Earth-Like Planets
Outline
1. Insides
* Average ==> bulk composition
* Core-mantle-crust structure
* Parts of Earth
> Differentiated interior
> Radioactive + accretion heating
* Seismic S and P waves
* Lithosphere
> Thickness depends on internal temperature
> Small planets cool faster
2. Surfaces
* Surface processes
> Tectonics
- Volcanism, plate motion
- Driven by escape of heat
> Weathering (erosion)
> Impact Cratering
* Tectonics
> Plate tectonics
- Earths lithospheric plates ride on upper mantle
- Driven by convection in mantle
- Seafloor spreading
- Plate collision
- Subduction
- Plate slippage
- Earthquakes, volcanos occur along plate boundaries
- No plates elsewhere
> Moon: lava flood plains (maria)
> Venus
- Lava flood plains
- Compressional mountains & chasms
- Volcanoes (extinct?), coronae, arachnoids & pancake domes
> Mars
- Lava flood plains
- Tharsis bulge
- Valles Marineris
- Tharsis volcanoes (extinct)
^ Olympus Mons
* Weathering
> Earth: wind, water & ice erosion
> Venus: wind erosion
> Mars
- Wind erosion: dunes
- Water erosion: ancient riverbeds (?) & outflow channels
^ Mars Pathfinder site: outflow channel
- Sediment layers
- Seepage & ponding in craters
- Polar ice (carbon dioxide + water)
* Impact Cratering
> Craters everywhere
> Moon
> Mercury
> Mars
- Fluidized craters
> Venus
- Lavina 'crater farm'
> Earth
- Barringer crater
- Lake Manicouagan
> Crater formation is explosive
> Largest craters: impact basins
- Moon: Imbrium & Mare Orientale
- Mercury: Caloris basin
- Mars: Hellas basin
> Cratering & age
- Heavily cratered surfaces tend to be old
- Cratering history: heavy bombardment early
3. Atmospheres
* Atmospheres of terrestrial worlds: composition, pressure, weather
> Earth atmosphere thin
> Pressure-gravity balance
* Temperatures of naked planets
> Incoming sunlight: absorbed & reflected
> Absorbed light heats planet
* Temperatures of real planets
> Atmospheres tend to raise temperature
* Temperature structure of Earth's atmosphere
> Exosphere, thermosphere, stratosphere, troposphere
* The Greenhouse Effect
> Atmospheric trapping of IR from surface
* Clouds on Earth (water) & Venus (sulfuric acid)
* Mars
> Low pressure cannot sustain water
> Clouds: water, carbon dioxide
> Dust storms
> Frosts
> Cyclones
* Atmospheric origin
> Outgassing
> 'Delivery'
* Atmospheric destruction/escape
> Thermal escape
- molecules escape when temp high, gravity weak, molecules light
> Atmospheric cratering
> Bombardment
> Condensation
> Chemical reactions
* Atmospheric evolution
> A matter of temperature & gravity
> Moon & Mercury: too warm & gravity too weak
> Venus: too hot
- Water never liquid
- Carbon dioixide retained
> Mars: too cold
- Atmosphere denser in past
- Today water frozen
> Earth: just right
- Water always liquid
- carbon dioxide absorbed by water
- Life supplies oxygen
4. Magnetic Fields
* Earth's magnetic field
* Magnetic fields created by flowing charge
* Earth: flowing charge in liquid outer core
* Others: negligible fields
* Magnetosphere of Earth
- Solar wind
-Aurora
Questions
1. Ques. #2, pg. 167.
2. Ques. #3, pg. 167.
3. Based on the model provided by Earth, what 'ingredients' are necessary for the existence of a planetary magnetic field?
4. Venus probably has a partly-liquid core, like Earth. Why doesn't it also have a magnetic field?
5. What weathering (erosion) processes are at work on Earth? Mercury? Venus? Mars?
6. Ques. #6, pg. 167.
7. Compare the patterns of impact cratering found on the Moon, Earth, Mercury, Venus and Mars. What do these patterns tell us about the age of the surfaces of
these objects?
8. Prob. #1, pg. 168.
9. Summarize the various types of tectonic activity that can occur on Earth and on other terrestrial planets. What ultimately drives this activity?
