Exam 2 Tuesday

Covers Chapters 7-10, & 14

One sheet of notes with writing on one side only

Terrestrial planet uniqueness

Apollo 12

Lunar Samples

Apollo missions returned over 840 pounds of rock and soil samples collected from different areas of the Moon. Three unmanned Soviet landers also returned samples.

The Moon is made of igneous rock – no cheese!

The entire Lunar surface was once molten.

4 types of Lunar material

1. Regolith – lunar soil2. Anorthosite – highlands, density 3.0 g/cm3

3. Mare basalts – lowlands, density 3.5 g/cm3

4. Impact breccias – composite rocks

Not quite the same!

Mare basalts are nearly devoid of water and other hydrated minerals. Yet mare basalts contain about 10 times more titanium than Earth basalts.

The lunar regolith

The lunar surface is covered with rock (silicates) pulverized to a fine grain called the regolith or lunar soil. In the maria this soil is 2 to 8 meters deep while in the highlands it may exceed 15 meters.

Glass spherules

Tiny beads of glass formed from volcanic fountains. Drops of lava thrown from the surface 3.7 billion years ago during volcanic eruptions that solidified before returning to the surface. Orange color comes from their high (9%) titanium content.

Discussion

Why is the regolith thicker in the Lunar highlands.

3 Origin Theories for the Moon before Apollo

1. Binary accretion – Earth and Moon formed together.

2. Capture – Moon formed elsewhere and was later captured by the Earth.

3. Fission – Moon material ejected from Earth during an early period of rapid rotation.

Discussion

If the Moon was tossed off a rapidly spinning Earth, where above the Earth would the Moon orbit?

Discussion

If the Moon formed from material thrown off from a rapidly spinning Earth, what would you expect the composition of the Moon to be?

Discussion

Why is the binary accretion theory, that the Earth and Moon were both formed from a single over density in the early solar nebula, unlikely?

Capture of the Moon

Isotopes of oxygen, 16O, 17O, and 18O, are the same on Earth and the Moon. This is not true for asteroids and Mars rocks. This indicates that the Moon did form in the same part of the solar nebula as the Earth.

Also, capture of such a large Moon by Earth in a nearly circular orbit is almost impossible.

Giant Impact Theory

4.5 billion years ago the Earth received an off-center blow from a Mars sized object. This impact vaporized of large chunk of the Earth’s and the impactor’s mantles throwing it into Earth orbit. The iron rich core of the impactor was incorporated into the Earth’s mantle while the Moon coalesced from material from the crusts and mantles.

Discussion

What could you cite as evidence for or against the giant impact formation theory of the Moon?

The case for further Lunar exploration

Study of the Lunar geology perfectly compliments the study of geology on Earth. On Earth, rocks older than 3.5 billion years have been completely destroyed, while most active Lunar geology was ended 4.5 to 3 billion years ago.

Discussion

Mercury is the least studied terrestrial planet. Why do you suppose that is?

Mercury is too close to the Sun

Average distance from Sun is 0.387 AU.

Observationally, Mercury is never more than 28 degrees from the Sun. Thus Mercury is only visible at most two hours before sunrise or two hours after sunset. This means that it is only seen low on the horizon.

Best orbital configurations to view Mercury

Greatest eastern elongation – Mercury seen just after sunset.

Greatest western elongation – Mercury seen just before sunrise.

Discussion

What time of the year will we get the most favorable viewing angle for Mercury?

Best seasons to view Mercury

Evening elongations are best in the Spring.

Morning elongations are best in the Fall.

Mercury from Earth

Discussion

Why don’t we just observe Mercury with the Hubble Space telescope from above the Earth’s atmosphere?

Why not observe Mercury from above atmosphere with the

Hubble Space Telescope?

Mercury is too close to the Sun. Pointing errors could allow glare from Sun into the telescope destroying the light sensitive detectors.

Why not send on space probe?

It is difficult to get to Mercury.

Average orbital speed, 47.5 km/s, is too fast for current rocket technology. Placing a probe in Mercury orbit was once thought impossible.

