Planets of the Solar System What s Initially Available: Solar Nebula - Composition
Size versus Mass depends on composition - fight between gravity & pressure Differentiation causes the picture to be more complex Gas & Ice Giant Interiors
Liquid Metallic H Molecular H Ices Rock Interiors of Jovian Planets
Interior Conditions Schematic representation of the interiors of Jupiter and Saturn. The range of temperatures is estimated using homogeneous models and including a possible radiative zone indicated by the hashed regions. Helium mass mixing ratios Y are indicated. The size of the central rock and ice cores of Jupiter and Saturn is very uncertain (see text). In the case of Saturn, the inhomogeneous region may extend down all the way to the core which would imply the formation of a helium core. from Guillot & Gautier 2015, Treatise on Geophysics, 2nd Ed.
Model solutions are not unique... Schematic representation of the interiors of Uranus and Neptune. The ensemble of possibilities for Neptune is larger. Two possible structures are shown. from Guillot & Gautier 2015, Treatise on Geophysics, 2nd Ed.
Terrestrial Planets Chemical Boundaries Core - solid center & liquid envelope - mostly Fe with some Ni, S - motion in liquid core generates Earth s magnetic field Mantle - rock - mostly olivines 1 & pyroxenes 2 - location below the crust is detected by a change in seismic wave speed Crust -lighter rock - rich in feldspars 3 Oceanic Crust - thinner crust under ocean floors Continental Crust - thicker - floats on mantle due to isostatic equilibrium 1 Mg2SiO4 - Fe2SiO4 * 2 MgSiO3 - FeSiO3 - CaSiO3 ** 3 KAlSi3O8 NaAlSi3O8 CaAl2Si2O8 * Can be (Mg,Fe)2SiO4 solid solution ** XYSi2O6 where X and Y can be Fe, Mg, Ca, K, and rarely Zn
Chemical & Physical Boundaries Crust & upper mantle - solid lithosphere Below is partly liquid aesthenosphere There seems to be a very fluid boundary layer (discovered in 2015)
Comparison of Terrestrial Planets
Atmospheres E esc = GM p m R p and KE = mv2 2 mv 2 2 = GM p m R p so v esc = 2GM p R p mv 2 2 = 3 2 kt and v = 3kT m use v rms Note that even if v avg < v esc, there will be some molecules at the high-speed tail of the distribution that will have v > vesc. The usual rule of thumb is that if v avg ( 1 6)v esc the gas molecules will not escape quickly enough. Notice that depends on the composition of the gas molecule more massive ones move more slowly at a given temperature, and are retained by the planet more easily! For Earth, Oxygen (O 2 ): Hydrogen (H 2 ): v esc = 11.2 km s 1 v rms = 0.48 km s 1 so 6v rms = 3 km s 1 < 11.2 km s 1 So the Earth retains v rms = 1.9 km s 1 so 6v rms = 11.4 km s 1 > 11.2 km s 1 oxygen but not hydrogen
The atmospheres of terrestrial planets are not the same O 2 + hν O + O O + O 2 + M O 3 + M O 3 + hν O 2 + O ( a 3 body collision) ozone production O + O 3 2O 2 CO 2 + silicate rock liquid water! SiO 2 + carbonate rock Urey Reaction These atmospheres have evolved, and the presence of liquid water on Earth has transformed it
Power absorbed by a planet: Planet Cross section $ L* π R 2 4πd 2 p (1!"# a) p!"# Fraction Energy Absorbed Reaching Planet Power radiated by a planet: In thermal equilibrium: Planetary Temperatures where a = reflectivity ( "albedo" 4π R 2 σt* 4 * 4πd 2 p 4π R * 2 σt* 4 4πd p 2 Planet Surface Area m!"# 2 4π Rp 2 σtp 4 $ Radiated W /m 2 π R p 2 (1 a) = 4π Rp 2 σtp 4 π R p 2 (1 a) or: T p = 1 2 R d p (1 a) 1/4 T or T p = 1 2 1 d p ( ) σπ L 1 a 1 4 or more simply: Tp = 278 1 d AU L Lsun 1 4 1 a ( ) 1 4 Kelvins
For our own solar system: Planet d (AU) a Predicted T Observed T Mercury 0.39 0.056 440 100-620 Venus 0.72 0.76 230 750 (and very uniform!) Earth 1.00 0.39 250 180-330 (290 avg.) Mars 1.52 0.16 220 130-290 (Sub-Solar Equatorial) Jupiter 5.2 0.51 104 160 (cloud tops) Saturn 9.5 0.61 81 90 (cloud tops) The freezing point of water is 273 K, and the boiling point is 373 K, under 1 Atm pressure. Venus is currently too hot for liquid water. Mars is too cold. The Earth is "just right". We can do the same calculation for other types of stars as well. Question: Why are most of the planets hotter than this? J & S - internal heat source - emit more than they absorb! V & E - Greenhouse effect
Greenhouse Effect!! The greenhouse effect warms a planetary surface by warming the atmosphere above it. Due to the Sun s surface T, Light from the Sun peaks near wavelengths where the Earth s atmosphere is transparent, so little heating of it occurs. The light that is absorbed by the ground heats the ground, when then radiates at its characteristic T, which is a lot cooler, and peaks in the IR. Unlike visible wavelengths, the IR is filled with absorption bands due to various molecules. These molecules absorb the IR, effectively depositing that energy in the atmosphere, and warming it. The warm molecules radiate at those same wavelengths that they absorb at, and half of the radiation is back in the direction of the surface of the planet.
