Astronomy Class Notes

Monday, April 06, 2009

- read chapter 9 and 10

Chapter 9

- the interior of the planets and the effect on the crusts

o geologically dead, thick lithosphere, crusts, interior is hot but lithosphere is so thick that even though the heat from the interior works its way outwards, there is no geological activity on the surface

§ Moon

§ Mercury

o Mars, geologically dead now but for the first 2-3 billion years it was geologically active

§ Lithosphere thick but not as much as the moon and mercury

o Alive Geologically, thin lithosphere, tectonic activity, volcanism

§ Venus

§ Earth

o The Earth

§ 2 cores

· Solid in the center

o Pressure at the center is so great that it makes the metal solid

· Liquid core

· Mostly composed of iron and nickel

§ Mantle

· Primarily rock

§ Heat transfer

· In core transferred by conduction

o Adjacent atoms transfer energy

· In mantle transferred by convection

o Mantle is turning

o Convection, hot material physically transporting itself

§ Material moves up to the surface, loses heat, then sinks back down

§ Tremendous stresses, different parts of crust feel pressure in different directions

· Leads to cracking, tectonics

§ How know this?

· Study earthquakes

· Certain waves don’t travel through liquid, detect only on certain areas of the earth and then learn geometry from that

· After know what made out of calculate with pressure the temperature

§ Tectonics

· Crust is pulled in two ways, easy for molten material to come out to the surface of the earth

· These are called ridge zones

o Most of these are in the ocean

§ Solidify immediately

§ Creating new crusts

· Earth is brittle convective cells tear apart, these are called tectonic plates

· Sometime pushes them together

o One goes under the other

o Subduction zones

o This destroys some of the crust, so as much that gets created the same amount gets destroyed so it is in equilibrium

§ Volcanism

· Vents that are channels from the molten material to the surface

· Limit to the size before the stresses in the mountain breaks it up

§ 200-250 million years for earth to be resurfaces, about 20 complete resurfacings

o Venus

§ Should work the same way but doesn’t, mystery

§ Doesn’t have plate tectonics

§ Has tectonics, but not plates

§ In the mantle of Venus convention cells are set-up like that of earth

· Earth’s crust is more brittle so it breaks up easily to plates

· Venus is less brittle

o One theory is that the surface is too hot, 700K, makes the rocks more pliable

o Because it is not brittle may get volcanic vent but no plates, possibly hot spots at base of volcano, but we don’t know

§ Resurfaces by volcanism at about ¼ billion years

§ Hard to study Venus surface because landing crafts are inoperable within 1 hour of landing because of temperature and air pressure

- Magnetic fields

o 2 conditions to have one on terrestrial planet

§ Need metal core

· In metal it is easy to make electrons move along the surface by hooking up a battery

o This starts a current flowing (electrons moving)

§ Relatively rapid rotation

· When a metal core is moving rapidly, a current is set-up

o Law of electromagnetism

§ Moving currents produce electric fields

o Planets that have a strong electric field

§ Earth and Mercury

o Importance

§ Solar wind, charged particles streaming off the sun at relativistic speeds

· Protons

· Electrons (beta particle)

· Helium nuclei (alpha particle)

§ This is a radioactive source

· The earth’s magnetic field protects us from this

o The earth’s magnetic field is a dipole field, such as one set-up by a bar magnet

o An electric field accelerates charged particles

o A magnetic field accelerates charged particles but changes the direction

o Without these fields higher life on earth could not happen

§ Our magnetic field funnels the charged particles to the poles

· Earth’s north and south magnetic poles are not lined up with the north south poles we know

§ This explains why we are stuck in low earth orbit, this is because the space shuttle people need to be protected by earth’s magnetic field from the solar wind

Wednesday, April 08, 2009

Chapter 10: Planetary Atmosphere

- Exam 4 Friday April 24^th

	Venus	Earth	Mars
CO2	96.5	0.033	95.3
N2	3.5	78	2.7
O2	None	20	None
Natural Greenhouse Effect	400K	30K	6K
Pressure	90	1	0.01

(Note: moon and mercury have no “real” atmosphere)

- mars and Venus have a significant amount of carbon dioxide and a little bit of nitrogen, while we have a little bit of carbon dioxide and a lot of nitrogen and earth also has oxygen, which the other two don’t

- Venus and mars are similar in composition

- Pressures are all very different

o Venus has pressure that is 90 times denser than ours (comparable to being under water)

o Mars has very little

o Earth has the pressure we observe

- Greenhouse effect

o Natural and necessary to life on the planet

o Venus has one that has changed that planet, causes Venus’s extreme heat

- Atmospheric pressure

o Definition, the pressure is proportional to the density and the temperature

o Pressure, is the force per area

§ Produced when particles run into a surface

· Can be increased by

o Increasing the number of particles so that more particles are hitting

o Raising the temperature, makes particles move faster, more of an impact when hit

o Pressure has an effect on surface because:

§ Allows what can be on the surface

· For example a glass of water can be liquid on the earth but not on mars because pressure is too low on mars

· Tissues would bulge painfully if a human was on mars

§ Wind and weather patterns

§ Erosion due to the atmosphere

§ The natural greenhouse effect

· Optical light and UV light come in from the sun, 80% gets absorbed

o Blackbody theory, we pretend that the earth is a perfect absorber and perfect emitter

o A lot of the absorbed light comes out in the infrared

o This causes the greenhouse effect

§ Most common greenhouse gas is water vapor, then CO2, then Methane

· Absorb infrared radiation, these gases cannot leave the atmosphere they are trapped for a little while, have a harder time getting out than coming in.

