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planetary geology.notebook
1
April 05, 2013
Planetary Geology
With the exception of the Apollo missions to the Moon, humans have not visited any other celestial body (in person). We have, however, been able to send probes, rovers, and other scientific machines to other planets and natural satellites in our solar system.
Scientists believe that we can use our knowledge of geology (the study of the Earth) to interpret the observations we make about other planets.
Geological Features and Processes on Earth
What features on Earth do we associate
with geological processes?
What features do we see on the Moon?
(This is a hint)
Geological Features and Processes on Earth
Why is the surface of the Earth different from the Moon?
What is responsible for the differences?
Terrestrial Planets –
How we know what we know • Telescope observations
• Hundreds of probes have visited. They take photographs, chemical samples, temperatures, and even drive around.
• Geology, meteorology, and climate science are well developed here on Earth
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BBC Video: "The Planets: Terra Firma"
Terrestrial Planets GeologyWhen the inner planets formed, the collisions that built them up generated a tremendous amount of heat.
This heated them up to the point where they were mostly liquid.
As they swirled and churned, the heaviest materials tended to sink to the middle.
We say that the planets became “chemically differentiated”
This is in contrast to small asteroids with are chemically similar throughout
Terrrestrial Planets Geology
Solid metal core
Liquid metal layer
Over time, the planets radiated away a lot of their heat and cooled to the point where they had solid surfaces
Rocky mantle
Solid crust
Surface: Temperature = 300 K
Pressure = 1 atmosphere
Middle: Temperature = 8000 K
Pressure = 10,000 atmospheres
Terrrestrial Planets GeologySome planets cooled faster than others. Why?
While the planets were cooling, the Solar System was still a very active place. The process of accretion by the planets did not suddenly end, and it took many (millions) of years for the planets to clear their orbits of 'leftover' material or establish (semi) stable orbits.
The last major period of accretion occurred approximately 4 billion years ago, and is referred to the Late Heavy Bombardment. By this point, the planets had formed (mostly) as they are now, and had cooled sufficiently to form solid crusts. The majority of craters on the Moon and Mercury occurred during this time. All the Inner Planets would have received a similar (i.e., same order of magnitude) bombardment, so why do the Moon and Mercury have more craters?
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Terrestrial Planets GeologyEven though the planets have largely lost the heat from their formation, there is another source of heating in the interior – Radioactive decay
This heating drives convection within the interiors – movement of material from hot spots to cooler spots
On Earth, this moving convection causes the plate tectonics at the surface –
the movement of the giant plates that form the crust over thousands of years
This is what creates Earthquakes, Volcanoes, mountain ranges, and continental drift
Mountain ranges like the Himalaya and Rockys are formed by the tectonic plates crashing into each other
Terrestrial Planets Geology
The other terrestrial planets show evidence of having processes similar to Earth's (at least at some point), like volcanoes and fissures.
Olympus Mons – giant volcano on Marsgiant fissure on Mars
MagnetismWhat produces a static magnetic field? Current in a loop!
As a planet rotates, any charged particles in the core are kind of making a loop
Planets have magnetic fields!
The details are quite complicated (dynamo theory) and we won’t get into it here
For instance the field poles can be offset from the rotational poles
And the polarity can ‘flip’ every so often
(26,000 years for the Earth)
Terrestrial Planets Water When the planets were forming, they were bombarded with waterrich comets from the accretion disk and elsewhere
Mercury and Venus had high daytime temperatures so the water boiled away
Mars was just a little too cold, so the water was frozen away in ice caps and (maybe) subsurface layers
Water is only liquid in a small range of temperature (273 373 K)
The Earth was like Goldylocks – just right
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Inner Planets Atmospheres
Planet Major Atmosphere Component
Surface Pressure (Atm. Thickness)(bar)
Mean Surface Temperature (K)
Predicted Surf Temp. from orbit dist. (K)
Venus Carbon Dioxide (96.5%)
92(very thick!) 737 300
EarthNitrogen (78.1%)Oxygen (20.9%)Carbon Dioxide (.03%)
1 288 253
Mars Carbon Dioxide (95.3%)
.007(very thin!) 210 200
Venus, Earth, and Mars are massive enough to retain some gasses by gravity near the surface Atmospheres
Atmospheric properties are largely responsible for the surface temperature deviation
Inner Planets AtmospheresCarbon Dioxide (CO2) is a greenhouse gas – it traps heat
Venus: large atmosphere with mostly CO2 = lots of CO2 and heat trapping
Earth: large atmosphere with little CO2 = some CO2 and heat trapping
Mars: tiny atmosphere with mostly CO2 = little CO2 and heat trapping
Inner Planets AtmospheresWhy does Earth have so little CO2 in the atmosphere?
1) Since the Earth has water, rain and rivers erodes surface minerals and move carbon around, allowing other substances to react with CO2 and form compounds. Carbon is locked up in rocks!
2) Plants take carbon out of the atmosphere and make organic compounds. Carbon is locked up in life!
Unfortunately, we are now releasing this carbon by burning fossil fuels! This is increasing the CO2 in our atmosphere and creating global warming.
So much carbon
Earth’s atmosphereNitrogen 78%Oxygen 21% (from plants!)Argon 1%CO2 0.4%
Mt. Everest
High altitude balloon
Water 0.4%Neon traceHelium traceMethane trace
Decreasing temp. with height due to decreasing pressure
Increasing temp. with height due absorption of sunlight by Ozone layer
Decreasing temp. with height due to decreasing pressure
Increasing temp. with height due absorption of cosmic rays
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