

- interpreted as thrust faults

- indicate compression (global shrinkage) over long time periods, occurred near end of bombardment phase, one tenth of 1% crustal shortening.

- not seen any other place except Earth & perhaps Ganymede.

- probably formed during cooling & contraction of planet
(heated to melting point during bombardment)

- during cooling, bombardment caused large flat craters

Mercury cooled more slowly than Moon due to its larger size.

- surface of Mercury may be younger than Moon's
more craters due to higher gravity.

Future Exploration

• on hiatus
• expensive
• question of priorities
• need to do radar altimetry, topography and gravity