The Terrestrial Planets: Earth, Moon, and Their Relatives

1
Chapter 6

• The Terrestrial Planets Earth, Moon, and Their
  Relatives

2
Introduction

• Mercury, Venus, Earth, and Mars share many
  similar features.
• Small compared with the huge planets beyond
  them, these inner planets also have rocky
  surfaces surrounded by relatively thin and
  transparent atmospheres, in contrast with the
  larger, gaseous/liquid planets.
• Together, we call these four the terrestrial
  planets (from the Latin terra, meaning earth),
  which indicates their significance to us in our
  attempts to understand our own Earth.
• In this chapter, we discuss each of these rocky
  bodies, as well as their moons.

3
Introduction

• Venus and the Earth are often thought of as
  sister planets, in that their sizes, masses,
  and densities are about the same (see figure).
• But in many respects they are as different from
  each other as the wicked stepsisters were from
  Cinderella.

4
Introduction

• The Earth is lush it has oceans of water, an
  atmosphere containing oxygen, and life.
• On the other hand, Venus is a hot, foreboding
  planet with temperatures constantly over 750 K
  (900F), a planet on which life seems unlikely to
  develop.
• Why is Venus like that?
• How did these harsh conditions come about?
• Can it happen to us here on Earth?
• The one Solar-System body other than Earth that
  humans have visited is our Moon.
• It is so large relative to Earth that it joins us
  as a type of a double-planet system.
• We will see how space exploration has revealed
  many of its secrets.

5
Introduction

• Mars is only 53 per cent the diameter of Earth
  and has 10 per cent of Earths mass.
• Its atmosphere is much thinner than Earths, too
  thin for visitors from Earth to rely on to
  breathe.
• But Mars has long been attractive as a site for
  exploration.
• We remain interested in Mars as a place where we
  may yet find signs of life or, indeed, where we
  might encourage life to grow.
• Marss two tiny moons are but chunks of rock in
  orbit.
• Mercury, the innermost planet, is more like our
  Moon than like our own Earth.
• Its atmosphere is negligible and its surface is
  seared by solar radiation.
• An American spacecraft is en route there.
• A European /Japanese spacecraft is to be launched
  to Mercury in 2011 or 2012.

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6.1 Earth Theres No Place Like Home

• On the first trip that astronauts ever took to
  the Moon, they looked back and saw for the first
  time the Earth floating in space.

• Nowadays we see that space view every day from
  weather satellites, so the views from the
  Jupiter-bound Galileo spacecraft and the
  Saturn-bound Cassini spacecraft as they passed
  allowed us to test the instruments on known
  objects, the Earth and Moon (see figure).

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6.1 Earth Theres No Place Like Home

• The realization that Earth is an oasis in space
  helped inspire our present concern for our
  environment.
• Until fairly recently, we studied the Earth only
  in geology courses and the other planets only in
  astronomy courses, but now the lines are very
  blurred.
• Not only have we learned more about the interior,
  surface, and atmosphere of the Earth but we have
  also seen the planets in enough detail to be able
  to make meaningful comparisons with Earth.
• The study of comparative planetology is helping
  us to understand weather, earthquakes, and other
  topics.
• This expanded knowledge will help us improve life
  on our own planet.

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6.1a The Earths Interior

• The study of the Earths interior and surface
  (see figure) is called geology.
• Geologists study, among other things, how the
  Earth vibrates as a result of large shocks, such
  as earthquakes.
• Much of our knowledge of the structure of the
  Earths interior comes from seismology, the study
  of these vibrations.

• The vibrations travel through different types of
  material at different speeds.

9
6.1a The Earths Interior

• From seismology and other studies, geologists
  have been able to develop a picture of the
  Earths interior.
• The Earths innermost region, the core, consists
  primarily of iron and nickel.
• Outside the core is the mantle, and on top of the
  mantle is the thin outer layer called the crust.
• The upper mantle and crust are rigid and contain
  a lot of silicates, while the lower mantle is
  partially melted.
• Such a layered structure must have developed when
  the Earth was young and molten the denser
  materials (like iron) sank deeper than the
  less-dense ones, as discussed below.
• But from where did Earth get sufficient heat to
  become molten?

10
6.1a The Earths Interior

• Such a layered structure must have developed when
  the Earth was young and molten the denser
  materials (like iron) sank deeper than the
  less-dense ones, as discussed below.
• But from where did Earth get sufficient heat to
  become molten?
• The Earth, along with the Sun and the other
  planets, was probably formed from a cloud of gas
  and dust.
• Some of the original energy, though not enough to
  melt the Earths interior, came from
  gravitational energy released as particles came
  together to form the Earth such energy is
  released from gravity between objects when the
  objects move closer together and collide.
• The water at the base of a waterfall, for
  example, is slightly (unnoticeably) hotter than
  the water at the top part of the falling waters
  energy of motion, gained by the pull of gravity,
  is converted to heat by the collision on the
  rocks or water at the waterfalls base.

11
6.1a The Earths Interior

• Also, the young Earth was subject to constant
  bombardment from the remaining debris (dust and
  rocks), which carried much energy of motion.
• This bombardment heated the surface to the point
  where it began to melt, producing lava.
• However, scientists have concluded that the major
  source of energy in the interior, both at early
  times and now, is the natural radioactivity
  within the Earth.
• Certain forms of atoms are unstablethat is, they
  spontaneously change into more stable forms.
• In the process, they give off energetic particles
  that collide with the atoms in the rock and give
  some of their energy to these atoms.
• The rock heats up.

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6.1a The Earths Interior

• The Earths interior became so hot that the iron
  melted and sank to the center since it was
  denser, forming the core.
• Eventually other materials also melted.
• As the Earth cooled, various materials, because
  of their different densities (density is mass
  divided by volume and freezing points (the
  temperature at which they change from liquid to
  solid), solidified at different distances from
  the center.
• This process, called differentiation, is
  responsible for the present layered structure of
  the Earth.

