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Geology 130F

Lecture Thirteen


Mercury


Orbital and Rotational Parameters of the Planets

MercuryVenusEarthMarsJupiter SaturnUranusNeptunePluto
eccentricity
0.206
0.007
0.017
0.093
0.048
0.056
0.046
0.009
0.248
inclination (rad)
0.122
0.059
0 by def.
0.032
0.023
0.043
0.013
0.031
0.299
Obliquity (deg)
2
177.3
23.45
25.19
3.12
26.73
97.86
29.6
122.46
Both Mercury and Pluto have irregular orbits compared to the other planets in the solar system. We will see later that Pluto is trapped in a 3/2 orbit-orbit resonance with Neptune, which is probably related to Pluto's peculiar orbit. Mercury exhibits a resonance between its spin period or length of day and its orbital period or year; the day is 2/3s as long as the year. This is belived to be a result of Mercury's large orbital eccentricity and Solar tides, begging the question, Why does Mercury have such a large eccentricity?

Geologic Characterstics

Bulk Properties of Mercury
MercuryMoon
Density5.44g/cm33.34
Radius2425 km1738
Area/Mass2.3x10-9cm2/g5.17x10-9
VolatilesH, Na, Henone

  1. Mercury is strikingly like the Moon, but
  2. There are significant differences:
    1. Crater ejecta do not extend as far on Mercury
    2. Mercury has old but non-crater-saturated terrain
    3. Old impact basins on Mercury are shallow and ill defined
  3. We have no solid evidence of extensive lava flows on Mercury
    1. This implies that Mercury cools by conduction (since there is also no evidence of tectonic motion)
    2. This suggests that there is no significant asthenosphere on Mercury.
  4. Mercury has planet-wide lobate scarps
    1. This suggests that the planet is contracting as it cools.
  5. Mercury's high bulk density suggests that it has a very large core
  6. Mercury has a magnetic field, which suggests that the core is liquid.
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