Chapter 8: The Moon and Mercury

• General Characteristics

• Surface Features

• Interior Structure

• Formation Theories

The Moon

Physical Properties of the Moon

What was known about the Moon before

the space program?• Relative size of Moon

– Aristarchus (3rd century B.C.) • study of lunar eclipses• Moon’s diameter ~ 1/4 Earth’s diameter

– Ptolemy• measured parallax: dmoon = 0.273 d = 3476 km

• Angular diameter of Moon ~0.50 • Distance to Moon

– angular diameter/3600 = diameter/distance to moon– Earth-Moon distance = 384,400 km– most accurate method: laser ranging off mirrors left on

Moon’s surface during Apollo missions

More about the Moon

•Mass of Moon– ~ 1/80 Earth’s mass

•Average density of Moon = 3.34 gm/cm3 •Surface gravity of Moon: gmoon = 1/6 g •Escape velocity = 2.4 km/sec•Average surface temperature –day 375K (+2160F) –night 125 K (-2340F) – lack of atmosphere -- extreme variation in surface T

Moon’s Orbit• The Moon orbits the Earth in an elliptical orbit, that

is almost circular, e = 0.05• Semi-major axis = 384,400 km

perigee = 363,300 km apogee = 405,500 km

Moon - Orbital Properties

• Synodic orbital period = 29.5 days (full phase to full phase)

• Sidereal orbital period = 27.3 days = sidereal rotation period

• Same side of Moon always faces Earth

Differential Forces

• Differential gravitational force results in tidal bulges.

• Tidal force effect on Moon: ~20 x greater than that on Earth.

Moon: Orbit and Tidal Forces

Tidal Forces and

Synchronization

•No accident that rotational period of Moon and orbital period of Earth-Moon system are of same length.

•Tidal coupling of the Earth and the Moon has led to this synchronization.

•Earth-Moon system synchronization not yet complete. – Earth slowly decreasing its rotational period as Moon

moves further from Earth (to conserve angular momentum for entire system)

– Eventually, Earth and Moon will have exact same rotational period which will equal orbital period of Moon about Earth.

Angular Momentum

• Objects executing motion around a point possess a quantity called angular momentum.

• Angular momentum is rigorously conserved in our Universe.

• Angular momentum is L = mvr, where • L = angular momentum, • m = mass of small object, • v = speed, and • r = separation between the objects.

Question: Motions of the Moon

• What does it mean to say that the Moon is in a synchronous orbit around the Earth?

• How did the Moon come to be in such an orbit?

• An album by Pink Floyd is titled “Dark Side of the Moon.” Is one hemisphere of the Moon continuously in darkness? Explain.

Lunar Atmosphere

• The Moon has no atmosphere. • The combination of low surface gravity and

relatively high temperature causes atmospheric gases to escape into interplanetary space.

• All gases are moving at speeds greater than escape velocity, so they eventually leave the Moon.

• Generally depleted in volatiles, including water.

Lunar Hydrosphere

• The Moon is generally depleted in volatiles, including water, but it has been suggested that some frozen water might exist at the bottoms of permanently shaded craters near the Moon's poles.

• This water would have been the result of impacts of comets long ago.

• The Lunar Prospector space mission has now confirmed the existence of water ice in the polar regions of the Moon.

Water in Moon’s Polar Regions

The Lunar Prospector space mission results strongly suggest the existence of water ice in the polar regions of the Moon.

The Visible Surface of the Moon• Visible features permanent, implying a solid surface.• Dark areas looked like water to Galileo

who named them mare or seas.• Reflectivity of surface - albedo

–surface texture• smooth - reflects almost all light• rough - reflects in many directions

–composition• different materials reflect

different colors

• Craters–meteorite impact–volcanic

View of the Lunar Surface

far side near side

The Moon’s Lithosphere

• SURFACE FEATURES

– Maria (“seas") or Lowlands - 15% • Dark, flat lava plains.

