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The Terrestrial Planets: Mercury• Viewed from the
Earth – it never strays far from the Sun.
• Maximumdeviation from Sun ~28° .
• Orbit: 0.4 AU.• Seen at sunrise or
sunset.Mercury has no ‘moons’
• Best images are at maximum elongation (~28°) = ‘Half- Mercury’. (Masks filter Sun’s glare)
• Angular Diameter at closest approach: 13 arc secs.
Viewing Mercury
Property Mercury MoonRadius 2440km 1738km
Mass 3.3x1023 kg= 0.055 ME
7.3x1022 kg = 0.0125 ME
Average Density 5430kg/m3 3340kg/m3
Gravity 0.38 Earth’s 0.167 Earth’s
Atmosphere None None
TemperatureNoon: ~700K /
427CNight: 100K
-173C
Noon: 400K / 127C
Night:100K / -173C
Why No Atmosphere?• Escape speed is only 4.2km/s.
This critical speed depends on mass and radius of the Planet
• This means hot, light (v > vescape) atoms and molecules can escape gravity’s pull.
Earth’s 11.2km/s , Moon’s 2.4km/s(No Helium or Hydrogen in the earth’s Atmosphere.)
rGMvEscape /2=
Consequences of No Atmosphere
• As the Moon and Mercury have no atmosphere, there is nothing to protect them from the harsh protection of interplanetary space.
• Surface is cratered. Scarred forever as there is no erosion
– except by other meteoritic impacts.(Crater is Greek for “bowl”.)
Mercury: From Messenger
Mercury: Close-up
Bach crater (and others) are
‘double ringed’: reason unknown
No ‘rivers’.Few Lava flows.
20,000km above Planet surface from
Mariner 10
Compare to the Lunar Surface
• 14 Maria – ‘seas’,extensive flat areas, and Highlands.
• Largest mare is 1100km diameter Mare Imbrium
• Maria are Solidified lava.
Moon’s AgeLack of craters in the
Maria indicate that most of the craters – formed
by meteor impact –occurred earlier in
Lunar history.
Lunar Rock Dating:Maria 3.2-3.9 billion years
Highlands: 4-4.4 billion years
The Rotation of the Moon
• Moon’s spin is “synchronised” with its orbital motion.
i.e :We only see ~one face of the Moon.• Due to gravitational interaction with the Earth –
“Tidal” effect.• Same effect is causing the Earth’s day to slowly
lengthen: 1.5millisecs per century!• But as Earth mass >> Moon Mass, the Moon’s spin
is already “gravitationally locked” to its orbital motion.
Incidently: Moon is Drifting Away!
• Laser Pulses bounced of the Moon show it is receding at a rate of ~3.8cm/year.
• Earth rapid spin causes a tidal bulge – oceans depth offset by ~10°.
• Gravity tugs Moon away and oceans-ground friction slows Earth’s rotation.
The Spin of Mercury
• By viewing surface features of Mercury one could determine its rotation rate.
• Once believed to be ‘synchronous’ with Suni.e.: rotates on its axis once every 88 earth days.
• In 1965 – Radio telescope studies – using the Doppler effect - found the rotation rate to be 58.6 days.
300m Radio Telescope in Puerto Rico
Reminder of Doppler Effect
• Radar pulse of a singlefrequency sent from Earth.
• Reflected pulse is Red or Blueshifted in frequency due to planet’s spin.
3:2 Spin-Orbit Resonance
• In 2 Mercury years, it rotates on its axis exactly 3 times! Its No Accident!
• Due to Mercury’s elliptical orbit – high eccentricity of 0.206.
• Gravity varies with distance: Aphelion: 0.47AU, Perihelion 0.31AU
• Tidal force is strongest at perihelion, where the orbital speed is fastest. Hence ‘locked’ to instantaneous rotation rate at perihelion.
2/1 r∝
Furthermore….
• Sun’s tidal pull also makes Mercury’s axis of rotation be exactly 90° to orbital plane.
• Due to 3:2 coupling, this means some locations on Mercury’s surface get much hotter than others.
• Equator: at Noon: (Hot enough to melt lead!)At perihelion to ~700K / 427At aphelion ~550K / 277C
• At the Poles could be much colder: Ice?
Lunar Craters
• Meteoroids are strewn throughout the solar system, orbiting the Sun until they hit something!
• Explosion ejects material around the crater: “ejecta blanket”: dust and debris.
• Impact eventually gives a crater diameter ~10x the meteoroid size, and 2x in depth.
• Note: Most craters not volcanic in origin.
Craters of Various Sizes
How Frequent are the ‘Hits’?
• Rate of cratering rapidly decreases with crater size . (Not much big stuff around!)
• At present, average rates are:~1 new 10km (dia) crater per 10 million years~1 new 1m crater every month~1 new cm-sized crater every few minutes.
• Now believe that the Moon (the whole solar system?) had a sudden drop in meteoritic hits ~3.9 Billion years ago, and is slowing declining.
Mega-craters to Micro-craters
Orientale Basin Concentric rings – outermost is 1000km diameter. Craters ~0.01mm across!
