Astronomy 330

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Astronomy 330

• Lecture 9

Apollo 16 image courtesty of NASA
http//images.jsc.nasa.gov/
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Astronomy 330 Lunar History

• Early in its history the Moon underwent many
  impact events of different sizes, many were very
  large
• Impacts of the Moon and other S.S. bodies are
  much less frequent today.
• Catastrophism - geological term used to describe
  events which change surface of a body in a short
  time (volcanoes, impacts)
• Uniformitarianism - opposite of catastrophism,
  describe surface features in terms of very slow
  changes over low time spans (erosion and
  sedimentation on Earth).

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Astronomy 330

• Uniformism and catastrophism both operate in the
  Solar System
• Cratering on the Moon, along with lunar samples,
  and measurements of the number of near Earth
  asteroids allow us to construct a rough history
  of inner Solar System
• Lunar impact evidence suggests that the cratering
  rate has not changed much for the past 3.8
  billions years. During this period impacts
  probably due to near-Earth asteriods and comets
  like we see today.

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Astronomy 330

• Lunar cratering evidence can be applied to
  understand cratering histories of inner planets
  as well.
• However, before 3.8 billions years ago, quite
  different conditions (in terms of cratering) must
  have prevailed.
• The rate of impacts must have been much higher to
  explain the high crater densities in the Lunar
  highlands. (Recall the dates for the formation of
  the Maria).
• Crater counts indicate that the cratering rate
  must have been at least 1000 times greater 4
  billion years ago than at 3.8 billion years ago.

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Astronomy 330

• The period at about 3.9 billion years ago is
  called the late heavy (or terminal) bombardment
  period.
• The impacting objects then were probably from a
  different source than impacting objects today.
• Objects at this time could have come from the
  collision and breakup of asteroids have been left
  over accreting junk from final stage of planet
  formation

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Astronomy 330

• Before 3.9 billion years ago we dont have much
  information about impact rates because evidence
  has been destroyed by subsequent impacts on the
  Moon.
• E. Shoemaker, by carefully counting the number of
  near-Earth objects, showed this would produce the
  observed number of craters on the Lunar Maria.

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Astronomy 330 Lunar Highlands

• Oldest part of the Lunar crust, heavily cratered,
  craters are one on top of another.
• This being the case, we dont have a good idea
  how many impacts took place in the pastimpact
  craters have reached a saturation pointnew
  craters not created, just replace old ones
• Seismometers placed on Lunar surface by Apollo
  indicate that Lunar crust in highlands is
  shattered to 25 km, surface rubble depth is
  hundreds of meters thick (maria its about 10 m)

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Astronomy 330

• In short the highlands are heavily fragmented
• Rocks in highlands are breccias - rocks formed by
  cementing together of fragments of and pieces of
  other rocks shattered by impacts
• Breccias are complex and indicate 3 and maybe 4
  generations of shattering and them reformation
• Breccias often contain frozen droplets of
  impact-melted material (breccias in asteroids do
  not show this).

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Astronomy 330

• Oldest dated (from radioactive dating) materials
  on the Moon come from pieces within breccias.
• Indicates ages of 4.2 - 4.4 billion years
• Dating incates Lunar catastrophism probably
  lasted up until 3.8 billions years ago.
• Composition of highland rocks is mostly devoid of
  volatile elements (not ices, but elements which
  easily are evaporated, N, C, S, Cl, and K) like
  the rest of the Moon.
• The Moon lacks both metals and volatile elements
  (a clue to its formation)

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Astronomy 330

• Most common material in highland cust
  isanorthosites.
• Anorthosites - igneous silicate rocks made up
  mainly of Si, Al, Ca, and Mg.
• Their presence suggests that the Moon
  differentiated early in its history since these
  type of rock might have formed out of originally
  molten material, before crust solidified

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Astronomy 330

• Bombardments in the early history of the Moon
  must have caused melting and also punctured the
  crust to allow molten interior to flow out,
  indicated by oldest basalts of 4.2 million years.
• Conventional also probably occurred in early
  history of Moon as crust thickened, eventually
  ceased as heat from Moon dissipated.

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Astronomy 330 Lunar Basins

• Remember these are the results of HUGE impacts on
  the Moons surface.
• Basin refers to an impact crater larger than
  300 km in diameter, must be caused by an object
  on the order of 30-100 km in diameter.
• Largest basin is near the Lunar south pole and is
  about 2,200 km in diameter.
• Youngest basins are Imbrium and Orientale.

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Astronomy 330

• Apollo obtained samples from Imbrium which
  indicate that the time of impact was about 3.9
  billion years ago. Ejecta from this impact
  provide a marked for other Lunar features.
• Ejecta from Orientale are on top of ejecta from
  Imbrium indicating Orientale is younger.
• Imbrium was later flooded by lavas
• Large mountains surround these basins, probably
  formed by uplift during the impacting event and
  later subsidence. Other mountains on the Moon
  appear to be just piles of ejecta.

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Astronomy 330 Lunar Volcanism

• Principle period of Lunar volcanism did not start
  until Orientale and Imbrium formed as indicated
  by lava flows filling Imbrium.
• Maria indicate volcanism
• Much information gained from Apollo missions
  since this is where they predominantly landed.
• Maria basalts resemble Earths basalts and are
  believed to orginate in the same way subsurface
  melting of rocks and outpouring of lava

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Astronomy 330

• Lunar basalts have more Fe and show no signs of
  interacting with H2O.
• All lunar rocks are igneous, have a simpler
  composition than rocks on Earth since they formed
  in an environment without water and oxygen.
• 100 minerals identified on Moon, 2,000 on Earth
• Oldest basalts on Moon are 3.8 billion to 3.7
  billion years old, youngest are 3.2 billion years

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Astronomy 330

• Lunar volcanic eruptions did not create mountains
  like on Earth.
• Instead erupted from long fissures and flowed
  over large areas, creating relatively flat planes
  (the Maria).
• Eruptions happened many times, with one outflow
  covering the last
• A single flows depth was measured to be 30 - 50
  m, total depth is 5 km indicating repeated flows.

