ASTR 330: The Solar System

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ASTR 330 The Solar System

• Announcements

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Lecture 13
• The Moon
• and
• Mercury

Dr Conor Nixon Fall 2006
Photos Moon - John French, Abrams Planetarium,
Michigan State Univ. Mercury - NASA.
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ASTR 330 The Solar System

• Apparent Motion of Mercury

• What about epicycles and retrograde motion, and
  do they just apply to the apparent motions of the
  outer planets?
• The answer is yes and no! Mercury and Venus also
  exhibit retrograde motion, as illustrated in the
  graphic below for Venus. Mercury is similar.
• The only difference between the inner and outer
  planets is that part of the retrograde path is
  unseen, because the planet is in front of the Sun.

• In the Ptolemaic system (geocentric), epicycles
  were used to explain the motion of Mercury,
  including the direction reversals.

Dr Conor Nixon Fall 2006
Figure credit Geometry Technologies, Science
U.Com
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ASTR 330 The Solar System

• Mercury and the Sun

• Due to its innermost position in the solar
  system, Mercury never strays far from the Sun, as
  seen from Earth.

• In fact, the largest possible angular separation
  between the Sun and Mercury is 28 on the sky.

• Hence, Mercury is always seen near sunrise or
  sunset.

Dr Conor Nixon Fall 2006
Figure credit Richard Pogge, Ohio State
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ASTR 330 The Solar System

• Mercurys Day

• Mercurys period of orbit about the Sun, its
  year was easy to observe and determine it is
  88 Earth days long.
• Much harder was to determine its rotational
  period how long it takes to spin once on its
  axis.
• Until several decades ago, the only way to
  determine this was to try to observe features on
  the surface, and see how long they took to rotate
  all the way around.
• These observations appeared to show that Mercury
  rotated once per orbit, i.e. its day and its year
  were the same.
• This is the same situation as the Earth-Moon
  system. The Moons rotation is tidally locked
  to its orbit about the Earth, and so we always
  see the same face. Lets take a look at tides

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• What Are Tides?

• Until now we have always been able to think of
  planets as point masses the size was irrelevant
  (see textbook Chapter 1 Question 1).
• But the fact that planets are not point masses
  is important.
• Tides are caused by differential gravity, the
  fact that the side of a planet towards a
  companion is more strongly pulled than the side
  away from the companion.
• For a completely solid, rigid planet, tides
  would have little effect.
• But most solar system bodies have either some
  liquid interior, or are flexible enough to deform
  somewhat.
• Lets look at the familiar tides between the
  Earth and Moon.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Tides two tidal bulges

• Moon's Gravity Pulls Oceans - Near-side Bulge is
  Easy to Understand
• Moon and Earth actually orbit around the
  Earth-Moon Center of Mass (about 1500 km beneath
  the surface of the Earth)
• Motion of Earth Around Center of Mass Creates a
  Bulge on the Far Side of the Earth

Dr Conor Nixon Fall 2006
Figures and text Steve Dutch, Univ. Winconsin,
Green Bay
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ASTR 330 The Solar System

• Tides from Sun and Moon

• Neap Tides
• First or Last Quarter
• Sun and Moon Pulling At Right Angles
• Lunar and Solar Tides Partially Cancel
• Unusually Small Tidal Range

• Spring Tides
• New or Full Moon
• Sun Moon Pulling in Parallel Directions
• Lunar and Solar Tides Add Up
• Unusually Large Tidal Range

Dr Conor Nixon Fall 2006
Figures and text Steve Dutch, Univ. Winconsin,
Green Bay
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ASTR 330 The Solar System

• Tidal Friction

• Rotation and Friction Causes Tides to Lead Moon
• Bulge Pulls Moon, Throws into Larger Orbit
• Friction Slows Earth
• Precambrian (900 m.y.) Year 500 Days,
• Day 18 Hr.,
• Month 23.4 Days
• Cambrian (500 m.y.)
• Year 400 Days
• Day 22 Hr.