• Roughly comparable to Earth's ocean basins.

• Rilles appear to be collapsed lava tubes.

• Relatively few craters.

• Relatively young surface (3.5 billion years old).

– Terrae (“land") of Highlands - 85% • Light, mountainous regions.

• Roughly comparable to Earth's continents.

• Heavily cratered.

• Relatively old surface (> 4 billion years old).

Highlands and Maria

• Maria

– dark colored

– less cratered than highlands

– ~15% of lunar surface

– mostly on near side

• Highlands

– light colored

– heavily cratered

– ~85% of lunar surface

Surface Features: Volcanic

•Volcanic domes– not circular– formed by high viscosity lavas

•Sinuous rilles– collapsed lava channels

•Maria– lava in-fill of giant impact craters– no observable domes, flows from long

fissures

Volcanic Rille

Photograph on left shows Hadley rille meandering through the Hadley-Apennine area.One of the Apollo landings was close enough to Hadley rille, to allow the astronauts to explore it.

Lunar Mare• Lunar "seas." This is a broad vista encompassing portions of three maria.– Mare Crisium

(foreground) – Mare Tranquilitatis

(beyond Mare Crisium)– Mare Serenitatis

(on horizon, upper right)

•These relatively smooth areas are younger than most of the lunar surface, having been formed by lava flows after much of the cratering had already occurred.

(NASA)

Surface Features: Impact Craters•Almost all lunar cratering has been caused by impacts.

•By studying overlapping craters, relative ages of events can be established.

•Crater densities are used to estimate the ages of planetary surfaces throughout the Solar system.

•The cratering record shows that there was a time of intense bombardments and cratering in the early years of the Solar System.

Moon’s surface, as seen from Apollo 8

Recent Meteorite

Impacts on the Moon

Leonid Meteorite Impacts, 2001At least 6 Leonids hitting the Moon in 1999 caused

explosions bright enough to see from Earth.http://science.nasa.gov/headlines/y2001/ast30nov_1.htm?list52322

Surface Features: Impact Craters

•Characteristic features–generally circular shape–surrounded by ejecta blanket

• rays of light colored material–symmetric impact

–asymmetric oblique impact

–secondary craters• formed from excavated material

–central peak• rebound of compressed surface

after impact

–terraced walls

Meteor impact crater on lunar far side

Crater Ejecta: Rays

Moon

This crater on the lunar far side is a good example of a case in which material

ejected by the impact has created rays of light-colored ejecta. (NASA)

Mercury

Impact Basins

• Impact basins are largest examples of craters.

• Caused by huge impacts on the Moon.

• Typical basin features are:

– ringed mountain ranges

– lava flooded interiors

– sizes about 1,000 miles across

• Prime examples: Orientale, Imbrium

Orientale Basin• Image provides an overview of

Orientale Basin. Unlike most other basins on the Moon, Orientale is relatively unflooded by mare basalts, exposing much of the basin structure to view.

• As a result, study of the Orientale Basin is important to our overall understanding of the geology of large impact basins.

• There are three prominent basin rings in this image. From the inside out, they are – the Inner Rook Mountains, – the Outer Rook Mountains, and – the Cordillera Mountains.

• The Cordillera Mountains are regarded as the rim of the basin, defining the basin's 930-kilometer diameter.

(Lunar Orbiter image IV- 187M.)

Imbrium Basin• This image provides an overview of

the Mare Imbrium region, which occupies the upper left portion of the image. Part of Mare Serenitatis is visible in the upper right.

• Imbrium and Serenitatis are separated by the Apennine Mountains, part of the main basin ring of the Imbrium Basin.

• On the northeast side of Imbrium are the Alpes Mountains, which are another part of the main Imbrium Basin ring.

• The Alpine Valley cuts through the Alpes Mountains near the 1 o'clock position around the Imbrium Basin.

• Copernicus Crater is prominent in the central portion of the image, just below Mare Imbrium.