13
6.1a The Earths Interior

• Geologists have known for decades that the
  Earths iron center consists of a solid inner
  core surrounded by a liquid outer core.
• The inner core is solid, in spite of its high
  (5000C) temperature, because of the great
  pressure on it.
• A new study of 30 years worth of earthquake waves
  that passed through the Earths core revealed in
  2002 that the inner core has a different inner
  region, like the pit in a peach.
• This inner region is less than 10 per cent of the
  diameter of the inner core, to continue to use
  that technical term.

14
6.1a The Earths Interior

• Why does this peach pit innermost core make
  earthquake waves act differently than they do in
  the surrounding inner core?
• It could be because this innermost core is a
  remnant of the original ball of material from
  which the Earth formed 4.6 billion years ago.
• Less exciting is the possibility that iron
  crystals deposited on it had a different
  orientation after the innermost core reached a
  critical size.
• Perhaps the temperature and pressure in that
  innermost region pack iron crystals differently.

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6.1a The Earths Interior

• The rotation of the Earths metallic core helps
  generate a magnetic field on Earth. (The
  discovery in 2005 that the Earths inner core
  spins 0.009 seconds per year faster than the rest
  of our planet, giving it an extra full revolution
  in about 900 years, may affect models of how the
  magnetic field is generated.)
• The magnetic field has a north magnetic pole and
  a south magnetic pole that are not quite where
  the regular north and south geographic poles are.
• The Earths magnetic north pole is in the Arctic
  Ocean north of Canada.
• The location of the magnetic poles wanders across
  the Earths surface over time.
• The north magnetic pole is currently moving
  northward at an average speed of 15 km /year.

16
6.1b Continental Drift

• Some geologically active areas exist in which
  heat flows from beneath the surface at a rate
  much higher than average (see figure).
• The outflowing geothermal energy, sometimes
  tapped as an energy source, signals what is below.

• The Earths rigid outer layer is segmented into
  plates, each thousands of kilometers in extent
  but only about 50 km thick.
• Because of the internal heating, the top layers
  float on an underlying hot layer (the mantle)
  where the rock is soft, though it is not hot
  enough to melt completely.

17
6.1b Continental Drift

• The mantle beneath the rigid plates of the
  surface churns very slowly, thereby carrying the
  plates around.
• This theory, called plate tectonics, explains the
  observed continental driftthe drifting of the
  continents, over eons, from their original
  positions, at the rate of a few centimeters per
  yearabout the speed your fingernails grow.
  (Tectonics comes from the Greek word meaning
  to build.)
• Although the notion of continental drift
  originally seemed unreasonable, it is now
  generally accepted.
• The continents were once connected as two
  super-continents, which may themselves have
  separated from a single super-continent called
  Pangaea (all lands).

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6.1b Continental Drift

• Over the past two hundred million years or so,
  the continents have moved apart as plates have
  separated.
• We can see from their shapes how they once fit
  together (see figure).
• We even find similar fossils and rock types along
  two opposite coastlines that were once adjacent
  but are now widely separated.
• Remnants of the magnetic field as measured in
  rocks laid down when the Earths magnetic poles
  had flipped, north and south magnetic poles
  interchanging (as they do occasionally), are also
  among the strongest evidence.

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6.1b Continental Drift

• Pangaea itself probably formed from the collision
  of previous generations of continents.
• The coming together and breaking apart of
  continents may have had many cycles in Earths
  history.
• In the future, we expect part of California to
  separate from the rest of the United States,
  Australia to be linked to Asia, and the Italian
  boot to disappear.

• The boundaries between the plates are
  geologically active areas (see figure).

20
6.1b Continental Drift

• Therefore, these boundaries are traced out by the
  regions where earthquakes and most of the
  volcanoes occur.
• The boundaries where two plates are moving apart
  mark regions where molten material is being
  pushed up from the hotter interior to the
  surface, such as the mid-Atlantic ridge (see
  figure).

• Molten material is being forced up through the
  center of the ridge and is being deposited as
  lava flows on either side, producing new seafloor.

21
6.1b Continental Drift

• The motion of the plates relative to each other
  is also responsible for the formation of most
  great mountain ranges.
• When two plates come together, one may be forced
  under the other and the other rises.
• The great Himalayan mountain chain, for example,
  was produced by the collision of India with the
  rest of Asia.
• The ring of fire volcanoes around the Pacific
  Ocean (including Mt. St. Helens in Washington)
  were formed when molten material made its way
  through gaps or weak points between plates.

22
6.1c Tides

• It has long been accepted that tides are most
  directly associated with the Moon and to a lesser
  extent with the Sun.
• We know of their association with the Moon
  because the tideslike the Moons passage across
  your meridian (the imaginary line in the sky
  passing from north to south through the point
  overhead) occur about an hour later each day.
• Tides result from the fact that the force of
  gravity exerted by the Moon (or any other body)
  gets weaker as you get farther away from it.
• Tides depend on the difference between the
  gravitational attraction of a massive body at
  different points on another body.

23
6.1c Tides

• To explain the tides in Earths oceans, suppose,
  for simplicity, that the Earth is completely
  covered with water.
• We might first say that the water closest to the
  Moon is attracted toward the Moon with the
  greatest force and so is the location of high
  tide as the Earth rotates.
• If this were the whole story, high tides would
  occur about once a day.
• However, two high tides occur daily, separated by
  roughly 12½ hours.

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6.1c Tides

• To see why we get two high tides a day, consider
  three points, A, B, and C, where B represents the
  solid Earth (which moves all together as a single
  object and is marked by a point at its center), A
  is the ocean nearest the Moon, and C is the ocean
  farthest from the Moon (see figure).
• Since the Moons gravity weakens with distance,
  it is greater at point A than at B, and greater
  at B than at C.

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6.1c Tides

• If the Earth and Moon were not in orbit around
  each other, all these points would fall toward
  the Moon, moving apart as they fell because of
  the difference in force.
• Thus the high tide on the side of the Earth that
  is near the Moon is a result of the water being
  pulled away from the Earth.
• The high tide on the opposite side of the Earth
  results from the Earth being pulled away from the
  water.
• In between the locations of the high tides the
  water has rushed elsewhere (to the regions of
  high tides), so we have low tides.