Here is an example of the effect, taken from Grant Petty s wonderful (but technical) book A First Course in Atmospheric Radiation. The upper panel shows the IR spectrum looking down through the atmosphere from 20 km above the surface. In this case, the location is the north polar ice sheet. Looking down through the transparent wavelengths one sees the blackbody spectrum of the ground (the high parts of the curve). At 15 µm it is fainter because you are looking at a wavelength where CO2 is absorbing that radiation. Essentially no ground radiation gets through at those wavelengths. What you see is the top of the layer only, and it is high in the atmosphere, and hence colder than the ground so it is less bright than the ground. The band between 9 and 10 µm is O3. In the bottom panel you are looking up. At the wavelengths where the atmosphere is pretty transparent, you are seeing the cold of outer space so to speak, so it is dark at those wavelengths. In the CO2 band, the sky is brighter, since the molecule is radiating photons down on you (and the rest of the ground, of course). Notice that grass at wavelengths both longward of 16 µm and shortward of 8 µm? Most of that is water vapor!
These are both satellite spectra i.e. looking down on the Sahara Desert and the Antarctic Ice Sheet. The Sahara is hot, and has a blackbody spectrum probably pretty close to the upper dashed curve. But some of it doesn t reach the satellite because of the absorption by greenhouse gases. On the other hand, when looking down on the Antarctic, the surface is very cold, and the CO2 is actually warmer!!
2 more cases. The upper one is over the tropical ocean. In the clear sky case, we see the usual things. But note the curve labeled Thunderstorm Anvil. Here the bands of H2O are saturated, blending in across the entire spectrum. And because the top of the anvil is high in the atmosphere, it is colder than the surface, and hence does not radiate as brightly as the surface. Below is a similar spectrum of Iraq. Note how weak the H2O bands are compared to the upper one. (I guess that the Sahara one from the previous page must have been a really high-humidity day!!) I found a nice little description, apparently from Channel 4 News in Chicago, which I saved, and you should look at the figures here: http://homepages.uc.edu/~sitkoml/advancedastro/archer.ch4.greenhouse_gases.pdf
At what distance will water freeze & boil? dau = 278 2 T L Lsun 1 a Set L * = L sun and calculate d for T = 273 K (water freezes) and T = 373 K (water boils, std atm pressure). With a greenhouse effect, need an additional term - ε ε = 1 means no greenhouse effect. Otherwise ε < 1. T p = 278 1 d AU L L sun 1 4 1 a ε 1 4 Kelvins dau = 278 2 T L L sun 1 a ε
If a = 0 and ε = 1 (blackbody planets) If a = 0.39 (Earth s reflectivity) but ε = 1 (no greenhouse) If a = 0.39 and ε = 0.5 (add some greenhouse) The HZ ( ecoshell ) depends on the properties of the planet As star s L changes and planet s atmosphere evolves, the HZ MOVES!! - Related to Faint Sun Problem - how was life on Earth possible when L sun was 25% less??? We will return to this topic later...
Reflectance & Emittance Spectra - Terrestrial Planets
SpeX Spectra of Jovian Planets http://irtfweb.ifa.hawaii.edu/~spex/irtf_spectral_library/index_files/planets.html
AKARI Spectrum of Neptune methane ethene = ethylene ethane