(Lunar Orbiter image IV-121M.)

Impact Basin: South Lunar Pole

This view of the south polar region of the Moon, obtained by the Clementine spacecraft, reveals a large, previously unknown impact basin near the pole, at lower right in this view. (NASA)

Meteorite Speed at Impact •In text, average impact speed = 10 km/sec.•Average rifle bullet speed = 1 km/sec, max speed of car on freeway < 0.03 km/sec.

•Earth/Moon orbit Sun at 30 km/sec.•If Earth/Moon has head-on collision with an object moving at 20 km/sec, relative speed on impact is 50 km/sec.

•Energy released speed2; at 50 km/sec, 25 times the energy released as 10 km/sec impact.

•1-kg meteoroid at 50 km/sec = 250 kg of TNT

Crater Formation and Ejecta

Lunar craters: diameter ~ 10 x diameter of incoming meteorite

depth ~ 2 x diameter of incoming meteorite

Similar pattern for formation on Moon and Mercury.

Crater Counts and Dating of Surface• Possible to use # of impact craters counted on surface to estimate the age of the surface, IF planet has little erosion or internal activity.

• Assumes rate of impacts ~ constant for several billion years.

• Then # of craters proportional to the length of time the surface has been exposed.

• From Earth and Moon data, impact rate has been almost constant for > 3 billion years and much higher prior to 3.8 billion years ago.

Geology: Earth vs. Moon

EARTH MOON

Continents

29%

Highlands

85%

Ocean basins

71%

Lowlands

15%

Very few craters visible

Many craters visible

Very active geology

Very inactive geology

Plate tectonics No plate tectonics

Unmanned Space Missions• Soviets made first attempts to photograph, land, and return samples from the Moon.

• U.S. unmanned program in phases– missions (1966-1968)

• soft-land craft with experiments to analyze surface

– Lunar Ranger series (1961-1965)• photograph and crash

– Lunar Orbiter series (1966-1967)• orbit and image

– Surveyor Prospector (1998)• map surface, structure,

search for water ice near poles View from Clementine

Manned Space Missions

• Apollo program (1961-1972)– U.S. manned program– Apollo 11 (July 20, 1969) landed first human

on Moon in Mare Tranquilitatis.– Astronauts in program

• performed geological and scientific experiments samples of on surface

• collected surface rocks/materials (843 lb.) that were returned to Earth for study

• left nuclear-powered scientific instruments to –monitor solar wind–measure heat flow from interior–record lunar seismic activity

Man on the Moon

The Apollo missions, six of which included successful manned landings on the Moon, are humankind's only attempt so far to visit another world. (NASA)

Lunar Surface

A large boulder. Rocks on the lunar surface range in size from tiny pebbles to massive objects like this. (NASA)

Apollo 17 Lunar Seismic

Stations• Purpose: to acquire data on physical properties of

lunar near-surface materials. • Specific objectives included

– measuring the lunar seismic signals produced by detonation of explosive charges on surface,

– monitoring natural seismic activity resulting from moonquakes or meteorite impacts,

– recording the seismic signals resulting from the ascent of the spent LM ascent stage.

• This experiment yielded detailed information on lunar geologic characteristics to depths of 3 km.

Samples of the Lunar Crust• The general types of samples brought back

from the Moon are: – REGOLITH (SOIL) SAMPLES

• Composed of broken rock fragments. • No organic material. • No water. • Regolith is about 10 meters thick. • Pulverized rocks from meteorite impacts and solar

wind particle collisions.

– ROCK SAMPLES • Mare basalts that are relatively young and

composed of heavier elements. • Highland anorthosites that are relatively old and

composed of lighter elements. • Impact breccias that are conglomerates from

rock fragments that have been welded together.

Erosion and the Lunar Regolith

•Lunar regolith or dust

–Layer of pulverized ejecta (tiny, shattered rock fragments) from meteoriod

collisions with lunar surface.

