Todays Article:

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Todays Article Beyond the Principle of
Plentitude A Review of Terrestrial Planet
Habitability E. Gaidos, B. Deschenes, L.
Dundon, K. Fagan, C. McNaughton, L.
Menviel-Hessler, N. Moskovitz, M. Workman Univ.
of Hawaii - Honolulu
U. of Wisconsin
NASA JSC
Astrobiology Journal Club - 7 December 2005
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In some worlds there is no Sun and Moon, in
others they are larger than in our world, and in
others more numerous...There are some worlds
devoid of living creatures or plants or any
moisture. - Democritius (460-370 BC)
...after all, a basic resemblance doesnt
exclude infinite differences...but the difficulty
is to figure it out. - Bernard de Fontanelle,
1686
Summary Beyond Plentitude - primary
considerations 1) Astronomical - Planet
formation and orbital stability 2) Geological -
Surface activity (e.g. plate tectonics) 3)
Establishing a biosphere - delivery of volatiles,
carbon and nitrogen cycles, the onset of life
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• Part I Astronomical considerations
• Motivation The circumstellar Habitable Zone
  (HZ)
• Series of refinements of the definition of the
  HZ 1959 - 1995
• 1st definition
• Volume around a star where the stellar flux
  falls between a proscribed minimum and
  maximum (Huang 1959)
• Subsequent refinements
• Surface temperature of a planet is
  appropriate for human habitation (Dole
  1970), stability of liquid water and the HZ as a
  function of stellar mass (Hart 1979)
• More recent calculations include modeling of
  climate which
• include silicate weathering and CO2
  greenhouse feedback Kasting et al. 1993) .
  HZ confined to 0.95 (water loss) to
• 1.37 AU (CO2 clouds).

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The Habitable Zone (continued) Refinements
since 1995 - 1) Including geochemical
considerations in climate and habitability
models (Franck et al. 1999, 2000). 2)
Treatment of obliquity (axial tilt), eccentricity
and rotation rates and their effect on
climate Results from these refinements -
Geological activity The HZ of a planet spans a
greater range of orbital semimajor axes than a
geologically static planet in the early solar
system (low solar luminosity)(Franck et al.
2000) - Obliquity Without the large angular
momentum or the E-M system, the Earth would
exhibit high and variable obliquity (Laskar et
al. 1993), leading to large seasonal temperature
variations. Still habitable - heat transport
moderated by oceans and atmosphere (Williams
Kasting 1997).
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Recent HZ results (continued) - Eccentricity
High eccentricity planets likewise still
habitable for similar reasons (Williams
Pollard 2002) - Rotation rate Faster rotation
leads to stronger Coriolis effects, inhibiting
the poleward transport of heat planet more
susceptible to ice-albedo feedback, runaway
glaciation (Jenkins 1996).
Figure from Franck et al. (2000) showing the HZ
in semimajor axis radius (AU) vs. time Red A
geologically static planet Green A
geologically active planet including weathering
and degassing
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The Frequency of Habitable Zone Terrestrial
Planets A Consideration of Formation and
Orbital Stability In the absence of
observational data, theoretical modeling
(numerical simulations) remain the primary
approach to investigating these
topics. Terrestrial Planet Formation - a general
picture - Coagulation of circumstellar dust
into km size planetesimals on the timescale of
105 yrs. - Subsequent runaway growth
resulting in lunar-sized planetary embryos or
oligarchs. - Dynamical interactions between
embryos generate a high merger rate leading to
the formation of terrestrial planet(s) total
formation time 10 -100 Myr
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Terrestrial Planet Formation (continued) Simula
tions have been increasing in complexity
incorporating chaotic dynamics, close embryo
encounters, gas friction in the protoplanetary
environment (Noble et al. 2002 Cincotta Simo
2000 Kominami Ida 2004) see Kenyon
Bromley (accepted AJ, astro-ph/0503568) for a new
series of simulations and background
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Terrestrial Planet Formation (continued) Effects
of gas giant planets on terrestrial planet
formation - Inhibiting planetestimal to embryo
growth (runaway phase) by accelerating small
bodies where collisions destroy them rather
than lead to coagulation. - Facilitating
embryo to planetsimal growth (oligarchic phase)
by increasing orbital eccentricities and
increasing crossing/merger frequency (Whitmire
et al. 1998, Weidenschilling 2000). - Levison
Agnor (2003) - a series of simulations that
determined the effect of giant planets (with
different masses, orbital semimajor axes,
orbital eccentricities) on terrestrial planet
formation Initial conditions Used giant
planet configurations of (Levison et al.
