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.