–Covers the lunar surface to average depth of 20 meters

• ~ 10 m over maria

• > 100 m over highlands

–Consistency of talcum powder or ready-mix dry mortar

–Contains NO organic matter

Lunar Surface Rock Types• Chemical analysis of lunar samples shows 3 main types:

– basalts• igneous rocks formed by cooling of molten material

– breccias• formed from fusing of rock fragments, often occurs due to

impacts by external bodies increasing P, T in region

– KREEP• basalt that has unusually high concentrations of

K - potassiumREE - rare earth elements

P - phosphorous

• Most samples completely devoid of water and volatiles

• Oxygen isotope abundance similar to Earth’s.

The Age of Lunar Rocks• Radioactive elements spontaneously emit nuclear

particles and change from one element to another. – Too many protons are packed close together,

so the nucleus is unstable. – The parent nucleus decays into the lighter daughter

nucleus/nuclei plus nuclear particles.

• Radioactive decay cause heating of planetary interiors. • Can also be used to date from last time rock was molten. • The half-life is the length of time it takes 1/2 of the

parent nucleus to decay into the daughter nucleus/nuclei. • The lunar samples range in ages from

– 3.1 - 3.8 billion years old for the mare basalts to – 4.0 - 4.3 billion years old for the highland anorthosites.

Lunar Surface Composition and AgeMaria :

composed of dark basalts, formed from rapid cooling of molten rock in massive lava flows.

Highlands:composed of Anorthosite, igneous rock formed when lava cools more slowly than for basalt formation.

Implies that rocks of Maria and Highlands cooled at different rates from the molten state and were formed

under different conditions.

Maria rock samplesApollo 11, 12 ~ 3.5 billion yrs old Apollo 14 ~3.9 billion yrs old

Oldest material from Moon’s surface is almost as old as assumed age of Solar System and ~ 1 billion years older than oldest Earth rocks.

Highlands rock samples Apollo 16 ~4.0 billion yrs old Apollo 17 ~4.5 billion yrs old

oldest known lunar rock

Lunar and Terrestrial Rocks Compared

• All lunar rock are igneous or metamorphic. • Lunar rocks are roughly similar to terrestrial rocks. • Lunar rocks generally contain a higher percentage of

heavier minerals and refractory elements. – Depleted in volatiles.

• Lunar rocks contain no free or chemically-bound water and very few organic compounds.

• Lunar rocks have a generally low bulk iron content. • Lunar rocks are somewhat similar to

Earth's mantle rocks.

Questions: Lunar Lithosphere

• Describe three ways in which the lunar maria differ from the highlands.

• What is the primary source of erosion on the surface of the Moon? How does the erosion rate on the Moon compare to that on Earth?

• Name two pieces of evidence indicating that the lunar highlands are older than the maria?

• Name the two types of rock found on the lunar surface. How do lunar rocks compare to terrestrial rocks?

The Moon’s Interior• Interior structure

–crust ~100 km thick,

–mantle ~700 km thick,

–core ~300 km in radius

• Seismic data suggests outer core may be

molten. • Some differentiation

apparent.

• No magnetic field observed, but magnetization of lunar rocks suggests possibility of one in past.

• Most lunar seismic activity appears to be triggered by tidal forces induced by the Earth.

History of Interior

Exploration

• NASA's Apollo missions noted moonquake waves lost energy if they went deeper than 1,000 km (620 miles) or over halfway into the center of the Moon. – This could indicate that the Moon's depths

are at least partially melted. • After the Apollo measurements of moonquakes ended in 1977,

two decades passed without new measurements of the deep lunar interior.

• Researchers now looking at data gathered by the Lunar Laser Ranging Experiment, using retro-reflectors left on the Moon's surface 30 years ago by U.S. and Russian missions.

