Solar System Astronomy: The Moon

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Solar System Astronomy The Moon

• Announcements
• Homework due Monday (on Website) is
• p. 402 test 1-5 (Focus 2 Time)
• p. 425 prob. 4, p. 426 test 1-5 (Chap. 13 The
  Moon)
• Exam 2 grades emailed Tuesday Average 76.3
• Homework 5, 6, extra credit grades will be posted
  later this week
• Email addresses needed for A. Branch, C. Boyd, J.
  Kehoe
• Extra credit assignment 2 (10 pts possible).
  Moon observing project p.427. Hand in Monday.
  Note Moon is nearly full this weekend.

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Moon
Sea of tranquility (Apollo 11)
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Moon Chapter Learning Goals

• Be able to describe the surface of the Moon
  (both sides).
• Know what libration is and how it relates to the
  Moon's orbit around the Earth.
• Be able to explain how craters, maria, and
  highlands formed on the Moon.
• Know the major manned missions to the Moon and
  what they accomplished.
• Be able to describe the Moon's chemical
  composition and internal structure.
• Understand the Moon's role in producing tides on
  Earth.
• Know the major kinds of rocks found on the Moon.
  
• Be able to describe how scientists determine the
  ages of lunar rocks.
• Understand the collisional ejection theory of
  the Moon's creation.

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Preliminary Questions

• Why is only one side visible from Earth?
• A Tidal locking
• Why is the far side so different from the near
  side?
• A Far side is thicker, also a result of tidal
  forces.
• What are those dark patches anyway?
• A Theyre called Maria, and are ancient lava
  flows
• What kind of atmosphere does the Moon have?
• A None because the escape speed is too low.
  (actually a tiny bit of Helium from solar wind).
• Is the Moon the largest satellite in the solar
  system?
• A Nope, several satellites of the outer planets
  are much larger.
• How old is the Moon? Is it older than Earth?
• A The Moon and Earth (and Sun and all other
  planets) are about 4.5 billion yrs old.
• Did astronauts really visit the Moon or was it
  faked by NASA and Hollywood?
• A Probably, but its a BIG government secret.

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The Moons Vital Statistics

• Mass 1.2 of Earths mass
• Diameter 27 of Earths diameter
• Density 60 of the Earths
• Surface gravity 17 of the Earths
• Escape velocity 21 of the Earths
• Surface temperature varies -115? C (no sun)
  to 138? C (sunlit)
• A world that can be described largely based on
  the above properties For the Moon ( Mercury),
  it is the story of magnificent desolation

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Comparison of Lunar Near, Far side Features
(Images from Clementine Lunar orbiting Spacecraft
1994)
Far side (invisible from Earth) Note almost
complete absence of maria (dark areas)
Near side (visible from Earth)
Why are they so different?
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Lunar Geology

• The most dramatic feature of the Moon (visible
  with the naked eye!) is the dichotomy between the
  dark/smooth mare/maria and the lighter and
  heavily cratered highlands.
• The lunar maria (seas) are great expanses of
  lowland planes covered by dark basalt (laval)
  flows (17 of the surface).
• Maria are younger than the highlands
• Often near the edges of the maria are sinuous
  rilles narrow winding channels cut by flowing
  lava.
• Provide evidence of volcanism but different
  from Earths volcanism. On the Moon there are no
  volcanic peaks and no active volcanism of any
  kind. Lunar volcanism happened long ago and was
  a very fluid process.

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Moons Geological Features Highlands, Maria,
Craters

• Highlands are much older (3.4 Byr)
• Ancient Moon had molten surface (volcanism)
• Maria (latin seas) are ancient lava flows, made
  of basalt rocks
• Surface is covered by regolith, a powdery loose
  rocky surface, caused by meteoric impacts
• Craters formed by meteoritic impacts Most formed
  in first 1 Byr, more recent impacts have rays

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Highlands and Mare Example
The Apollo 15 landing site showing the Apennine
mountains and the thin, winding Hadley Rill
Movie of landing
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Moons Interior
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Theories for Origin of Moon

• Fission Originally part of Earth but torn free.
• Problem would have fallen back or been flung
  into space, not into orbit.
• Should orbit in Earth's equatorial plane
• Fails to explain why lunar chemistry differs from
  Earth's
• Co-Creation Formed in its present orbit.
• Should orbit in Earth's equatorial plane
• Fails to explain why lunar chemistry differs from
  Earth's
• Capture Formed as a separate planet but captured
  by Earth
• Explains why Moon orbits in same plane as other
  planets
• Conditions for successful capture very stringent
• Impact Formed from Mega-Impact of Mars-sized
  planet
• Computer modeling suggests solar system forms 100
  or so small planets which then collide to make
  larger objects.
• Explains why Moon orbits in same plane as other
  planets
• Can explain why lunar chemistry differs from
  Earth's
• Avoids fatal problems of other theories
• Currently favored model

