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Chapter 8: The Moon and Mercury


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Title: Chapter 8: The Moon and Mercury

Chapter 8 The Moon and Mercury
  • General Characteristics
  • Surface Features
  • Interior Structure
  • Formation Theories

After completing this chapter, you should be able
  • describe general physical properties of the Moon
    and compare them to Earth.
  • describe the orbital and rotational
    characteristics of the Moon.
  • describe the effect of tides on the Earth-Moon
  • describe lunar surface features and explain how
    each was formed.
  • describe the lunar atmosphere, hydrosphere,
    lithosphere, magnetosphere, and biosphere and
    compare to the Earth.
  • compare the geologic histories of the Earth and
  • describe the currently favored theory of the
    origin of the Moon.
  • describe our current knowledge of physical and
    chemical make-up of lunar rocks.
  • compare the general physical properties of
    Mercury to Earth and the Moon.
  • compare orbital and rotational properties of
    Mercury to Earth and the Moon.
  • describe the atmosphere, hydrosphere,
    lithosphere, magnetosphere, and biosphere of
    Mercury and compare to Earth and the Moon.
  • describe Mercury's cycle of visibility as seen
    from the Earth.
  • describe and explain Mercury's "spin-orbit"
    coupling with Sun.
  • describe the "geologic" history of Mercury.

The Moon
Physical Properties of the Moon
What was known about the Moon before the space
  • Relative size of Moon
  • Aristarchus (3rd century B.C.)
  • study of lunar eclipses
  • Moons diameter 1/4 Earths 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 Moons surface during
    Apollo missions

More about the Moon
  • Mass of Moon
  • 1/80 Earths 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 (216oF)
  • night 125K (-234oF)
  • lack of atmosphere -- extreme variation in
    surface T

Moons 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
  • Tidal coupling of the Earth and the Moon has led
    to this synchronization.
  • Earth-Moon system synchronization not yet
  • 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
  • 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 the
    same 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
  • 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 Moons 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
  • 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
The Moons Lithosphere
  • Surface Features
  • Maria (seas") or Lowlands
  • 15 of lunar surface
  • 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") or Highlands
  • 85 of the lunar surface
  • Light, mountainous regions.
  • Roughly comparable to Earth's continents.
  • Heavily cratered.
  • Relatively old surface ( 4 billion years old).

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

Lunar Maria
  • Lunar maria or "seas."
    This is a broad vista encompassing portions of
    three maria.
  • Mare Crisium (foreground)
  • Mare Tranquilitatis (beyond Mare Crisium)
  • Mare Serenitatis (on horizon, upper
  • 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
  • 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.

Moons surface, as seen from Apollo 8
Recent Meteorite Impacts on the Moon
  • Leonid Meteorite Impacts, 2001
  • At least 6 Leonids hit the Moon in 1999 causing
    explosions bright enough to see from Earth.
  • http//

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
  • terraced walls

Crater Ejecta Rays
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
  • 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
  • (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
  • 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
  • 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
  • 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
Exploration of the Moon
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
  • 10 m over maria
  • 100 m over highlands
  • Consistency of talcum powder or ready-mix dry
  • Contains NO organic matter

Lunar Surface Rock Types
  • Chemical analysis of lunar samples shows 3 main
  • basalts
  • igneous rocks formed by cooling of molten
  • breccias
  • formed from fusing of rock fragments, often
    occurs due to impacts by external bodies
    increasing P, T in region
  • basalt that has unusually high concentrations of
    K - potassium REE - rare earth
    elements P - phosphorous
  • Most samples completely devoid of water and
  • Oxygen isotope abundance similar to Earths.

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
  • Can also be used to date from last time rock was
  • 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

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 Moons 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
  • 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
  • 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 Moons Interior
  • Interior structure
  • crust 100 km thick,
  • mantle 700 km thick,
  • core 300 km in radius
  • Seismic data suggests outer core may be
  • 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
  • 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
  • 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

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
Moon's Heart Melted, Say Lunar
Love Numbers
  • February 13, 2002
  • http//
  • 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

Measuring the Magnitude of Tidal Distortions
  • The lunar Love number tells how Moons 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.
  • Moons crust is much thicker than Earths (2 x
  • Moons mantle is relatively thicker (80 of
    radius) than Earths (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
Lunar Origins
  • No definitive theory, but theory must predict
  • Moons mass relative to Earth
  • chemical composition of Earth and Moon
  • Moons depletion of volatile elements and
  • equality of oxygen isotopes between Earth and
  • 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
  • Capture hypothesis Moon gravitationally
  • 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
  • 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

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
  • During next few 100 million years, surface
    melted, fused to form breccias seen in
  • Meteoritic bombardment probably frequent enough
    to heat and re-melt most
    surface layers of Moon during first half billion
  • 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
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

  • 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
  • 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
Mercurys Visibility from Earth
  • Elongations away from the Sun (28o maximum)
  • Eastern Elongation - Visible in the evening at
  • Western Elongation - Visible in the morning at
  • Conjunctions alignments with the Sun.
  • Superior - Located on the far side of the Sun.
  • Inferior - Located between Earth and Sun.
  • Transits when Mercury crosses disk of the Sun.
  • Must occur at inferior conjunction.
  • Must occur in either May or November.

Inferior Planet Configurations
Question Mercury
  • Why is Mercury never seen overhead at midnight
    when viewed from Earth?

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

Mercurys Long Day
  • Rotation period 59 Earth days
  • Orbital period 88 Earth days
  • 3 rotations about own axis for every 2
    revolutions about Sun.
  • 32 spin-orbit resonance

1 Mercury solar day 2 Mercury years
Questions Mercury
  • What does it mean to say that Mercury has a 32
    spin-orbit resonance?
  • Why isnt Mercury in a 11 spin-orbit resonance?

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 Mercurys north pole.

Mariner 10 and Mercury
  • Launched November 3, 1973.
  • Completed 3 fly-by passes from 1974-1975, returned
    4000 photographs, covering 45 of Mercurys
  • First spacecraft to transmit high resolution
    digital color images.

Mercurys 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.

Mercurys Hydrosphere
  • Most all volatiles, including water,
    have evaporated and left the
  • 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
  • In contrast to the Earth,
    Mercury and the Moon undergo
    extremes in surface temperature.
  • The Moon and Mercury do not have no significant
    atmospheres (unlike Earth). Why?

Mercurys 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
  • Formed by huge impact (150 km asteroid).

Mercurys Surface Features
  • Meteorite Craters
  • similar to Moons, but less densely
  • 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.

Mercurys 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 Mercurys 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.

Mercurys Weird Terrain
Questions Mercurys 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?

Mercurys 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 Moons 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.
  • Mercurys core is much thicker than Earths,
  • Mercurys mantle is relatively thinner than
    Earths or Moons.

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

Mercurys Biosphere
  • Because of the lack of an atmosphere and
    hydrosphere, no biosphere is expected.

Spheres Earth, Moon, Mercury
Mercurys Geologic History
  • Condensation and accretion from
    solar nebula 4.6 billion years
  • 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 Earths size and 5 Earths mass.
  • Eccentric orbit, tilted to the ecliptic.
  • 2nd most eccentric and tilted in solar system.
  • Radar reflected from surface shows that Mercury
    has a 32 spin-orbit resonance with the Sun.
  • No natural satellites.
  • Magnetic field 1/100x Earths 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
  • 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