Lecture 6: Terrestrial Planets

    -- these are Mercury, Venus, Earth, Mars, all small and dense

Start at home by looking at the Earth and the Moon:

The Earth: Our Home Planet

Earth serves as a template for much of our knowledge of a planet (but must be cautious because the Earth may not be typical!)

Salient Features of Earth

The difference in composition between the core and the outer layers of the Earth is the result of differentiation, the effect of gravity pulling denser materials towards the Earth's center and lighter materials floating on top.

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Magnetic field manifests itself by trapping charged particles in belts around the Earth, aurorae are another phenomenon caused by the magnetic field.

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Equilibrium Temperature for Earth:

Energy received per sec = Energy lost per second

Energy received per sec = Cross-section of Earth X Solar Flux

To finish this calculation, we also need to know that the Earth reflects 39% of the incident sunlight and absorbs 61%. This leads to

Substituting d=1.5x1011m, LSun=4x1026 watts,

           

Why is this T so much less than the usual 290ºK that we experience as the Earth's average surface temperature?

Because some of the absorbed radiation is trapped in the lower atmosphere by the "Greenhouse" effect:

Current composition of Earth's atmosphere is

77% N2

21% O2

2% Argon, H20,CO2 etc.

Molecules like CO2 trap heat by being transparent at visible wavelengths where the bulk of the energy is absorbed from the Sun but are opaque at IR wavelengths where the Earth can radiate heat back into space. This greenhouse effect causes the Earth to be ~40ºK warmer than if there were no atmosphere.

Temperature that Earth equilibrates to is the result of the energy balance outlined above with the heat trapping added in (and the issue with global warming is whether or not the energy emitted to space has been altered by increasing the number of CO2 and similar molecules in the atmosphere).

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The Moon

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We must not forget that the Moon is very special - it is the only astronomical body outside of the Earth that people have visited and returned with rock and surface samples.

Does the Moon rotate?

Yes -- otherwise we would see different portions at different times! The Moon takes exactly the same amount of time to go around the Earth as it does to rotate once. This synchronous rotation is a minimum energy configuration and is common in the Solar System           

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Overview of Lunar Geography

Mare (plural maria) : dark, relatively smooth regions, roughly circular in shape, an average 3 km lower in elevation than the rest of surface, "mascons" = regions of enhanced density lie beneath the maria, the rocks from maria are mainly basaltic (like those from volcanoes on Earth) and the youngest of the lunar rocks.

Highlands: light colored, frequently rugged or heavily cratered, the oldest lunar rocks come from highland regions

Analyzing Lunar Rocks

Chemical analysis of lunar rocks revealed that

    1. They are extremely similar in composition to Earth rocks.
    2. Lunar rocks appear to have been heated to a higher temperature early in their histories as compared to Earth rocks.

The using radioactive age dating techniques revealed the significant age differences between the lunar highlands and maria:

Radioactive Age Dating

Some atomic nuclei are unstable and will spontaneously decay either by emitting an electron (b -decay) or by emitting an a -particle which is a bare He nucleus (2 protons and 2 neutrons). If a b -decay occurs, the atomic number weight doesn't change but the atomic number increases by one. If an a -particle is emitted, the atomic weight decreases by 4 and the atomic number decreases by two. The probability that a nucleus will decay is expressed by the concept of half-life: one half-life = the time for half of the nuclei to decay.

Examples of radioactive decays that are useful for making astronomical age measurements:

[these are summaries of reaction chains that start with the unstable parent and end at the first stable nucleus]

Half Life (109 yrs)

U238 -> Pb206 + 8 He4 4.49

U235-> Pb207 + 7 He4 0.713

Th232-> Pb208 + 6 He4 13.9

Rb87-> Sr87 + e- 47

2 K40 -> Ar40 + Ca40 + e- 1.3

Note that these ages refer to the last time that a sample (a rock) was heated. By measuring the relative numbers of parent and daughter products, the age of the rock can be inferred:

n0 = original quantity of atom

n = quantity present now T1/2 = half life

It is easier to work using base 10 logarithms (recall that log10x=log102.log2x):

Example:

A rock has a ratio of Ar40 to K40 of 10.6. The age of the rock can be computed:

Results for various rocks:

Lunar maria       3.0-3.1x109 yrs

Lunar highlands 3.8-4.0x109 yrs

Earth rocks      3.3x109 yrs

Meteorites       4.6x109 yrs

Several conclusions can be derived from these numbers:

    1. The age of the Solar System is 4.6x109 yrs
    2. The lunar maria formed after the highlands
    3. Virtually all rocks on Earth have been heated since the Earth formed.

Lunar Atmosphere

Moon has essentially no atmosphere at all which means that its surface suffers large temperature swings between the lunar night and lunar day.

