Internal Heating: Planets and Moons
July 21, 2005
Presented to teachers in TRUST
by Denton S. Ebel
Assistant 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 published
literature have been removed here)
Early Solar System:
Collisions of Small Bodies
to Make Bigger Bodies
and Eventually Planets
(image taken from published
literature has been removed here)
Chondritic meteorites contain radionuclides
Abundant Isotopes
Extinct:
26Al => 26Mg
720 K years
Present time:
40K => 40Ar, 40Ca
238U …. 208Pb
235U …. 207Pb
232Th …. 208Pb
1.27 G years
4.47 G years
704 M years
14.0 G years
From The New Solar System,
Beatty, Petersen & Chaikin (1999),
Cambridge U. Press, ch. 17 fig. 5
Tidal Heating of Io
(image taken from published
literature has been removed here)
Orbital resonance with Europa tugs Io, so Io’s Jupiter-facing side
wobbles slightly. These tidal forces generate heat by internal friction.
orbit R / primary equatorial g
160000
orbitR/priGeq
140000
120000
100000
80000
60000
40000
Io
20000
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Moon - Io - Europa - Titan
Comparing the orbital radius with the gravity of the primary gives
an idea of the tidal forces experienced by a Moon.
Jupiter’s major moons, seen by Galileo in 1610:
Io
Europa
Ganymede
Saturn’s major moon, Titan
Callisto
Earth’s moon, Moon
Io
Europa
Ganymede
Callisto
Voyager missions (1979)
showed that each of these
moons is a different world.
The moons are all ‘tidally
locked’, rotate in the same
direction in nearly circular
orbits in Jupiter’s equatorial
plane. They likely formed as
a ‘subnebula’ in the solar disk.
Moon
orbit density
Io
5.9
3.5
Europa
9.4
3.0
Ganymede
15.0 1.9
Callisto
26.4 1.8
(orbits are in Jupiter radii)
Our Solar System
Pluto-Kuiper belt
(short period comets)
Asteroid belt
(meteorites)
Io
Plan Patera plume, 140 km
(Galileo spacecraft 1997)
Pele’s plume,
300 km high
(Voyager 1, 1979)
Prometheus plume
(Galileo spacecraft 1997)
Pele volcano on Io (Galileo spacecraft image, 1997)
Io
April 1997
September 1997
400 km
Pillan Patera volcano outflow on Io, imaged by Galileo spacecraft, 1997
Inside Io
Io
(maybe)
Silicate sulfur crust
Silicate
Mantle
FeS?
Core
Europa
Crater
on
Europa
Streaks
on
Europa
Streaks fractures
filled with
ice.
Streaks
on
Europa
Ice Rafts on Europa
Great 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
Europa inside
on
(maybe)
Europa
Silicate + ice
silicate
FeS?
Core
water ice crust
The Moon
Photo #:
IV-138-M
Photo #:
Mission:
IV
IV-121-M
Lunar Orbiter
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 Theories
1) 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 published
literature 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)
Mare Imbrium
Erastothenes
Copernicus
Kepler
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
N2
CH4
Ar
surface T: 93.8 K (-180 C)
Artist rendering of Huygens probe descending into Titan
Saturn A Ring
UV Imaging Spectrograph
dirty = red; icy = turquoise
res = 60 miles (97km)
Photo # PIA05075
The End
PIA05076 C+B rings