Post on 23-Feb-2016
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11a. Sun-Scorched Mercury• Earth-based observations of Mercury• Mercury’s rotation & year• Mariner 10’s images of Mercury• Mercury’s interior
Mercury Data (Table 11-1)
Mercury Data: Numbers• Diameter: 4,878.km 0.38 . Earth• Mass: 3.3 . 1023 kg 0.055 . Earth• Density: 5.4 . water 0.99 . Earth• Orbit: 5.8 . 107 km 0.39 AU• Day: 58.65 days 58.65 . Earth• Year: 87.97 days 0.24 . Earth
Mercury Data: Special Features• Mercury is the closest planet to the Sun• Mercury is the smallest terrestrial planet• Mercury has essentially no atmosphere• 3 sidereal days = 2 sidereal years
– Mercury exhibits unique 3-to-2 spin-orbit coupling• Mercury is very heavily cratered• Mercury is a visual twin of the Moon except …
– Mercury does not have any maria (i.e., “seas”)• Mercury’s interior is dominated by an iron core
– 75% the diameter & 42% the volume• Mercury is very difficult to observe from Earth
– Mercury is never >28° from the Sun– This third week of February 2013
Earth-Based Observations of Mercury• Brighter than any star (at times)
– Albedo is only 0.12, the same as weathered asphalt• Difficult to observe from Earth
– Copernicus apparently never saw Mercury• Neither did I until April 2002 despite many attempts
– Elongation maxima of Mercury• Maximum eastern elongation of 18°
Evening sky• Maximum western elongation of 28°
Morning sky– Elongation favorability of Mercury
• Unfavorable Eastern & low angle to the horizon
• Favorable Western & high angle to the horizon
• Conjunctions Three inferior per year• Solar transits Crossing in front of the Sun
– Aphelion in May & perihelion in November
Transit of Mercury: 8 November 2006
2006 Transit
Mercury’s Elongations
Evening
Morning
Favorable & Unfavorable Elongations
Western elongation Eastern elongationHigh-angle to horizon Low-angle to
horizonMorning sky Evening sky
Mercury’s Greatest ElongationsEastern (Evening) Western (Morning)
Friday 26 October 2012 Tuesday 4 December 2012
Saturday 16 February 2013 Sunday 31 March 2013
Wednesday 12 June 2013 Tuesday 30 July 2013
Wednesday 9 October 2013 Monday 18 November 2013
Friday 31 January 2014 Friday 14 March 2014
Sunday 25 May 2014 Saturday 12 July 2014
Sunday 21 September 2014 Saturday 1 November 2014
Mercury Chaser's Calculator
Mercury’s Rotation & Revolution• Determining Mercury’s axial rotation rate
– 1880s Schiaparelli wrongly concludes 1-to-1 S.O.C.• Unable to see enough surface detail with his telescopes
– 1962 Radio noise emitted from MercuryPassive
• Sunlit side blackbody radiation curve ~623 K– Expected radiant temperature
• Sunless side blackbody radiation curve ~103 K– Unexpected radiant temperature Too high!– Implied that Mercury has no permanent sunless side
– 1965 Arecibo radio telescopeActive
• Transmitted 1 precise radio l to Mercury• Reflected radio signal analyzed for Doppler shift
– Mercury’s left side Very small blue shiftApproaching
– Mercury’s right side Very small red shift Receding
• Measured at very nearly 59 days– 2/3 of Mercury’s year– 3-to-2 spin-orbit coupling
Unique in the Solar System
Measuring Mercury’s Axial Rotation
Mercury’s 3-to-2 Spin-Orbit Coupling
Causes of 3-to-2 Spin-Orbit Coupling• Solid Sun tides distort Mercury into an ellipsoid
– There is a tidal bulge on opposite sides of Mercury• Mercury is in a highly elliptical orbit
– Mercury’s aphelion distance is ~ 1.52 x perihelion• The Sun’s gravitational force varies ~ 2.3 x• The Sun’s gravitational force cannot lock onto one side• The Sun’s gravitational force can lock onto one axis