10. List a few tectonic features found on: a) Mercury; b) Venus; c) Mars.
11. Ques. #19, pg. 167.
12. Ques. #10, pg. 167.
13. Ques. #15, pg. 167.
14. Ques. #20, pg. 167.
15. Prob. #3, pg. 167.
16. Why is it unlikely that we'll ever find lakes (or even ponds) of liquid water on Mars today?
17. What are Mars' polar caps made of?
18. In contrast to Earth, Mercury undergoes large temperature differences between night and day. Why?
19. Account for the large temperature difference between night and day on Mars.
20. Contrast the clouds of Venus with Earth's clouds.
21. What is the greenhouse effect, and what role does it play in the atmospheres of Venus and Earth?
22. How do we know about Earth's interior, given that the the deepest wells and mines extend only a few kilometers into its crust?
23. Describe Earth's magnetosphere.
Answers
1. Differentiation occurs when a planet is heated to the point that its entire interior is liquified. In this circumtance, the densest materials naturally sink to the center of the planet. The densest material ends up at the center (core); the next densest outside the core (mantle); and the least densest the surface (crust).
2. The lithosphere is the rigid outer shell of a planet: the crust + rigid upper mantle. Size of a planet determines how long a planetary interior remains hot: the larger the planet, the longer it remains hot inside. All planets were hot to begin with - large hot objects take longer to cool than small hot objects. But planets remain hot for another reason: radioactivity. More material means more radioactivity, so larger planets generate more heat via radioactivity.
3. i) A partially liquid core. ii) A hot interior. iii) Relatively rapid rotation. Earth possesses all of these properties; not all terrestrials do.
4. It rotates too slowly (once in 243 days) to generate much of a field.
5. Earth: water & wind erosion + weathering due to chemical reactions between atmosphere & water & rocks. Mercury: zero weathering. You need an atmosphere for wind erosion, and water will not exist as a liquid without an atmosphere. Mercury's atmosphere is extremely thin – thus no wind or water is available for weathering. Venus: weak wind erosion; no water erosion; probably some chemical erosion. Because the atmosphere is so dense and Venus' temperature so uniform, winds are very sluggish (though not absent). The very high temperature prevents the existence of liquid water. Mars: wind erosion; no water erosion. Winds on Mars can be remarkably strong. No water, of course, because the atmospheric pressure is too low to support
liquid water.
6. See Ques. #9 in the lecture guide for Structure and Formation of the Solar System. The lunar maria are believed to be younger than the highlands because the maria are much less heavily cratered than the highlands.
7. Moon: heavily cratered regions (highlands) and lightly cratered regions (maria). The maria are evidently younger than the highlands; the highlands are probably very old. Earth: very few impact craters, as a result of weathering and tectonics. All of the craters from the early era of heavy bombardment have been wiped from the face of the Earth. The craters that we do find are of much more recent vintage, and will eventually disappear. Mercury: largely heavily cratered; few maria-like regions. Like the Moon, the surface here is likely very old. Venus: surprisingly few craters, leading to suspicion that Venus' surface is relatively young (perhaps less than 500 million years). What caused all of the early bombardment craters to disappear is a mystery. Some speculate that Venus' surface melts entirely on occasion, wiping out all surface topography (including craters). Mars: Heavily cratered southern hemisphere; lightly cratered northern hemisphere. Northern hemisphere is likely lava flood plain (like lunar maria). Parts of the southern hemisphere very old, perhaps as old as lunar highlands. In all cases, lightly cratered terrains are younger than heavily cratered terrains.
8. Moon's surface has been geologically dead for billions of years - no tectonics, no weathering - which would tend to wipe out the craters. Earth's surface is geologically active; thus craters are easily erased. Difference has very little to do with composition, but does depend on the difference in size: The moon’s volume is about 51 times smaller than Earth’s volume; thus the moon’s interior cooled long ago; Earth is still quite warm inside; thus we find active tectonics on Earth, but not the Moon. They are both about the same distance from the Sun, so there's no difference there. The main difference is this: Earth has a substantial atmosphere; the Moon doesn't.