Dr Conor Nixon Fall 2006
Figures and text Steve Dutch, Univ. Winconsin,
Green Bay
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ASTR 330 The Solar System

• Tides Over Time

• The fact that there is friction between the
  tidal bulges of water, and the rotating seabed
  underneath, dissipates huge amounts of energy
  currently 2 billion horsepower (think 5 million
  Corvettes!)
• The pulling of the Moon on the nearest tidal
  bulge causes the Earths day to grow longer by 20
  secs per million years.
• Part of the frictional energy goes into heating
  the oceans and the Earths crust.
• As the Earth slows its spinning, the Moon moves
  farther away. This is due to conservation of
  angular momentum for the Earth-Moon system think
  of a skater stretching her arms and slowing down.
• In the past, a Earth day was as short as six
  hours, and a lunar orbit (month) was equivalent
  to only one week (of 24-hour days).

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Synchronous Rotation

• The Moon currently keeps the same face always
  towards the Earth called synchronous rotation.
• This is not coincidence. Tidal damping by
  flexing and energy dissipation in the Moons
  crust slowed its rotation in the past.
• When the Moon reached synchronous rotation with
  respect to its orbital period, it stopped
  slowing.
• Synchronous rotation is a minimum energy
  dissipation configuration, and hence is stable.
• As the Moon gradually recedes from the Earth,
  its rotation will slow further to match its
  lengthening orbital period.
• The same effect will eventually slow the Earth
  until the same side always faces the Moon!

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Synchronous rotation of Mercury?

• If Mercury was tidally locked to the Sun, like
  the Moon to the Earth, it would be easy to
  understand.
• Because of the huge mass of the Sun, and the
  proximity of Mercury to the Sun, a strong tidal
  locking effect was expected.
• Note that this would have a dramatic effect on
  the surface temperature at various longitudes
• On the Sun-facing side, it would be perpetual
  daytime, and intensely hot.
• On the dark side (a real dark side, not like
  the Moon), it would be perpetual night, and
  incredibly cold.
• Everything was as expected, until

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Radar determination of rotation

• In 1965, radar astronomy got seriously underway.
  Radar techniques gave a second way to determine
  the rotation period.

• A radar signal bounced off a rotating object
  becomes more spread out (or broadened) in
  frequency. The more broadening, the faster the
  body is rotating (see section 8.2 of the book for
  more details).

Dr Conor Nixon Fall 2006
Figure credit Nanjing University Astronomy
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ASTR 330 The Solar System

• Unexpected News

• The news from the radar astronomy team was
  shocking Mercury actually spinned faster than 88
  days, in fact, it took only 59 days to rotate!
• How could such a huge difference be overlooked?
  It turns out that Mercury is only in a good
  position for observation every two orbits, i.e.
  every 176 days.
• Astronomers had seen the same feature in the
  same place every 176 days. Synchronous rotation
  was expected, and so everyone assumed that
  Mercury had made two rotations. In fact, it had
  made three!
• So why the 59-day rotation, and not the 88-days
  as we expect from tidal locking?

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Elliptical Orbits and Tidal Locking

• The reason is Mercurys elliptical orbit
  (e0.206). As in all elliptical orbits, Mercury
  travels faster closer to the Sun, and slower
  further out (Keplers Second Law equal areas in
  equal times).
• Unlike more circular orbits, this gives Mercury
  a choice of which orbital speed to lock its
  rotation speed to.
• The strength of tidal forces is extremely
  sensitive to the distance between the bodies
  proportional to 1/D3, instead of 1/D2 as for
  gravity. So, tides are much stronger for Mercury
  when it is closest to the Sun (perihelion).
• So, the tidal forces at perihelion dominated,
  and locked Mercurys rotation to its fastest
  orbital velocity, not its average orbital
  velocity. Mercury hence has a 32 spin-orbit
  coupling.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Solar Day on Mercury

• The 32 resonance leads to some curious effects
  when we consider the concept of day.
• If we use day to mean, not a single rotation
  period, but the time from sunrise to sunrise, we
  find that a Mercurian day is 176 Earth days, or
  two Mercurian years long! Why?
• The reason is due to the fact that Mercury, as
  it spins on its axis, is also going around the
  Sun.
• By the time Mercury has spinned around once on
  its axis, the planet has moved 2/3 of the way
  around the Sun. The Sun is no longer where it was
  (relative to the stars) 59 days before!
• By the time Mercury catches up with the
  elusive Sun, it has actually turned three times
  around on its axis and two years have gone by.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Mercurys long, long day

• Lets imagine we were standing at the red mark
  on Day 1.
• Day 29 half a rotation period. On the Earth we
  would call it sunset. But on Mercury, it is only
  late morning.
• Now look at Day 58 by its rotation, we are
  looking again at the same stars (1 sidereal day),
  but the Sun is not rising, its late afternoon.
• Sunset actually occurs on Day 89, and sunrise
  again is not until Day 176.

