Mach 15 world?

It was long a staple in science-fiction that Mercury was locked into rotational resonance with its own orbit around the sun - although I doubt anyone serious ever imagined that this (visibly) tiny planet could hold an atmosphere. Some years back I read and enjoyed Comins' What if the earth had two moons; among which parallel Earths, was one locked into rotational resonance with its own orbit around the sun. Recently James Trefil and Michael Summers in Imagined Life have mooted the same. Now that we've been seeing plenty of close-in Earthlike planets, and even one around Proxima, it's time to constrain how a habitable such world would be possible.

The reason Mercury was assumed tidally-locked so long was because nineteenth-century models explained how such worked out for our Moon. (Mercury turns out to spin:revolve at 3:2, which isn't so far off.) We will rarely see tidal locking for further-out planets like Venus and Earth. So, for our tidally-locked hypothet: assume a tight Mercury-like day-year, and similar distance from a Proxima-like star.

Oh, they'll just be living on the prime-meridian, say the naïve. Even Clark Ashton Smith used to say that, back in the Mercury days. Trefil and Summers note that if the place has an atmosphere, that atmosphere will be moving - like Venus' atmosphere moves. On a sun-facing planet, the air on the sunny side gets hot and on the darker side, not. This leads to the hot air rising and flowing outward; the cold air sinking, and flowing inward.

Trefil and Summers pp. 120-1 claim that the planetary wind does this fast. Mach 15, they say.

This piqued my notice because I'm proposing (elsewhere) 11 AM Flotilla. Some amazing material scientist could run an scramjet against a Mach 15 stratosphere, even with Earthlike insolation (UPDATE 2/3 - material science is, yes, key here; X-43 is only at, what, Mach 9?). As for carrying that Orion nuclear rocket, I'd look into inlining its architecture with that of an aircraft - I see no way a Mach 15 perma-plane drags this thing behind on a net.

The question then boils down (heh) to: how long is this planet going to keep its atmosphere. The term is atmospheric escape (pdf). For thermal alone I think I'd take Figure 1.6, with a look at the planetary surface escape-velocity. From that I'd subtract the wind velocity. Given a speed of sound 340.3 ms^-1, Mach 15 is 5104.5. Earth-escape is 11200. Subtracting the one from th'other, I'm looking at atmospheric escape-velocity 6100 ms^-1 at the trailing planetary coma. That's also assuming exosphere 1000 K which I'm unsure we can. A few good flares would nuke this atmosphere but good. So we'd have to constrain Mach 15 World's sun that it not flare so bad. And that the planet has a decent spinning core, even if the planet itself isn't spinning on the outside.

I wonder if the whole question be moot. The authors forget exactly that these are close-in planets - their rotation is not as slow as they assume. For Super-Mercuries we must assume Coriolis. That distributes the wind more evenly around the planet.

I do not see in these Earthlike simulations where the wind reaches Mach 15. They reach instead a (handsome, admittedly) 75 ms^-1. The slower wind and the less-extreme heat differential further allow cloud buildup on the sunny side, reflecting that sun back away. Which brings us back to an ocean-world.

Maybe Trefil and Summers were thinking of the habitability of superMercury near an orange dwarf. That would explain why they speak of silicon rains on dayside. But as mentioned, I don't think they're keeping an atmosphere at this windspeed and heat.