26
6.1c Tides

• Since the Moon is moving in its orbit around the
  Earth, a point on the Earths surface has to
  rotate longer than 24 hours to return to a spot
  nearest to the Moon.
• Thus a pair of tides repeats about every 25
  hours, making 12½ hours between high tides.
• The Suns effect on the Earths tides is only
  about half as much as the Moons effect.
• Though the Sun exerts a greater gravitational
  force on the Earth than does the Moon, the Sun is
  so far away that its force does not change very
  much from one side of the Earth to the other.
• And it is only the difference in force from one
  place to another that counts for tides.

27
6.1c Tides

• Nonetheless, the Sun does matter.
• We tend to have very high and very low tides when
  the Sun, Earth, and Moon are aligned (as is the
  case near the time of full moon or of new moon),
  because their effects reinforce each other.
• Conversely, tides are less extreme when the Sun,
  Earth, and Moon form a right angle (as near the
  time of a first-quarter or third-quarter moon).
• The effect of the tides on the EarthMoon system
  slows down the Earths rotation slightly, by
  about 1 second per 100,000 years, as we can
  verify from the timing of solar eclipses that
  took place thousands of years ago.
• Also, the interaction is leading to a gradual
  spiraling away of the Moon from the Earth, though
  the rate is only centimeters per year.
• Thus, far in the future (about a billion years
  from now), it will not be possible to witness a
  total solar eclipse from Earth the Moons
  angular diameter will be less than that of the
  Suns photosphere.

28
6.1d The Earths Atmosphere

• We name layers of our atmosphere (see figure)
  according to the composition and the physical
  processes that determine their temperatures.
• The atmosphere contains about 20 per cent oxygen,
  the gas that our bodies use when we breathe
  almost all of the rest is nitrogen.
• Later, we will see how the small amounts of
  carbon monoxide, of carbon dioxide, of methane,
  and of other gases are affecting our climate.

• When we find a planet around another star
  (Chapter 9) whose spectrum shows such a high
  percentage of oxygen, we will infer the presence
  there of life-forms making the oxygen.

29
6.1d The Earths Atmosphere

• The Earths weather is confined to the very thin
  troposphere.
• The ground is a major source of heat for the
  troposphere, so the temperature of the
  troposphere decreases as altitude increases.
• The rest of the Earths atmosphere, as well as
  the Earths surface, is heated mainly by solar
  energy from above.
• A higher layer of the Earths atmosphere is the
  thermosphere.
• It is also known as the ionosphere, since many of
  the atoms there are ionizedthat is, stripped of
  some of the electrons they normally contain.
• Most of the ionization is caused by x-ray and
  ultraviolet radiation from the Sun, as well as
  from solar particles.
• Thus the ionosphere forms during the daytime and
  diminishes at night.
• The free electrons in the ionosphere reflect
  very-long-wavelength radio signals.
• When the conditions are right, radio waves bounce
  off the ionosphere, which allows us to tune in
  distant radio stations.

30
6.1d The Earths Atmosphere

• Observations from high-altitude balloons and
  satellites have greatly enhanced our knowledge of
  Earths atmosphere.
• Scientists carry out calculations using the most
  powerful supercomputers to interpret the global
  data and to predict how the atmosphere will
  behave.
• The equations are essentially the same as those
  for the internal temperature and structure of
  stars, except that the sources of energy are
  different, with stars heated from below and the
  Earths atmosphere mostly heated from above.

31
6.1d The Earths Atmosphere

• Winds are caused partly by uneven heating of
  different regions of Earth.
• The rotation of the Earth also has a very
  important effect in determining how the winds
  blow.
• Comparison of the circulation of winds on the
  Earth (which rotates in 1 Earth day), on slowly
  rotating Venus (which rotates in 243 Earth days),
  and on rapidly rotating Jupiter and Saturn (each
  of which rotates in about 10 Earth hours) helps
  us understand the weather on Earth.
• Our improved understanding allows forecasters of
  weather (day to day) and climate (long term) to
  be more accurate.

32
6.1d The Earths Atmosphere

• Comparison with the planet Venus (Section 6.4d)
  led to our realization that the Earths
  atmosphere traps some of the radiation from the
  Sun, and that we are steadily increasing the
  amount of trapping. (The word trapping is used
  here in a figurative rather than a literal sense
  the energy is actually transformed from one kind
  to another when air molecules absorb it, and no
  particular light photons are physically
  trapped.)

• The process by which light is trapped, resulting
  in the extra heating of Earths atmosphere and
  surface, is similar to the process that is
  generally (though incorrectly) thought to occur
  in terrestrial greenhouses it is thus called the
  greenhouse effect.
• It is caused largely by the carbon dioxide in
  Earths atmosphere (see figure), the amount of
  which is growing each year because of our use of
  fossil fuels.

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6.1d The Earths Atmosphere

• Section 6.4d more fully describes the greenhouse
  effect, which greatly affects the temperature of
  Venus.
• In brief, Earths atmosphere is warmed both from
  above by solar radiation, and from below by
  radiation from the Earths surface, which is
  itself warmed by solar radiation.
• Most solar radiation is in the visible,
  corresponding to the peak of the Suns black-body
  radiation (recall the discussion of this
  radiation in Chapter 2).
• The radiation from the Earths surface is in the
  form of infrared, which corresponds to the peak
  wavelength of the black-body curve at Earths
  temperature.
• Some of this infrared radiation is absorbed in
  the atmosphere by carbon dioxide, water, and, to
  a lesser extent, by other greenhouse gases such
  as methane.

34
6.1d The Earths Atmosphere

• The greenhouse effect itself is good it warms us
  by about 33C, bringing Earths atmospheric
  temperature to the livable range it is now.
• The important question is whether the amount of
  greenhouse warming is increasing, progressively
  raising the Earths temperature.
• Such a phenomenon is known as global warming.