§ N2 and O2 are not greenhouse gases

· This causes a higher temperature

· If Venus didn’t have an atmosphere is would be at 360K, actual temperature is 700K because of greenhouse effect

· Mars this only changes the temperature by 6 degrees Kelvin because the atmosphere is so low

§ Magnetosphere

· Ionization of the atmosphere

o Layer of ionized gas high up in the earth’s atmosphere

o How they interact with the magnetic field

· This has a multitude of effects, including how we can receive radio waves

o If there is no greenhouse effect then the temperature is given by this formula

§ T= 280 x fourth root((1-reflectivity)/d^2) *online how to derive this*

· 280 Kelvin (calibration factor)

· Distance has to be in AU

· Reflectivity runs from 0 to 1 where 0 is not reflective at all and 1 would be completely (mirror)

· T stands for temperature in Kelvin

· Earth’s is .2 reflectivity

o Without the greenhouse effect the earth would have been below freezing and the earth would be inhabitable to life

- Natural ways for climate to change

o Greenhouse effect

o Solar Variability

§ Even though the output is steady over time it does change by a few tenths of a percent

§ So low presently that it seems odd and could lead to something bad “solar minimum”

o Changes in reflectivity

§ Volcanic eruption, throws a lot of particles in the air makes reflectivity go up and temperature goes down

Friday, April 10, 2009

- Exam 4 Friday April 24^th Ch. 9, 10, 11

- Final Monday May 4^th

- Long term climate change

o Solar brightening

o Change in reflectivity

o Greenhouse gas abundance changes

o Changes in axial tilt

§ Earth is tilted 23 ½ degrees at present but the range is 22-25 degrees

· This affects the insolation (amount of radiation coming from the sun)

§ Combination changes in the tilt and changes in the elliptisity play a huge role in climate (Milankovitch cycles)

- How terrestrial planets gain gas

o Volcanoes

§ This is how secondary atmosphere (used to be thought) is formed

§ Tremendous amounts of water vapor and CO2

o Evaporation

§ Sheet ice evaporates and goes into the air

o Planetesimals

§ Controversial

· Some think it is negligible

· Some think it is really great

§ When planetesimals ran into planets it releases gases into the air

- How terrestrial planets lose gas

o Condensation

§ Oceans form (big role in the earth)

o Thermal escape

§ At a given temperature there can be fast moving particles and escape the atmosphere

o Solar wind

§ Ionize particles and kick them out of the atmosphere

§ Earth’s magnetic field protects earth atmosphere somewhat from this

o Chemical reactions

§ Rocks and such combine with gas

o Large impact

§ Large planetesimal colliding with a planet can cause a lot of the particles to reach escape velocity and escape the atmosphere

- Stratification (layers) of atmospheres

o Earth’s (complicated)

§ Troposphere

· Air is churned in convective cells

· Infrared energy causes the convection

· Heat has to move from the surface to space

§ Stratosphere

· Calm air

· Temperature is a function of height

· Heating is the direct absorption of ultraviolet light, this is some light that never reaches the ground from the sun

§ Ionosphere

· Density is low enough that x-rays and particles from the sun ionized particles

· Very little past this

§ Exosphere

· Where you get your loses from thermal escape

o Mars and Venus

§ Temperature just drops, not a function of height

· no ozone

§ no stratosphere, so there is no ultraviolet absorbing layer

- The goldilocks problem

o Few factors

	Venus	Earth	Mars
CO2	96%	.03%	95%
N2	3%	77%	3%
Pressure	90	1	0.006
O2	none	21%	None

§ natural occurring greenhouse effect makes the temperatures of the planets much higher in Venus and only slightly higher in Earth, hardly any effect on Mars

o Problem

§ Venus looks a the way it does because it is too hot, mars looks this way because it is too cold, and earth has the right temperature to be habitable for humans

§ Earth lost carbon dioxide because water can be liquid on earth so the carbon dioxide got dissolved in this water, made carbonate layers in the bottom of the sea

· This explains why we have a lot of nitrogen

· We don’t have the same bulky atmosphere of Venus because nitrogen is puny in comparison to the amount of carbon dioxide that we should’ve had in our atmosphere

§ Mars had some liquid water so it lost some carbon dioxide but it kept most in the atmosphere

· Why hasn’t earth lost carbon dioxide due to rain?

o Does, but it is counteracted by tectonic activity so it stays in equilibrium, this gives us a steady temperature

· Mars is smaller than the earth so it died geologically a long time ago

o Lack of geological activity killed the above mechanism for returning CO2

o So atmospheric pressure goes down, and greenhouse effect go down

o This was the popular explanation for a while

§ Recently some research has raised questions about this hypothesis

· How does the solar wind play into this?

o A lot of these numbers can be explained by the stripping of the particles by the solar wind

o Solar wind was mostly responsible for the drop in Mars’s atmospheric pressure

o This is not the whole story, the atmosphere works in a non-linear pattern, so we can’t simplify the interactions in the atmosphere down to this