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Habitable zone
F. Marzari,
Dept. Physics,
Padova Univ.
Earth: 0.95-1.15 AU
HABITABLE ZONE: DEFINITION
A habitable zone for a given star is defined by the range of
distances around a central star
at which Earth-like planets maintain conditions
sufficient for the existence of life (carbon based?) at the
surface.
A habitable planet is one on which liquid water is
stable at the surface (in between 273-373 K).
DELIVERY OF WATER AND OTHER VOLATILES TO THE
EARTH
Main volatiles: H2O, CO2, N2, HCl
Outgassing from Earth interior (vulcanic activity)
Impacts of water-rich bodies: scattered by outer planets
Total mass of water on Earth:
5× 10− 4 M E
Planetesimals in the terrestrial zone: less than 0.05% of water.
This could also be lost during the accretion phase due to
collisional heating
Planetesimals from the outer regions of the solar system
rich in ices: possible radial mixing
3 possible sources
Outer asteroid belt
Jupiter-Saturn planetesimals
TNO region
1) Outer asteroid belt
Formation of planetary embryos – Stirring due to mutual
perturbations and Jupiter – Compositional mixing and enrichment
of water with impacts of outer embryos (Chambers, Raymond
etc...) - asteroid belt depletion
2) Planetesimals from the Jupiter-Saturn region.
About 50-100 ME of planetesimals in this region.
Estimates assume that 2-20% of water could have
been supplied. This assuming that the Earth had
its present size, but when Jupiter (and Saturn)
were fully formed probably the Earth was small
and still losing water by accretional heating.
3) Planetesimals (cometesimals..) from the TNO regions
In the Nice model
cometesimals are scattered
inside by encounters with
Neptune and Uranus which
migrate outwards. However,
estimates of total mass
delivered by cometesimals
is about 2.5 x 10-5 ME Too
low, only about 10% of
ocean water mass.
All scenarios require Jupiter! If terrestrial planets
form in systems where Jupiter is not around 5 AU
can they be enriched with water like the Earth?
Most extrasolar planets
on eccentric orbits:
habitable planet needs
circular orbit.
Temperature and feedbacks
S
σ T = (1− A)
4
4
e
Te = 255 K
Greenhouse effects: IR emission is absorbed by
greenhouse gasses (CO2, H2O, CH4 ...)
Te = 255 +33 = 288 K
Climate feedbacks
If T decreases, saturation
pressure drops, less H2O in
atmosphere, less greenhouse
effect, farther decrease of T:
positive feedback
If T increases, the Earth emits
more IR radiation and cools down:
negative feedback
Carbonate-silicate cycle: CO2 +
H2O → H2CO3 (carbonic acid)
which dissolves silicate rocks and
goes to the bottom of ocean where
it is reprocessed and emitted by
vulcanos as CO2. If T grows CO2
precipitation increases (while
vulcanism is constant) and CO2
concentration falls.
The sun luminosity
changes with time,
it was cooler in the
past: higher
concentration of
greenhouse gases.