Habitable Moons and Planets Around Post-Main Sequence

Stars, or Titan Under a Red Giant Sun

Ralph D. LorenzSpace Department

JHU Applied Physics Laboratory

Scientific American 2010

Outline

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.

Guo et al., Astrophys Space Sci 2010

‘Delayed Gratification Habitable Zone’ (A. Stern, Astrobiology)

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 long-duration 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..

Apr 20, 2023Pre-Decisional For Planning Only Site Visit, 8

Known to Occur on Titan

Believed to Occur on Titan

Known to Occur on Earth

Pyrimidines e.g. Cytosine

Tholins, HCN oligomers

N-N

Poorly-Understood

Amino Acids e.g. Glycine

Photochemistry

Self-reproducing chemical systems e.g. DNA codes information using Purine and Pyrimidine bases to determine Amino Acid sequence in proteins Us

Hydrolysis by H2O in impact, cryovolcano

Autocatalytic systems, information storage & transfer, membrane formation, peptides, sugars

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Titan Surface

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.

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)

Lorenz et al. 1999 semiheuristic analytic fit to McKay RCM.

Ocean-Atmosphere equilibrium

Radiative- ConvectiveEquilibriumL/Lo

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, 1999But atmosphere controlled by coldest spot – equator/pole gradient becomes important

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.

Lorenz et al., GRL, 2001

Cassini radar mapping of seas is essentially complete. Inventory of surface liquids (~1% of surface area) is less than that in the atmosphere.

Sink (clathrates? haze?) required for ethane?

Atmospheric methane is not buffered (unless large hidden ‘groundwater’ reservoir)

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 magnetosphereAssume 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 NH3….

Thermal conduction time constant d~()0.5 ~10-6 m2s-1

~1000s d~ 3cm (hot potato)~105s d~ 30cm (diurnal heat wave)~3x107s d~ 5m(annual heat wave)~1010s d~ 100m (Little Ice Age)

~ 300Myr ~1016s 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.