Lunar Laser Ranging Experiment• A laser pulse is fired from Earth to the Moon, bounced

by a reflector and returned back to Earth. • The round-trip travel time gives distance between the

two bodies with accuracy better than 2 cm (0.8 inches). • Unlike the other scientific experiments left on the

Moon, reflectors require no power and are still functioning perfectly after 30 years.

• Scientists who analyze the data from the Lunar Laser Ranging Experiment have measured, among other things, – that the Moon is moving away from Earth – that the shape of Earth is changing and – used the experiment to test the validity of several predictions

of Einstein's Theory of Relativity.

McDonald Laser Ranging

StationA dedicated laser ranging

station capable of measuring round trip light travel times to a

constellation of artificial earth satellites and lunar

retro-reflectors to a precision of about 1 cm

and time of laser firing to ~ 35 picoseconds.

“Moon's Heart Melted, Say Lunar Love Numbers”

February 13, 2002 http://www.jpl.nasa.gov/releases/2002/release_2002_37.html

• Love numbers– measures of how much a planet's surface and

interior move in response to the gravitational pull of nearby bodies.

• New calculations of the lunar Love number may indicate that the Moon has something like a molten slush surrounding its core. – The idea was first suggested by Apollo program scientists.

Measuring the Magnitude of Tidal Distortions

• The lunar Love number tells how Moon’s gravity field changes due to tidal pull of Sun and Earth.

• The Moon's Love number is 0.0266.– Moon's surface, pulled by the Sun and Earth, may bulge

out and dip in as much as 10 cm (~4 inches) over 27 days. – Earth's is 0.3, showing that our planet's bigger, rocky

surface may move as much as a half a meter (~ 20 inches) in a day in response to the pull of Moon and Sun.

– Venus' surface, with a Love number of 0.3, may move as much as 0.4 meter (~1 foot) from the pull of the Sun.

• The Moon's Love number is tiny compared to Earth's, and it takes huge planetary bodies to stretch and squeeze the rocky Moon.

Interiors: Earth vs. Moon

• Moon is smaller in size than Earth, but similar in structure.• Moon’s crust is much thicker than Earth’s (2 x Earth’s).• Moon’s mantle is relatively thicker (80% of radius) than

Earth’s (45% of radius) and probably warm and plastic.• Heat flow from the interior is 1/3 that of Earth.

Lunar Magnetosphere

• No large, general magnetic field has been detected around the Moon. – This is supports the conclusion that the Moon

does not have a liquid core.

• However, the Lunar Prospector spacecraft has discovered the presence of local magnetic fields that create the two smallest magnetospheres in the Solar System.

Lunar Biosphere

• Because of the lack of an atmosphere and hydrosphere (liquid water), it is thought that no biosphere exists.

Spheres: Earth vs. Moon

REALM

EARTH MOON

AtmosphereVery Active

None

HydrosphereVery Active

Very inactive

MagnetosphereVery Active

None

LithosphereVery Active

Very inactive

BiosphereVery Active

None

Lunar Origins•No definitive theory, but theory must predict

–Moon’s mass relative to Earth

–chemical composition of Earth and Moon

• Moon’s depletion of volatile elements and iron

• equality of oxygen isotopes between Earth and Moon

–angular momentum of Earth-Moon system

–overall melting of lunar surface

–physical plausibility

Formation Hypotheses• Fission hypothesis: Moon spun off of rapidly spinning Earth.

–Earth's rotation rate was not fast enough.

–Moon's rocks are different than Earth's mantle rocks.

• Capture hypothesis: Moon gravitationally captured.

–Low probability of such an event.

–Some similarities between Earth and Moon rocks.

• Accretion hypothesis: Moon/Earth formed at same time, place.

–Differences between Earth and Moon rocks.

–Moon does not orbit in the plane of the Earth's equator.

• Giant impact theory: Moon formed from debris of huge impact.

–Explain both differences and similarities of Moon/Earth rocks.

–Circumstantial evidence of other impacts in Solar System.

Impact Theory Simulation

•Earth suffered major impact during earliest stages while still molten and forming a crust.

