The Innermost Planet MERCURY

Inferior conjunction – inferior planet and Earth on same side of Sun Superior conjunction – inferior planet and Earth on opposite sides of Sun Conjunction

Mercury ☿ Smallest of the planets (Pluto etc. does not count) Named for the swift messenger to the Roman gods

Little is known about Mercury 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

Mercury – Physical Properties Moon Mercury Earth Radius 1700 km 1440 km 6380 km Mass 7.3 × 1022 kg 3.3 × 1023 kg 6.0 × 1024 kg Density 3300 kg/m3 5400 kg/m3 5500 kg/m3 Escape Speed 2.4 km/s 4.3 km/s 11.2 km/s

Mercury’s Orbit 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

Mercury’s Orbit most eccentric orbit of all the planets orbit is inclined by 7 degrees to the plane of the ecliptic axial tilt is almost zero

Transits 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

Interior of Mercury Crust Mantle Core Core occupies 42% of Mercury (17% for Earth) Has higher iron content

Compared to Luna Mercury is much denser than the Moon Has a magnetic field – not well understood!

Mercury’s Magnetosphere Significant for size Planetwide Dynamo effect? MESSENGER will measure

Geology of Mercury Features of Mercury’s geology: Dorsa Impact basin’s and craters Pit floor craters Plains Scarps Volcanoes

Features of Mercury - Dorsa Ridges Caused by faults in crust

Features of Mercury - Craters Mercury is very heavily cratered. More so than the Moon. Predominant feature of Mercury Floors can be smooth plains

Caloris Basin Largest crater in solar system 1,550 Km in diameter

Weird Terrain Antipodal of Caloris Basin Where shock waves collided

Features of Mercury – Pit Floor Craters Crater floor has collapsed Possible magma chamber under crater?

Features of Mercury – Plains 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

Features of Mercury - Scarps Scarps are cliffs caused by the terrain moving vertically On Mercury, caused by crust shrinking as planet cooled.

Features of Mercury - Volcanoes evidence of lava flows Cinder cone volcanoes Second most important feature-former

Features of Mercury - Volcanoes

Ice? On Mercury? It’s more likely than you think. Craters at pole may have water ice Bottom of crater is in permanent shadow Spotted with radar, need confirmation from MESSENGER

Mercury’s Atmosphere 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

Exploring Mercury Only two missions have visited Mercury: Mariner 10 (1973) MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) (2004)

Mariner 10 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

MESSENGER Can get better pictures of Mercury Has performed 3 flybys so far Will enter orbit of Mercury in March 2011

Mercury - Summary 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