Mercury

Mercury basic data Semi-major axis = 0.39 AU Eccentricity = 0.206 Inclination = 7 ° (wrt ecliptic) Orbital period = 87.969 Earth days Spin period = 58.646 Earth days (sidereal) Solar day = 176 Earth days Diameter = 4880 km = 0.38 Earth diameter Mass = 3.3 x 1023 kg = 5.5% Earth mass Density = 5430 kg/m3 = 5.4 g/cm3 Vesc = 4.3 km/s Temp erature = 103 to 623 K

Appearance: Hard to see since max elongation is only 28o Angular size never more than about 10” Always too close to Sun for Hubble to observe Best ground-based telescope photos show little detail Terminator

Mercury transit in 2006 Transits do not occur every year, but only ~14 times a century, due to the 7° inclination of Mercury's orbit to the Ecliptic.

Transits of Mercury during 21st century: 2003 May 07 2052 Nov 09 2006 Nov 08 2062 May 10 2016 May 09 2065 Nov 11 2019 Nov 11 2078 Nov 14 2032 Nov 13 2085 Nov 07 2039 Nov 07 2095 May 08 2049 May 07 2098 Nov 10 Occur only in May and November (when orbit planes of Earth and Mercury intersect).

Mercury's orbit and rotation Mercury’s rotation was a challenge to measure optically from Earth because no features could be seen. So how to measure? Even if angular resolution not high enough to resolve approaching and receding hemispheres, the returned signal will still have a width in frequency due to the outgoing signal bouncing off both hemispheres. Do Doppler shift and angular res. calc for Mercury’s spin. Demonstrates how you bring different physics concepts into solving astro problems. Can we resolve the planet? If not, how to interpret reflected sp. line. Can actually use radar delay to “resolve” it. Longest delay from limbs. Differences are microsecs but measurable. width depends on twice the rotation speed outgoing signal returned signal signal strength wavelength wavelength

Mercury's orbit and rotation Mercury has a tidal bulge due to Sun. IF Mercury had a circular or slightly elliptical orbit, it would be in synchronous rotation (a 1:1 spin-orbit period ratio). But because of high eccentricity, can’t be tidally locked over orbit. Tidal force so much stronger at perihelion, ended up being tidally locked there only. Requires spin period < orbital period because orbits faster near perihelion. Spin period = 2/3 or orbital period. Orbital period = 3/2 spin period exactly, so alternating ends of bulge always line up at perihelion. A “3:2” resonance orbit.

Spacecraft missions Previous mission: US Mariner 10 flew past Mercury 3 times in ’74 & ’75. Imaged 45% of surface only (sunlight direction)

Mariner 10 (1974 – 75): looks similar to far side of Moon: Mercury has craters, mountains, plains, long cliffs called scarps (indicating contraction of whole planet), and the enormous Caloris Basin (large, early impact). Mercury Far side of Moon

Current mission NASA spacecraft MESSENGER launched in Aug. 2004.

Mercury's surface Mariner 10 mosaic – 1km resolution

Mercury's surface MESSENGER has imaged entire surface, at 18-m resolution.

Mercury's surface compared to Moon No significant atmosphere (like the Moon) Heavily cratered (like the Moon) 3.8 - 4 Gyrs old (similar to lunar highlands) No plate tectonics, water or wind erosion (like the Moon) Surface well preserved (like the Moon) No large-scale maria (unlike the Moon) No large hemispheric differences (unlike the Moon) Surface rock rich in “volatiles” and poor in iron (unlike the Moon)

Typical region as seen by Messenger Smooth plains Intercrater plains “Smooth plains and “Intercrater plains”. Smooth plains probably old lava flows - cratering rate indicates age of 3.8 Gyr. Intercrater plain origin less certain. MESSENGER image

Structure of Mercury Denser than expected if just a smaller version of Earth (Earth’s slightly higher density partly due to compression by higher gravity). Hence, relatively large iron core (most iron-rich planet), probably molten. Has magnetic field about 1% of Earth’s. Solar wind would have stripped its low density mantle. Impact may have removed it too. May have had a lower density mantle that got chipped away by late collisions during formation stage. But that should have got rid of the volatiles, like on Moon.

Surface temperatures No significant atmosphere (please read), so no greenhouse effect => large day and night time temperature variations (103 – 623 K) Also, long days and nights increase temperature differences Very little tilt of rotation axis => poles in constant twilight Polar surface cold, 125 K tilt <1/30 degrees. Atmos due to trapped solar wind or bombardment of surface by wind. Water found in it by MESSENGER.

Mercury evolution Accumulation of solid material, not much light elements in the inner Solar Nebula. Differentiation, then cooling. Was less dense mantle chipped away by collisions? Heavy bombardment, ending close to Caloris event. Volcanism created plains, ending about 3.8 Gyr ago. Small, like the Moon, Mercury lost internal heat quickly. Contraction, creating scarps, started about 3.8 Gyr ago. Duration of this period seems unknown.

One unique feature: scarps One unique feature: scarps. These are long cliffs, up to 3 km high though to be caused by a cooling, contracting planet.

Caloris Basin – from large impact, about 3. 9 Gyr ago Caloris Basin – from large impact, about 3.9 Gyr ago. Created rings of mountain ranges (color heavily enhanced) Caloris basin is just off to right of full Messenger image earlier. Created “jumbled” terrain on opposite side (due to convergence of seismic waves).

Ice evidence from radar reflection 1991: Polar regions highly reflective 'Normal' ice usually absorbs radio waves, but at cold temperatures water ice is highly reflective Similar to what is seen on Mars and icy moons of Jupiter Map made with the VLA telescopes, 8.5 GHz. Mapping radar echo from Goldstone. Sources of ice may be bombardment, outgassing. Map made with the VLA.

MESSENGER evidence Ice in polar craters seen? Strong radar echo. Radar image

Standing on Mercury This odd rotation can make an observer on the surface of Mercury to observe the Sun to rise, stop, move backwards, stop and then set: ~ 4 days prior to perihelion, vorb = vrot The Sun's apparent motion ceases Then vorb > vrot : Sun appears retrograde 4 days after perihelion, the Sun's normal apparent motion resumes. Animation?

Precession of the perihelion Estimates including effects of other planets etc were still 43 arcseconds/century off! A planet, Vulcan, was searched for but never found. Answer: Einsteins general theory of relativity

Proto-Mercury collided head-on with a large body (although smaller than Mercury). Impact blew off most of the mantle: left-over planet is a huge iron core. Impacts are essential elements in the Solar System history!

No significant atmosphere No primordial atmosphere retained Thin atmosphere with pressure <10-12 that of Earth's Partly due to H and He captured from the Solar wind (retained by B-field) Also sodium, calcium, magnesium from Solar wind particles and meteoric impacts knocking atoms out of rocks.

Is the core liquid or solid? Large number of ancient craters indicate a cold, solid interior. However, a weak (1% of Earth's) global magnetic field detected: Liquid material Rotation of core Energy source (to keep it molten) Yet core though to be too small and slowly rotating to create field. Remains a puzzle.

Future mission ESA mission BepiColombo, to be launched 2017 Two orbiters, one to study planet, and one further out to study magnetosphere