•Surfaces of both objects vaporized, jets of material from Earth re-form in Earth orbit, coalescing into the Moon.

Lunar History

•Apparently formed ~ 4.6 billion years ago, with planets.•During next few 100 million years, surface melted, fused to form breccias seen in highlands – Meteoritic bombardment probably frequent enough to heat and

re-melt most surface layers of Moon during first half billion years.– Internal radioactive decay produces heat, melts interior, but not entire planet;

possible source of molten surface material.•After some cooling, crust forms, continued meteorite bombardment, large size impacts made deep cracks in crust.

•Between 3.9 and 3.2 billion years ago, lunar volcanism filled mare. •Last 3 billion years, Moon cool, quiescent, and geologically dead.

Lunar Geologic History

EVENT

BILLIONS OF YEARS AGO

PROCESS

Formation 4.6 Accretion

Melting of Crust 4.4 Impacts

Highlands form 4.1Solid crust and

impacts

Impact Basins form

3.9 Large impacts

Maria form 3.5Heating of

interior

Geologic Activity Ends

3.5Occasional

impacts

Map of the Moon

Earth vs.

Moon

• Earth– internal heat, motion

– moving crustal plates

– atmosphere

– oceans

– known life

• Moon– little interior heat

– no crustal motion

– no atmosphere

– no oceans

– lifeless

Mercury

Mercury•Smallest terrestrial planet

– radius = 0.38 x r

•Closet planet to Sun– semi-major axis = 0.39 AU

•Similar to Moon– small mass

(0.055 x mass )

– no permanent atmosphere

– extreme temperature variations: 700K - 100K

– heavily cratered, ancient surface, covered with boulders and dust

– geologically dead

Phases of Mercury

• Similar to lunar phases, but cannot view full cycle from Earth.

• Angular distance between Sun and Mercury never > 280.

• Best viewing (without filters) just before dawn of after sunset.

Favorable and unfavorable orientations of Mercury's orbit result from different Earth

orientations and observer locations.

At the most unfavorable orientations, Mercury is close to both the Sun and the horizon.

Observation of Mercury from Earth

Mercury Time-Lapse

Mercury’s Visibility from Earth

1. ELONGATIONS: Away from the Sun (28o maximum) – Eastern Elongation - Visible in the evening at sunset.

– Western Elongation - Visible in the morning at sunrise.

2. CONJUNCTIONS: Alignments with the Sun. – Superior - Located on the far side of the Sun.

– Inferior - Located between Earth and Sun.

3. TRANSITS: When Mercury crosses disk of the Sun.– Must occur at inferior conjunction.

– Must occur in either May or November.

Earth-Sun line

Inferior Planet Configurations

Inferior conjunction

Maximum elongation

Superior conjunction

Measurement of Mercury’s Rotation

• As Mercury rotates, radiation reflected from the side of the planet moving toward us returns at a slightly higher frequency (bluer) than the radiation reflected from the receding side (redder).

• Doppler Effect - very similar to rotational line broadening, but in this case, light is not emitted by the planet but reflected from its surface.

Mercury’s Long Day

• Rotation period= 59 Earth days

• Orbital period = 88 Earth days

• 3 rotations about own axis for every 2 revolutions about Sun.

• 3:2 spin-orbit resonance

1 Mercury solar day = 2 Mercury years

View from Mercury•Sun appears 2.5 x larger

than on Earth.•Sky appears black.

•Seasonal variation with longitude

– spin-orbit resonance results in regions near 0o , 180o longitude receive 2.5 x overall radiation from Sun as those near 90o, 270o.

•Observe planetary wanderers– Earth blue ;Venus beige

ViewFrom

Space

• In 1974 , Mariner 10 arrived near Mercury and sent back images of 45% of the surface.– Photographed features as small as 150 m across.– No great volcanoes, but

rimless pits that may be volcanic vents.– Cliffs several km high and often 100s km long.

