Internal Heating: Planets and Moons

July 21, 2005

Presented to teachers in TRUST

by Denton S. EbelAssistant Curator, Meteorites

Department of Earth and Planetary Sciences

Heat Sources of Planetary Bodies

Primordial Gravitational potential energy (differentiation) Accretion or collision energy (external source)

Contemporary Decay of radioactive elements (all rocky planets)

(probably 60-80% of Earth’s heat flow: 40K, 232Th, 235U, 238U)

Tidal friction (only in some cases, e.g.-Io) Solar heating (restricted to surfaces)

Complex and Simple Cratering

(images taken from publishedliterature have been removed here)

Early Solar System:Collisions of Small Bodies

to Make Bigger Bodiesand Eventually Planets

(image taken from publishedliterature has been removed here)

Chondritic meteorites contain radionuclides

Abundant Isotopes

Extinct: 26Al => 26Mg 720 K years

Present time:40K => 40Ar, 40Ca 1.27 G years238U …. 208Pb 4.47 G years235U …. 207Pb 704 M years232Th …. 208Pb 14.0 G years

Orbital resonance with Europa tugs Io, so Io’s Jupiter-facing sidewobbles slightly. These tidal forces generate heat by internal friction.

From The New Solar System,Beatty, Petersen & Chaikin (1999),Cambridge U. Press, ch. 17 fig. 5

Tidal Heating of Io

(image taken from publishedliterature has been removed here)

Comparing the orbital radius with the gravity of the primary givesan idea of the tidal forces experienced by a Moon.

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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Moon - Io - Europa - Titan

orbi

t R /

prim

ary

equa

tori

al g

orbitR/priGeq

Io

Jupiter’s major moons, seen by Galileo in 1610: Io Europa Ganymede Callisto

Earth’s moon, MoonSaturn’s major moon, Titan

Io

Europa

Ganymede

Callisto

Voyager missions (1979)showed that each of thesemoons is a different world.

The moons are all ‘tidallylocked’, rotate in the samedirection in nearly circularorbits in Jupiter’s equatorialplane. They likely formed asa ‘subnebula’ in the solar disk.

Moon orbit densityIo 5.9 3.5Europa 9.4 3.0Ganymede 15.0 1.9Callisto 26.4 1.8 (orbits are in Jupiter radii)

Asteroid belt(meteorites)

Pluto-Kuiper belt(short period comets)

Our Solar System

Io

Pele’s plume,300 km high(Voyager 1, 1979)

Plan Patera plume, 140 km(Galileo spacecraft 1997)

Prometheus plume(Galileo spacecraft 1997)

Pele volcano on Io (Galileo spacecraft image, 1997)

Io

April 1997 September 1997

Pillan Patera volcano outflow on Io, imaged by Galileo spacecraft, 1997

400 km

Io

SilicateMantle

Silicate - sulfur crust

FeS?Core

Inside Io

(maybe)

Europa

Crater on Europa

Streaks on Europa

Streaks -fractures filled with ice.

Streaks on Europa

Ice Rafts on EuropaGreat rafts of ice in re-frozen surface (view width ~70 km)

Deformation of Europa

Four possible processes:

1 - upwarping

2 - surface fractures

3 - upwelling & fluid flow

4 - collapse to chaotic terrain

Crater on Europa

FeS?

Core

silicate

Silicate + ice

water ice crust

Europa inside(maybe)

Photo #:    IV-121-M Mission:    Lunar Orbiter IV Date:    1967

Photo #:    IV-138-M

The Moon

The Moon• Galileo Galilei (1564-1642) - observed the

moon through a telescope and called the dark smooth areas maria (latin for seas) and the lighter colored, rugged terrain, he called terrae (latin for lands).

• Aside from the Earth the moon is the best understood planetary body in the solar system.

• Many of our current theories and hypotheses of how the Earth and other planets formed were developed and tested by studying the moon.

Dr. Harrison Schmitt,astronaut on Apollo 17

Large split boulder at Taurus-Littrow landing site

Moon Formation Theories1) Co-accretion in orbit while Earth formed.2) Capture - Moon formed elsewhere in the

nebula but was captured by Earth’s gravity.

3) Giant Impact.

• Observations that need to be explained• Chemically the moon is similar to Earth’s mantle• The moon lacks the more volatile elements• Moon’s metal core, if present< is relatively small• Oxygen isotopes are similar to the earth.

Schematic of Moon Forming Impact

(image taken from publishedliterature has been removed here)

Radiometric Dates for the Moon

• Absolute ages determined by radiometric dating of rocks from the moon.

• Basaltic lavas are 3.65 to 4.0 billion years old.

• Lunar highlands are more than 4.5 billion years old.– Indicates that the terrae formed shortly after

accretion of the moon.

• Some ray material from Copernicus is less than 1 billion years old.

• Integrating these ages into the relative scale allows the development of an absolute scale.

Rate of Cratering and Volcanism with Time

• Rate of cratering was much more intense in the earlier periods of lunar history.

• The decline in the amount of impact events was rapid after about 3 billion years ago.

• It is assumed that this is representative of the cratering history of all planets including the Earth.

• Based on radiometric ages, volcanism lasted about one billion years between 4.0 and 3.2 billion years ago.

• Some lavas are 2.5 billion years old but may be melt generated by impact.

Lunar chronology of Crater Copernicus region.

Shoemaker and Hackman (1962)

Copernicus

Erastothenes

Kepler

Mare Imbrium

Crater Copernicus• Copernicus

– Bright rays extend over 300 km.– Rays extend across Procellarum and Mare

Imbrium.– Rays cut across the floor of Erastothenes. – Craters Kepler and Aristarchus have similar

patterns of rays to those of Copernicus.

•Therefore Copernicus ( and Kepler and Aristarchus) are younger than Erastothenes and the basalts of Mare Imbrium).

Crater Erastothenes• Erastothenes

– Found on the lunar maria– Terraced walls– Circular floor– Central peak – Small secondary craters– No visible rays

•Therefore, Erastothenes is younger than the maria and older than the rayed craters.

Imbrium Basin• Imbrium Basin

– Large multi-ring basin– Filled by lunar maria

•Craters like the Imbrium Basin are older than the lunar maria and craters like Erostathenes

– This period of muilti-ring craters and extrusions of the lunar maria is known as the Imbrium Period

Ejecta from the Imbrium Basin overlap craters like the Nectarian Basin in the lunar highlands.

Titan

Artist rendering of Huygens probe descending into Titan

N2

CH4

Ar

surface T: 93.8 K (-180 C)

The End

Saturn A RingUV Imaging Spectrographdirty = red; icy = turquoise

res = 60 miles (97km)Photo # PIA05075

PIA05076 C+B rings