Astronomy 330

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Astronomy 330

• Lecture 21

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Astronomy 330 The Large Satellites

• Jupiter - Io, Europa, Ganymede, Callisto
• Saturn - Titan
• Neptune - Triton
• We could consider these planets if they orbited
  the Sun.
• The are about the size of Mercury
• One has an atmosphere denser than Earths

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Astronomy 330 Jupiters Large Satellites
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID2098
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Astronomy 330 Saturns large satellite, Titan
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID3023
http//solarsystem.nasa.gov/planets/profile.cfm?Ob
jectSaturnDisplayMoons
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Astronomy 330 Neptunes Large Satellite, Triton
http//solarsystem.nasa.gov/planets/profile.cfm?Ob
jectNeptuneDisplayMoons
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Astronomy 330 Overview (Impact craters)

• The most important property for each satellite is
  its bulk composition (interior composition and
  structure)this has a major affect on what we see
  when we observe them.
• This also is the major factor in determining
  their geologic histories.
• These statements are true in the inner solar
  system, but it is even more true for the solid
  bodies in the outer solar system.

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Astronomy 330

• In the outer solar system there are many
  different substances present out of which we can
  imagine a body forming. Especially ices (that is
  volatile compounds) in addition to the metals and
  silicates which are common in the inner solar
  system.
• So, we might expect a wide difference in observed
  behavior from one outer solar system body to
  another.
• The dominant ice is water ice. Why? Because it
  is made with 2 H atoms!

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Astronomy 330

• Many of these outer solar system satellites are
  composed of up to 1/2 ices with the other half
  being rock (that is mixtures of silicates and
  metals).
• As a result, their bulk densities are lower than
  the Moons.

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Astronomy 330 Bulk properties of Jupiters
Satellites
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Astronomy 330 Bulk properties of Saturn and
Neptunes satellites
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Astronomy 330

• Based on this information, which of these bodies
  have more metals and siliactes than ices?

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Astronomy 330 Impact histories

• Yes, even more about craters!
• As we have seen, craters and counting them are
  very important for studying the Moon, Mercury,
  Venus, the Earth, and Marsthey give us a way
  constructing the geologic history of the surface
  of planet.
• Does the same apply for the outer solar system?
• Are the craters preserved in the same way on
  these icy bodies or do they erode due to
  different processes occurring there?

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Astronomy 330

• In other words, since these bodies have much
  larger icy contents than the inner solar system
  bodies, how would craters form and how would they
  evolve.
• On Earth we know that ice flows (as in
  glaciers) and can be deformed over time due to
  gravity.
• But, we also know that at very low temperatures
  ice becomes very stiff and strong (and more
  brittle as well).

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Astronomy 330

• At about the distance to Saturn and further out,
  ice should be about as strong as rocktherefore
  craters should be retained indefinitely (unless
  some other process such as heating or erosion
  occurs).
• However, at Jupiter the ice may not be as strong.
  
• Saturns satellite Rhea looks very much like the
  Moon.
• Callistos craters are significantly modified.

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Astronomy 330 Saturns Moon Rhea looks much like
the Moon
http//photojournal.jpl.nasa.gov/catalog/PIA06575
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Astronomy 330 Some of Jupiters Moon Callistos
Craters
http//photojournal.jpl.nasa.gov/catalog/PIA02277
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Astronomy 330

• Craters can be influenced by other processes
  unique to ice as well.
• Sublimation - ice to gas phase caused by constant
  bombardment of solar wind particles and from the
  magnetosphere of Jupiter.
• Also, the rate of impacts in the past and in the
  present may have been different in the outer
  solar system.

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Astronomy 330

• There probably are or have been fewer asteriods
  in the outer atmosphere but maybe more comets
  (remember SL 9?).
• Jupiters and the other gas giants gravities
  will attract more such bodies, so the impact rate
  might be higher in the outer solar system.

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Astronomy 330

• Inner Solar System sources for projectiles
• Asteriods and comets (this applies to the present
  and might have been different in the past)
• Outer Solar System
• Jupiter-family comets from the Kuiper belt
  (important for Jupiter)
• Long period comets are the primary source beyond
  Jupiter

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Astronomy 330

• The total impact rate near Jupiter is estimated
  to be about a factor of 2 lower than for the
  Earth-Moon system.
• It is lower by a further factor of 2 for Saturn.
• Comets and asteroids have different size
  distributions. There are fewer small comets than
  for asteroids, so there are fewer small craters
  in the outer solar system on the satellites of
  the gas giants
• Aside Maybe fewer small comets are produced by
  impacts between large comets or they evaporated
  away.

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Astronomy 330

• The size of a crater depends on the impact speed
  as well on the size of impacting object.
• Therefore, the mass of a planet or satellite will
  have an effect on size of the craters on its
  surface (large masses will accelerate impacting
  objects to higher speeds).

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Astronomy 330

• In the outer solar system both the mass of the
  body being impacted are important as well as the
  mass of the planet they are orbiting.
• Impacting objects are initially attracted mostly
  by the gravitational effect of the gas giant and
  only as they approach more closely are they
  attracted by the smaller satellites.
• This also has the effect of creating more impact
  events for satellites which orbit their planets
  at smaller distances.

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Astronomy 330

• As an example, Mimas the innermost large
  satellite of Saturn, should receive 20x the
  impacts of Iapetus (far from the planet).
• So, the outer satellites should have impact rates
  less than the Moon and the Earth, but the
  satellites near the planet will have higher
  impact rates.

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Astronomy 330 Mimas and Iapetus (moons of Saturn)
Mimas
Iapetus
http//photojournal.jpl.nasa.gov/
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Astronomy 330 Crater retention age

• Recall that crater retention age is the average
  amount of time a crater will be present on a
  surface before it is lost due to some process
  (remember what some of those processes are?).
• As it turns out, even though there are fewer
  impactors in the outer solar system, on average,
  the gravity of the gas giants compensates for
  this and the cratering on the satellites gives us
  a good picture of the ages of the surfaces of
  these satellites (provided we make the proper
  corrections).

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Astronomy 330

• So, if we see a crater density on some surface
  which is like that on the Lunar Maria that
  surface is probably at least several billion
  years old.
• Also, some of the satellites in the outer solar
  system have very high crater densities. As in
  the inner solar system these crater densities
  cannot be explained by the current rate of
  cratering and therefore there was a higher
  cratering rate in the past.

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Astronomy 330

• Therefore, it can be deduced that there was a
  rate of intense cratering (i.e. lots of debris
  was flying around) early in the history of outer
  solar system as well as in the inner solar
  system.