• Some effects– 1.00 Mercury day = 2.00 Mercury years– Occasional retrograde Sun motion in Mercury’s sky
• Slow East -to-West sunset• Slow West-to-East sunrise• Slow East -to-West sunset
Mariner 10 at Mercury (1974 & 1975)
Mariner 10’s Images of Mercury• Only three passes of Mercury
– Mariner 10 orbited the Sun, not Mercury– 1.00 Mariner 10 orbit every 2.00 Mercury years
• March 29, 1974~704 km above Mercury• September 21, 1974~47,000 km above Mercury• March 16, 1975~327 km above Mercury
– Same hemisphere toward the Sun each time• Mariner 10 obtained images approaching & leaving• Detailed mosaics of only one hemisphere
Mercury & the Moon Compared
Mercury’s Surface• Casually, Mercury looks much like the Moon• Mercury is heavily cratered but…
– Crater density is not as high as on the Moon– Mercury has gray intercrater plains, not black maria
• Mercury has long, irregular ridges & scarps– Probably shrinkage features as Mercury cooled
• Most materials shrink as they solidify– The surface solidifies before the interior– When the interior solidifies, the surface gets compressed
• Only ½ of Mercury’s surface was well-known– The Mariner 10 spacecraft went past three times
• Precisely the same face toward the Sun both times
Mercury & Moon: Subtle DifferencesMercurian craters & plains Lunar highland craters
Mercury’s Shrinkage Scarps (Cliffs)
Mercury’s Caloris Basin• Very similar to the Moon’s Mare Orientale
– Much larger than any other impact crater– Multi-ringed– Not flooded with lava
• Jumbled terrain on opposite side of Mercury– Seismic wave energy focused by Mercury’s core
• Similar to 17 October 1989 Loma Prieta earthquake– Seismic wave energy focused on San Francisco Bay area– Much stronger shaking than closer to the quake epicenter
– Strong enough to fracture the surface• Chaotic hills ~100 to ~1,800 m high• Large smooth-floor crater superimposed on hills
– Impact after formation of the Caloris Basin
Mercury & Moon: Impact BasinsCaloris BasinMare OrientaleMercury Moon
The Caloris Basin: A Second Look
http://en.wikipedia.org/wiki/File:Mercury_Double-Ring_Impact_Basin.png
The Caloris Basin: A Third Look
Mercury’s Interior• Dominated by a very large iron core
– Mercury ~75% of diameter~42% of volume
– Earth ~55% of diameter~17% of volume
– Moon ~20% of diameter~ 1% of volume
• Proposed causes– Too hot for condensation of low-density minerals– Strong solar wind removed low-density materials– Head-on impact with a planetesimal
• Computer simulations favor this hypothesis
Planetary Interiors: Mercury & Earth
Mercury Messenger Spacecraft
Mercury’s de Graft Crater
http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/EW1017384139G.3band.mapped.png
Mercury’s Jumbled (Weird) Terrain
http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/Caloris_antipode.jpg
Mercury’s Formation: Head-On Impact
• Mercury seen from Earth– Very bright yet very elusive
• Always close to Earth’s horizon– Maximum E. & W. elongations
• Never more than 28° from the Sun• Mercury’s unusual axial rotation
– 3-to-2 spin-orbit coupling• Solid tides distort Mercury’s shape• Radically changing solar gravity
• Mariner 10 at Mercury– Made three passes
• Exactly 2 Mercurial years apart– Imaged only one-half of its surface
• Result of 3-to-2 spin-orbit coupling– Remarkably Moon-like surface
• Heavily cratered • Caloris Basin & jumbled terrain• Intercrater plains & no maria• Ridges & scarps
• Mercury’s interior– Completely core dominated
• Comparison with the Earth & Moon– Probable head-on planetesimal
impact
Important Concepts