Descriptive Astronomy 101

General Characteristics

Closest planet to the Sun.
1 revolution = 88 days (87.969)
1 rotation = 59 days (58.65)
Mass is 1/18th of Earth's
Radius = 2,439 km
Density = 5.43 g/cm3
Surface Temperature
- daytime = 740 K (872°F)
- nighttime = 90 K (-297°F)

Internal Structure

Believed to have a differentiated core, mantle and crust
Core - iron and nickel
Comparison of core with Earth:
- Mercury - 42% of volume, 75% of radius
- Earth - 16% of volume, 52% of radius
Mantle and crust composed of silicate rocks
Atmosphere - very tenuous hydrogen and helium derived from the solar wind

Surface Features

Craters, hilly terrain, smooth plains, lobate scarps and intercrater plains
Superficially the surface resembles the Moon - three differences
1) greater extent of old intercrater plains
2) widespread distribution of lobate scarps
3) crater ejecta characteristics are somewhat different - smaller extent


Radial extent of ejecta blankets is smaller than that found on the Moon
Larger gravitational pull reduces the ballistic range
Mercury 2.74 m/s2
Moon 1.66 m/s2
Earth 9.78 m/s2

Large Craters

Large Craters:
Largest structure on Mercury is the Caloris Basin (1300 km diameter)
Similar structure to large basins on the Moon
- multi-ringed
- smooth plains often fill and surround the basin

Smooth Plains

Large scale features
- often associated with basins
Resemble lunar maria
Believed to be large volumes of basalt lava flood plains

Intercrater Plains

Gently rolling plains with a high density of craters
Some of the rolling hills show forms similar to volcanic landforms
Believe to be the result of extensive volcanic activity earlier in the planets history
Later covered by impact ejecta

Hilly Terrain

Peculiar hills and depressions which disrupt existing landforms
Located antipodal to the Caloris Basin
Computer modeling indicates that focused seismic waves from the Caloris impact could cause this disruption

Lobate Scarps

Tectonic landforms with widespread distribution
Relatively steep escarpment
- lobate outline
- 20-500 km lengths
- 100 m to 1-2 km height
Terrain gently slopes away behind the scarp
Geometry indicates thrust faults formed by compressive stress
Estimated 1-2 km decrease in planetary radius resulted in formation of scarps

  • Example 1
  • Example 2
  • Planetary History

    Surface history is a reflection of the thermal history
    1) Composition of the surface:
    Indicates widespread melting and differentiation followed by cooling

    Planetary History - Stage 1

    Mercury has recently accreted
    Composed of a large percent of iron, uniformly distributed throughout the planet

    Planetary History - Stage 2

    Heating of the planet results in melting and differentiation of core, mantle and crust

    Planetary History - Stage 3

    Mantle cooling results in global contraction
    Planetary contraction causes crust to shrink and develop thrust faults (lobate scarps)

    Planetary History - Stage 4

    New volcanic eruptions fill the Caloris basin and other low areas

    Planetary History - Stage 5

    Further cooling causes more planetary contraction

    Planetary History - Stage 6

    Cooling of the mantle and crust is complete

    Additional Reading

    Geological History of Mercury by Paul Spudis (PDF file)