Dr Conor Nixon Fall 2006
Figure credit ESO Education and Outreach Office
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ASTR 330 The Solar System

• Mercurys day and year

• The animation (left) shows the day and year of
  Mercury.
• Note that the red dot is facing the Sun when the
  Sun is also closest (perihelion).
• This is called a hot longitude. Because a
  Mercurian day takes 2 years, there are two
  different hot longitudes on either side of the
  planet.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Hot Longitudes

• Imagine we were able to spend several years
  living on the surface of Mercury. Depending on
  which longitude we stayed at, we would see very
  different sorts of days.
• At a hot longitude, we would see
• The Sun would rise small, then grow in size as
  mid-day approached.
• Racing through perihelion, the Sun would
  apparently hang motionless and large in the sky.
• The Sun would then speed up as it sinks towards
  sunset.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Cool Longitudes

• An observer at a cool longitude, 90 to either
  side of a hot longitude, would see the exact
  opposite
• The Sun would be large and stationary near
  sunrise and sunset.
• At noon, the Sun would be small and moving
  quickly.
• On average, the temperature at a cool longitude
  is 550 K, and 100 K more than that at the hot
  longitudes!

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Exploration of Mercury

• Almost all our knowledge of Mercurys geology
  comes from a single spacecraft Mariner 10.
• Mariner 10 was the first spacecraft to
  successfully use a gravity assist maneuver at one
  planet (Venus) to reach another.

• Mariner 10 made 3 close flybys of Mercury in
  1973 and 1974, confirming that Mercury had no
  atmosphere, and a heavily cratered surface not
  unlike the Moon.
• However, even after Mariner 10, only 45 of
  Mercurys surface had been mapped in detail.

Dr Conor Nixon Fall 2006
Image NASA/NSSDC
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ASTR 330 The Solar System

• Mercury as Seen By Mariner 10

Dr Conor Nixon Fall 2006
Images NASA/NSSDC
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ASTR 330 The Solar System

• Mercury Geology Overview

• The surface of Mercury could easily be mistaken
  at a casual glance for the Moon! The surface of
  Mercury bears an uncanny resemblance to the
  ancient lunar highlands.
• Mercury might have been expected to have more
  geologic activity than the Moon, being larger,
  but in fact only one lava-flooded impact basin
  was found (like the Lunar maria).
• The surface appears to be largely igneous
  silicate rocks, like the lunar anorthosites
  (highlands) but we do not see much evidence for
  dark basalts like the lunar maria.
• Mercury is hence brighter than the Moon.
• We do not have any returned samples from
  Mercury, so we are still ignorant of the exact
  composition of the rocks. Unless reports of a
  Mercurian meteorite are true!

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Temperatures and poles

• At noon at the hot longitudes, the temperature
  rises to 675 K, hot enough to melt some metals,
  while during night the temperature falls to only
  90 K, cold enough to freeze CO2 (dry ice).
• Do you think Mercury has much volatiles, or any
  polar caps?
• In fact, radar experiments since Mariner 10 have
  showed the distinctive echo of frozen material at
  Mercurys poles, in complete contrast to
  expectations!
• Because Mercurys spin is almost 90 to its
  orbit (unlike the Earth, which tilts at 23) the
  poles do not get much sunlight, and remain cool
  enough for ices. But, all our formation theories
  indicate that Mercury would not have retained any
  volatiles when it formed.
• So how do you think the ices got there?

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• The Lunar Poles

• In fact, the Moon has also been shown quite
  recently to have some polar ices.
• The first evidence came from a DoD spacecraft
  called Clementine in 1996, which saw reflections
  at the bottom of craters near the lunar south
  pole.
• A second mission was soon launched to make sure.

