ASTR 330: The Solar System
Announcements
•
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 13:
The Moon
and
Mercury
Photos: Moon - John French, Abrams Planetarium, Michigan State Univ. Mercury - NASA.
Dr Conor Nixon Fall 2006
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.
Figure credit: Geometry Technologies, Science U.Com
Dr Conor Nixon Fall 2006
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.
Figure credit: Richard Pogge, Ohio State
• Hence, Mercury is
always seen near
sunrise or sunset.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Mercury’s Day
• Mercury’s 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 Moon’s
rotation is ‘tidally locked’ to its orbit about the Earth, and so we always
see the same face. Let’s take a look at tides…
Dr Conor Nixon Fall 2006
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.
• Let’s look at the familiar tides between the Earth and Moon.
Dr Conor Nixon Fall 2006
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
Figures and text: Steve Dutch, Univ. Winconsin, Green Bay
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
• Spring Tides
• New or Full Moon
Tides from Sun
and Moon
• Neap Tides
• First or Last Quarter
• Sun & Moon Pulling in Parallel Directions
• Lunar and Solar Tides Add Up
• Unusually Large Tidal Range
Figures and text: Steve Dutch, Univ. Winconsin, Green Bay
• Sun and Moon Pulling At Right Angles
• Lunar and Solar Tides Partially Cancel
• Unusually Small Tidal Range
Dr Conor Nixon Fall 2006
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.
Figures and text: Steve Dutch, Univ. Winconsin, Green Bay
Dr Conor Nixon Fall 2006
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 Earth’s
day to grow longer by 20 secs per million years.
• Part of the frictional energy goes into heating the oceans and the
Earth’s 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
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 Moon’s 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
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
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).
Figure credit: Nanjing University Astronomy
Dr Conor Nixon Fall 2006
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
ASTR 330: The Solar System
Elliptical Orbits and Tidal Locking
• The reason is Mercury’s elliptical orbit (e=0.206). As in all elliptical
orbits, Mercury travels faster closer to the Sun, and slower further out
(Kepler’s 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 Mercury’s
rotation to its fastest orbital velocity, not its average orbital velocity.
Mercury hence has a 3:2 spin-orbit coupling.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Solar Day on Mercury
• The 3:2 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
ASTR 330: The Solar System
Mercury’s long,
long day
• Let’s 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.
Figure credit: ESO Education and Outreach Office
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Mercury’s 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
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
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
ASTR 330: The Solar System
Exploration of Mercury
• Almost all our knowledge of Mercury’s 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 Mercury’s surface had
been mapped in detail.
Image: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Images: NASA/NSSDC
Mercury as Seen By
Mariner 10
Dr Conor Nixon Fall 2006
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
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 Mercury’s poles, in complete
contrast to expectations!
• Because Mercury’s 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
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.
Image: Clementine DSPSE/NRL
Dr Conor Nixon Fall 2006
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
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
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
ASTR 330: The Solar System
Compression Scarps
• The most distinctive identifying
features of Mercury’s 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.
Image: NASA/NSSDC Figure: Scott Hughes, Idaho State
Dr Conor Nixon Fall 2006
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 extracompressed interiors. E.g. the iron in the Earth’s 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.
Figure: Nanjing University
Dr Conor Nixon Fall 2006
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
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 (seismometers) 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.
Figure: Nanjing University
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Mercury’s 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
Earth’s.
• 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.
Figure: Scott Hughes, Idaho State
Dr Conor Nixon Fall 2006
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.
• Mercury’s 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
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
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
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 Mercury’s 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
ASTR 330: The Solar System
Mysteries Of Lunar Formation
•
Although lunar rocks have many similarities to terrestrial rocks:
•
•
There are also important differences:
•
•
•
Isotopic composition, some similar minerals,
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:
1. ‘Daughter’ Theory.
2. ‘Sister’ Theory.
3. ‘Capture’ Theory.
Dr Conor Nixon Fall 2006
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
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
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
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 Earth’s
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
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
ASTR 330: The Solar System
Summary - Quiz
1. Why are the inferior planets (Mercury and Venus) never seen very
far from the Sun?
2. Do these planets exhibit retrograde motion?
3. What causes tides on the Earth. When are they strongest?
4. What are some of the effects of tides today?
5. Describe the orbital and rotational evolution of the Earth-Moon
system over time.
6. What was the expected rotation period of Mercury, and what was
actually measured in 1965? Why was this a surprise?
7. What is the explanation for Mercury’s solar day length?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Summary-Quiz (contd)
8. Describe a day on Mercury (i) at a hot longitude (ii) at a cool
longitude. Are the cool longitudes really cool by our standards?
9. Which spacecraft has given us the most knowledge about
Mercury?
10. Describe the similarities and differences between Mercury’s
surface and that of the Moon.
11. What was discovered by the Clementine spacecraft, and has it
been verified?
12. Describe some of the geologic features seen on the Mercurian
surface. Are they the same as the Moon?
Dr Conor Nixon Fall 2006
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 Moon’s 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