Tidal Constraints on Planetary Habitability

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Tidal Constraints on Planetary Habitability
Rory Barnes U. of Washington Virtual Planetary Lab Sean N. Raymond, Brian Jackson, Richard Greenberg

Distance from Star (AU)
Sun From Selsis et al. (2007) Mass of Star (M) GJ 581 Likely habitable Possibly habitable Distance from Star (AU)

Distance from Star (AU)
Tides Important Sun Mass of Star (M) GJ 581 Distance from Star (AU)

Distance from Star (AU)
Tidal Locking Synchronous Rotation! Sun Porb Mass of Star (M) Prot ∞ 1 + e2 GJ 581 obliquity 0 Distance from Star (AU)

Distance from Star (AU)
Sun Porb = 66 d e = 0.38 Prot = 28 – 39 d Mass of Star (M) GJ 581 ? High e => more habitable Distance from Star (AU)

GJ 581 No Clouds Total cloud coverage
Williams & Pollard (2002) + Selsis et al. (2007) GJ 581 No Clouds Total cloud coverage Barnes et al. (2008)

But tides circularize orbits…
semi- major axis HZ Orbit 0.2 M

But tides circularize orbits…

Orbital Decay -> Tidal Heating
Io has ~2 W/m2 heat flux Such heating could be bad for life

But Some Heating is Good…
Plate tectonics requires ~40 mW/m2 (Williams et al. 1997) Prevents runaway greenhouse

If heating > 2 W/m2 => uninhabitable If 0.04 < heating < 2 W/m2 => habitable If heating < 0.04 W/m2 => uninhabitable (need radiogenic sources)

“Tidal Habitable Zone”

d c e

Super- Earth? “Super- Ios”

or Super- Mars? “Super- Ios”

e = 0.01 e = 0.5 10 M

Summary Tidal locking ≠ Synchronous rotation Rotation rates are faster
Obliquities go to zero Tides may remove planets from HZ Tidal heating may be severe (super-Io) But tidal heating may drive plate tec. “Tidal Habitable Zone” First-detected rocky planets in HZ will be affected by tides

ec = 0 ec = 0.01