35
6.1d The Earths Atmosphere

• Quite a separate problem is a discovery about the
  ozone (O3) in our upper atmosphere.
• This ozone becomes thinner over Antarctica each
  Antarctic spring, a phenomenon known as the
  ozone hole.
• The ozone hole apparently started forming only in
  the mid-1980s, but its maximum size has been
  larger almost every year since then (see figure).

36
6.1d The Earths Atmosphere

• The ozone hole is caused by the interaction in
  the cold upper atmosphere of sunlight with
  certain gases we give off near the ground, such
  as chlorofluorocarbons used in air conditioners
  and refrigerators.
• International governmental meetings have arranged
  cutbacks in the use of these harmful gases.
• Some success has been achieved, but we have far
  to go to protect our future atmosphere.

37
6.1e The Van Allen Belts

• In 1958, the first American space satellite
  carried aloft, among other things, a device to
  search for particles carrying electric charge
  that might be orbiting the Earth.
• This device, under the direction of James A. Van
  Allen of the University of Iowa, detected a
  region filled with charged particles having high
  energies.
• Two such regionsthe Van Allen beltswere found
  to surround the Earth, like a small and a large
  doughnut, containing protons and electrons (see
  figure).

• They start a few hundred kilometers above the
  Earths surface and extend outward to about 8
  times the Earths radius.
• A more recently discovered third, innermost belt
  contains mainly ions of heavier elements from
  interstellar space.

38
6.1e The Van Allen Belts

• The particles in the Van Allen belts are trapped
  by the Earths magnetic field.
• Charged particles preferentially move in the
  direction of magnetic-field lines, and not across
  the field lines.
• These particles, often from solar magnetic
  storms, are guided by the Earths magnetic field
  toward the Earths magnetic poles.
• When they interact with air molecules, they cause
  our atmosphere to glow, which we see as the
  beautiful northern and southern lightsthe aurora
  borealis and aurora australis, respectively (see
  figures).

39
6.2 The Moon

• The Earths nearest celestial neighborthe
  Moonis only 380,000 km (238,000 miles) away from
  us, on the average.
• At this distance, it appears sufficiently large
  and bright to dominate our nighttime sky.
• The Moons stark beauty has captured our
  attention since the beginning of history.
• Now we can study the Moon not only as an
  individual object but also as an example of a
  small planet or a large planetary satellite,
  since spacecraft observations have told us that
  there may be little difference between small
  planets and large moons.

40
6.2a The Moons Appearance

• Even binoculars reveal that the Moons surface is
  pockmarked with craters.
• Other areas, called maria (pronounced mar'ee-a
  singular mare, pronounced mar'eyh), are
  relatively smooth and dark.
• Indeed, the name comes from the Latin word for
  sea (see figure).
• But there are no ships sailing on the lunar seas
  and no water in them the Moon is a dry, airless,
  barren place.
• The gravity at the Moons surface is only
  one-sixth that of the Earth.

• Typically you would weigh only 20 or 30 pounds
  there if you stepped on a scale!
• The gravity is so weak that any atmosphere and
  any water that may once have been present would
  long since have escaped into space.

41
6.2a The Moons Appearance

• The Moon rotates on its axis at the same rate
  that it revolves around the Earth, thereby always
  keeping the same face in our direction. (To
  understand this idea, put a quarter on your desk
  and then slide a dime around it, keeping both
  flat on the desk and keeping the top of the head
  on the dime always on the side that is away from
  you. Notice that though the dime isnt rotating
  as seen from above, a viewer on the quarter would
  see the dime at different angles. Then move the
  dime around the quarter so that the same point on
  the dime always faces the quarter. Notice that
  as seen from above the dime rotates as it
  revolves around the quarter.)
• Over time, the Earths gravity locked the Moon in
  this pattern, pulling on a bulge in the
  distribution of the lunar mass to prevent the
  Moon from rotating freely.
• As a result of this interlock (known as
  synchronous rotation) we always see essentially
  the same side of the Moon from our vantage point
  on Earth.

42
6.2a The Moons Appearance

• When the Moon is full, it is bright enough to
  cast shadows or even to read by.
• But a full moon is a bad time to try to observe
  lunar surface structure, for any shadows we see
  are short, and lunar features appear washed out.
• When the Moon is a crescent or even a quarter
  moon, however, the sunlighted part of the Moon
  facing us is covered with long shadows.
• The lunar features then stand out in bold relief.
  
• Shadows are longest near the terminator, the line
  that separates day from night.
• Note that nature photographers on Earth,
  concluding that views with shadows are more
  dramatic, generally take their best photos when
  the Sun is low.

43
6.2a The Moons Appearance

• Six teams of astronauts in NASAs Apollo program
  landed on the Moon in 19691972.
• In some sense, before this period of exploration,
  we knew more about bright stars than we did about
  the Moon.
• As a relatively cold, solid body, the Moon
  reflects the spectrum of sunlight rather than
  emitting its own optical spectrum, so we were
  hard pressed to determine even the composition or
  the physical properties of the Moons surface
  (such as whether you would sink into it!).

44
6.2b The Lunar Surface

• The kilometers of film exposed by the astronauts,
  the 382 kg of rock brought back to Earth, the
  lunar seismograph data recorded on tape,
  meteorites from the Moon that have been found on
  Earth, and other sources of data have been
  studied by hundreds of scientists from all over
  the world. (Meteorites are rocks from space that
  have landed on Earth see Chapter 8 for more
  details.)
• The data have led to new views of several basic
  questions, and have raised many new questions
  about the Moon and the Solar System.

45
6.2b The Lunar Surface

• The rocks that were encountered on the Moon are
  types that are familiar to terrestrial geologists
  (see figure).
• Almost all the rocks are the kind that were
  formed by the cooling of lava, known as igneous
  rocks.
• Basalts are one example.
• The Moon and the Earth seem to be similar
  chemically, though significant differences in
  overall composition do exist.
• Some elements that are rare on Earthsuch as
  uranium and thoriumare found in greater
  abundances on the Moon. (Will we be mining on the
  Moon one day?)
• None of the lunar rocks contain any trace of
  water bound inside their minerals.

