Information about Jupiter and Saturn

Jupiter

I. General Characteristics

Mass				1.898 X 10²⁷ kg = 317.7 Earth
Radius at 1 bar 		71492 km = 11.21 Earth
g = GM/r²			23.12 m/s²
Mean Density	 		1.33 g/cm³
inclination to ecliptic		1.3^o
inclination of equator to orbit	3.12^o
Rotation Period			9.925 hours
Mean orbital distance	 	5.2 AU  
Orbital period			11.86 years
Mean orbital velocity		13.07 km/s
eccentricity			0.048
temperature at 1 bar		165 K
effective temperature 		124 K

Saturn

Mass				5.69 X 10²⁶ kg = 95.2 Earth
Radius at 1 bar 		60268 km = 9.45 Earth
g = GM/r²			8.96 m/s²
Mean Density	 		0.69 g/cm³
inclination to ecliptic		2.49^o
inclination of equator to orbit	26.73^o
Rotation Period			10.656 hours
Mean orbital distance	 	9.555 AU  
Orbital period			29.42 years
Mean orbital velocity		9.67 km/s
eccentricity			0.056
temperature at 1 bar		134 K
effective temperature 		95 K

At first glance, the 4 Gas Giants appear very similar in many ways. In other ways, however, they are quite different. An analogy is to a family of 4 siblings -- each have similarities, but often the differences are striking, and have to do with genetic makeup as well as environmental factors such as birth order. I hope this material can help to convince you that the differences in the Gas Planets are rooted in how and where they formed. These planets comprise 99.5% of the planetary mass in our solar system -- understanding them is intimately linked to the understanding of how they formed.

II. Composition in Context

Summary of formation

Condensation likely between 100-200 K

Outside "frost line" ~170 K -- mostly ices, some silicates

Nucleus accretes to about 5 M_earth

Nebular gas is attracted to this core -- forms thick envelope

Later accreting planetesimals dissolve in the atmosphere

"ices" (as Darby said) can be methane, ammonia, not just water

Composition with respect to solar nebula/primitive atmosphere

when we say "solar" composition, that is solar nebula (before fusion)

H 71%

He 27%

2% "condensable" -- C, N, O (in water, ammonia, methane, rock)

Figure 14.3 from text: rock/water/H/He planets

Retention of primitive atmosphere

escape velocity is ~60 km/s (sqrt(2 G M/R))

mean molecular weight is 2.3, so V_mean = sqrt(3kT/m) ~= 0.133 km/s

define "retentivity" as V_esc/Vmean_mean (for H) retentivity "R"
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250
170
147
120
30
25
15
12
11
10
10
9
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6

Planet
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Jupiter
Saturn
Uranus
Neptune
Earth
Venus
Mars
Titan
Ganymede
Triton
Io
Mercury
Europa
Pluto
Callisto
Moon

so we see from this that the Gas Giants should easily retain H! Do they?

Trace primitive atmospheres with noble gases

Ne -- heavy, not from decay

estimate Ne, add proportions of others to estimate "primitive" atmosphere

Jupiter is remarkably close to primordial composition

Saturn is missing more H/He

Uranus & Neptune are missing most H/He

Chemistry/Composition still an active research area

in equilibrium, most C in methane, N in ammonia, and O in water

ethane (C₂H₆), acetylene (C₂H₂), CO imply dis-eqilibrium (could be photodissociation products)

vertical mixing & inhibited chemical reactions

III. Interior (Ch. 14)

Bulk density close to 1 g cm^-3 -- clue to composition

Increase in pressure with depth balances the increased pull of gravity: HYDROSTATIC EQUILIBRIUM

Bulk density is not the whole story because the interior is compressed

More mass only increases size a small amount, if any, may decrease (ionization or D fusion)

High compression results in high temperatures as well

Phase diagram for Hydrogen -- H₂ gas & liquid/metallic

A good part of giant planets are "liquid" -- from 1/6 radius down

Oblateness

Non-rotating planet is a perfect sphere, which we know from Newton acts as all the mass concentrated at center

A spinning ball of gas spreads out at the equator and flattens at the poles (r_eq-r_pole)/r_eq

Saturn greatest at 0.098, then Jupiter 0.065, Uranus 0.023, Neptune 0.017

Models of gravitational field (compared to observations) yield info about interiors

observed by satellite (natural & artificial) orbits

probe of nonuniformities within

summary wedge showing probable structure

core (1% by mass) 10-15 Earth masses of rock/ice, but only 1.5 Earth radii

Internal heat sources

Jupiter emits twice as much energy as it receives from the Sun

Where does this energy come from?

primordial heat -- left over from planet's formation/gravitational collapse

still contracting -- gravitational collapse generates heat (together Kelvin-Helmholtz model)

chemical or radioactive processes -- insufficient to explain current heat

Jupiter has the largest internal energy source in Solar System

hot interior causes convective currents which affect the visible atmosphere

IV. Clouds and Circulation

First concept: Opacity is wavelength dependent

familiar from Greenhouse effect -- atmospheric gases are opaque at IR only

an atmosphere can be entirely clear except at a few particular wavelengths

see multiwavelength Jupiter images

Condensation temp. determine cloud layers

absolute abundance exceeds abundance at dew point

cloud structures determined by temp/pressure & each type of cloud forms accordingly

lowest water ice or drops

ammonium hydrosulfide (NH₄SH = NH₃ + H₂S)

