Download Mercury

Spacecraft Dawn
Orbits Ceres
•  Discovered in 1801, Ceres was a
planet, then an asteroid & now a
dwarf planet
•  Dawn spacecraft orbited Vesta &
now orbits Ceres
•  Infrared telescope detected water
•  Could white spots be cryovolcanoes
Mercury = Hermes Mythology
•  Planet Mercury, Mercredi
•  God of Commerce, Messenger
God, guide to Hades
•  Winged sandals and staff
Observing Mercury
•  Mercury never seen more than 28° from
the sun= Greatest Elongation East/West
•  Greatest Elongation East = Evening star
best viewing evening 06May15 ~21°
Mercury’s Orbit
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Revolves/orbits 0.4 AU from sun in 88 days
Rotates in 59 days (radar redshift/blueshift)
3 rotations to 2 revolutions
3 to 2 Resonance = spin to orbit coupling/resonance
Frame where
Sun is still
Frame where
Mercury is still
Mercury’s Physical Properties
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⅓Earth’s radius; 1/27volume
But 1/20 Earth’s mass
So more dense than Earth
Big Iron core – 75% of radius
But very weak magnetic field
Small changes so not a totally
solidified core
•  Greatest temperature
variation of all planets
Noon=~700K &
Night=~100K
Atmospheric Escape
•  Atmosphere can leak into space over eons of time
•  More massive planet=higher Escape Speed/Velocity
Temperature of the Atmosphere
Lighter
Molecules
Lost First
Magnetic Field Helps Protect
Planet from Solar Wind
•  Mercury’s weak magnetic field not much protection
•  Mars has no magnetic field so H2O+solar UV=2H+ + O+
•  Ions at top of atmosphere swept away by solar wind
MAVEN Results Sep2014
•  Low mass hydrogen atoms escaping while
more high mass oxygen is held close to Mars
•  Depends on
1.  Planet’s Mass
=Escape speed or
escape velocity
2.  Temperature=
Speed of molecules
3.  Mass of molecules
4.  Magnetic field =
protection from
solar wind
Escape
Retention of an Atmosphere
Messenger First Mission
to Orbit Mercury
•  Launched Aug04; loops around
sun 15 times, past Earth once,
Venus twice & Mercury 3 times
•  Finally orbited 2011- End 2015
Impact Craters
•  Heavy bombardment =
cratered surface
•  Strange dark rimmed crater
•  Crater with gullies run to/
from
Intercrater Plains
•  More craters than the Lunar Maria
=3.5 Billion years, but less than
Highlands=4.5 Billion so
Intercrater plains~4Billion years
•  More massive than moon =
cooled more slowly
Caloris Basin
•  Giant impact creates ringed
basin ~1500km diameter
•  Later flooded by lava
•  Jumbled/wierd terrain on
opposite hemisphere of
planet
Volcanism on Mercury
•  Bright volcanic vents
•  Lava flows nearly hide craters
•  No Plate Tectonics
Mercury Hollows
•  Flat shallow bright spots
unique to Mercury
•  Volatile material lost to
space. Solar wind??
Lobate Scarps
•  Mercury cooled and shrunk
forming surface wrinkles = cliffs
•  Which pass through a previously
(109 years) existing crater
A planet or moon is more likely
to retain an atmosphere if:
a)  It is cooler so the molecules move more slowly
b)  It is more massive so it has a larger escape
speed/velocity
c)  The atmosphere is made up of heavy molecules
d)  It has a magnetic field to protect its atmosphere
from the solar wind
e)  All of these
Polar Ice?
•  Similar to moon polar craters have
areas in permanent shadow
•  Radar echoes indicate ice