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Moons Angular Size

• About ½ degree, change because orbit is elliptical

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Libration Elliptical orbit allows view of more
than 50 of surface
Libration is the ability to see more than 50 of
the lunar surface (56 actually). It is a result
of the the moons orbit being significantly
elliptical (e 0.
Moon at Perigee
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The Moon
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Lunar Exploration Apollo Missions

• Apollo 1-10 were pre-landing missions (Apollo 1
  module fire killed 3 astronauts while being
  tested)
• Apollo 11-17 landed (except Apollo 13, which had
  an explosion and returned to Earth and orbiting).
  (1969- 1972)
• Principal Scientific results of the Apollo
  missions

• The Moon is ancient and still preserves an early
  history (the first billion years) that must be
  common to all terrestrial planets.
• The youngest Moon rocks are virtually as old as
  the oldest Earth rocks. The earliest processes
  and events that probably affected both planetary
  bodies can now only be found on the Moon.
• Early in its history, the Moon was melted to
  great depths to form a "magma ocean." The lunar
  highlands contain the remnants of early, low
  density rocks that floated to the surface of the
  magma ocean.
• The Moon is not a primordial object it is an
  evolved terrestrial planet with internal zoning
  similar to that of Earth.
• The Moon is lifeless it contains no living
  organisms, fossils, or native organic compounds

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Eclipses and the Moons Orbit

• Important points
• Moons orbit is inclined w.r.t ecliptic by 5?
• Intersection of ecliptic with moons orbit is
  called line of nodes
• Eclipses only occur when line of nodes is along
  the Earth-Sun line.

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Lunar Eclipse Geometry
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Lunar Eclipse Animation
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Solar Eclipses
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Eclipses during 2003
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The Saros Cycle
The Saros Cycle is a pattern of eclipses of the
Sun and Moon that repeats itself every 6585.32
days (18 yrs, 11 days, 8 hrs).

• If an solar eclipse is seen at a given location,
  when will the next eclipse be seen at that
  location? Three separate independent
  requirements
• Requires moon be at same phase (new) Synodic
  month 29.53059 days (29d 12h 44m)
• Requires that Moon be in the ecliptic plane
  (i.e., on the line of nodes) Draconian month
  27.21222 days (27d 05h 06m)
• Requires that Moon be in the same part of the
  orbit Anomalistic Month 27.55455 days (27d 13h
  19m )

It turns out that (almost exactly) 223 synodic
months 242 Draconic months 239 Anomalistic
months 18 yrs 11 days, 8 hrs (Saros Cycle)!
Note the 8 hour offset means that need 3 Saros
cycles to repeat eclipses at any given longitude
(54 yr, 34 days)
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Solar Eclipses and the Saros Cycle in History

• 585 BCE The most famous eclipse of ancient
  times ended a five-year war between the Lydians
  and the Medes. These two Middle Eastern armies
  were locked in battle when "the day was turned
  into night." The sight of this total solar
  eclipse (the date is fixed as May 28, 585 B.C.)
  was startling enough to cause both nations to
  stop fighting at once. They agreed to a peace
  treaty and cemented the bond with a double
  marriage. The eclipse was predicted by Thales,
  the celebrated Greek astronomer and philosopher,
  but the prediction was probably not known to the
  warring nations.
• The solar Eclipse was considered as an evil
  omen in the Chinese, Greek, Incas and Indian
  cultures. It is an indication from God of his
  anger towards 'man'. The ancient Chinese would
  produce great noise and commotion with all they
  could banging on pots and drums to frighten away
  the dragon.

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Columbus (1492) Eclipse Prediction saves him
from starvation in Jamaica

• Christopher Columbus, on his voyage
  attempting to discover a western passage to the
  Indies, is stranded in Jamaica, where he and his
  crew have stopped to gather supplies. The local
  people are unwilling to provide the food and
  supplies Columbus demands, and his crew is
  growing hungry and restless.
• Stuck in this awkward position, Columbus (it
  is said) hits on an ingenious solution from his
  astrological charts, he knows that a total lunar
  eclipse will happen in a few days. When the day
  arrives, he gathers the local people, tells them
  that he is very angry with them for withholding
  supplies, and that he will show his wrath by
  causing the moon to disappear.
• As if on cue, the moon begins to fade away
  behind the shadow of the earth. The local people
  are struck with terror, and they offer Columbus
  whatever he wishes, if only he will return the
  moon to its place in the sky. Columbus relents,
  the moon reappears in a few minutes, and Columbus
  and his crew are lavishly resupplied and sent on
  their way by the grateful Jamaicans.

