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Lecture #15:
The Main Point
• Mercury:
• General properties.
Mercury is a small, atmosphereless
terrestrial planet with a large metallic
core and evidence for a complex
geologic history.
• Surface characteristics.
• Interior and evolution.
• Reading:
– Chapter 7.1, 9.3, 10.3.
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Basic Properties of Mercury
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Observing Mercury in the Sky
• Average Distance from Sun: 58,000,000 km (0.39 AU).
• But: Orbital eccentricity = 0.206.
• So: Distance varies from 46,000,000 to 70,000,000 km.
• Orbital period: 88 days.
• Period of Spin around axis: 58.65 days (2/3 orbit period!)
• Mass = 3.3x1023 kg = 0.055 ME.
• Radius = 2440 km = 0.38 RE.
• Density = 5.4 g/cm3 (recall that Earth ~5.5 g/cm3).
• Surface Gravity = 3.7 m/sec2 (38% of Earth's).
• Surface Temperatures: -185°C (night), +430°C (day) [!]
• Surface superficially lunar, but important differences.
• Mercury is the innermost planet.
• It never gets more than ~28° from the Sun.
• Hard to see!
– Twilight.
– Haze/Smog.
– Starlike.
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What's up with that Density?
Is the core molten?
A 30-year old question in planetary science, with important
implications for the internal structure, thermal evolution, and
magnetic field generation of terrestrial planets.
• Mercury's density vs. size is distinctly different
from that of the other terrestrial planets.
• Implies a large metallic core!
Planet %Core
Earth ~54%
Moon ~10%?
Mercury ~74%
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And that Weather!
What's up with that Spin?
Mercury rotates 3 times on its axis for every 2
revolutions around the Sun (i.e. 3 Mercury
days = 2 Mercury years)! • Yet another example of a
system influenced by tides.
• Mercury, close to the Sun,
feels strong solar tidal forces.
• But Mercury's orbit is eccentric
(Kepler: orbital speed varies),
so Moon-like synchronous
rotation is difficult.
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• 2 years : 3 days is the most
favorable resonance.
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Telescopic Observations
Mariner 10
• Mercury studied by the
Mariner 10 space mission.
• Flew past Mercury 3 times in 1974 and 1975 (after gravity
assists from the Earth/Moon and Venus...)
• Only about 50% of the surface imaged by Mariner.
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•
•
•
Difficult because Mercury low in the sky.
Also, Mercury is small.
Contrasts are weak.
Result: very little was
known based on
visible telescope data.
• More later on radar...
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Geology of Mercury
Mercury's Surface Properties
• Crater shapes different from the Moon:
• Surface appears generally MoonMoon-like, covered
by dark gray regolith.
• Brightness, roughness similar to lunar
highlands.
• No water! (but more about polar regions later)
• Small, localized mare deposits.
• Many craters, several large impact basins.
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• Gravity on Mercury is ~
twice that on the Moon.
• Crater bowls shallower
than on the Moon.
• Ejecta blankets less
extensive than on the
Moon.
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Geology of Mercury
Planetary Evolution
• Evidence of tectonic forces at work in the past.
• Mercury has a weak magnetic field:
– Is there a dynamo in a molten metallic core?
– Or is the magnetic field frozen in the crust?
• Surface evidence for compression, but no extension.
• Mercury's crust appears to have shrunk from a
once-larger size:
– Contraction after cooling from a molten state?
Lobate Scarp
• Indicates
compression of
the planet's crust.
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Cliff is 2 km high and 100 km in length
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The Incredible Shrinking Planet?
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Radar Observations
• Arecibo radio telescope used to discover the ~ 59
day spin period of Mercury (1965).
• More recently: Evidence for polar ice caps found
from radar data!
– Something very "radar bright" near the poles.
– Ice reflects radar very strongly.
– But it sounds crazy--ice on such a hot planet?
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Related Issue: Ice on the Moon?
• Radar-reflective deposits
appear to occur in shadowed
craters.
• Mercury's axis tilt is ~0°
(no seasons!)
• Some deep craters near the
poles never see sunlight!
• Hypothesis: ice from
cometary impacts.
• Other less likely hypotheses:
sulfur, silicates.
• A recent mission called Lunar Prospector has
reported possible evidence for ice on the Moon:
– The Moon has permanently shadowed polar craters too.
– Data showed slightly enhanced Hydrogen in these areas.
– Not confirmed by radar images as on Mercury.
• Controversial!
– H could be from solar wind or some other process and
might not be the "H" in H2O.
• But big implications for space exploration.
Harmon et al., Arecibo radar data.
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Summary
NASA’s Return to Mercury
Many questions are being answered by NASA’s
MESSENGER Mercury orbiter:
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• Mercury is an enigmatic terrestrial planet:
– Strange relationship between spin and orbit.
– Evidence for an enormous core.
– Evidence for shrinking of the crust.
Launched in 2004.
First flyby on Jan 14, 2008.
Orbit insertion in 2011.
Orbits, maps for 1 Earth year.
Imaging, spectroscopy,
magnetic fields, altimetry.
• Mariner 10 & recent MESSENGER data provide most
of our detailed knowledge of Mercury's geology:
– Outwardly Moon-like, but important differences.
• Radar data indicate possibility of ice in shadowed
craters near the poles!
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Next Lecture...
• Venus:
•General properties.
•Telescopic
observations.
•Space missions.
•Atmospheric
characteristics.
• Reading:
–Chapters 7.1 (Venus), 9.5, 10.5.
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