The Innermost Planet
• Inferior conjunction – inferior planet and Earth on same side of Sun
• Superior conjunction – inferior planet and Earth on opposite sides of Sun
Smallest of the planets (Pluto etc. does not count)
Named for the swift messenger to the Roman gods
Bright in appearance but not easily seen:• Only visible at morning or evening• At most, 28.3º from the Sun
Ground based telescopes give only limited detail
MoonMoon MercuryMercury EarthEarth
RadiusRadius 1700 km1700 km 1440 km1440 km 6380 km6380 km
MassMass 7.3 × 107.3 × 102222 kgkg
3.3 × 103.3 × 102323 kg kg 6.0 × 106.0 × 102424 kg kg
DensityDensity 3300 kg/m3300 kg/m33 5400 kg/m5400 kg/m33 5500 kg/m5500 kg/m33
Escape Escape SpeedSpeed
2.4 km/s2.4 km/s 4.3 km/s4.3 km/s 11.2 km/s11.2 km/s
Long thought to be tidally locked to the Sun; measurements in 1965 showed this to be false.
Mercury’s day and year are in a 3:2 resonance; Mercury rotates three times while going around the Sun twice.
Spin-orbit resonance – ratio of the periods can be expressed as simple integers
most eccentric orbit of all the planets
orbit is inclined by 7 degrees to the plane of the ecliptic
axial tilt is almost zero
when Mercury comes between the Sun and the Earth
Mercury is seen as a small black dot moving across the face of the Sun
2016 May 9
1. Crust2. Mantle3. Core Core occupies
42% of Mercury (17% for Earth)
Has higher iron content
Mercury is much denser than the Moon
Has a magnetic field – not well understood!
Significant for size Planetwide Dynamo effect? MESSENGER will
measure
Features of Mercury’s geology:• Dorsa• Impact basin’s and craters• Pit floor craters• Plains• Scarps• Volcanoes
Ridges Caused
by faults in crust
Mercury is very heavily cratered. More so than the Moon.
Predominant feature of Mercury
Floors can be smooth plains
Largest crater in solar system
1,550 Km in diameter
Antipodal of Caloris Basin Where shock waves collided
Crater floor has collapsed
Possible magma chamber under crater?
Inter-crater plains are the oldest visible surface• occur in the regions
between larger craters• Gently rolling hills
Smooth plains are widespread flat areas • resemble lunar maria • fill depressions
Scarps are cliffs caused by the terrain moving vertically
On Mercury, caused by crust shrinking as planet cooled.
evidence of lava flows
Cinder cone volcanoes
Second most important feature-former
Craters at pole may have water ice
Bottom of crater is in permanent shadow
Spotted with radar, need confirmation from MESSENGER
Very tenuous• about 2 trillionths of the
atmospheric pressure on Earth
containing hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor
Solar wind gives Mercury a tail
Only two missions have visited Mercury:• Mariner 10 (1973)• MESSENGER (MErcury Surface, Space ENvironment,
GEochemistry and Ranging) (2004)
Last of the Mariner missions Explored both Venus and Mercury Could only map about 45% of
Mercury• Same side was sunlit on each of the three
flybys
Can get better pictures of Mercury Has performed 3 flybys so far Will enter orbit of Mercury in March
2011
Smallest planet 3:2 orbital resonance; most eccentric
orbit Molten iron core Small but significant magnetic field Impact craters are biggest feature Ice in polar craters Very thin atmosphere