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