1998)...
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Formation of Terrestrial Planets (continued)
Levison Agnor (2003)
No giant planets
Initial conditions 100 planetesimals, each with
m 0.04 Mearth A surface density profile that
varied as r-3/4 with 8.0 g/cm2 at 1
AU Comparable to minimum mass solar nebula
models with 2 Mearth interior to 1.5 AU
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Formation of Terrestrial Planets (continued)
Levison Agnor (2003)
Giant planet configurations
Resulting terrestrial planets
General trend More massive giant planets on
more eccentric orbits will produce fewer, more
massive terrestrial planets.
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Orbital Stability of Terrestrial
Planets Dynamical habitability - Which known
exosolar systems allow for stable orbits of
terrestrial planets within the habitable
zone? Addressed by Menou Tabachnik (2003)

• Each point is one outcome of the simulation with
  the initial conditions
• test particles distributed evenly throughout the
  HZ of the star
• A giant planet with a semimajor axis location
  given by the x-position of the point, scaled to
  the HZ of the star

Result Survivability of test particles highest
when giant planets do not enter the HZ
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Orbital Stability of Terrestrial Planets
(continued) Simulations of Binary
systems Benest et al. (1988, 1989, 1996, 1998,
2003) Simulations of a single hypothetical
terrestrial planet in s-type orbit in a binary
system. Result stable roughly circular
orbits exist when the orbital radius is up to
one-half of the binarys periastron separation.
Figures from Benest (1988) showing the orbital
stability of a terrestrial planet orbiting ? Cen
B Left Binary barycenter frame Right ? Cen B
frame
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Orbital Stability of Terrestrial Planets
(continued) Other studies of even more complex
binary systems show similar regions of orbital
stability, some even in the stars HZ see
Dvorak (2003a,b) study of ? Cephei (as system
with a binary star (a 21.4 AU) and a giant
planet (a 2.15 AU)
Zones of terrestrial planet stability (white) in
the HZ of the ? Cep system as calculated by
Dvorak (2003) by numerical integrations of a
4-body system. Below Existence of jumping
orbits (semimajor axis vs. time)
Semimajor axis (AU)
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Orbital Stability of Terrestrial Planets
(continued) Gas giant migration and its effect
on orbital stability Mandell Sigurdsson
(2003) found that terrestrial planets can
survive the migration of a Jupiter-like planet
through the HZ, though the rate is low (1 -
4) Gomes et al. (2005) showed that giant
planet migration may cause stochastic
bombardment events (analogous to the solar
systems LHB) later in the evolution (several
100 Myr) of a planetary system
Left Numerical simulations of Gomes et al.
(2005) showing disruption of a planetesimal disk
caused by Saturns 12 MMR crossing. Right
Evolution of giant planet orbits (top) and
cumulative lunar impact mass (bottom)
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Part II Geological Considerations Motivation
Geological activity is necessary to sustain may
processes essential for the habitability of
Earth The Long-Term Carbon Cycle (a.k.a. abiotic
carbon cycle, inorganic carbon cycle, or the
carbonate-silicate cycle)
In the inorganic carbon cycle, removal of
atmospheric CO2 by weathering is balanced by
degassing of CO2 by subduction and subsequent
volcanism.
from Kasting Catling (2003)
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The Long-Term Carbon Cycle (continued) In the
absence of geologic activity (i.e. volcanic
outgassing), weathering would remove all carbon
from the modern atmosphere, sequestering it in
the ocean at carbonates (Schlesinger 1997) The
Magnetic Field The magnetic dipole produced by
the convecting liquid metal core protects the
earth from atmospheric sputtering from the solar
wind (Yung DeMore 1999) and interstellar
material (Pavlov colloq.) A brief digression
into geomagnetic reversal Periodically, the
Earths magnetic field will decrease to almost
nothing and sometimes switch polarity
(geomagnetic excursions and reversals). The
most recent occurrence is the Brunhes- Matuyama
reversal, about 780,000 years prior.
Geomagnetic excursions and reversals
attributed to chaotic motion in the core and
correlated (?) to speciation...
