Editing Openai/6946e998-79f0-8007-9a65-19828450838d (section)

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These are icy moons with subsurface oceans today. As insolation rises:

===== • Ice surfaces warm substantially. =====
• For luminosities of ~20–100 L☉, these moons receive Earth-like fluxes.
• Surface ice melts, creating global oceans for tens to hundreds of millions of years.
• BUT: thin atmospheres and low gravity → rapid atmospheric escape.
• These worlds cannot maintain Earth-like conditions for more than a fraction of the red-giant timescale.

===== • Same physics as above, but even lower gravity → extremely fragile atmosphere; any surface ocean is temporary. =====

===== Titan is uniquely promising because: =====

• It already has a dense N₂ atmosphere (1.5 bar).
• Surface temperature is ~94 K now, but warming will vaporize its methane and ethane, forming a volatile-rich greenhouse.
• At luminosities around 20–30 L☉, Titan approaches 273 K surface temperatures.
• Atmospheric pressure is high enough for stability; Titan could support long-livedsurface oceans of water-ammonia, and eventually water.

Titan is often cited as the best candidate for a transient post-main-sequence habitable world in our system.

Timescale: tens to a few hundreds of millions of years—still temporary.

==== • Moons like Triton, Oberon, Titania could also warm. ====
• The Sun's HZ during late RGB and AGB phases could momentarily place these moons in a temperature regime allowing surface liquid water.
• Conditions remain transient and atmospherically unstable due to low gravity and solar-wind stripping (the Sun becomes very active late in giant phases).

==== • During the Sun’s AGB phase, luminosity can reach ~2,000–3,000 L☉. ====
• HZ at this point may briefly sweep out to 25–40 AU.
• Pluto could warm to near melting temperatures for a few million years.
• Pluto-like objects cannot hold thick atmospheres; habitability is extremely brief.