We are now geologists, having been astronomers and then chemists

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• We are now geologists, having been astronomers
  and then chemists
• Relative age dating using relationships between
  features on a surface to decide which is older
  and which is younger
• We will start with relative age dating using
  craters on planet surfaces--then move to other
  geologic features.

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Cratered highlands on the Moon (a), mare on the
Moon (b), and craters on Ganymede (c)

• Craters Holes in the ground made by natural or
  man-made events. Kinds of craters volcanic,
  sinkholes, explosion, impact.
• Impact craters are common on most solid bodies in
  the solar system. Even the Earth has some impact
  craters, though the effects of wind, water and
  other geologic processes we will talk about
  obliterate craters on Earth.

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• The source of the impacts that cause craters are
  comets and asteroids.

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• Impact cratering is a violent process that
  shatters the ground and fires material off the
  surface.
• http//www.classzone.com/books/earth_science/terc/
  content/investigations/esu801/esu801page07.cfm
• http//janus.astro.umd.edu/astro/impact/

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Earth
Jupiters moon Callisto

• Impact craters have different forms depending on
  size and the surface they've impacted, as well as
  their age.

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• Using craters to date surfaces as more and more
  impacts occur surface becomes more and more
  covered with craters. If a geologic event renews
  the surface it covers or erases the craters.
• The first application of this principle is to the
  Moon. Lunar highlands are heavily cratered while
  the mare are only lightly so. Thus the mare have
  been resurfaced and are thus termed "younger".

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• With the Moon we have samples of rocks from the
  highlands and mare courtesy of US and Russian
  missions. They date the highlands to 4.5 billion
  years and the mare to 3.8-4.0 billion. From this
  one can tell that the rate of impacts has not
  been constant with time--it was very great during
  the 1st half billion years of Earth history. It
  then declined very rapidly to a much lower value
  where it has roughly remained since.
• We can use this information to estimate how old
  various surfaces are on planets for which we have
  no samples. And those features that have impact
  craters superimposed on them can be dated as
  "old" or "recent".

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Dried tributary eminating from Gusev crater, Mars

• Thus we can use craters to date the relative
  occurrence of events, and even tell crudely "how
  old" certain features are.

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• But the rate of cratering may have been different
  far from the Earth, in the outer solar system.
  There, a different mix of impactors may have
  dominated cratering--more comets than asteroids,
  and icy debris surrounding the giant planets
  themselves. This is a significant uncertainty in
  using craters to date surfaces in the outer solar
  system.
• Another uncertainty is that thick atmospheres
  (Venus, Titan, Earth) tend to shield their
  surfaces from smaller impactors. And, on the
  Earth (and maybe Titan), erosion obliterates
  craters.

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Mars from Themis (ASU/JPL image)