If you compute Tesc for the Moon for the types of molecules that constitute the Earth's atmosphere, you will discover that they could all escape from the Moon due to its lower surface gravity and somewhat higher surface temperature.

Formation of the Moon

Several theories have been proposed:

  1. Co-formation theory states that the Earth and Moon formed as separate blobs of material
  2. Capture theory states that the Moon formed elsewhere in the Solar System and then was propelled into an orbit where the Earth captured it. This theory suffers from the fact that the Moon is large and would have so much kinetic energy that only very special (and unlikely) orbits would allow it to be captured by the Earth.
  3. Fission theory proposes that the Earth was rotating rapidly enough in its youth that a large chunk split off and became the Moon (this notion is supported by te observation that the width of the Pacific Ocean is about the same as the Moon's diameter). However, there is no evidence that the Earth ever spun rapidly enough nor would the Moon go into a stable orbit as a result of this event.
  4. Impact theory proposes that an object of roughly the size of Mars collided with Earth and knocked some of the Earth's surface material into space. This material then formed the Moon. This explains why the composition of the Moon is so similar to that of the Earth's crust and why the Moon rocks appeared to have been heated to a higher temperature than those of the Earth. This theory is now the preferred theory for the formation of the Moon.

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Mercury

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This planet is much smaller than the Earth (only 5.5% as massive as the Earth) but about a factor of 4 more massive than the Moon. Its density is similar to the Earth's. Due to its synchronous rotation (in two revolutions around the Sun, Mercury rotates three times on its axis), Mercury's surface suffers from temperature extremes with the hottest points reaching 700ºK while the poles may be as cold as 125ºK.

The surface of Mercury is very similar to that of the Moon - covered by many craters suggesting that Mercury has been geologically dead for the last 4 billion years.

Venus

A first glance, Venus looks like a near twin of the Earth:

                          Venus     Earth

Diameter(km)    6052          6378

Density(kg/m3)  5240          5520

But in fact it is very different from Earth. We have had to observe Venus mainly through radar techniques because of its dense and cloudy atmosphere.

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Venus' rotational properties are unusual - it takes 243 days to rotate once, and what is even more unusual is that its rotation is retrograde meaning east to west, opposite of the prevailing trend in the rest of the Solar System.

Determination of rotational period using radar:

Normally one measures a rotation period by measuring how long it takes for a surface feature to return to the same orientation relative to the stars. The only cloud features that are apparent from the Earth but rarely reflect high winds in the Venusian atmosphere. However, by bouncing a radar beam off the surface, a straightforward measure of the rotational velocity is obtained because the return radar beam is Doppler-shifted by the moving surface.

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We do not know why Venus rotates in this exceptional fashion. We can only speculate that a collision early in the Solar System's history must have imparted a large angular momentum change.

Surface Temperature: The Dominant Parameter for Venus

Because of the thick, light-colored clouds in its atmosphere, Venus reflects 65% of the sunlight hitting it, nearly twice as much as the Earth reflects which partially compensates for the smaller distance between Venus and the Sun. Computing the equilibrium temperature as we didi for the Earth would suggest that Venus should have a surface temperature of 302ºK, only slightly warmer than Earth [note that proper allowance for Venus' slow rotation must be made].

Measurements show that the actual surface temperature on Venus is 730ºK!!! This is the highest for any planet. Why so hot? An extreme case of the greenhouse effect, driven by the 96.5% of the Venusian atmosphere that is CO2, accounts for this high temperature. Most of the rest of the atmosphere is N2 while the clouds are largely sulfuric acid. The atmospheric pressure is about 90x that on Earth.

Why has the atmosphere and consequently, the surface temperature, turned out so differently on Venus than on the Earth? One would have naively expected Earth and Venus to be somewhat similar.

One clue comes from realizing that if all the CO2 that is either dissolved in the ocean or trapped in rocks were released, the Earth's atmosphere would be 98% CO2, 2% N2 with a pressure 70x that of our actual atmosphere. Apparently early in its history, Venus was somewhat warmer than the earth simply do being closer to the Sun. This higher temperature reduced the degree to which CO2 either dissolve in a liquid ocean or be trapped in rocks. This lead to more CO2 being present in the atmosphere as compared to the earth and hence to more greenhouse effect. A runway greenhouse effect occurred -- the hotter it got, the more CO2 was present in the atmosphere and the stronger the greenhouse effect became.

Surface of Venus

Because of thick clouds in its atmosphere, the surface of Venus can neither be observed from Earth or by an orbiting satellite. Our knowledge of the surface comes from two sources: Venera landers sent as part of the Soviet space program and radar mapping, especially the Magellan mission which mapped the entire Venusian surface while it orbited the planet.