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Astronomy 330

• As lava cooled it shrunk and it formed cracks,
  also wrinkled ridges.
• Rilles are long, sinuous valleys resembling
  stream beds. Formed by rivers of lava.
• Some volcanic mountains may be present on the
  Moon, the domes in Oceanus Procelarum, and Marius
  Hills, where not visited by Apollo, so we dont
  know conclusively

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Astronomy 330 Wrinkle ridges
Image from consolidated lunar atlas
http//www.lpi.usra.edu/resources/cla/
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Astronomy 330 Rilles
Image courtesy of NASA http//images.jsc.nasa.gov
/index.html
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Astronomy 330 Rilles
Image courtesy of NASA http//images.jsc.nasa.gov
/index.html
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Astronomy 330

• Maria confined mainly to the near side of the
  Moon
• Seems to be because surface elevation is lower
  there, hence internal pressure could more easily
  force lava to the surface there
• Lunar basalts are 3 times fractionated or
  chemically separated relative the solar nebula
  material
• 1) Loss of water, volatiles
• 2) differentiation
• 3) partial melting

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Astronomy 330

• Volcanic activity stopped about 3 billion years
  ago, Moons appearance has changed little since
  then (except for young craters being formed).
• Earth would appear quite different at that time

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Astronomy 330 The Lunar Surface

• No air, sky appears black
• Fine dust covers the surface along with
  fragmented rock, the ejecta from impact
  cratersthis is called regolith
• Lunar soil refers to the fine dust
• On maria regolith depth is 10 m
• In highlands depth is hundreds of m
• Core samples show layers a few cm thick, ejecta
  from different impacts

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Astronomy 330

• Most of the regolith at any one location come
  from nearby impacts
• Roughly 2 of the regolith is meteoritic
• Regolith accumulates at about 2 mm per million
  years (after the late heavy bombardment period).
• Microscopic examination indicates regolith is
  composed of spherical glass (melted silicates)
  particles, indicating impact origin

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Astronomy 330

• Since the Moon has no atmosphere, it undergoes
  large temperature changes (day and night are each
  2 weeks long).
• Ranges from 110o C to -170o C
• Mountains on the Moon are relatively round, not
  sharp as on Earth due to regolith blanket and
  lack of erosion.

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Astronomy 330 The Lunar Surface
Image courtesy of NASA http//images.jsc.nasa.gov
/index.html
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Astronomy 330 The Lunar Surface
Image courtesy of NASA http//images.jsc.nasa.gov
/index.html
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Astronomy 330 The Lunar Surface
Image courtesy of NASA http//images.jsc.nasa.gov
/index.html
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Astronomy 330 Lunar Missions

• Pioneer, U.S.
• Early series of Spacecraft designed to go to Moon
• Later Pioneers went to other parts of SS.
• Pioneer 4 was first successful U.S. mission to
  Moon (1959) and first to escape Earths gravity.

Image courtesy of NASA http//solarsystem.nasa.go
v/missions
Image courtesy of NASA http//solarsystem.nasa.go
v/missions
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Astronomy 330

• Luna Series, USSR
• 17 or 45 mission succesful
• Robotic missions
• First to impact Moon and also land on Moon
• First to photograph far side
• Some mission returned samples

Image courtesy of NASA http//solarsystem.nasa.go
v/missions
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Astronomy 330

• Ranger program
• Took high quality images of Moon in preparation
  for Apollo
• Designed to crash into Moon while photographing it

Image courtesy of NASA http//solarsystem.nasa.go
v/missions
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Astronomy 330

• Surveyor, U.S.
• Robots designed to land on the Moon
• Tested landing techniques for Apollo
• Surveyor 1 first to make soft landing (1967)
• 5 successful landings (theyre still there)

Image courtesy of NASA http//solarsystem.nasa.go
v/missions
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Astronomy 330

• Lunar Orbiter, U.S., 1966-1967
• Took high resolution photographs of Lunar surface
  in preparation for Apollo
• Mapped 99 of Lunar surface

Image courtesy of NASA http//solarsystem.nasa.go
v/missions
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Astronomy 330

• Apollo, U.S., landed people on the Moon, 1969 -
  1972.
• Primarily engineering/poltical missions
• Placed seismographs on moon, other instruments
  (ALSEP)
• 6 successful landings
• Returned Lunar samples (382 kg)

Image courtesy of NASA http//solarsystem.nasa.go
v/missions
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Astronomy 330 An ALSEP
Image courtesy of NASA http//images.jsc.nasa.gov
/index.html
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Astronomy 330

• Hiten-Hagomoro, First Japanese mission
• First to use aero-braking

Image courtesy of NASA http//solarsystem.nasa.go
v/missions
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Astronomy 330

• Clementine
• A mission to test cheap technology
• Found evidence of ice at poles of Moon

Image courtesy of NASA http//solarsystem.nasa.go
v/missions
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Astronomy 330

• SMART-1, European
• Currently in operation
• Test new technologies, including an ion engine
• Will explore dark regions of South Pole for the
  first time

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Astronomy 330 Future Missions

• Lunar-A, Japanese, will carry penetration probes
  w. seismometers to better determine the interior
  structure of the Moon
• SELENE - Japanese, will map moons surface, study
  lunar origins and evolution, test new
  technologies
• Lunar Reconnaissance Orbiter - U.S., map lunar
  topography, radiation fields near Moon,
  preparation for future landings