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Lunar Rocks Basalts

• Basalts are rocks solidified from molten lava.
• On Earth, basalts are a common type of volcanic
  rock and are found in places such as Hawai'i.
  Basalts are generally dark gray in color
• When one looks at the Moon in the night sky, the
  dark areas are basalt. T
• The basalts found at the Apollo 11 landing site
  are generally similar to basalts on Earth and are
  composed primarily of the minerals pyroxene and
  plagioclase.
• One difference is that the Apollo 11 basalts
  contain much more of the element titanium than is
  usually found in basalts on Earth.
• The basalts found at the Apollo 11 landing site
  range in age from 3.6 to 3.9 billion years and
  were formed from at least two chemically
  different magma sources.

Apollo 11 basalt 10049. This sample has a mass of
193 grams and is up to 10 centimeters across.
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Lunar Rocks Breccias

• Over its long history, the Moon has been
  bombarded by countless meteorites. These impacts
  have broken many rocks up into small fragments.
• The heat and pressure of such impacts sometimes
  fuses small rock fragments into new rocks, called
  breccias. Many fragments can be seen in the
  breccia photograph shown at right.
• The rock fragments in a breccia can include both
  mare basalts as well as material from the lunar
  highlands.
• The lunar highlands are primarily a light-colored
  rock known as anorthosite, which consists
  primarily of the mineral plagioclase.
• It is very rare to find rocks on Earth that are
  virtually pure plagioclase.
• On the Moon, it is believed that the anorthosite
  layer in the highland crust formed very early in
  the Moon's history when much of the Moon's outer
  layers were molten.
• This stage in lunar history is known as the magma
  ocean. The plagioclase-rich anorthosite floated
  on the magma ocean like icebergs in the Earth's
  oceans

Apollo 11 breccia 10018. This sample has a mass
of 213 grams and is 8 cm across.
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Water on the Moon?

• How can ice survive on the Moon?
• The Moon has no atmosphere, any substance on
  the lunar surface is exposed directly to vacuum.
  For water ice, this means it will rapidly sublime
  directly into water vapor and escape into space,
  as the Moon's low gravity cannot hold gas for any
  appreciable time.
• Over the course of a lunar day (29 Earth
  days), all regions of the Moon are exposed to
  sunlight, and the temperature on the Moon in
  direct sunlight reaches about 395 K (395 Kelvin,
  which is equal to about 250 degrees above zero
  F). So any ice exposed to sunlight for even a
  short time would be lost.
• Hence, the only possible way for ice to exist
  on the Moon would be in a permanently shadowed
  area.

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Lunar Prospector (1998) Discovers evidence of
Water Ice

• On 5 March 1998 it was announced that data
  returned by the Lunar Prospector spacecraft
  indicated that water ice is present at both the
  north and south lunar poles.
• The ice originally appeared to be mixed in with
  the lunar regolith (surface rocks, soil, and
  dust) at low concentrations conservatively
  estimated at 0.3 to 1 percent.
• Subsequent data from Lunar Prospector taken over
  a longer period has indicated the possible
  presence of discrete, confined, near-pure water
  ice deposits buried beneath as much as 18 inches
  (40 centimeters) of dry regolith, with the water
  signature being stronger at the Moon's north pole
  than at the south (1).
• The water may be concentrated in localized areas
  (roughly 1850 square km, or 650 square miles, at
  each pole) rather than being spread out over
  these large regions.
• The estimated total volume of ice is 6 trillion
  kg (6.6 billion tons).

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But is evidence for lunar water convincing?
Crash of Lunar Prospector finds NO evidence of
water

• The controlled crash of NASA's Lunar Prospector
  spacecraft into a crater near the south pole of
  the Moon on July 31, 1999, produced no observable
  signature of water, according to scientists
  digging through data from Earth- based
  observatories and spacecraft such as the Hubble
  Space Telescope.
• Worldwide observations of the crash were
  focused primarily on using sensitive
  spectrometers tuned to look for the ultraviolet
  emission lines expected from the hydroxyl (OH)
  molecules that should be a by-product of any icy
  rock and dust kicked up by the impact of the 354-
  pound spacecraft.
• This lack of physical evidence leaves open the
  question of whether ancient cometary impacts
  delivered ice that remains buried in permanently
  shadowed regions of the Moon, as suggested by the
  large amounts of hydrogen measured indirectly
  from lunar orbit by Lunar Prospector during its
  main mapping mission.

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Death of Lunar Prospector July 1999
Lunar prospector crash-landed in the crater
marked by the smaller red circle, about 50 miles
wide and 2½ miles deep.
No water plume was seen, but only 40-10 kg of
water vapor was expected. This would have been
very difficult to detect, even with HST.
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Lunar question 1
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Lunar question 2
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Lunar question 3
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Lunar question 4
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Lunar question 5
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Lunar question 6
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Lunar question 7
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Lunar question 8
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Lunar question 9
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Lunar question 10