Astonishing Astronomy 101With Doctor Bones (Don R. Mueller,

Ph.D.)

EducatorEntertainer

JU

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RPLANETARY

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Chapter 8 Earth’s Moon and Mercury

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.

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.

Both Sides of the Moon

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.

Formation of Maria

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.

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.

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 .

Tidal Braking

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

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.

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…

Birth of the Moon

Collision Theory

Computer Simulation of the Birth of the Moon

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

• 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.

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

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.

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

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.

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.

Will the “real” Mercury please stand up!

Mercury? Mercury?

Mercury?