12/9/2010
Mercury vs. the Moon
Lost planet? Lost moon? What’s
th d
the
deall with
ith M
Mercury?
? A
And
d what
h t
makes it like our moon anyway?
Mercury Highlights
•Named for the Roman messenger of the
gods, because it’s quick
•88 day long year
•57 day long day (that’s 3 days every 2
years—how old would you be?)
•4878 km in diameter
•Temp. ranges from -183 C at night to 427
C during the day
•58,000,000 km from the sun (1/3 earth’s
distance)
•Smaller than earth and less massive but
almost as dense
dense—what
what does this tell us?
•No satellites
•Orbits at 30 mi/s (48 km/s) (earth is 18
mi/s – Kepler was right!)
•No atmosphere?
(2008)
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12/9/2010
Mercury Re-viewed
1973
2008
Probes
Messenger
Achieving Orbit in March 2011
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12/9/2010
Surface Features - Craters
Boethius
Polygnotus
83km
Craters, complete with ejecta rays.
Rays are smaller than on the moon
because the higher gravity reduces
the distance debris can be thrown.
(named for artists/scientists: Shakespeare, Bach,
Tolstoy, Mozart, Goethe)
More Craters
Smooth craters
(MESSENGER)
Sullivan (Mariner)
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12/9/2010
Surface Features –
Scarps on a Plain
Scarps
Plains
Cliffs
That’s one big scarp.
125 miles
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12/9/2010
Mercury Geology
• Scarps – small ridges caused by fast contraction of the
planet when semiliquid
– Stretch across the planet
– 100m to 1.5km high
– Sometimes cut through craters, indicating they formed after the
craters
– Formed when the semimolten planet cooled quickly, shrinking its
diameter by 4km.
• Plains – large, smooth areas. Not like lunar maria (not
volcanic in nature),
) but Mercury’s
y highlands
g
are not so
extensively cratered as the moon’s highlands.
• Volcanoes? Maybe once, but not recently. Some
craters appear volcanic in nature rather than the result of
impact.
• Mountains? Most are the result of sizeable crater
formation. There is no evidence of tectonics on Mercury
Caloris Basin
Caloris Basin, at the south pole, is a
massive impact crater
•Note the circular rings around the “crater”.
•Crater interior is smooth; rim is as much as
2km tall.
•Could mean old
•Could mean it was so large it didn’t
make an obvious rim
•Impact was large enough to partially melt
the crust and resurface itit, like the lunar
maria.
•Impact was so great that a shockwave
propagated through the planet causing hills
to develop antipodal to the basin (the socalled “Weird Terrain” or “Chaotic Terrain”
(technically, the “Hilly and Lineated Terrain”)
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12/9/2010
The “Weird Terrain”
MESSENGER image of CB
The Inner Mercury
•Density is highest of all but earth
•Since it is so small, it must be
composed mostlyy of heavy
y
elements, particularly Fe.
•Mariner measuers Mercury as
having a magnetic field (1% that
of earth)
•implies that core is partially
molten?
•Field comes from sun, was
frozen in rocks?
•Some volcanism seen in the past,
so maybe core is or was partially
molten.
•Perhaps crust was vaporized in
an impact?
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12/9/2010
The Outer Mercury
• Surface is covered with dust, similar to lunar
g
regolith
• Most surface material is similar to lunar
highlands (deficient in Ti, Fe)
• Surface is highly reflective (high albedo)
indicating brighter minerals (unlike maria)
• Atmosphere
– Hardly worth mentioning, but present (about 5x10-11
that of earth)
– Mostly H, He, Ne, Na vapor; probably mostly made of
solar wind particles trapped.
– Not enough to affect the planet (no wind erosion, etc.)
Magnetic Mercury
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12/9/2010
Rotation and Revolution
• Radio measurements determined rotation
rate
t by
b D
Doppler
l shift
hift
• Rotation is the result of gravitational
influence of the sun
– Mercury and the sun are gravitationally locked
– Mercury
Mercury’s
s day looks a little stranger than
earth’s
Mercury vs. the Moon
•Both have similar
“atmospheres”
•Both are covered with
fine dust
•Similar albedoes
•Both have metallic cores
•Both are geologically
dead (no tectonics)
•Both show maria and
lava flooding
•Both show many impact
craters
•Both have orbits that are
affected by tidal forces
from their parent bodies
Different?
•Moon has little iron;; mostlyy
crust material (refractory
metals)
•Mercury is mostly iron, with
few light refractory metals
(because their origins are
different)
•Mercury has a magnetic
field; the moon does not
•Mercury
M
iis d
denser and
d llarger
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