Mercury

Closest to sun: Orbit very eccentric: 0.308 - 0.467 AU

No moons

2nd smallest planet; Ganymede, Titan, (moons) larger

Bright, but never observed in darkness from Earth.

Seen as "evening star" or "morning star" Mariner 10, 1974, passed Venus, then into solar orbit encountering Mercury every other orbit

Mariner 10, 1974, passed Venus, then into solar orbit encountering Mercury every other orbit

Always saw same hemisphere of Mercury - only 45% of surface seen

Looks superficially like the moon!

Night temps. like moon: - 274^oF

Day temps. much hotter: 756^oF

Radar Astronomy

· Used to accurately measure orbits for later planetary missions

· Determined rotation of Mercury and Venus

Returning radar signal:

broadened spectral lines due to

reflections from entire hemisphere

Sidereal rotational period = 58.7 d. (sidereal day)

Period of revolution = 88 d. (sidereal year) 2:3 ratio, spin orbit coupling

Solar day = 2 yrs. = 176 Earth days

Solar day = 3 sidereal days

1:1 resonance not possible because orbit very eccentric

Kepler’s second law: orbital speed greatest at perihelion - least at aphelion

Cannot remain synchronous around orbit:

· If synchronous near perihelion, it would be too rapid at aphelion,

· Synchronous rate at aphelion would be too slow at perihelion.

Tidal forces try to synchronize the rotation rate with instantaneous orbital speed

Tidal effects diminish very rapidly with increasing distance: M / D³

\ much greater at perihelion than aphelion

Þ orbital and rotational rates synchronous at perihelion only

Mercury rotates 180° between one perihelion and the next Þ 3:2 resonance

Resonance important in orbits of many planets, moons, rings, asteroid belt

\ alternate hemispheres face sun each perihelion

Mercury’s rotational axis perpendicular to its orbital plane

Þ Two (diametrically opposite) points on the equator where the Sun is directly overhead at perihelion get hottest (hot longitudes) - 675 K

Polar temperatures as low as 125 K may be covered with sheets of water ice

Tidal heating from sun creates internal friction and heating as tidal bulge alternately increases and decreases around orbit

Surface Features - Seen by Mariner 10:

Resembles lunar highlands - probably mostly anorthosite, perhaps with more metal

No large lava - flooded basins like lunar maria

But cratered plains seen with 1/10 crater density of lunar highlands

Þ early vulcanism, before end of heavy bombardment

Craters - not quite as steep as on moon due to higher gravity (.38 g.)

Caloris Planitia: hot, faces sun @ alternate perihelia, huge "bulls eye" crater (1,400 km.) Floor mottled, not dark like lunar maria Þ possibly surface material melted from impact, not flooded later

Weird or Chaotic Terrain: wavy, rippled terrain - opposite Caloris Planitia on planet

Caused by seismic waves from impact; Made Mercury heavier on one side

Lobate scarps - system of compressional scarps of "cliffs" all over planet - up to 3 km high - not found on moon

Scarps explained by 2% radial shrinkage of interior Þ 4% shrinkage of surface

Relatively high uncompressed density (5.2% compared to 4.5% for Earth) implies dense metallic core - 60% metal

Origin of Mercury: Perhaps 100 lunar to Mars-sized protoplanets in inner solar system Þ many giant impacts

Mercury lost muck of rocky crust and mantle from impacts, leaving metallic core with relatively thin rocky shell

Weak magnetic field-due to slow rotation?

Very thin atmosphere - probably trapped solar wind particles