• The Lunar Prospector mission in 1998 was able to
  confirm, using a neutron detection technique to
  detect hydrogen, that H20 ice was likely.

Dr Conor Nixon Fall 2006
Image Clementine DSPSE/NRL
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ASTR 330 The Solar System

• Polar Ices

• Polar ices on both Mercury and the Moon are only
  possible in the cold shadows of craters, places
  which are permanently protected from sunlight.
• The only plausible explanation for the ices to
  be there is from cometary impacts the craters
  act as cold traps to condense and retain the
  volatile H2O (and any other ices).
• Even a few km2 of water ice is exciting for
  space enthusiasts who want to send humans back to
  the Moon on a permanent basis they would not
  need to take their water with them!
• Also, water ices can be electrolyzed using solar
  energy to produce H2 and O2 a possible fuel
  source.
• In January 2004, the President announced a
  permanent return to the Moon in his State of the
  Union address made possible by this discovery.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Mercury Craters

• The crater density on Mercury varies from about
  1000 per million km2 (like the lunar highlands),
  down to 100 per million km2. The density never
  goes as low as the lunar maria.
• The ejecta blankets are smaller as expected
  from the higher gravity.
• None of the lunar geological features associated
  with volcanism flow fronts, sinuous rilles, etc
  have been spotted on Mercury.
• This leaves the question as to why the surface
  is not cratered evenly something must have
  covered the regions of lower density at some
  point in the past.
• Was it some volcanism we do not yet recognize?
• Was it a very large covering of ejected crater
  material?
• No-one knows for sure.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Caloris Basin

• The largest impact basin is located near one of
  the two hot longitudes, and accordingly named
  Caloris (heat) 1300 km across.
• The image (right) shows half of Caloris as seen
  by Mariner 10. It bears a resemblance to
  Orientalis.
• However, the floor of Caloris is cracked, rather
  than smooth like the lunar basins, filled with
  basalt.
• One hypothesis is that Caloris shows impact
  melting, rather than later in-filling.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Compression Scarps

• The most distinctive identifying features of
  Mercurys crust are compression scarps (cliffs)
  which are not seen on the Moon.
• The scarp called Discovery is 500 km long and up
  to 3 km high.
• Scientists believe these were caused by cooling
  shrinking of Mercury, millions of years after
  the crust formed, perhaps associated with tidal
  friction and de-spinning.
• Mercury has lost an estimated 4 of its surface
  area in this way, and 2 of its radius.

Dr Conor Nixon Fall 2006
Image NASA/NSSDC Figure Scott Hughes, Idaho
State
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ASTR 330 The Solar System

• Densities and Interiors

• The bulk densities of the Earth, the Moon and
  Mercury are 5.5, 3.3 and 5.4 g/cm3 respectively.
  However, we have learned before (in the Asteroids
  class) that densities can be misleading. Remember
  Psyche?
• As well as porous asteroids, we must also beware
  of extra-compressed interiors. E.g. the iron in
  the Earths core is at a density of 15 g/cm3,
  when it would only be 9 g/cm3 at the surface
  pressure.

• A more useful indicator for the planets is the
  uncompressed densities which are 4.5, 3.3 and
  5.2 g/cm3 for the Earth, Moon and Mercury.
• This shows that the Moon is mostly rock, the
  Earth is rock and some metal, and Mercury has
  even more metal inside.

Dr Conor Nixon Fall 2006
Figure Nanjing University
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ASTR 330 The Solar System

• Interior Compositions

• The difference between the composition of the
  Moon and the Earth is one fact we must explain,
  if they formed together.
• For Mercury, our calculations indicate that 60
  of the mass comes from metal.
• Mercury is hence basically an iron ball (3500 km
  across) covered by a 700 km layer of dirt!
• Mercury is extremely deficient in silicate rocks
  compared to the Earth. There are two possible
  scenarios for this
• Mercury formed without the silicates, due to
  fractionation in the solar nebula.
• Mercury formed with some silicates, but later
  lost them somehow.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Solid or liquid?

• It is also pertinent to ask whether the cores of
  the Moon and Mercury show any signs of being
  liquid, which could cause a magnetic field.
• For the Moon, we have direct evidence, from the
  ALSEP (seismo-meters) left by the Apollo
  astronauts.