46
6.2b The Lunar Surface

• Meteoroids, interplanetary rocks that we will
  discuss in Chapter 8, hit the Moon with such high
  speeds that huge amounts of energy are released
  at the impact.
• The effect is that of an explosion, as though TNT
  or an H-bomb had exploded.
• As a result of the Apollo missions, we know that
  almost all the craters on the Moon come from such
  impacts.
• One way of dating the surface of a moon or planet
  is to count the number of craters in a given
  area, a method that was used before Apollo.
• Surely those locations with the greatest number
  of craters must be the oldest.
• Relatively smooth areaslike mariamust have been
  covered over with molten volcanic material at
  some relatively recent time (which is still
  billions of years ago, though).

47
6.2b The Lunar Surface

• Obvious rays of lighter-colored matter splattered
  outward during the impacts that formed a few of
  the craters.
• Since these rays extend over other craters, the
  craters with rays must have formed more recently.
  
• The youngest rayed craters may be very young
  indeedperhaps only a few hundred million years.
• The rays darken with time, so rays that may have
  once existed near other craters are now
  indistinguishable from the rest of the surface.
• Crater counts and the superposition of one crater
  on another give only relative ages.
• We found the absolute ages only when rocks from
  the Moon were physically returned to Earth.

48
6.2b The Lunar Surface

• Scientists worked out the dates by comparing the
  current ratio of radioactive forms of atoms to
  nonradioactive forms present in the rocks with
  the ratio that they would have had when they were
  formed. (Varieties of the chemical elements
  having different numbers of neutrons are known as
  isotopes, and radioactive isotopes are those
  that decay spontaneously that is, they change
  into other isotopes even when left alone. Stable
  isotopes remain unchanged. For certain pairs of
  isotopesone radioactive and one stablewe know
  the proportion of the two when the rock was
  formed. Since we know the rate at which the
  radioactive one is decaying, we can calculate how
  long it has been decaying from a measurement of
  what fraction is left.)
• The oldest rocks that were found on the Moon
  solidified 4.4 billion years ago.
• The youngest rocks ever found solidified 3.1
  billion years ago.

49
6.2b The Lunar Surface

• The observations can be explained on the basis of
  the following general sequence (see figure)
• The Moon formed about 4.6 billion years ago.
• From the oldest rocks, we know that at least the
  surface of the Moon was molten about 200 million
  years later.
• Then the surface cooled.

• From 4.2 to 3.9 billion years ago, bombardment by
  interplanetary rocks caused most of the craters
  we see today.
• About 3.8 billion years ago, the interior of the
  Moon heated up sufficiently (from radioactive
  elements inside) that volcanism began.

50
6.2b The Lunar Surface

• Lava flowed onto the lunar surface and filled the
  largest basins that resulted from the earlier
  bombardment, thus forming the maria (see figure).
• By 3.1 billion years ago, the era of volcanism
  was over.
• The Moon has been geologically pretty quiet since
  then.

51
6.2b The Lunar Surface

• Up to this time, the Earth and the Moon shared
  similar histories.
• But active lunar history stopped about 3 billion
  years ago, while the Earth continued to be
  geologically active.
• Almost all the rocks on the Earth are younger
  than 3 billion years of age the oldest single
  rock ever discovered on Earth has an age of 4.5
  billion years, but few such old rocks have been
  found.
• Erosion and the remolding of the continents as
  they move slowly over the Earths surface have
  taken their toll.
• So we must look to extraterrestrial bodiesthe
  Moon or meteoritesthat have not suffered the
  effects of plate tectonics or erosion (which
  occurs in the presence of water or an atmosphere)
  to study the first billion years of the Solar
  System.

52
6.2b The Lunar Surface

• Not until the 1990s did spacecraft revisit the
  Moon.
• The Clementine spacecraft (named after the
  prospectors daughter in the old song, since the
  spacecraft was looking for minerals) took
  photographs and other measurements.
• Photographs of the far side of the Moon (see
  figure) have shown us that the near and far
  hemispheres are quite different in overall
  appearance.
• The maria, which are so conspicuous on the near
  side, are almost absent from the far side, which
  is cratered all over.
• We shall see in the next section that the
  difference probably results from the different
  thicknesses of the lunar crust on the sides of
  the Moon nearest Earth and farthest from Earth.
• The difference was first seen in the fuzzy
  photographs of the far side that were taken by
  the Soviet Lunik 3 Spacecraft in 1959.

53
6.2b The Lunar Surface

• In the 1990s, NASAs Lunar Prospector and
  Clementine spacecraft mapped the Moon with a
  variety of instruments (see figure).

• Lunar Prospector confirmed indications from the
  Clementine spacecraft that there is likely to be
  water ice on the Moon, by detecting more neutrons
  coming from the Moons polar regions than
  elsewhere.
• Clementine and Lunar Prospector scientists think
  that these neutrons are given off in interactions
  of particles coming from the Sun with hydrogen in
  water ice in craters near the lunar poles, where
  they are shaded from the Suns rays.

54
6.2b The Lunar Surface

• But the detection is not of water directly, and
  Apollo 17 astronaut Harrison Schmitt, the only
  geologist ever to have walked on the Moon, told
  one of the authors (J.M.P.) in 1999 that the
  neutrons may instead have come from the solar
  wind (see figure), though as of 2005 most
  scientists do not agree.
• He would love to find water there, because it
  would increase the chance that a crewed Moon base
  could be supplied on the Moon itself, much easier
  than bringing everything from Earth.
• In his estimation, it would take about 10 years
  to set up such a base, once basic funding is
  available on Earth.

• An attempt to test the idea of water in the
  crater was made by crashing Lunar Prospector into
  the most likely spot in 1999, but none of the
  spectrographs on Earth viewing the event detected
  any signal from the crash.

55
6.2b The Lunar Surface

• The European Space Agency has a spacecraft in
  orbit around the moon.
• Their SMART-1 (Small Mission for Advanced
  Research and Technology) spacecraft is basically
  meant to test new technology, but they may as
  well test it in lunar orbit.