Earth - Moon Erosion Rates
• Barringer Crater, Arizona 1.2km Dia, 0.2 km deep, ~50,000 years old – meteor~50m diameter.
• Will disappear in ~1million years, due to erosion!• But would survive billions of years on the Moon.
Astronaut footprints will remain for millions of years.
Lunar Ice?
• No biological organisms on the Moon!• No water ‘locked’ in rock samples…• Possible Ice in polar craters
– shielded from sunlight.• November 1996 Clementine
reported radar echossuggesting ice a fewmeters deep.
Lunar South Pole.
Lunar Prospector’s Crash Landing!• Prospector’s instruments detect Hydrogen,
presumed to be in the form of water: H20.• Deliberately crashed into the bottom of a
polar crater, in the hope that water vapour would be released which would be spectroscopically detected by the Hubble Space Telescope….
• No signs of water vapour was found!(But mission only ~10% chance of success…)
Where did the water come from?(IF it is there!)
• Possibly via comets and meteoroidsBoiled off everywhere except the poles..
Mystery: More Questions than answers!
• Important for lunar space stations! Costs ~$2,000-$20,000 to move 1kg of water to the Moon!
• If on the Moon, why not elsewhere?
Evidence of Lunar Volcanism
Crater ‘Chain’thought to indicate a
old ‘fault’ line.
Similar features seen on Venus.
Moon’s volcanic activity ended
~ 3 billion years ago.
Further Evidence of Lunar Volcanism: ‘Rille’
Volcanic Rille :a ditch where molten
lava once flowed.
Mercury’s Craters
• Crater walls lower than Moon.• Debris not spread as far.
Due to Mercury's greater gravity.
• Most Geologists think there was volcanic activity in the past….
• But no rilles observed.…therefore, different history
from that of the Moon.
Some Craters Make Big Impressions!
…with a little peak in the middle
Dark ‘Halos’ Around Other Craters
…and craters with ‘double-rims’
Vivaldi craterLocal melting
on impact?
Mercury from
Messenger
Mercury’s “Scarps”
Wrinkled surface –looks like a
shrunken orange!Hence ‘scarps’ or
cliff, ridges…..formed about 4 billion years ago
..are now cratered.
Mercury’s Southern Hemisphere
Scarps
The Interior of the Moon• Average density is
3300kg/m3.
• Similar to surface rocks.
• Hence no large dense core.
• Seismic evidence of a small iron core -partly molten?
• Very weak magnetic field ~0.1% of Earth
The Far (“Dark”) Side of the Moon
• No large dark Maria.
• Moon crust is thicker on this side!
• Less volcanism.• Due to Earth’s
gravity distorting the Moon’s shape!
The Interior of the Mercury
• Average density ~ that of Earth.
• Large iron-rich core: 40% Vol or 60% Mass
• Solid or Liquid?
Surprise Find:Mercury has a weak magnetic field (1% of Earth), that interacts cosmic particles. Origin unknown.
The Origin of the Moon?
• Several theories proposed….A) Conformation Theory
Double system, orbiting each other, formed at the same time from a coalescing gas dust cloud.
Problem: Why so different in density and composition?
The Origin of the Moon II
B) Capture Theory
Moon formed far from the Earth and later captured it.
• Good: Explains different composition• Bad: Very difficult ‘capture’ event for
such a large mass.• Conclusion: Unlikely.
The Origin of the Moon III
C) Daughter or Fission Theory.
Moon originated out of the Earth itself, possibly from the Pacific Ocean basin.How could the proto-Earth be spinning so fast that such a large object was ejected?Would it have resulted in a stable lunar orbit?
• Conclusion: Unlikely
The Origin of the Moon IVD) Impact Theory
• Mars-sized object hit young Earth with a glancing blow – ejecting material.
• Earth’s core remained largely intact.• Moon’s composition similar to Earth’s mantle• Further coalescence resulted in Earth-Moon.• Conclusions:
Favoured Theory as Collisions likely.
The Formation of the Moon
The Formation of the Moon II
Successive images cover a larger volume of space.
Blue : Core
Red & Yellow: Mantle
Earth’s Core re-unitesin this simulation.
History of the Moon
• Formed ~4.6 Billion years ago.
• Oldest Highland rocks ~ 4.4 Billion years.
• Meteoric heating re-melts much of lowland surface.
• Volcanism 3.9-3.2 billion years (Maria formed)
• Moon has been dead for3 billion years.
Lunar Evolution
• Heavy meteoric bombardment
• Molten lava seas fill low lying basins
• Later impacts pit maria
History of the Mercury
• Formed ~4.6 billion years ago.• Possible collision could have ripped off its
mantle in its early formation – hence heavy core.
• Similar meteoric heating history as the Moon
• Cooled slowly with ‘shrinkage’• Thick solid mantle prevents volcanism
and tectonic activity.• Geologically dead for last ~4 billion years.
Last 2 points to be revised!
Recent Discoveries:Mercury’s
Tail!
Ions blown away from a tenuous
atmosphere by the solar wind.
CalorisBasin
….And signs of past and present volcanic activity.