Titan atmosphere and evolution

Chris McKay NASA Ames

chris.mckay@nasa.gov

With Kevin Zanhle Jeff Moore, Brian Toon, Feng Tian

Talking points

• Titan’s current thermal and chemical structure is adequately understood.

• The central problem in extrapolating this over time is CH4.

• There is good evidence for ~10x CH4.

• Two scenarios- continuously wet Titan- recently wet Titan

• Both have significant issues.

Organic haze

N2-N2, CH4-N2, CH4-CH4, H2-N2 collision induced opacity

Anti-greenhouse

Greenhouse

The Greenhouse Effect on Titan is due to N2 and CH4

From McKay et al. Science 1991

The Antigreenhouse Effect due to Haze

From McKay et al. Science 1991

Net global averaged solar flux normalized to total incident solar flux. DSIR results compared to model of McKay et al. 1989. Figure from Tomasko et al (2007).

Energy balance in Titan’s atmosphere Chemistry

3.8 W m-2

From McKay et al. Science 1991

C6H6N

C6H6

N2-N2, N2-CH4, CH4-CH4 C2H6

Cold trap limits CH4

~1%

H2

~1010 cm-2s-1

~108 years

CH4

T(z)

Methane on Titan today

RH CH4 0.43 (Nieman et al. 2005)

Atmosphere 400 gm-C/cm2 (Lorenz et al. 2008)

Lakes 20 – 200 gm-C/cm2

Dunes 200 – 640 gm-C/cm2

Fluvial activity at the Equator

Large lakes at the Equator in the past

Storms require high RH at the equator.

Flooding the equator to form seas implies ~ 10x atmospheric CH4.

N2 Clouds

Titan’s surface temperature over time with lakes

With N2 condensation (preliminary, but no obvious way to deep freeze)

CH4 limiting flux (the loss rate of H2) is reduced in the past but only ~4x.

Continuously Wet Titan• Outgassing of 40x atmospheric CH4 (and 40Ar)

provides enough CH4 to last to the present time.

• Photochemistry produces 39x C2H6 and 1x dune material.

• Dunes are visible.

• C2H6 peculates into the surface.

• CH4 photochemical lifetime implies we are coincidentally close to the end of CH4. Waiting for cryovolcanios or at the end of the wet era.

• Not clear how to hide the C2H6 (> 200 m is hidden, with or without CH4).

1 Gyr ago10x current CH4

Today1.2x atmospheric CH4

100 Myr No CH4

Recently wet Titan: an initial cold inventory, no cryovolcanism, recent atmosphere & past rain,

Recently Wet Titan• Titan spends most of its history in a Triton-like state. • Runaway greenhouse nx108 Gyr ago with production

of nx atmospheric CH4 (and 40Ar) provides enough CH4 to last to the present time. Minimum n ~ 5 , implies wet age is > 5x108 years

• Photochemistry produces C2H6 and tholin, C2H6 converts to dunes. Dunes have nx C.

• No hidden C2H6

• CH4 photochemical lifetime implies we are coincidentally close to the end of CH4.

• Not clear how to keep a Triton-like state cold enough to shut off CH4 photolysis.

• Not clear how C2H6 converts to dune material.

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