Internal structure of planets

Internal structures of planets (not at scale). The three sub-families on the left are part of the

terrestrial family. Giant planets (Jupiter-like) are on the right.

Neptune–like planets, are on the fourth position from the left.

Overview

Except for the Earth and the Moon, there is no direct measurements of the deep structure of the planets.

This investigation requires a network of seismometers for terrestrial planets, or techniques similar to the asterosismology for gaseous giants.

Nonetheless, the internal structure of planetary bodies in the solar system is, even if not very precise, relatively well understood.

Overview

Planetary bodies can be split into three main families:

i) the terrestrial planets (or solid planets)Subclasses: Mercury-like, Silicate, Ocean planets

ii)the giant planets (or gaseous)iii)the icy giants which are in between the

two extreme cases.

Overview

Planetary bodies can be split into three main families:

i) the terrestrial planets (or solid planets)Subclasses: Mercury-like, Silicate, Ocean planets

ii)the giant planets (or gaseous)iii)the icy giants which are in between the

two extreme cases.

Giant Planets

The G

iant P

lanets

Formation theories

• Core accretion:a rocky core forms through the coagulation of planetesimals until it is sufficiently massive to accrete a gaseous envelope

• Gravitational instability: Same mechanism as stars form

The jovian planets can be roughly divided into two groups:

• Jupiter and Saturn are similar in size (large, ~ 10 Rearth), with similar reddish and brownish colour.

• Uranus and Neptune are smaller in size (~ 4 Rearth), with similar bluish colour.

The GAS GIANTS in Solar System

Internal Structure of the Jovian Planets• Computer models fully differentiated interiors all four jovian planets have cores of rock, metal, and hydrogen compounds,

with a total mass of about 10 Mearth.

• Jupiter and Saturn captured large amount of hydrogen gas… Uranus and Neptune, being further away from the Sun where the density of

the solar nebula is smaller smaller amount of gas before solar wind cleared the gas supply for planet formation.

M = 318 Mearth M = 95.2 Mearth M = 14.5 Mearth M = 17.1 Mearth

Ionic `Ocean’+ Hydrocarbons

(H30)

Rocky core

MolecularEnvelope (H, He, methane)

Uranus & Neptune

• While the surface temperature of the jovian planets are very low,

the temperature

inside the planets increases rapidly with depth.

Mass-RadiusDiagram

Energy budget• Incident solar radiation much less than

that at Earth• So surface temperatures are lower• We can compare the amount of solar

energy absorbed with that emitted. there is usually an excess! - except

Uranus

5.4

8.1

13.5

48

2.0

2.6

4.6

14

0.6

0.6

3.5

0.3

0.3

0.6

1.4

incidentreflected

All units in W/m2

Jupiter Saturn Uranus Neptune

Sources of Energy • One major one is contraction – gravitational energy converts

to thermal energy…• Gravitational energy of a uniform sphere is

• So the rate of energy release during contraction isRGMEg /6.0 2

dt

dR

R

GM

dt

dEg2

2

6.0

e.g.Jupiter is radiating 3.5x1017 W in excess of incident solar radiation. it is contracting at a rate of 0.4 km / million years

• Another possibility is tidal effects in the interior turns out to be small.

• Radioactive decay is a minor contributor.

• Why is Uranus’ heat budget so different? – maybe due to compositional density

differences– inhibiting convection at levels deeper than

~0.6Rp

Uranus tilted on its side…– a possible explanation is an oblique impact

with a large planetesimal (c.f. Earth-Moon)

– This impact might even help to explain the compositional gradients

which (may!) explain Uranus’ heat budget

Hydrogen phase diagram

• Jupiter – interior mostly metallic hydrogen

Hydrogen undergoes a phase change at ~100 GPa to metallic hydrogen (conductive)

It is also theorized that He may be insoluble in metallic H. This has implications for Saturn.(rainout)…

• Saturn – some metallic hydrogen

• Uranus/Neptune – molecular hydrogen only

Summary• Jupiter - mainly metallic hydrogen >> self-compression. Rock-ice core.

• Saturn - mix of metallic and molecular hydrogen;

helium may have migrated to centre due to insolubility.

Mean density lower than Jupiter because of smaller self-compression effect.

• Uranus/Neptune – pressures too low to generate metallic hydrogen.

Densities require large rock-ice cores in the interior.