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The landers revealed that

    1. The lower 30km of the atmosphere are essentially clear and with low wind speeds.
    2. Some of the surface rocks are basaltic in nature while others are igneous suggesting that some of the surface has been covered by lava flows.
    3. Surface is littered with varying sizes of rocks, some showing signs of erosion and others note (but must keep in mind how little of the surface has been inspected).
    4. Unfortunately none of the landers carried seismographs so we don't if there are "Venusquakes".

The radar mapping revealed that

  1. Venus is very smooth as compared to the Earth with much smaller excursions around the mean surface elevation.
  2. Many surface features which would result from volcanoes are seen.venus_lava.gif (34345 bytes)
  3. Impact craters are also present, and counts of these show that the lowlands on Venus are older than the highlands. The oldest surfaces appear to be about 109 yrs while the youngest have ages in the 200-300 million year range. Note that there are no small impact craters -- this is the result of shielding by the dense atmosphere.venus_crater.gif (50912 bytes)
  4. There is no unambiguous evidence of plate tectonics. No single answer has been found for the lack of plate tectonic activity -- maybe the crust has never completely solidified due to the high surface temperature or may be the high temperature and soft crust has lead to more volcanism and less convective motion that can move plates or may be water is need to lubricate plate motion.

Mars

Seasonally varying surface features on Mars as viewed from Earth led to early speculation (especially by Percival Lowell) that Mars could be habitable.

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Such features included

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The similarity of Mars polar axis tilt (24º) to Earth's and the similarity in the length of its day (24.6hrs) to Earth's also fueled the speculation that life could exist on Mars.

The reality is that 1) the polar caps are frozen CO2, not frozen water ; 2) the so-called vegetated areas are actually dark red regions that alternately get covered with dust and then swept clean in a seasonal cycle; 3) the canals were an optical illusion; 4) the surface temperature and atmospheric pressure are too low to support any but the simplest forms of Earth-like life.

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Surface Features

The Mariner series of Mars orbiters and the Viking landers and orbiters revealed much about the Marian surface that could not be discerned from Earth:

1. The Martian surface sports a large number of impact craters. Many of these should evidence of wind erosion and filling by dust. Many craters also have features suggestive of liquids being thrown out of the impact area; this is explained by postulating a permafrost layer.       

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2. Mars has many volcanoes, some are extremely large shield volcanoes. Their extreme size is primarily the result of a lack of plate motion so that lava coming out of a hotspot piles up on a single mountain.

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3. Many channels indicative of dry river beds or flash flood areas are seen. Clearly Mars had much more liquid water than it does now (and in fact, free water would not be stable on the surface now because of the low atmospheric pressure). 

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4. A large canyon exists suggesting cracking of the crust early in Mars' history.

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5. An as yet unexplained asymmetry exists between the northern and southern hemispheres with the south having higher elevations and more craters.

Features such as the large canyon, Valle Marinaris, can be explained as the result of Mars nearly developing plate tectonics. Why didn't Mars develop a full blown case of plate tectonics?

Plate tectonics depends on the interior of the planet staying warm enough for en elastic layer to exist beneath the crust. We can estimate the relative time for a planet or moon to cool:

so smaller, less mass objects cool more rapidly. Mars cooled too quickly to develop plate tectonics.

Atmosphere

Surface pressure is .005 that of the Earth's.

Composition is very similar to Venus' atmosphere:

95.3% CO2

0.13% O2

traces of CO, water 2.7% N2 1.6% Ar

Where Did the Water Go?

Mars must have been warmer and must have had a denser atmosphere when the river beds were etched into surface. If molecules like CO2 and H2O are dissociated by UV radiation from the Sun, Mars is small enough that the constituent atoms can escape. Mars atmosphere apparently leaked away which accelerated cooling because the greenhouse effect was reduced in intensity. Some water froze either in the polar caps or as a permafrost. The current total amount of water on Mars is difficult to estimate but could be only 1/10th of what it once was.

Is There Life on Mars?

The Viking Landers carried a variety of experiments designed to look for microbial life on Mars. These experiments were predicated on the concept that these life forms would be chemically similar to life on Earth. These experiments only revealed activity that could result from inorganic chemicals. Neither lander nor the recent Pathfinder mission have seen any evidence for macroscopic life (moving creatures like bugs or any larger).

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Study of a Martian meteorite found in the Antarctic may contain fossil bacteria:

Some facts are not in dispute:
-- that the rock came from Mars is generally agreed to
-- that the rock contains a certain collection of chemical compounds is agreed to
-- that the rock is very old is agreed to
BUT
--although the structures seen in the pictures look a lot like Earth bacteria, the Martian structures are ~100-1000x smaller than any on Earth
--the possibility that the structures and combination of chemical compounds arose through straightforward geologic processes cannot be ruled out

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