• Radar images in 1991 revealed a possible ice cap at Mercury’s north pole.

Mariner 10 and Mercury

•Launched November 3, 1973.•Completed 3 fly-by passes from 1974-1975,

returned >4000 photographs, covering 45% of Mercury’s surface.

•First spacecraft to transmit high resolution digital color images.

Mercury’s Atmosphere

• A few helium, hydrogen, sodium, and potassium atoms have been detected in Mercury's vicinity, giving it a very thin atmosphere.

• Probably does not retain its atmosphere intact.

– Instead, atmosphere constantly being replaced by interaction of solar wind with its surface rocks.

• Density of the "atmosphere" is ~10-12 x Earth's.

Mercury’s Hydrosphere• Most all volatiles, including water,

have evaporated and left the planet. • Mercury is the most volatile depleted planet

in the Solar System.• No hydrosphere is expected to exist.

• Mercury has the highest refractory element concentration in the Solar System.

• It is possible that, like the Moon, Mercury could have some ices at the bottom of polar craters that

are continuously shaded from the Sun.

Questions: Mercury• Why is Mercury never seen overhead at

midnight when viewed from Earth?• What does it mean to say that Mercury has a

3:2 spin-orbit resonance? – Why isn’t Mercury in a 1:1 spin-orbit resonance?

• In contrast to the Earth, Mercury and the Moon undergo extremes in temperature. Why?

• Why do the Moon and Mercury have no significant atmospheres, unlike Earth?

Mercury’s Lithosphere: Surface Features• HIGHLANDS

– Similar to Moon's. – Older cratered terrain. – Possibly some volcanic craters. – Craters have some differences from lunar craters.

• LOWLANDS – Smooth plains similar to lunar maria. – Scarps (cliffs) perhaps formed as planet

cooled and shrank. • IMPACT BASINS

– Caloris Basin (1,300 km across).– Ringed mountain ranges (1.5 km high). – Central lava flooded basin. – Jumbled terrain on the opposite side of the planet. – Formed by huge impact (150 km asteroid).

Mercury’s Surface Features• Meteorite Craters

– similar to Moon’s, but less densely packed

– crater walls not as high as on Moon

– ejecta closer to impact site

• Intercrater Plains– light colored

– probably volcanic, large scale flows, no rilles

– composition unknown

– scarps cut craters / plains

Craters on Mercury

Like the Moon, Mercury has a heavily cratered surface. Because Mercury has a greater surface gravity than the Moon, however,impact craters have lower rims and are shallower, and ejecta do not travel as far. (NASA/JPL)

Crater Formation on Mercury

• Top: A Mariner 10 image showing a cratered region on the surface of Mercury. (NASA)

• Bottom: This drawing illustrates the contrasts between craters on Mercury and those on the Moon: on Mercury, the crater walls are lower and the ejecta do not travel as far due to Mercury's higher surface gravity.

Mercury’s Unusual Surface Faults, Scarps

Scarps: cliffs that cut across surface; formed after cratering events; do NOT seem to be of volcanic or plate tectonic origin;

probably formed as interior cooled and shrank Age: ~ 4 billion years Mariner 10 images (NASA)

Caloris Planitia

This photo shows half of the immense impact

basin known as Caloris Planitia.

This region is directly facing the Sun at

perihelion on every other orbit.

(NASA/JPL)

Seismic Effects on Mercury’s Terrain• Caloris Basin

diameter = 1300 km

• On opposite side of Mercury from Caloris Basin is a region of oddly rippled and wavy surface features, “weird terrain.”

• Scientists believe terrain produced when seismic waves from Caloris impact traveled around planet and converged on diametrically opposite point, causing large-scale disruption of surface.

Mercury’s Weird Terrain

Questions: Mercury’s Surface

• The surface of Mercury is often compared with that of the Moon. List two similarities and two differences between the surfaces of Mercury and the Moon.

• Compare and contrast impact craters on the Moon and Mercury.

