Transcript Chapter 11a: Sun-Scorched Mercury PowerPoint

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
• Orbit:
5.8 . 107 km
0.39
.
Earth
AU
• Day:
58.65 days
58.65
.
Earth
• Year:
87.97 days
0.24
.
Earth
Mercury Data: Special Features
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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°
• Maximum western elongation of 28°
Evening sky
Morning sky
– Elongation favorability of Mercury
• Unfavorable
• Favorable
Eastern & low angle to the horizon
Western & high angle to the horizon
• Conjunctions
• Solar transits
Three inferior per year
Crossing in front of the Sun
– Aphelion in May & perihelion in November
Transit of Mercury: 8 November 2006
Mercury’s Elongations
Evening
Morning
Favorable & Unfavorable Elongations
Western elongation
Eastern elongation
High-angle to horizon Low-angle to horizon
Morning sky
Evening sky
Mercury’s Greatest Elongations
Eastern (Evening)
Western (Morning)
31 January 2014
14 March 2014
25 May 2014
12 July 2014
21 September 2014
1 November 2014
14 January 2015
24 February 2015
7 May 2015
24 June 2015
4 September 2015
16 October 2015
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 Mercury
• Sunlit
Passive
side blackbody radiation curve ~623 K
– Expected
radiant temperature
• Sunless side blackbody radiation curve ~103 K
– Unexpected radiant temperature
– Implied that Mercury has no permanent sunless side
– 1965 Arecibo radio telescope
• Transmitted 1 precise radio l to Mercury
• Reflected radio signal analyzed for Doppler shift
– Mercury’s left side
– Mercury’s right side
Very small blue shift
Very small red shift
Too high!
Active
Approaching
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 Differences
Mercurian 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 Basins
Caloris Basin
Mercury
Mare Orientale
Moon
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’s Formation: Head-On Impact
Important Concepts
• 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
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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