9. Tectonics: plate motion/disruption of crust; volcanism; earthquakes; mountain-building, and so on. Tectonics means in general any activity originating within a planet that disrupts its crust. This activity is ultimately driven by escape of heat from inside a planet. Where tectonics are absent/weak, we generally assume the planet has cooled off.
10. a) Mercury: we mentioned only the planet-wide scarps (cliffs) thought to have formed when the Mercury's interior shrank as it cooled.
b) Venus: Large volcanoes (e.g., Sif Mons); pancake domes (where thick lava has flooded the surface); chasms (where crust has been stretched) ; mountains resulting from compression of crust; coronae and arachnoids (where molten rock has moved up through the crust without quite penetrating the surface).
c) Mars: Tharsis Bulge, where crust has been bulged outward; Valles Marineris, where crust has been stretched to form a broad & deep chasm; large volcanoes (mainly associated with Tharsis Bugle), most notably Olympus Mons.
11. Tectonic processes of various kinds and, to a lesser extent, impact cratering are the most important processes occuring on Venus' surface. Venus apparently does not have plate tectonics, although some tectonic process of a global nature evidently wipes out craters from time to time.
12. Earth: dense nitrogen & oxygen atmosphere; well defined zones: troposphere, stratosphere, thermosphere, exosphere; water clouds routinely form.Mercury: virtually no atmosphere.
Venus: very dense atmsophere of carbon dioxide; high-altitude sulfuric acid clouds. Mars: thin carbon dioxide atmosphere; thin clouds of water & carbon dioxide; occasional dust storms. Moon: virtually no atmosphere.
13. Thermal escape generally requires high temperature and weak gravity. High temperature means molecules move fast; low gravity means escape velocity is low. At any given temperature, light molecules move faster than heavy molecules. The Moon's gravity is weak, and its temperture is moderately high (on the daytime side). Mercury's gravity is also weak, and it is very hot on the daytime side. Thus both the Moon and Mercury have lost whatever atmosphere they might have originally had. The other terrestrial planets are warm enough that light hydrogen molecules can escape.
14. Mars must have had a denser atmosphere the past because there is abundant evidence in its surface topography that water once flowed. Today, the atmosphere is far too thin to support water in the liquid state. It is possible that much of the atmospheric gas disappeared via atmospheric cratering (this would have been effective on Mars due to its relatively weak gravity). Some of its atmosphere may now be locked-up as ice, both at the poles, and underground.
15. a) A planet the size of Venus probably has a metallic core that is partly liquid. Given this, and the relatively rapid rotation rate, this planet should have a magnetic field.
b) Without radioactivity, the planet's surface is probably tectonically dead, and has been so for a very long time. We would thus likely find no plate motion and no active volcanoes. Without plates, we would find no mountain chains like those on Earth. In general, few features we associate with tectonic activity would be apparent.
c) A substantial atmosphere would probably manifest itself in weathering features on the surface.
16. Mars' atmosphere exerts too little pressure to allow liquid water to exist at the surface today.
17. Mars' polar caps apparently consist of a mix of frozen carbon dioxide and frozen water.
18. The large temperature differences are mainly due to i) the lack of any significant atmosphere, which would tend to trap heat at night; ii) a very slow rotation rate: the Mercurian day (sunrise to sunrise) is 176 Earth days in length. Slow rotation leaves a lot of time for cooling at night.
19. Mars' rotation rate is fast (a Mars day is about 24 hours), but its atmosphere is very thin; the thin atmosphere provides little insulation against the loss of heat at night.
20. Earth's clouds: droplets of liquid water & crystals of frozen water at low altitude. Venus' clouds: droplets of sulfuric acid at high altitude.
21. Greenhouse effect: the trapping of infrared radiation emitted by a surface of a planet by its atmosphere; IR radiation is trapped when it is absorbed by molecules of certain greenhouse gasses, e.g., carbon dioxide and water. The greenhouse effect warms Earth's atmosphere to moderate temperatures; on Venus, the greenhouse effect has created oven-like temperatures.
22. Knowledge of Earth's deep interior comes mainly from the study of seismic (earthquake) waves.
23. Earth's magnetosphere is the region of space surrounding Earth dominated by Earth's magnetic field. The magnetosphere prevents the solar wind from penetrating too close to Earth.