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.km0.38. Earth Mass: kg Earth Density:5.4. water0.99. Earth Orbit: km0.39 AU Day:58.65 days Earth Year:87.97 days0.24. 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 –Elongationmaximaof Mercury Maximum eastern elongation of 18°Evening sky Maximum western elongation of 28°Morning sky –Elongationfavorabilityof Mercury UnfavorableEastern & low angle to the horizon FavorableWestern & high angle to the horizon ConjunctionsThree inferior per year Solar transitsCrossing in front of the Sun –Aphelion in May & perihelion in November

Transit of Mercury: 8 November Transit

Mercury’s Elongations Evening Morning

Favorable & Unfavorable Elongations Western elongationEastern elongation High-angle to horizonLow-angle to horizon Morning skyEvening sky

Mercury’s Greatest Elongations Eastern (Evening)Western (Morning) Friday 26 October 2012Tuesday 4 December 2012 Saturday 16 February 2013Sunday 31 March 2013 Wednesday 12 June 2013Tuesday 30 July 2013 Wednesday 9 October 2013Monday 18 November 2013 Friday 31 January 2014Friday 14 March 2014 Sunday 25 May 2014Saturday 12 July 2014 Sunday 21 September 2014Saturday 1 November 2014 Mercury Chaser's Calculator

Mercury’s Rotation & Revolution Determining Mercury’s axial rotation rate –1880sSchiaparelli wrongly concludes 1-to-1 S.O.C. Unable to see enough surface detail with his telescopes –1962Radio noise emitted from MercuryPassive Sunlitside blackbody radiation curve ~623 K –Expected radiant temperature Sunlessside blackbody radiation curve ~103 K –Unexpected radiant temperatureToo high! –Implied that Mercury has no permanent sunless side –1965Arecibo radio telescopeActive Transmitted 1 precise radio to Mercury Reflected radio signal analyzed for Doppler shift –Mercury’s left sideVery small blue shiftApproaching –Mercury’s right sideVery small red shift Receding Measured at very nearly 59 days –2/3 of Mercury’s year –3-to-2 spin-orbit couplingUnique 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 & plainsLunar 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 BasinMare Orientale MercuryMoon

The Caloris Basin: A Second Look

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

Mercury’s Jumbled (Weird) Terrain

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