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Habitable Moons and Planets
Around Post-Main Sequence
Stars, or
Titan Under a Red Giant Sun
Ralph D. Lorenz
Space Department
JHU Applied Physics Laboratory
Scientific American 2010
Habitability / Origins of Life in outer solar system and
especially of Titan-like-worlds
Features of the Titan climate system – condensible
greenhouse (runaway) and haze antigreenhouse effects.
Effects of evolving insolation, evolving solar spectrum
Effects of solar mass loss.
‘Delayed Gratification Habitable Zone’ (A. Stern, Astrobiology)
Guo et al., Astrophys Space Sci 2010
Titan Climate Studies
1970s – speculations. Nitrogen greenhouse – Habitable conditions? (Sagan, Hunten)
1980s - Voyager data in hand. Overall radiative balance of red haze/greenhouse world laid
out analytically by Samuelson (Icarus, 1983). Detailed photochemical models developed
~1990 - Cassini mission formulated. Wavelength-resolved radiative-convective model
(RCM) developed by McKay (Icarus, 1990; Science, 1991)
Coupling of photochemical evolution of volatile reservoir and climate change due to solar
evolution (Lunine and Rizk, 1989; McKay et al. 1993)
Mid-late 1990s - Explorations with McKay RCM - (partial) collapse of atmosphere in case
of methane depletion, investigation of feedbacks, Titan under a Red Giant Sun (Lorenz et
al., 1997a,b; 1999)
2006 - Exploration with GCMs – cloud patterns, dune latitudes, orientations, precipitation
Mid-2010s Informed by Cassini results (dunes, seas, river channels) Titan
Paleoclimate studies are now entering a new post-Cassini era with application of
GCMs to different orbital configurations (Croll-Milankovich cycles, e.g. Lora et al.)
and volatile inventories/insolation (Charnay et al..; Wong and Yung)
Baxter’s Titan, timed opportunistically, to
come out in 1997, when Cassini and
Huygens were launched
Paints a grim picture of human longduration spaceflight…
Rather accurate depiction of Titan
conditions (drawing closely on the
literature at the time) plus has astronauts
executing the measurement functions of
the Huygens Surface Science Package
(dunking a refractometer into the sea, etc.)
Speculatively considers emergence of life
on Titan as sun evolves into a red giant
phase – contemporaneously and apparently
independently of Lorenz GRL paper of the
same year…
Titan’s Surface-Atmosphere Interactions give many similarities
with the terrestrial planets : Titan is an outstanding laboratory
for comparative planetology and climatology.
“Titan is to Earth’s hydrological cycle what Venus is to its
greenhouse effect” … a process of vital importance to our home
planet taken to a frightening extreme..
Exploration with Titan Flagship Mission
Pyrimidines e.g.
Self-reproducing chemical
systems e.g. DNA codes
information using Purine
and Pyrimidine bases to
determine Amino Acid
sequence in proteins
Pre-Decisional For Planning Only
Hydrolysis by
H2O in impact,
Amino Acids
e.g. Glycine
Autocatalytic systems,
information storage &
transfer, membrane
formation, peptides,
Site Visit, 8
Known to
Occur on Titan
Believed to
Occur on Titan
Known to
Occur on
May 17
Troposphere warmed by
condensible (CH4, N2*) and
noncondensible greenhouse
gases (H2, N2) – cf. H2O, CO2
on Earth
Tropopause cold trap limits
(CH4, H2O) abundance in
stratosphere and thereby
photolysis rate
Stratosphere warmed by
photolytic haze (cf ozone)
*N2 doesn’t condense in present
climate directly, though it does
dissolve in CH4..
Global average, annual average radiative-convective energy balance by
Mckay etal (1991). NB dramatic seasonal changes at high latitude, so
surface energy deposition and convection are stronger than shown here.
Lorenz et al. 1999
semiheuristic analytic
fit to McKay RCM.
Methane amount declines with time due to photolysis (~10Myr)
If not buffered then there may have been cold spells in Titan’s
past. These may have been cold and methane-deprived, but may
have been wet due to N2 condensation (B. Charnay)
Volatile-poor Titan is well-behaved in 1-D model. Progressive
solar forcing gives warmer conditions, higher pressure
atmosphere (feedback), until oceans boil dry.
Volatile-rich Titan has ocean P-T relation that is parallel with
RCM. Multiple equilibria exist !
Volatile-rich Titan shows hysteresis (a la Budyko-Sellers EBM
ice-albedo feedback)
Lorenz et al., Planet Space Sci, 1999
But atmosphere controlled by coldest spot – equator/pole
gradient becomes important
Lorenz et
al., GRL,
Equator-Pole gradient parameterized in Budyko-Sellers models as a
heat diffusion term D (ignores phenomenology). Naïve scaling
Earth value by P, rotation, radius doesn’t work, but selecting D to
maximize entropy (or work) production does, as for Earth….
Controversial idea, still needs work.
Cassini radar mapping
of seas is essentially
complete. Inventory of
surface liquids (~1% of
surface area) is less
than that in the
Sink (clathrates?
haze?) required for
Atmospheric methane
is not buffered (unless
large hidden
Lorenz and Sotin, Scientific American, March 2010
Hydrological cycle as relaxation oscillator Cloud climate feedbacks? Like on Earth, hard to judge.
Without invoking stronger greenhouse, surface temperatures increase
with stellar evolution. Initial rise is small (hazy atmosphere ‘puffs
up’) but changing solar spectrum reduces haze production
Effect of Solar Mass Loss
Depends on state of Saturnian magnetic field ! Does the
field periodically reverse like Earth’s ? Is there a secular
effect? Does warming of Saturn change rotation period?
Has rotation period changed due to orbital evolution of
satellites or due to stochastic impact in the Gyr between now
and then…?
End member approaches
1. No effect, assume mass loss zero as system protected by
Saturnian magnetosphere
Assume atmosphere is stripped. Might remove present
inventory of N2, but Secondary atmosphere could include
CH4, C2H6, CO2 and in case of extreme heating H2O and
Thermal conduction time constant d~(kt)0.5 k~10-6 m2s-1
t~1000s  d~ 3cm
t~105s  d~ 30cm
t~3x107s  d~ 5m
t~1010s  d~ 100m
t~ 300Myr ~1016s 
(hot potato)
(diurnal heat wave)
(annual heat wave)
(Little Ice Age)
d~ 105m
Large icy moons with ~100km thick crusts take too long to
respond conductively to changing surface conditions on solar
evolution timescales - will melt at the top (or ablate) while ice
beneath remains unaffected (unless other effects take over –
meltwater leads. Difficult to model ! )
Closing Remarks
Titan makes a great prototype exoworld. Exotic yet instructive –
climate modeling entering a new era.
Evolving solar luminosity makes the outer solar system even
more interesting ! Spectrum changes are important as well as
intensity changes.
Wide range of possible feedbacks can exist. Impact of stellar
mass loss will be profound, but depends on parent planet
magnetic field etc.