Lecture 6: Mercury History (God of travel, commerce & thieves) • Oldest observation (in 235 B.C.), noted by Ptolemy. • Greeks at first observed two planets – “Apollo” and • History “Hermes” – the “morning” and “evening” stars. • Orbit • Schiaparelli recorded “markings” (1881-1889). • Structure • Lowell (1896) claimed “canal-like features”. • Surface • Atmosphere • Evolution

Mercury - vital statistics Orbit precession Orbital semi-major axis 0.387 AU Orbital period 87.969 days

Mercury’s orbit “precesses” by 574" per century. Rotational period 58.65 days (3:2 spin/orbit Newtonian mechanics only predicts 531". resonance) o Difference of 43" per century far above observational error. Eccentricity of orbit 0.206 at 7 tilt (both large) Why? Inclination of rotation axis 0.0º Diameter (at equator) 0.383 d (4880 km) • Another planet – Vulcan (Le Verrier – predicted Neptune) Earth Mass 0.055 M • General Relativity gives right value ⇒ Einstein a genius! Earth Mean density 5430 kg m -3 (0.98 Earth)

1 Seasons on Mercury Hard to see details from Earth

Orbit, axial inclination and lack of a dense atmosphere result in a uniquely large range in surface temperature: Best possible spatial resolution from Earth ~ 100 km. Day : T equator ~ 700 K (825 K at perihelion) In practice, typical images have a resolution ~ 200 km Night : T equator ~ 100 K (so no “canals” could have been seen…)

Mariner 10 Interior of Mercury • Observed density of Mercury (5430 kg m -3) is 98% of Until 2011 most of our best data for Earth. But, we must allow for gravitational compression. Mercury came from Mariner 10, which imaged 45% of the surface. • Earth’s uncompressed density is 4400 kg m -3 compared to Mercury’s 5300 kg m -3. Mercury is the densest planet. Launched: 3/11/73 • Thus, Mercury must have a very large iron core and a Arrival(s): 20/3/74 (703 km) silicate outer shell. 21/9/74 (48069 km) 16/3/75 (327 km) First mission to use gravity assist – also flew by Venus (5/2/74). Now we have Messenger. (c.f. Earth’s core 1/6 by volume)

2 Density of Mercury The surface of Mercury 3 kg/m 3 10

Mercury lies far from the trend of other inner worlds. Heavily cratered with some “plains/basins”

Craters (False) Colour imaging from Messenger

Multiple impacts Volcanic vent

Degas “ray crater” Large “double-ring” • Scarcity of impact craters > 50 km size implies soft crust during the period of most massive bombardment. • Large craters subsided through viscous relaxation. • Ejecta closer to crater and lower crater walls cf. Moon.

3 The shrinking planet – lobate scarps Discovery Rupes Messenger 550 km long, 1.5 km high

See many examples of scarps and ridges (rupes): thought mainly due to planet shrinkage as Mercury cooled. The radius may have decreased by 1-2 km.

Caloris Basin Caloris Basin: Mariner 10 & Messenger

Huge impact structure with outer rim diameter ~1550 km. Ejecta reach as far again. Created three concentric mountain ranges. Surrounding smooth plains may be 2.5 km lower.

4 Interior of Caloris Basin “Weird” terrain

Messenger

Directly opposite the Caloris basin (the antipodial point) is Young (smoother) surface ridges/fractures suggest lava a jumbled, hilly region. flooding followed by contraction and settling. Mercury’s large core acts like a lens to focus the seismic Messenger data suggest many smooth plains – more waves, generating huge forces which fractured the surface. than on the Moon – so more volcanic/tectonic activity. Similar basin/hill pairings exist on Moon and Mars.

Present-day “Atmosphere” of Mercury The Atmosphere of Mercury Any initial atmosphere was lost due to thermal escape and O 44000 (n cm -3) o was not replenished due to lack of recent tectonic activity. Na 30000 Thermal escape: He 6000 Balance between planetary escape speed and gas speed: K 500 v(escape) = 11.2 M (in Earth masses) km s -1 H 23000 √ R (in Earth radii) Mercury has a very tenuous atmosphere – column mass For gas, mean speed of Maxwellian speed distribution: of 10 -10 kg m -3 (about 10 -14 Earth atmospheres!). v(ms) = 0.157 T (Gas temp in K) km s -1 Composition very different as it is due to solar wind, √ m (molecular mass in H units) ejection from surface (and volatile capture from comets?).

5 Maxwellian speed distribution If v(ms) < v(escape)/6 then the total fractional escape of that gas will be small over the age of Solar System.

Only 1 molecule in 2x10 6 has speed >3 x mean Only 1 molecule in 10 16 has speed >5 x mean

Water Ice on Mercury? The present and the future Strong reflection of radio waves suggest water ice near both poles covered with a thin dust layer. How can ice survive on the hottest planet? Messenger BepiColombo Nasa mission – 1 orbiter Esa/Japan mission – 2 Mercury has a zero axial inclination to its orbit and no Launched: 3 rd Aug. 2004 orbiters (MPO, MMO) atmosphere. Thus, can stay very cold (<100-150 K) at Arrival: in orbit March Launch: 2015 the poles, particularly if in shadow inside a crater rim. 2011 after flyby of Venus Arrives: 2022 Messenger+Arecibo data (red=shadow, yellow=radio). and of Mercury (3 times) (Leicester X-ray instrument)

6 BepiColombo MIXS + SIXS Will measures X-ray emission from the surface of Mercury: – X-ray Flourescence induced by solar coronal X-rays – Bremsstrahlung (and fluorescence) induced by electron and charged particle impacts on the surface The End – Sensitivity range ~ 0.5 – 7 keV

7