• Natural moonquakes are very small compared to
  earthquakes the energy released is 100 billion
  times less in any one year.
• Moonquakes are due to heat flowing from the
  core mostly due to natural radioactivity. The
  Moon may have some molten rock in the core.
• The Moon has no metal core, hence (almost) no
  magnetic field.

Dr Conor Nixon Fall 2006
Figure Nanjing University
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ASTR 330 The Solar System

• Mercurys Core

• No-one knew what to expect from Mercury!
• In fact, Mariner 10 did discover a magnetic
  field at Mercury, about 1 as strong as the
  Earths.
• Thus is presented a paradox on the one hand, a
  magnetic field indicates a moving, fluid metallic
  core.
• But, if the core is hot enough to be molten, how
  come there has been so little geologic activity,
  in the last several billion years.

• A mystery indeed, which forthcoming space
  missions (Messenger, BepiColumbo) will try to
  solve.

Dr Conor Nixon Fall 2006
Figure Scott Hughes, Idaho State
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ASTR 330 The Solar System

• History and Evidence

• We now turn our attentions to origin and
  evolution of the Moon and Mercury. We have much
  more data for the Moon than Mercury.
• We have about 4 billion years worth of evidence
  in the form of the exposed lunar surface to
  constrain our models.
• Mercurys surface is probably a similar age, but
  we do not know for sure, in the absence of dated
  samples. But we can apply stratigraphy.
• Beyond 4 Gyr in the past, the high impact rates
  of the early solar system have destroyed any
  detailed evidence of the lunar surface, although
  we have rock samples dated to 4.4 Gyr.
• We believe that both the Moon and Mercury were
  largely molten and fully differentiated as our
  story begins 4.4 Gyr ago

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• A Lunar Timeline (i)

• 4.4 billion years ago
• the Moon is mostly molten and differentiated.
• The lunar highland crust is beginning to form.
• The oldest rocks we have evidence for solidified
  at this time.
• Heavy impacts are continuing breaking and
  remaking the crust, and forming breccias.
• 4.2 billion years ago
• The magma ocean is beginning to cool and
  solidify.
• The end of the heavy bombardment is happening.
• 3.8 billion years ago
• the huge Imbrium Basin is formed by a massive
  impact, blanketing the nearside in Fra Mauro
  material.
• The Orientale basin is formed soon after.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• A Lunar Timeline (ii)

• 3.4-3.3 billion years ago
• parts of the interior, heated by radioactivity,
  are still molten.
• expansion of the crust causes fractures lava
  seeps out, flooding the Imbrium Basin.
• 3.0 billion years ago
• Large-scale volcanism has now ceased, the Moon
  continues to cool.
• 3.0 billion years ago to present day
• Cratering continues at a slow rate. Large
  impacts are rare.
• 1.0 billion years ago
• The crater Copernicus is created.
• 0.1 billion years ago
• The youngest major crater, Tycho is formed.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Mercury Timeline

• Our historical timeline is mostly guesswork
• 4.4 billion years ago
• Mercury is molten and differentiated, like the
  Moon.
• 4.2 billion years ago
• Heating and expansion are occurring during the
  end of the heavy bombardment period there may
  have been widespread volcanism.
• 3.4 billion years ago
• Perhaps due to Mercurys nickel-iron core
  Mercury began to shrink. Any cracks in the crust
  were squeezed shut, and scarp ridges formed.
• 3.0 billion years ago to present
• Mercury continues to cool and solidify, but
  still may be much hotter and more fluid inside
  than the Moon.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Mysteries Of Lunar Formation

• Although lunar rocks have many similarities to
  terrestrial rocks
• Isotopic composition, some similar minerals,
• There are also important differences
• Less volatiles almost no water, depleted
  potassium.
• Higher abundance of Ca, Ti, Al.
• How did these differences occur, if the Moon and
  Earth formed at the same place and time? There
  are three traditional theories
• Daughter Theory.
• Sister Theory.
• Capture Theory.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• The Daughter Theory