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The Magnetic Timeline Excursions and Reversals
(continued)
Credit USGS and Guyodo and Valet (1999)
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Geological Considerations - Mantle
activity Geological activity in a terrestrial
planet is intimately tied to interior
convection Gaidos et al. explores the diversity
of terrestrial planet interiors... Mantle
convection in terrestrial planets is the result
of heating from the decay of radiogenic isotopes
(e.g. 40 K, 232Th, 235U, 238U) and latent heat
from a solidifying core. Subsurface convection
is thought to be nearly universal among
terrestrial planets larger than a given
size The convection criteria (Raleigh
number) is a function of h5 where h is the
scale height (depth) of the mantle region.
Other dependences, such as the abundance of
radioiotopes are negligible. However, this
subsurface convection has diverse manifestations
on the lithosphere (crust)...
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Geological activity - The Lithosphere

• Three distinct modes
• 1) Plate tectonic regime
• (Earth)
• 2) Stagnant lid
• (Moon, Mercury, Mars)
• Magma ocean
• (impact induced)
• Also, an intermediate regime between 1) and 2)
  (Venus)

(Heat loss through crust)
(Heat production in the mantle)
The figure (Sleep 2000) describes the three
lithosphere modes in terms of mantle temp, heat
loss through crust, and heat production in
mantle. Transitions can occur between modes.
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Geological activity - The Lithosphere
(continued) The existence of plate tectonics
is limited to a range of temperatures (Sleep
2000) Low temp Disappearance of mantle
melting and cessation of sea floor
spreading High temp Greater mantle melting
and the formation of a crust too thick for
subduction to occur Activity in the absence of
Plate Tectonics Hot-spot volcanism - a
consequence of deep mantle material (core- mantle
boundary) ascending to the lithosphere. Accounts
for only a small fraction of outgassing in
modern Earth, but may be responsible for surface
volatile budgets on Venus and Mars (Phillips et
al. 2000). Question Relative frequency of
geological modes?
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Part III Establishing a Biosphere Motivation
Even given a terrestrial planet in the HZ of a
star, with the necessary geological activity,
other ingredients are necessary to establish a
viable biosphere. Among these, the presence of
volatiles, including water. Special Delivery -
The introduction of water to Earth Important
not only for life processes, but for geologic and
atmospheric processes Planet forming material in
the HZ experience high temperatures, leaving the
inner system depleted of volatiles (Stevenson
1988, Cyr er al. 1999). Meteroric material
derived from inner system planet bodies have a
low water content 0.1 by weight. Therefore -
Must deliver water from elsewhere in the solar
system...
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Special Delivery (continued) Any successful
theory of water delivery to Earth must
explain 1) The abundance of water (2.8 x 10-4
Mearth on the surface and in the crust, and
0.8 - 8 x 10-4 Mearth in the mantle (Lecuyer
1998) 2) The ratio of deuterium to hydrogen
isotopes in seawater (1.53 x 10-4) Several
candidates for water delivery...
One possible delivery mechanism The so-called
six pack
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Special Delivery - Candidates for Water delivery
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Special Delivery (continued)
Raymond et al. (2004) simulations modeling the
formation and delivery of water to terrestrial
planets in the HZ of a hypothetical
system. Mass, orbital semimajor axis, orbital
eccentricity of a outer giant planet, radius of
water condensation, and density of solids were
varied between simulations. Only bodies outside
2.5 AU contained water. Of the 45 terrestrial
planets formed in the HZ, 35 received as much or
more water than the Earth (solid dot).
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The Retention of Water Habitability dependent
not only on the initial water budget, but also
the long term retention of water against both
loss to space and the planet interior Factors
affecting retention of water Rate of water
return to mantle via incomplete dewatering during
subduction - in 1 billion years, 1/4 of the
Earths water will be sequestered in the
mantle Stellar luminosity - the ultimate loss
of water on Earth due to the increase in solar
luminosity (in about 2.5 billion yrs).
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Other Elements of Habitability - Carbon and
Nitrogen Functions Both - Essential
building blocks of life, and necessary for life
processes Carbon - maintenance of surface
temperatures (long-term C cycle) via CO2
greenhouse Nitrogen - Inert buffer gas
permitting gas exchange between organisms,
pressure broadens IR absorption lines of
greenhouse gasses Again geology is important
the amount of C and N involved in geochemical
cycles exceeds that in biotic processes by 106...
