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The Seven Ages of Terrestrial Planetary
Science and the Pervasive Contributions
of William Kaula
Kaula’s Seven Ages of a Planet
1.
2.
3.
4.
5.
6.
7.
Nebular condensation
Planetesimal growth
Planetary formation
Vigorous convection
Plate tectonics
One-plate volcanism
Quiescence
Seven Ages of
Terrestrial Planetary Exploration
1.
2.
3.
4.
5.
6.
7.
Early Earth satellites
Planetary reconnaissance (Mariner flybys)
Apollo
Global survey of Mars (Viking, MGS)
Global survey of Venus (PVO, Magellan)
Global survey of the Moon (Clementine, LP)
Present era (Mars Surveyor and Discovery Programs
First Age of Terrestrial Planetary
Exploration: Early Earth Satellites
In the 1960s, William
Kaula pioneered the
conceptual and analytical
tools that established the
field of space geodesy.
“And then formation: sweeping up
The bodies in its way”
High-school photograph
(Kaula, 1966)
Second Age of Terrestrial Planet
Exploration: Reconnaissance by Flybys
Venus
1962
1967
Mariner 2
Mariner 5
Mars
1965
1969
Mariner 4
Mariner 6, 7
Mercury
1974-75
Mariner 10
“The first of these is condensation; dust grains drifting to
The nebula plane in chondrite clods.”
Detail from AAS Division of
Dynamical Astronomy meeting
photo, 1970
ICARUS 28, 429-433 (1976)
Mercury May Have Accreted Material
from a Range of Solar Distances
One scenario for orbital evolution and planetesimal
provenance for “Mercury” [Wetherill, 1988].
Competing Hypothesis for the Origin of
Mercury’s High Metal/Silicate Ratio
1. Preferential removal of
silicate grains by gas drag.
2. Vaporization of silicate outer
layers by an early active Sun.
3. Stripping of silicate outer
layers by a giant impact.
Simulation of an off-axis collision of protoMercury with a protoplanet one sixth its mass
[Benz et al., 1988].
Third Age of Terrestrial Planetary
Exploration: Apollo
William Kaula served as
Principal Investigator for the
laser altimeter experiment
on Apollo 15, 16, and 17,
which provided the first
quantitative information on
lunar nearside-farside
differences.
“But heating slows;…mare
To surface, ending fractionation.”
Fourth Age of Terrestrial Planetary
Exploration: Global Surveys of Mars
(Viking, Mars Global Surveyor)
William Kaula was the first
to show clearly the effect of
Tharsis on the global shape
of Mars and how to remove
its contribution to J2 to infer
C/MR2.
“Final volcanism: no
More lithospheric spreading,only vents
For magma, Nix Olympica.”
Issues in Martian Internal Structure
• Core composition,
radius, and physical
state
• Nature of core-mantle
boundary
• Implications for
mantle plumes
Bertka and Fei (1998)
Global Influence of Tharsis
The Tharsis rise must have been substantially in place by the
Late Noachian, on the basis of its influence on deformation
and valley network downslope directions.
(Phillips et al. 2000)
Fifth Age of Terrestrial Planetary
Exploration: Global Surveys of Venus
(PVO, Magellan)
William Kaula served on the
science teams for both the Pioneer
Venus Orbiter and Magellan
missions. He made many
contributions to an understanding
of the comparative evolution of
Venus and Earth.
“And then comes plate tectonics: cooling leads
To lithosphere, with many marginal breaks.”
Detail from Magellan Radar
Investigation Group photo, 1990
Crater Retention Ages on Venus Imply
Past Lithospheric Recycling
Possible recycling mechanisms:
•Lithosphere stabilizes by melt
extraction then becomes
unstable after cooling
[Parmentier and Hess, 1992].
•Boundary-layer instability for
high-Rayleigh-number thermal
convection [Turcotte et al.,
1999].
Climate Change May Have Influenced
Tectonic Deformation on Venus
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Magellan image of ridged plains,
Rusalka Planitia (image 150 km
wide).
Surface thermal stress induced by climate
change following emplacement of ridged
plains lavas, according to the climate
models of Bullock and Grinspoon (2000).
Sixth Age of Terrestrial Planetary
Exploration: Global Surveys of the Moon
(Clementine, Lunar Prospector)
In the 1990s, William Kaula
continued to press for key
threads that linked the distinct
evolutionary tracks of the
bodies in the inner solar
system.
“A planet in its time plays
many parts.”
Global Geophysical and Geochemical Mapping
Indicates the Moon is Strongly Heterogeneous
Neumann et al. (1996)
Joliff et al. (2000)
The Seventh Age is Upon Us
The NASA MESSENGER mission, to be
launched in 2004, will flyby and orbit
Mercury and return geochemical
information diagnostic of the planet’s
formational processes.
The Seventh Age is Upon Us
Determining the interior structure
of Mars requires a surface seismic
network.
The CNES NetLander mission will send four
geophysical and meteorological stations to
Mars in 2007.
Kaula 5485
“Last scene
That ends this history is quiescence: time
Sans melt, sans plates, sans almost everything”
William Kaula, 1926 - 2000
The Seven Ages of a Planet
Our system is a stage,
And both the Sun and planets merely players.
They had their birth and’ll have their fiery end.
A planet in its time plays many parts,
Its acts being seven ages. The first of these
Is condensation: dust grains drifting to
The nebula plane in chondrite clods. And then
The planetesimals: breaking sometimes, but
Most growing, though the Sun’s hot breath blows gas
Away. And then formation: sweeping up
The bodies in its way, in fierce infalls
To bring then full convective vigor, too hot
For crust to form, though iron may sink and seas
Outgas, by radioactive energy driven.
And then comes plate tectonics: cooling leads
To lithosphere, with many marginal breaks.
Convective thrusts a crust create in belts
Complex. But heating slows; the sixth age shifts
Into the final volcanism: no
More lithospheric spreading, only vents
For magma, Nix Olympica or mare
To surface, ending fractionation. Last scene
That ends this history is quiescence: time
Sans melt, sans plates, sans almost everything.