Chapter 8:
The Moon and Mercury
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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
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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%
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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%
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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, 2001
At 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
Mercury
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)
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
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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%
Ocean basins
71%
Highlands
85%
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
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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 - potassium
REE - 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
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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 Age
Maria :
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 samples
Apollo 11, 12 ~ 3.5 billion yrs old
Apollo 14
~3.9 billion yrs old
Highlands rock samples
Apollo 16 ~4.0 billion yrs old
Apollo 17 ~4.5 billion yrs old
oldest known lunar rock
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.
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
Station
A 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
Atmosphere
Very Active
None
Hydrosphere
Very Active
Very inactive
Magnetosphere
Very Active
None
Lithosphere
Very Active
Very inactive
Biosphere
Very 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
4.1
Solid crust and
impacts
Impact Basins
form
3.9
Large impacts
Maria form
3.5
Geologic Activity
Ends
3.5
Heating of
interior
Occasional
impacts
Map
of the
Moon
Earth
vs.
Moon
• Earth
• Moon
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–
–
–
–
–
–
–
–
internal heat, motion
moving crustal plates
atmosphere
oceans
known life
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.
Observation of Mercury from Earth
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.
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.
Inferior Planet Configurations
Superior conjunction
Maximum elongation
Inferior conjunction
Earth-Sun line
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
View
From
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 Mercury
Moon
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
Density
0.6 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