Mariner 10 was the first probe to visit Mercury. Used a close flyby of Venus to place it in a 176 day elliptical orbit about the Sun which brought it close to Mercury 3 times in 1974-1975.

MESSENGER route to Mercury

Discussion

It was once thought that Mercury’s rotation period was synchronous with its orbital period.

How did we measure the rotation rate of Mercury?

Mercury’s rotation

It was once thought that Mercury’s rotation period was synchronous with its orbital period.

But when Mercury’s rotation period was measured using the Doppler effect it was found to rotate once every 58.646 days, 2/3rds of its orbital period.

Mercury’s rotation

3:2 spin orbit coupling – Mercury rotates on its axis 3 times for every 2 orbits, or each orbit Mercury rotates 1 ½ times.

Mercury is not perfectly spherical

Tidal forces from the Sun try to keep Mercury’s long axis pointed toward it.

Why doesn’t Mercury rotate synchronously?

If Mercury’s orbit were more nearly circular, Mercury would keep the same face toward the Sun, just as Earth’s Moon does.

Discussion

How does the fact that Mercury’s orbit has an ellipticity of 0.2 prevent the long axis of Mercury from always pointing toward the Sun?

Mercury’s orbit is elliptical

The tidal forces on Mercury are much stronger at perihelion than they are at aphelion.

Mercury’s hot poles

At successive perihelion passages Mercury presents one hemisphere, and then the one opposite it, toward the Sun. When Mercury is closest to the Sun its surface reaches a temperature of 740 K. The longitudes 90 degrees away from these “hot poles” face the Sun only at aphelion and thus only get to a temperature of 525 K.

Mercury’s hot poles

Solar day vs. sidereal day

The solar “day” on Mercury is very different from the rotation period. Mercury rotates once in 59 earth days. But it takes two Mercurian years (176 Earth days) between two successive meridian crossings.

Mass, size, and density

Mercury is the smallest of the major planets. Both Ganymede and Titan are larger than it, though they have less mass.

Mercury has a high density at 5.4 g/cm3, indicating a large fraction of metals.

Planet Density Uncompressed Density

Mercury 5.44 5.4

Venus 5.24 4.2

Earth 5.50 4.2

Mars 3.93 3.3

Moon 3.36 3.35

Discussion

Based on our formation theory of the solar system, why would we expect Mercury to have a higher iron content than Mars?

Discussion

Can you think of any other reason why Mercury might have an oversized core, or perhaps an undersized mantel?

Composition

We would expect the mantle and crust to contain iron-silicates. The current evidence indicates the opposite is true. The surface appears to be devoid iron compounds, yet rich in volatile elements such as potassium and sulfur.

Iron may be locked up in the mineral ilmenite, not visible to Messenger.

Iron deficiency, or detector deficiency

Discussion

The sidereal rotation rate of Venus is 243 days, the slowest in the solar system and 19 days longer than the venusian year. Why do you think that might be?

Venus has no magnetic field

Because Venus rotates 243 times more slowly than Earth we would expect Venus to have a weaker magnetic field.

None has been detected. Venus’s magnetic field has to be at least 10,000 times weaker than Earth’s.

Perhaps Venus is undergoing magnetic pole reversal

Lava flows on Earth become magnetized by the Earth’s magnetic field when they solidify. Lava flows 30,000 years ago show that the Earth’s magnetic field was oriented opposite of what it is today. During reversal, Earth has no magnetic field.

Consequence of not having a magnetic field

The atmosphere on Venus is thick enough to protect the surface from the solar wind.

The solar wind interacts directly with Venus’s upper atmosphere and carries some of it away.

Discussion

Although Magellan mapped Venus at a resolution of 100 m, it found no impact craters with diameters less than 3 km. Why not?

Mars appears the most like Earth

Rotation period – 24 h 37 min 22 s

Inclination of equator to orbit – 25.19 degrees

Has polar ice caps

Has white water vapor clouds

Schiaparelli’s “canali”

The Search for life on Mars

Early Mars flyby and orbiter missions indicated there were no vast forests, no large bodies of water, and no canal systems linking population centers.