• It uses, for the first time, an electric engine,
  in which a weak but steady puff of ions ejected
  out the back of the spacecraft accelerated the
  spacecraft at a slow but steady rate, eventually
  bringing it into its final lunar orbit in 2005.
• It is sending back images that include
  perpetually shaded polar regions where water ice
  may be found (see figure).

56
6.2c The Lunar Interior

• Before the Moon landings, it was widely thought
  that the Moon was a simple body, with the same
  composition throughout.
• But we now know it to be differentiated (see
  figure), like the planets.
• Most experts believe that the Moons core is
  molten, but the evidence is not conclusive.
• The lunar crust is perhaps 65 km thick on the
  near side and twice as thick on the far side.
• This asymmetry may explain the different
  appearances of the sides, because lava would be
  less likely to flow through the far sides
  thicker crust.

57
6.2c The Lunar Interior

• The Apollo astronauts brought seismic equipment
  to the Moon (see figure).
• One type of earthquake wave moves material to the
  side.

• Only if the Moons core were solid would the
  material move and then return to where it
  started, continuing the wave.
• Since that type of wave doesnt come through the
  Moon, scientists deduce that the Moons core is
  probably molten.
• New computer analysis methods were applied in
  2004 to the old data, and thousands of additional
  moonquakes were discovered.

58
6.2c The Lunar Interior

• Tracking the orbits of the Apollo Command Modules
  and more recently the Clementine and Lunar
  Prospector spacecraft that orbited the Moon told
  us about the lunar interior.
• If the Moon were a perfect, uniform sphere, the
  spacecraft orbits would have been perfect
  ellipses.
• But they werent.
• One of the major surprises of the lunar missions
  of the 1960s, refined more recently, was the
  discovery in this way of mascons, regions of mass
  concentrations near and under most maria.
• The mascons may be lava that is denser than the
  surrounding matter, providing a stronger
  gravitational force on satellites passing
  overhead.

59
6.2c The Lunar Interior

• We also find out about the lunar interior by
  bouncing powerful laser beams off the Moons
  surface.
• This laser ranging (where ranging means
  finding distances) uses several sets of
  reflectors left on the Moon by the Apollo
  astronauts.
• The laser ranging programs find the distance to
  the Moon to within a few centimeterspretty good
  for an object about 400,000 km away!
• Variations in the distance result in part from
  conditions in the lunar interior.

60
6.2d The Origin of the Moon

• The leading models for the origin of the Moon
  that were considered at the time of the Apollo
  missions were as follows.
• 1. Fission. The Moon was separated from the
  material that formed the Earth the Earth spun up
  and the Moon somehow spun off
• 2. Capture. The Moon was formed far from the
  Earth in another part of the Solar System, and
  was later captured by the Earths gravity and
• 3. Condensation. The Moon was formed near to (and
  simultaneously with) the Earth in the Solar
  System.

61
6.2d The Origin of the Moon

• But work over the last three decades has all but
  ruled out the first two of these and has made the
  third seem less likely.
• The model now strongly favored, especially
  because of computer simulations, is
• 4. Ejection of a Gaseous Ring. A planet-like body
  perhaps twice the size of Mars hit the young
  Earth, ejecting matter in gaseous (and perhaps
  some in liquid or solid) form (see figure).
• Although some of the matter fell back to Earth,
  and part escaped entirely, a significant fraction
  started orbiting the Earth, probably in the same
  direction as the initial incoming body.
• The orbiting material eventually coalesced to
  form the Moon.

62
6.2d The Origin of the Moon

• Comparing the chemical composition of the lunar
  surface with the composition of the Earths
  surface has been important in narrowing down the
  possibilities.
• The mean lunar density of 3.3 grams/cm3 is close
  to the average density of the Earths major upper
  region (the mantle), and the Moon seems
  especially deficient in iron.
• This fact favors the fission hypothesis had the
  Moon condensed from the same material as Earth,
  as in the condensation scenario, it would contain
  much more iron.
• However, detailed examination of the lunar rocks
  and soils indicates that the abundances of
  elements on the Moon and Earths mantle are
  sufficiently different from each other to
  indicate that the Moon did not form directly from
  the Earth.
• In the collision hypothesis, on the other hand,
  such differences are expected because of
  contaminant material from the impactor.

63
6.2d The Origin of the Moon

• Calculations considering angular momentum (recall
  the discussion of this concept in Chapter 5)
  strongly suggest that the fission mechanism
  doesnt work.
• Plate-tectonic theory now explains the formation
  of the Pacific Ocean basin.
• Before the ejection-of-a-ring theory was
  considered most probable, it seemed possible that
  the Pacific Ocean could be the scar left behind
  when the Moon was ripped from the Earth according
  to the fission hypothesis.
• We are obtaining additional evidence about the
  capture model by studying the moons of Jupiter
  and Saturn.
• The outermost moon of Saturn, for example, is
  apparently a captured asteroid (small bodies
  orbiting the Sun, mostly between Mars and
  Jupiter see Chapter 8).
• However, the similarities in composition between
  the Moons and the Earths mantles argue against
  the capture hypothesis for the EarthMoon system.
• Thus as of now, ejection of a gaseous ring is the
  most accepted model.

64
6.2e Rocks from the Moon

• A handful of meteorites found in Antarctica,
  Australia, and Africa have been identified by
  their chemical composition as having come from
  the Moon (see figure).
• They presumably were ejected from the Moon when
  craters formed.
• So we are still getting new moon rocks to study!
• A few other meteorites have even been found to
  come from Mars, as we shall discuss later in this
  chapter.

65
6.3 Mercury

• Mercury is the innermost of our Suns nine
  planets.
• Its average distance from the Sun is of the
  Earths average distance, or 0.4 A.U.
• Except for distant Pluto, its elliptical orbit
  around the Sun is the most elongated (eccentric).
• Since we on the Earth are outside Mercurys orbit
  looking in at it, Mercury always appears close to
  the Sun in the sky (see figure).
• At times Mercury rises just before sunrise, and
  at times it sets just after sunset, but it is
  never up when the sky is really dark.