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Other Elements of Habitability - Carbon and
Nitrogen (continued)
biosphere
Inventories of surface carbon and nitrogen on the
Earth (units of 1018 mol) (Schlesinger
1997) Abiotic sources dominate by a factor of
106. Primitive chondrites have C and N
abundances that are depleted by factors of 60 and
400 relative to the bulk modern earth (Newsom
1995) Again, how were the current inventories of
C N delivered to the early Earth?
biosphere
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Other Elements of Habitability - Carbon and
Nitrogen (continued) Possible delivery
mechanisms 1) Meterorites - early CC have
isotopic compositions in C and N similar to
Earth (Sephton et al. 2003 Alexander et al.
1998) 2) Comets - possible source of N, would
explain high terrestrial ratios of Kr and Xe as
well (Owen et al. 2001) 3) Interplanetary dust
- IDP from Antacrtic ice contain a carbon
abundance similar to CM (Merchison) meteorites
(Maurette et al. 2000) Origin of Organics in
the Inner Solar System (see Review by
Ehrenfreund Charnley (2000))
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Organics in the Inner Solar System
(continued) Possible sources of terrestrial
organic molecules 1) Interstellar organics -
Meteorite organics show D enrichment indicative
of low-temp interstellar origin of meteorite
organics (Sandford et al. 2001) 2)
Fischer-Tropsch conversion of CO and H2 into
methane and heavier hydrocarbons on the surface
of metal grains in the solar nebula (Prinn
Fegley 1989) (n 0.5m)H2(g) nCO(g) --
CnHm(s) nH2O(g) Though does not explain
enrichments of D and 15N in meteorite organic
matter 3) Photochemical processing of solar
nebular gas - Ion-molecule reactions driven by
x-ray irradiation (Robert 2002)
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Factors Affecting Surface Inventories of C
N - Rate of impact degassing - Impacts release
C and N sequestered in the mantle. Efficiency
of degassing of mantle due to impacts increases
with planetary size (Matsui Abe 1986) - Rate
of atmospheric loss - Hydrodynamic loss to space
of H, C and N due to UV heating of the upper
atmosphere and/or giant impacts. Larger bodies
are less susceptible to this loss (Ahrens
1993) - Efficiency of C and N sequestering in
the mantle and core - C is siderophilic and can
be incorporated into Fe melts during impacts and
core formation (Abe 1993 Wood 1993). Affected
by size and composition of core (Kuramoto
1997) - Solubility of N2 in silicate melts -
highly dependent on the oxidation state of the
mantle (Libourel et al 2003 Miyazaki et al.
2004). A change in oxidation state of the
mantle could have led to the initial
sequestering and later release of N into the
atmosphere
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Factors Affecting Surface Inventories of C N
(continued) The C and N cycles - Volatiles
not sequestered in the core or lost to space
cycle between the surface and mantle via the
aforementioned long-term C cycle and the global
N cycle - Surface inventory of N includes only
atmospheric N2 and N in sedimentary and
crystalline rocks - N released into atmosphere
at mid-oceanic ridges and subduction zones via
degassing - Current rates of degassing
integrated of the age of the Earth only account
for 7 of the surface inventory, indicating
higher rates of degassing in the early
Earth - Nitrogen removed from the atmosphere
only by biological fixation into organic forms,
or lighning conversion into NOx
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Global N Cycle (continued) - Thus, N remains
in the mantle due to incomplete degassing
rather than efficient recycling (e.g. the
Venusian atmosphere has a much higher N2 content
due to the lack of recycling) The Early
Hydrosphere - Jack Hills zircons (4.4 Gyr old)
show signs of interaction with water,
indicating an early hydrosphere, though the
nature of the water is not known (temperature,
pH, salinity) - The chemistry of the early
hydrosphere would be dominated by interactions
with the atmosphere (see Garrels Mackenzie
1971, Kempe Degens 1985 for a discussion on
early ocean pH) - Salinity another factor in
debate (see Morse Mackenzie 1998, Hardie 2003)
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Timescales for the formation and evolution of the
Earth (Gaidos et al. 2005)
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The Emergence of Life - When did the earliest
life occur? - The oldest purported microfossils
(of cyanobacteria-like organisms are 3.5 Gyr
old (Schopf 1993) - Other early microfossils
include the Barberton tubes (3.5-3.2 Gyr Furnes
et al 2004) and pyritic filiments from deep-sea
volcanogenic deposits (3.2 Gyr Rasmussen
2000) - Chemofossils - Isotopically light
carbon depleted in 13C found in graphite
inclusions in apatite grains in Greenland (3.8
Gyr Mojzsis et al. 1996) Oil inclusions in 3
Gyr old Archean sandstone (Dutkiewicz et al.
1998) - Implications for the selection of stars
for future terrestrial planet finding missions