• This theory was proposed in 1880 by the
  astronomer George Darwin (son of Charles Darwin),
  and is sometimes called the fission theory.
• In brief, the theory suggests
• The Earth was once rapidly spinning and molten.
• Sometime after differentiation, the Earth split,
  and a piece was flung off into orbit, forming the
  Moon.
• The piece was largely composed of Earth mantle
  material, and so is depleted in metals.
• There are many technical problems with this
  theory, and it also does not explain why the Moon
  is so depleted in volatiles compared to the
  Earth.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• The Capture Theory

• The capture theory proposes that the Moon and
  Earth initially formed in separate places in the
  solar system.
• Subsequently, the Moon was disturbed into an
  Earth-crossing orbit, and was somehow captured
  into Earth orbit.
• The capture theory explains why the volatile and
  metal inventory could be different from the
  Earth, but not why the isotope ratios (e.g. in
  oxygen 18O/16O) are the same.
• Also, the Earth could not easily have captured
  such a large body, in the manner in which Phobos
  and Deimos were captured by Mars without the
  intervention of a third body.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• The Sister Theory

• Very simple this theory supposes that the Earth
  and Moon formed together out of a spinning cloud
  of dust.
• This is the same theory used today to explain
  the large satellites of Jupiter and Saturn.
• The main problem is composition why did the
  Moon form with less metals than the Earth?
• None of the three traditional theories entirely
  works we must somehow modify our model and use
  the best parts of each

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• The Giant Impact Hypothesis

• The currently favored scenario is as follows
• The Earth had just formed, and had largely
  differentiated.
• When along came a protoplanet the size of
  Mars, about 10 the mass of the Earth, and
  smashed into the Earth at an oblique angle.
• The impact was just small enough not to destroy
  the Earth entirely, but about 10 of Earth
  material was ejected into orbit. The ejected
  material was metal-poor, coming from the Earths
  mantle.
• Some fraction (about 10) of the ejecta
  aggregated in orbit to form the Moon subsequent
  impact heating drove off most of the volatiles.

Dr Conor Nixon Fall 2006
43
ASTR 330 The Solar System

• Origin Of Mercury

• Computer calculations suggest that, in the early
  solar system, as many as 100 protoplanets the
  size of Mars existed, many on eccentric orbits.
• Over time, collisions whittled the number down
  to just 4, the four surviving terrestrial planets
  we see today. Collisions may have been the norm,
  rather than the exception!
• This gives us an alternative theory for the very
  metal-rich composition of Mercury, rather than
  just supposing it formed that way.
• Mercury may have originally been 50 as large as
  Earth or Venus.
• After Mercury had differentiated into a metal
  core and rocky mantle, it may have been impacted
  by a Mars-size projectile, losing most of the
  mantle material, but retaining the iron-nickel
  core. That would explain the disproportionately
  large metal center Mercury has today.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Summary - Quiz

• Why are the inferior planets (Mercury and Venus)
  never seen very far from the Sun?
• Do these planets exhibit retrograde motion?
• What causes tides on the Earth. When are they
  strongest?
• What are some of the effects of tides today?
• Describe the orbital and rotational evolution of
  the Earth-Moon system over time.
• What was the expected rotation period of
  Mercury, and what was actually measured in 1965?
  Why was this a surprise?
• What is the explanation for Mercurys solar day
  length?

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Summary-Quiz (contd)

• Describe a day on Mercury (i) at a hot longitude
  (ii) at a cool longitude. Are the cool longitudes
  really cool by our standards?
• Which spacecraft has given us the most knowledge
  about Mercury?
• Describe the similarities and differences
  between Mercurys surface and that of the Moon.
• What was discovered by the Clementine
  spacecraft, and has it been verified?
• Describe some of the geologic features seen on
  the Mercurian surface. Are they the same as the
  Moon?

Dr Conor Nixon Fall 2006
46
ASTR 330 The Solar System

• Summary-Quiz (contd)

• Compare and contrast the interiors of the Earth,
  the Moon and Mercury. What sources of information
  do we have for the Lunar and Mercurian interiors?
• Was the Moon a geologically active world prior to
  3 Gyr ago? What about Mercury?
• What are the compositional differences between
  the Earth and Moon?
• What are the three failed theories to explain
  the Moons origin?
• What features of these were incorporated into
  the generally accepted theory today?
• What insight does the current theory of lunar
  origin give us into the possible origin of
  Mercury?

Dr Conor Nixon Fall 2006