• How do scarps on Mercury differ from geologic faults on Earth?

Mercury’s Interior Structure • Radius = 2439 km • Ave. density = 5.4 g/cm3 • Metallic iron-nickel core is

believed to make up about 75% of this distance (~1800 km).

• Measurements of magnetic field (1/100 magnetic field) indicate– hot, fluid interior w/slow rotation or – solid core w/ frozen remnant field

• Overlying the core is a mantle of lighter silicate rocks. – solid, rocky mantle similar to Moon’s

mantle (~500 km thick).

• Mantle topped with a thin crust (~100 km thick).

Interiors: Earth and Mercury

• Mercury is smaller in size than Earth, but similar in structure and in average density.

• Mercury’s core is much thicker than Earth’s, proportionately.• Mercury’s mantle is relatively thinner than Earth’s or Moon’s.

Mercury’s Magnetosphere

•Mercury has a weak magnetic field.

0.01 x Earth's • May be caused by motions in a partially liquid metallic core.

• However,

– Mercury's rotation rate is very slow, and the planet may not even have enough mass to retain a molten core.

– Lack of recent surface geologic activity suggests outer layers solid to considerable depth.

• Possible that Mercury's magnetic field is a remnant field, frozen into a solid metallic core.

Mercury’s Biosphere

• Because of the lack of an

atmosphere and

hydrosphere, no biosphere is

expected.

Spheres: Earth, Moon, Mercury

REALM

EARTH MOON MERCURY

Atmosphere

Very Active

None Very thin

Hydrosphere

Very Active

Very inactive None

Magnetosphere

Very Active

None Very weak

Lithosphere

Very Active

Very inactive Very inactive

Biosphere

Very Active

None None

Mercury’s Geologic History• Condensation and accretion from

solar nebula 4.6 billion years ago. • Completely molten from numerous

impacts and gravitational collapse. • Differentiation form iron core,

less dense mantle, and low density crust.– Cooled more slowly than Moon, leading to thinner crust

and increased early volcanic activity.

• Crust cools and contraction may form scarps.– May have prematurely terminated volcanic activity by

squeezing shut cracks and fissures on surface. • Heavy meteoroid bombardment forms most craters,

3.9 billion years ago. • Formation of Caloris Basin. • Lava plains form 3.8 billion years ago. • Present inactivity.

Overview of Mercury• Difficult to observe from Earth:

– Planet nearest to Sun.– Maximum elongation of 280.– Small angular size.

• 40% Earth’s size and 5% Earth’s mass.• Eccentric orbit, tilted to the ecliptic.

– 2nd most eccentric and tilted in solar system.• Radar reflected from surface shows that Mercury

has a 3:2 spin-orbit resonance with the Sun.• No natural satellites.• Magnetic field 1/100x Earth’s magnetic field

– shields planet from solar wind– if caused by dynamo effect, must have large metallic core

Overview of Mercury• Surface features:

– Numerous craters, similar to Moon.– Inter-crater plains, probably volcanic.– Scarps, steep cliffs perhaps caused by stresses

in crust as interior cooled.– Large multi-ringed basin, Caloris Planitia,

with weird terrain on opposite side of planet.– Lack of mountain ranges similar to those on the Moon.– Possible polar ice cap .

• Largest difference in average surface temperature for any planet: 700K to 100K.

• Low mass and high temperature preclude maintenance of substantial atmosphere.– atmosphere from Sun and gasses emitted from planet

surface.

The Moon and MercuryMoon Mercury

Distance from Sun 1 A.U. 0.39 A.U.

Surface gravity 0.17 x Earth 0.38 x Earth

Radius 0.27 x Earth 0.38 x Earth

Mass 0.012 x Earth 0.055 x Earth

Density0.6 x Earth

(3.34 g/cm3)

0.98 x Earth

(5.43 g/cm3)

Mean surface temp 100-400 K 100-700 K

Magnetic field None detected None detected