NH₃ ammonia layer

CH₄ methane condenses at REALLY cold temps (Uranus & Neptune)

unless supported by updrafts, condensates collect and "rain out"

same structures on each planet -- lie deeper on colder planets

2 probes, one planned & one unplanned

Galileo probe -- fell into hotspot, was relatively "dry"

Comet Shoemaker-Levy 9 -- high energy, dredged up things from below

atmospheric motions fundamentally different from terrestrial planets

temperatures depend on depth primarily, not latitude
faster planetary rotation: smears circulation into horizontal bands -- more of them
no planetary surface: circulation doesn't slow and lose energy (results in long lived storms)
planet's internal heat: contributes to atmospheric motion, setting up vertical convective cells (cf. Earth's molten mantle)

Colored banded structure (reds, yellows, browns, whites -- composition)

rotation spreads solar energy along a latitude band

bands are very stable (compared to Earth's -- named in text)

move in opposite directions, light & dark

light zones (upwelling, warm clouds)

dark bands (cooler sinking gases; see deeper into atmosphere)

strong equatorial winds

equatorial winds about 300 km/hr

east and west wind patterns mirrored about equator

powerful jet streams result

Big storms

convective currents, caused by rising hot material, are deflected by Coriolis effect (force due to planets rotation)

this produces powerful jet streams & bands of clouds

swirling patterns (vortices, eddies) in between these -- big storm systems

find long lived storms - high pressure systems (cf. Earth, low pressure storm systems)

no land mass to slow them down

e.g. Great Red Spot:

at least 300 years old

40,000 km by 15,000 km - size of two Earths!

anticlockwise rotation in southern hemisphere --> high pressure system

six day rotation period

Magnetic Field

As well as atmospheric circulation, material also circulates in the interior

Jupiter has rapid rotation rate

large convective metallic liquid hydrogen mantle yields huge magnetic field!

need rapid rotation and electrically conducting material for a magnetic field

How big is Jupiter's magnetic field?

at cloud top, field 20 times that at Earth's surface

accounting for size, this means Jupiter's field is 20,000 times that of Earth's!!

Orientation of Jupiter's magnetic field

the field is tilted by 10^o to Jupiter's rotation axis

has opposite polarity compared with Earth (magnetic N is in the south)

field is offset from the centre of the planet by about 10%

The magnetosphere one of largest objects in Solar System

confined by the solar wind -- not so strong further from the Sun, resulting in a huge magnetosphere, larger than the Sun itself

Jupiter's magnetotail (away from Sun) appears to reach all the way to Saturn's orbit!

charged particles get trapped in the field, spiral along field lines and thus auroras are seen at Jupiter's poles

Jupiter's Rings

More about rings next week, but all gas planets have some level of rings

Jupiter's are small and faint, but interesting and illuminating nonetheless

"Gossamer" ring structures are related to "source" moons

Saturn

Giant ringed planet; 85% the size of Jupiter but a third the mass;

also no solid surface; banded atmosphere; rapid rotation; emits own energy source; large magnetic field

Similar composition to Jupiter (and the Sun and Solar Nebula):

89% H and 11% He - where has the He gone?

also ammonia, methane, and water traces

plus a few complex carbon compounds

Like Jupiter, has a highly compressed interior possibly superfluid

large mantle of liquid hydrogen under gas atmosphere

also has an inner region of metallic liquid hydrogen

smaller mass than Jupiter so need to go deeper for high pressures needed for liquid metallic hydrogen

so smaller volume of metallic hydrogen than Jupiter

similar small rocky/ice core

similar size as Jupiter's (thus greater percentage of Saturn's total mass)

``rock'' = Fe, Si, O and ``ice'' = C, N, O and H compounds

Saturn's interior is mostly liquid hydrogen

Internal Energy

Saturn emits three time the energy it receives from the Sun (cf. Jupiter emits twice solar)

but being further from the Sun it receives less energy than Jupiter

so the energy emitted is less

Being a smaller body, it will have retained less primordial heat

energy source thought to come from differentiating helium

cooler atmospheric clouds (since further from Sun)

helium condenses and "rains out"

falls towards centre

releases heat from gravitational energy

Saturn is still differentiating!

explains lower % of helium in atmosphere

falling helium drops convert gravitational energy into thermal energy

Atmosphere

Similar to Jupiter with more blurred yellowish banded structure

cooler temperatures (further from Sun)

thinner and more extended upper atmosphere (lower ``surface'' gravity)

cold enough to freeze ammonia in clouds

veils deeper layers

less distinct markings than Jupiter (more blurred)

the colder temperatures also inhibits chemical reactions; results in duller cloud colours than Jupiter

Large storms quite rare

they appear to be seasonal (due to 27^o obliquity)

equatorial storms seen about every thirty years

caused by hot upwelling plumes

Still see banded structure however, caused by rapid rotation and interior convection cells due to internal heat

Also very rapid wind speeds 1300 km/hr

Magnetic Field

Saturn also has a large magnetic field, but not as strong as Jupiter's

similar rotation speed (a little slower)

smaller metallic liquid hydrogen content

thus smaller magnetic field than Jupiter

Magnetic field is aligned with planet's rotation axis

Also has a very large magnetosphere

Auroras also seen; but fewer charged particles are trapped in the magnetosphere (charged particles are affected by the ring system...)

Titan

Atmospheric Pressure = 1.6 Bars

Surface near triple point of methane -- suspected ocean

HST images -- maybe?? markings suggest "dark" parts of surface

Huygens Probe!