Chapter 8: The Moon and Mercury

1
Chapter 8 The Moon and Mercury

• General Characteristics
• Surface Features
• Interior Structure
• Formation Theories

2
After completing this chapter, you should be able
to

• 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
  system.
• 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
  Moon.
• 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.

3
The Moon
4
Physical Properties of the Moon
5
What was known about the Moon before the space
program?

• 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

6
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 (2160F)
• night 125 K (-2340F)
• lack of atmosphere -- extreme variation in
  surface T

7
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
8
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

9
Differential Forces

• Differential gravitational force results in
  tidal bulges.
• Tidal force effect on Moon
  20 x greater than that on Earth.

10
Moon Orbit and Tidal Forces
11
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.

12
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.

13
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.

14
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.

15
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.

16
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.

17
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

18
View of the Lunar Surface
19
The Moons 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 (gt 4 billion years old).

20
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

21
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

22
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.

23
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)

24
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.

Moons surface, as seen from Apollo 8
25
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

26
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

27
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)
28
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

29
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.)

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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.)

31
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)
32
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 lt 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

33
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.
34
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 gt 3
  billion years and much higher prior to 3.8
  billion years ago.

35
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
36
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
37
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

38
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)

39
Lunar Surface

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

40
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.

41
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.

42
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
• gt 100 m over highlands
• Consistency of talcum powder or ready-mix dry
  mortar
• Contains NO organic matter

43
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 Earths.

44
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.

45
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.
46
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.

47
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?

48
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
  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.

49
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.

50
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.

51
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.
52
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.

53
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.

54
Interiors Earth vs. Moon

• Moon is smaller in size than Earth, but similar
  in structure.
• Moons crust is much thicker than Earths (2 x
  Earths).
• 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.

55
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.

56
Lunar Biosphere

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

57
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
58
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
  iron
• equality of oxygen isotopes between Earth and
  Moon
• angular momentum of Earth-Moon system
• overall melting of lunar surface
• physical plausibility

59
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.

60
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.

61
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.

62
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 Heating of interior
Geologic Activity Ends 3.5 Occasional impacts
63
Map of the Moon
64
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

65
Mercury
66
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

67
Phases of Mercury

• Similar to lunar phases, but cannot view full
  cycle from Earth.
• Angular distance between Sun and Mercury never gt
  280.
• Best viewing (without filters) just before dawn
  of after sunset.

68
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.
69
Mercury Time-Lapse
70
Mercurys Visibility from Earth

• ELONGATIONS Away from the Sun (28o maximum)
• Eastern Elongation - Visible in the evening at
  sunset.
• Western Elongation - Visible in the morning at
  sunrise.
• 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.

71
Inferior Planet Configurations
72
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
  (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.

73
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
74
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

75
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.

76
Mariner 10 and Mercury

• Launched November 3, 1973.
• Completed 3 fly-by passes from 1974-1975, returned
  gt4000 photographs, covering 45 of Mercurys
  surface.
• First spacecraft to transmit high resolution
  digital color images.

77
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.

78
Mercurys 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.

79
Questions Mercury

• Why is Mercury never seen overhead at midnight
  when viewed from Earth?
• What does it mean to say that Mercury has a 32
  spin-orbit resonance?
• Why isnt Mercury in a 11 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?

80
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
  planet.
• Formed by huge impact (150 km asteroid).

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Mercurys Surface Features

• Meteorite Craters
• similar to Moons, 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

82
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)

83
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.

84
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)
85
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)

86
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.

87
Mercurys Weird Terrain
88
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?

89
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).

90
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,
  proportionately.
• Mercurys mantle is relatively thinner than
  Earths or Moons.

91
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
  depth.
• Possible that Mercury's magnetic field is a
  remnant field, frozen into a solid metallic
  core.

92
Mercurys Biosphere

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

93
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
94
Mercurys 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.

95
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

96
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.

97
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