Chapter 8: “Earth’s Moon” and “Mercury” With Doctor Bones aka Don R. Mueller, Ph.D.

1. Astonishing Astronomy 101
With Doctor Bones (Don R. Mueller, Ph.D.)
Educator
Entertainer
J
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G
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L
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Scientist
Science
Explorer

2. Chapter 8
Earth’s Moon and Mercury

3. Surface Features • Our Moon is one of the
better objects for novice
astronomers to view.
With much to see:
• Maria: are smooth, dark
gray lowland regions.
• Craters: impact sites
where planetesimals and
other bodies struck the
moon.
• Highlands: are lighter
gray regions, that are
mostly mountains and
craters.

4. Lunar Highlands and Maria
• The lunar highlands are the lighter, heavily cratered
regions on the Moon.
• Mostly lower-density rock.
• Aside from craters, many mountains are found here.
• The lunar maria are low-lying regions formed by the
flow of lava billions of years ago.
• Very few craters are located within the maria.
• This implies that the maria formed after the later
bombardment, early in the solar system’s history.
• Rock in the maria: primarily basalt, a dark lava rock.

5. Both Sides of the Moon

6. Rilles (German for 'groove‘)
• If we look closely at the Moon’s surface, we see long
river-like canyons running along the surface.
• These rilles are most likely ancient lava tubes that ran
just beneath the surface.

7. Formation of Maria

8. Interior of
the Moon
• Astronauts put
seismometers on
the Moon back
during the 1970s.
• Seismometers let us
examine the interior
of the Moon using
moonquakes.
• The outermost layer
is powdered rock,
called regolith.
• The Moon’s crust is
thicker on the far
side than it is on the
near side.
• The Moon’s mantle is thick, but too cool to be
stirred by convection.
• The iron core may be molten and is located not
quite at the center of the Moon.

9. Lunar Atmosphere • The lunar atmosphere is practically
non-existent.
• Too cool to have volcanic activity, a
good source for an atmosphere.
• The gravitational pull of the Moon
is small, therefore gases can
escape easily.
• Some gases (notably hydrogen)
have been found, but they are
very thin and probably came from
the solar wind.
• The lack of an atmosphere means
that there is no erosion. The
footprints that the astronauts left
are still there.
• There may be water ice in craters
at the lunar poles.

10. The Moon’s Rotation: Dark side of the Moon
• The Moon rotates
about its axis once
every revolution
around the Earth.
• This is called a
synchronous rotation,
and means that the
Moon only shows one
face to the Earth.
• A person sitting on the
Moon would see the
Sun rise every 28 days .

11. Tidal Braking
Tidal braking results in the Earth’s day increasing and the
Moon moving farther away from the Earth.

12. TIDAL BRAKING
• TIDAL BRAKING slows the Earth's rotation and speeds up
the Moon's motion in its orbit. As the Earth spins, friction
between the ocean and the solid Earth below drags the tidal
bulge ahead of the line joining the Earth and the Moon. The
Moon's gravity pulls on the bulge and holds it back. The
resulting drag is transmitted through the ocean to the Earth,
slowing its rotation the way your hand placed on a
spinning bicycle wheel slows the wheel. Tidal braking
lengthens the day by 0.002 seconds every century. As the
Earth's rotation slows, the Moon accelerates in its orbit,
moving farther from the Earth (Kepler's third law). This
acceleration makes the Moon move away from Earth at a
rate of 3 cm per year. Thus, the Moon was once much
closer to the Earth and the Earth spun much faster.

13. Theories for the Formation of the Moon
• Capture theory: The Moon
was an independent planet,
gravitationally captured by the
Earth. However, the Moon’s
composition is too similar to
that of the Earth’s for this to
have happened.
• Twin Theory: The Moon
formed out of the same cloud
of particles as the Earth.
However, again, the
composition of the Moon is
different enough from the
Earth for this to be possible.
• Expulsion Theory: The Earth
spun very quickly early in its
history and threw out a chunk
of itself, forming the Moon. Yet
again, the composition of the
Moon is different enough from
the Earth for this to be likely.
• Finally, we have the
Collision Theory…

14. Birth of the Moon
Collision Theory

15. Computer Simulation of the Birth of the Moon

16. Mercury - the Innermost Planet
• Mercury is one of the least
explored planets in the Solar
System
– Mariner 10 (1970’s) provided most
of the information we have .
– NASA’s MESSENGER returned to
Mercury in 2008 on its first flyby.
– MESSENGER went into orbit
around Mercury in 2011.
• Planet of Extremes
– On the dayside, surface
temperatures exceed 700K.
– On the nightside, temperatures
drop as low as 70 K.
– Very eccentric orbit, coming as
close as 0.29 AU and as far as 0.41
AU from the Sun

17. • Surface is covered
with craters and
lava flows.
• Fewer of Mercury’s
craters overlap.
• Craters are similar
to the Moon’s,
with rays and
crater walls.
• Lower crater rims
due to Mercury’s
higher gravity.
Mercury’s surface is similar to the Moon’s surface.

18. Caloris Basin
Early in Mercury’s history, a huge impact occurred on the
surface: Forming the Caloris Basin, an 800 km wide crater.
The impact was so great that shock waves traveled
through the planet and along its surface. Shaking up the
terrain on the opposite side!
Odd terrain: opposite
the Caloris Basin

19. Scarps
• Running across Mercury’s
surface are Scarps:
kilometer-tall cliffs, the
result of the shrinking of
the planet as it cooled.
• The presence of these
cliffs suggests that the
core of Mercury is large
and metallic. Shrinking
greatly as it cooled.
• The surface slopes as the
core cools resulting in a
contracting crust.

20. The Interior of Mercury • Mercury’s density is
unusually high: 5.4
kg/liter.
• This indicates a very
large iron core
covered by a thin
silicate mantle.
• Mercury may have a
molten iron (mixed
with sulfur) core,
despite its size.
• This core may be the
cause of the weak
magnetic field: similar
in shape to the
Earth’s, but much
weaker.
Please insert figure 38.6

21. Why is Mercury so dense?
• It is possible that Mercury once had a thicker mantle, however a
collision could have removed much of this material, leaving only
the core and the lighter mantle material behind.

22. 3:2 Orbital Resonance
• Mercury is tidally locked
to the Sun.
• Similar to the way the
Moon is locked with the
Earth.
• During one orbit around
the Sun (88 days) the
planet has rotated on its
axis 1.5 times.
• It takes two orbits for
Mercury to rotate three
times.
• This is called a 3:2 orbital
resonance.

23. Will the “real” Mercury please stand up!
Mercury?
Mercury?
Mercury?