66
6.3 Mercury

• The Sun always rises or sets within an hour or so
  of Mercurys rising or setting.
• As a result, whenever Mercury is visible, its
  light has to pass obliquely through the Earths
  atmosphere.
• This long path through turbulent air leads to
  blurred images.
• Thus astronomers have never had a really clear
  view of Mercury from the Earth, even with the
  largest telescopes.
• Even the best photographs taken from the Earth
  show Mercury as only a fuzzy ball with faint,
  indistinct markings.
• Most people have never seen it at all.

67
6.3 Mercury

• On rare occasions, Mercury goes into transit
  across the Sun that is, we see it as a black dot
  crossing the Sun.
• Transits of Mercury occurred in 1999 (see figure)
  and 2003.
• The next transit of Mercury will occur on
  November 28, 2006.
• The entire transit will be visible from the
  U.S.s west coast, and the Sun will set during
  the transit for observers on the east coast.
• Understanding what we see as Mercury transits
  helps us understand the much rarer transits of
  Venus, which we will discuss later in this
  chapter.

68
6.3a The Rotation of Mercury

• From studies of ground-based drawings and
  photographs, astronomers did as well as they
  could to describe Mercurys surface.
• A few features seemed to be barely distinguished,
  and the astronomers watched to see how long those
  features took to rotate around the planet.
• From these observations they decided that Mercury
  rotates in the same amount of time that it takes
  to revolve around the Sun in its orbit, 88 Earth
  days.
• Thus they thought that one side always faces the
  Sun and the other side always faces away from the
  Sun. (Recall that one side of the Moon always
  faces the Earth, a similar phenomenon.)
• This discovery led to the fascinating conclusion
  that Mercury could be both the hottest planet (on
  the Sun-facing side) and the coldest planet (on
  the other side) in the Solar System.

69
6.3a The Rotation of Mercury

• But when the first measurements were made of
  Mercurys radio radiation, the planet turned out
  to be giving off more energy than had been
  expected.
• This meant that it was hotter than expected.
• The dark side of Mercury was too hot for a
  surface that was always in the shade. (The
  visible light we see is merely sunlight reflected
  by Mercurys surface and doesnt tell us the
  surfaces temperature. The radio waves are
  actually being emitted by the surface, as part of
  its thermal or nearly black-body radiation see
  the discussion in Chapter 2.)

70
6.3a The Rotation of Mercury

• Later, we became able to transmit radar from
  Earth to Mercury. (Radarradio detection and
  rangingis sending out radio waves so that they
  bounce off another object, allowing you to study
  their reflection.)
• Since one edge of the visible face of Mercury is
  rotating toward Earth, while the other edge is
  rotating away from Earth, the reflected radio
  waves were slightly smeared in wavelength
  according to the Doppler effect (recall the
  description of this effect in Chapter 2).
• This measurement allowed astronomers to determine
  Mercurys rotation speed, similarly to the way
  that radar is used by the police to tell if a car
  is breaking the speed limit.
• Knowing the rotation speed and Mercurys radius,
  we could determine the rotation period.

71
6.3a The Rotation of Mercury

• The results were a surprise it actually rotates
  every 59 days, not 88 days.
• Mercurys 59-day period of rotation with respect
  to the stars is exactly ? of the 88-day orbital
  period, so the planet rotates three times for
  every two times it revolves around the Sun.
• Mercurys rotation and revolution combine to give
  a value for the rotation of Mercury relative to
  the Sun (that is, a mercurian solar day) that is
  neither 59 nor 88 days long (see figure).

72
6.3a The Rotation of Mercury

• If we lived on Mercury we would measure each
  day (that is, each day/night cycle) to be 176
  Earth days long.
• We would alternately be fried for 88 Earth days
  and then frozen for 88 Earth days.
• Since each point on Mercury faces the Sun at some
  time, the heat doesnt build up forever at the
  place under the Sun, nor does the coldest point
  cool down as much as it would if it never
  received sunlight.
• The hottest temperature is about 700 K (800F).
• The minimum temperature is about 100 K (280F).

73
6.3a The Rotation of Mercury

• No harm was done by the scientists original
  misconception of Mercurys rotational period, but
  the story teaches all of us a lesson we should
  not be too sure of so-called facts.
• Dont believe everything you read in this book,
  either.
• It should be fun for you to look back in 20 years
  and see how much of what we now think we know
  about astronomy actually turned out to be wrong.
• After all, science is a dynamic process.

74
6.3b Mercury Observed from the Earth

• Even though the details of the surface of Mercury
  cant be seen very well from the Earth, other
  properties of the planet can be better studied.
• For example, we can measure Mercurys albedo, the
  fraction of the sunlight hitting Mercury that is
  reflected (see figure).
• We can measure the albedo because we know how
  much sunlight hits Mercury (we know the
  brightness of the Sun and the distance of Mercury
  from the Sun).
• Then we can easily calculate at any given time
  how much light Mercury reflects, knowing how
  bright Mercury looks to us and its distance from
  the Earth.
• Comparison with the albedoes of materials on the
  Earth and on the Moon can teach us something of
  what the surface of Mercury is like.

75
6.3b Mercury Observed from the Earth

• Let us consider some examples of albedo.
• An ideal mirror reflects all the light that hits
  it its albedo is thus 100 per cent. (The very
  best real mirrors have albedoes of as much as 96
  per cent.)
• A black cloth reflects essentially none of the
  light that hits it its albedo (in the visible
  part of the spectrum, anyway) is almost 0 per
  cent.
• Mercurys overall albedo is only about 10 per
  cent.
• Its surface, therefore, must be made of a dark
  (that is, poorly reflecting) material (though a
  few regions are very reflective, with albedoes of
  up to 45 per cent).
• The albedoes of the Moons maria are similarly
  low, 6 to 10 per cent.
• In fact, Mercury (or the Moon) appears bright to
  us only because it is contrasted against a
  relatively dark sky if it were silhouetted
  against a white bedsheet, it would look
  relatively dark.

