If a Europa-like body were in the Sun's habitable zone, let's say in an orbit between Earth and Mars, would the body become and remain a water ocean planet? In the habitable zone, the Sun would warm up and melt Europa's icy surface which will first turn into water vapor because Europa has no atmosphere, but the water vapor might form an atmosphere around Europa (which would contain oxygen as well) and eventually Europa might get a liquid water surface. Would that be possible? And would the atmosphere remain or couldn't it remain due to Europa's low gravity? But what if the Europa-like body had a Mars-like gravity and/or an Earth-like magnetosphere?

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    $\begingroup$ With no magnetic field the solar wind will soon blow off the atmosphere and the water will partly freeze and partly evaporate. $\endgroup$ – Yellow Sky Jul 25 at 10:11
  • $\begingroup$ @YellowSky And if it had an Earth-like magnetosphere? $\endgroup$ – Ioannes Jul 25 at 10:19
  • $\begingroup$ Then it would be something in between Mars and Moon since Europa is smaller than Moon and even the core of much bigger Mars froze billions of years ago. If Europa were the size of Earth with magnetosphere like Earth's, it would be like Earths with a much deeper ocean. $\endgroup$ – Yellow Sky Jul 25 at 10:36

According to Arnscheidt et al. (2019) "Atmospheric Evolution on Low-Gravity Waterworlds", the transition between "planet-like" and "comet-like" (escaping) atmospheres occurs at a surface gravity of around 1.48 m/s2. As a caveat, the waterworlds being considered in the paper are objects which have water reservoirs of 40% of the total mass, which is a much higher fraction than Mars or Europa, such worlds would be more like scaled versions of Ganymede.

The surface gravity on Europa is 1.315 m/s2, putting it on the comet-like side of the transition. According to this, it seems unlikely that Europa would be able to maintain long-term habitable conditions in the face of atmospheric escape.

Martian gravity is 3.71 m/s2, putting it on the planet-like side of the transition, so a "super-Ganymede" with Mars-like gravity would likely be able to maintain long-lived habitable conditions.

They also note that the high albedo of ice means that deglaciating ice worlds tend to skip the long-lived habitable state entirely, going straight from frozen to runaway greenhouse/atmospheric escape:

The ice-albedo feedback can hamper transitions from snowball states to temperate states: this has already been demonstrated for terrestrial-mass worlds (Yang et al. 2017). We can incorporate the ice-albedo feedback into our model using the simple albedo step function $$ A(T_s) = \begin{cases} \alpha_I & T_s < 273\,\mathrm{K} \\ \alpha_L & T_s \ge 273\,\mathrm{K} \end{cases} $$ where $\alpha_I$ is the albedo of the icy (snowball) state, and $\alpha_L$ is the albedo when there is surface liquid water. Hysteresis plots for different choices of $\alpha_I$ are shown in Figure 5. We observe that a snowball state experiencing a stellar flux-driven deglaciation generally bypasses the long-lived state entirely, except for very low $\alpha_I$ values. Although the mechanism setting the inner edge of the habitable zone is different, the conclusion of the habitable state likely being bypassed upon stellar flux-driven deglaciation is the same as that of Yang et al. (2017).

(emphasis mine)

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    $\begingroup$ If any civilization develops in a habitable planet with 1.48 m/s^2 surface gravity and density similar to Earth, they surely could bacame spacefaring on the cheap-cheap, as a bonus. Low escape velocity would make Verne-style spaceguns and single-stage-to-orbit easy to implement. $\endgroup$ – ksousa Jul 25 at 19:21
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    $\begingroup$ @ksousa yes, though the low Δv to get to their own orbit would still help them only to limited degree with getting to other bodies – that would anyway require big multistage rockets. More of a gamechanger is that they might actually be able to build a space elevator with slingshot-extension, and that would then allow even interplanetary launches with only modest energy requirements. $\endgroup$ – leftaroundabout Jul 25 at 19:53
  • $\begingroup$ @leftaroundabout: The lower mass of their homeworld (1.48 m/s^2 surface gravity - about 15% that of Earth - on a body with Earth's density requires a body much less massive than Earth; as the surface gravity of a body with a given density is proportional to the cube root of the body's mass, this would translate into a homeworld with a mass of about one-third of a percent of Earth's) would also translate into a shallower gravity well, making it much easier to escape the homeworld's sphere of influence - and, once in interplanetary space, they'd be able to make use of gravity assists (1/2) $\endgroup$ – Sean Jul 25 at 21:37
  • $\begingroup$ (2/3) to get to other bodies in their solar system essentially for free (starting, if necessary, with gravity assists from the homeworld itself). On the other hand, a body with the same composition as Earth, but less massive, would have a considerably lower density (due to the density-increasing effects of gravitational compression - the reason Earth is the densest gravitationally-rounded body in our solar system, denser even than Mercury - being far weaker), increasing its radius and weakening its surface gravity further, and, thus, requiring a somewhat more massive body in order to (2/3) $\endgroup$ – Sean Jul 25 at 21:44
  • $\begingroup$ (3/3) avoid dropping below the critical 1.48 m/s^2 surface-gravity threshold; this would deepen the body's sphere of influence somewhat, partially counteracting some of the benefit gained from the first-mentioned effect. $\endgroup$ – Sean Jul 25 at 21:46

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