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

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 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 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

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

• 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