76
6.3c Mercury from Mariner 10

• In 1974, we learned most of what we know about
  Mercury in a brief time.
• We flew right by it.
• The tenth in the series of Mariner spacecraft
  launched by the United States went to Mercury
  with a variety of instruments on board.
• First the 475-kg spacecraft passed by Venus and
  had its orbit changed by Venuss gravity to
  direct it to Mercury.
• Tracking its orbit improved our measurements of
  the gravity of these planets and thus of their
  masses.
• The most striking overall impression is that
  Mercury is heavily cratered (see figure).
• At first glance, it looks like the Moon!
• But there are several basic differences between
  the features on the surface of Mercury and those
  on the lunar surface.

77
6.3c Mercury from Mariner 10

• Mercurys craters seem flatter than those on the
  Moon, and they have thinner rims (see figure).
• Mercurys higher gravity at its surface may have
  caused the rims to slump more.
• Also, Mercurys surface may have been softer,
  more plastic-like, when most of the cratering
  occurred.
• The craters may have been eroded by any of a
  number of methods, such as the impacts of
  meteorites or micrometeorites (large or small
  bits of interplanetary rock).
• Alternatively, erosion may have occurred during a
  much earlier period when Mercury may have had an
  atmosphere, undergone internal activity, or been
  flooded by lava.

78
6.3c Mercury from Mariner 10

• Most of the craters seem to have been formed by
  impacts of meteorites.
• The Caloris Basin, in particular, is the site of
  a major impact.
• The secondary craters, caused by material ejected
  as primary craters were formed, are closer to the
  primaries than on the Moon, presumably because of
  Mercurys higher surface gravity.
• In many areas, the craters appear superimposed on
  relatively smooth plains.
• The plains are so extensive that they are
  probably volcanic.
• Their age is estimated to be 4.2 billion years,
  the oldest features on Mercury.

79
6.3c Mercury from Mariner 10

• Smaller, brighter craters are sometimes, in turn,
  superimposed on the larger craters and thus must
  have been made afterward.
• Some craters have rays of higher albedo emanating
  from them (see figure), just as some lunar
  craters do.
• The ray material represents relatively recent
  crater formation (that is, within the last
  hundred million years).
• The ray material must have been tossed out in the
  impact that formed the crater.

80
6.3c Mercury from Mariner 10

• Lines of cliffs hundreds of miles long are
  visible on Mercury on Mercury, as on Earth, such
  lines of cliffs are called scarps.
• The scarps are particularly apparent in the
  region of Mercurys south pole (see figure).
• Unlike fault lines on the Earth, such as
  Californias famous San Andreas fault
  (responsible for the 1906 San Francisco
  earthquake), on Mercury there are no signs of
  geologic tensions like rifts or fissures nearby.
• These scarps are global in scale, not just
  isolated.
• The scarps may actually be wrinkles in Mercurys
  crust.

• Perhaps Mercury was once molten, and shrank by 1
  or 2 km as it cooled.
• This shrinking would have caused the crust to
  buckle, creating the scarps in the quantity that
  we now observe.

81
6.3c Mercury from Mariner 10

• Judging by the fact that Mercurys average
  density is about the same as the Earths, its
  core is probably iron and takes up perhaps 50 per
  cent of the volume, or 70 per cent of the mass, a
  much greater proportion than in the case of
  Earths core.
• Data from Mariners infrared radiometer indicate
  that the surface of Mercury is covered with fine
  dust, as is the surface of the Moon, to a depth
  of at least several centimeters.
• Astronauts sent to Mercury, whenever they go,
  will leave footprints behind.
• Part of Mercurys surface is very jumbled,
  probably from the energy released by an impact
  (see figure).

82
6.3c Mercury from Mariner 10

• The biggest surprise of the mission was the
  detection of a magnetic field in space near
  Mercury.
• The field is weak, only about 1 per cent of the
  Earths surface field.
• It had been thought that magnetic fields were
  generated by the rapid rotation of molten iron
  cores in planets, but Mercury is so small that
  its core should have quickly solidified after
  forming.
• So the magnetic field is probably not now being
  generated.
• Perhaps the magnetic field has been frozen into
  Mercury since the time when its core was molten.

83
6.3d Mercury Research Rejuvenated

• After decades with no additional images of
  Mercury, in 2000 two teams of scientists released
  their composite images.
• Each is better than any previous ground-based
  image (see figure).
• A dozen years after Mariner 10 sent back data
  about Mercury, an important new discovery about
  Mercury was made with a telescope on Earth
  Mercury has an atmosphere!

• The atmosphere is very thin, but is still easily
  detectable in spectra.
• It was a surprise that Mercury has an atmosphere
  because it is so hot, since it is relatively
  close to the Sun.
• Hot means that the individual atoms or
  molecules are moving rapidly in random
  directions, and to keep an atmosphere a planet
  must have enough gravity to hold in the particles
  moving in its atmosphere.
• Since Mercury has relatively little mass but is
  hot, the atmospheric particles escape easily and
  few are leftnone from long ago.

84
6.3d Mercury Research Rejuvenated

• Mercurys atmosphere contains more sodium than
  any other element150,000 atoms per cubic
  centimeter compared with 4500 of helium and
  smaller amounts of oxygen, potassium, and
  hydrogen.
• At first, it appeared that the sodium was ejected
  into Mercurys atmosphere when particles from the
  Sun or from meteorites hit Mercurys surface.
• Newer evidence that the potassium and sodium are
  enhanced when the Caloris Basin is in view
  indicates, instead, that Mercurys atmosphere may
  have diffused up through Mercurys crust the
  crust is thinner than average in the Caloris
  Basin.

85
6.3d Mercury Research Rejuvenated

• Mercurys surface features can be mapped from
  Earth with radar.
• The radar observations provide altitudes and the
  roughness of the surface.
• The radar features show, in part, the half of
  Mercury not imaged by Mariner 10.
• It, too, is dominated by intercrater plains,
  though its overall appearance is different.
• The craters, with their floors flatter than the
  Moons craters, show clearly on the radar maps.
• The scarps are obvious as well.
• The highest-resolution images (see figure) reveal
  probable water-ice deposits near Mercurys poles.
  
• They have been shielded from sunlight