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Mercury vs. the Moon

Lost planet? Lost moon? What’s th d l ith M ? A d h tthe deal with Mercury? And what 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 what does this tell us?almost as dense—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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Mercury Re-viewed

20081973

Probes

Messenger

Achieving Orbit in March 2011

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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

Sullivan (Mariner)

(MESSENGER)

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Surface Features –Scarps on a Plain

Scarps Plains

Cliffs

That’s one big scarp.

125 miles

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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 highlands are not so ) y gextensively 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 it like the lunarthe crust and resurface it, like the lunar maria.•Impact was so great that a shockwave propagated through the planet causing hills to develop antipodal to the basin (the so-called “Weird Terrain” or “Chaotic Terrain” (technically, the “Hilly and Lineated Terrain”)

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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 mostly of heavy y yelements, 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?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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The Outer Mercury

• Surface is covered with dust, similar to lunar regolithg

• 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 5x10-11Hardly worth mentioning, but present (about 5x10

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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Rotation and Revolution

• Radio measurements determined rotation t b D l hiftrate by Doppler shift

• Rotation is the result of gravitational influence of the sun– Mercury and the sun are gravitationally locked

– Mercury’s day looks a little stranger thanMercury s day looks a little stranger than earth’s

Mercury vs. the Moon

•Both have similar “atmospheres”

Different?•Moon has little iron; mostly

•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

; ycrust 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 notM i d d l•Both show many impact

craters•Both have orbits that are affected by tidal forces from their parent bodies

•Mercury is denser and larger