Ultra low pressure water worlds, and what they mean for Odyssey

[snesiscool]

So I recently stumbled across a water world with just 0.01 atm of pressure and a surface temperature of 264 K (-9.15° C). This is within the pressure cutoff for landable bodies in Odyssey (the limit is at least 0.02 atm). I checked the temperature and pressure numbers on a phase diagram for water, and it seems such a planet is actually physically possible, but just barely. Said planet also has thin, wispy clouds and no ice caps, despite the temperature (it probably has a lot of dissolved salts).

Spoiler: System Map

Spoiler: Atmosphere

Spoiler: Full Disk

While I don't think water worlds will be landable in Odyssey regardless of atmosphere (water worlds and lava worlds without atmospheres aren't landable either) it probably does mean there's a possibility that landables in Odyssey will have surface liquids and thin clouds.

Now I wonder what the record for the lowest atmospheric pressure for a water world is, disregarding the no atmosphere ones (which are apparently glitches).

 

[Sapyx]

The "no atmosphere" ones actually do have an atmosphere, it just rounds down to zero so the game classifies it as "no atmosphere". But if you look in the journal files - or at third-party sites which read the journals for you - it actually gives a minuscule atmospheric pressure for these "no atmosphere" water worlds, ammonia worlds and lava worlds that "should be landable" right now in Horizons because they're airless, but aren't landable because FD haven't given us liquid surfaces yet.

Example: here's a lava world, Croomaa PK-C d14-63 C 2. Reported in-game as "no atmosphere", but it's a lava world and therefore not landable.

And here's the EDSM page for it. Check out the atmosphere stats. Atmosphere type "No atmosphere", no atmospheric gas composition listed, just like the in-game stats, but look at the atmospheric pressure: "0.00009391 atmospheres ". You see exactly the same thing for "no atmosphere" water worlds and ammonia worlds.

Will we be able to land on these low-atmospheric-pressure surface-liquid worlds? I suspect not. Doing liquids is several orders of magnitude harder than atmospheres and plant life, especially if you're going to try for realistic hydrology and make the water flow downhill in lakes and rivers before flowing into the sea. FD have plenty of experience with human-scale water, as seen in their Planet Zoo and Planet Coaster games, but it's all sea-level hydrology (no flowing water) and the trick is going to be getting it to look right across the whole continuum from orbital-scale right down to on-foot scale. There's also the sensitive issue of what would happen if you were to try to land on a liquid surface. Few if any of our ships look either seaworthy or submersible. Would our ships float? Sink? Explode? Then what about on foot, or in SRVs? Do we go swimming, or scuba-diving?

As much as I'd like to land on an alien shore and go strolling along the beach and watching the low-gravity-slow-motion waves roll in, I think that's still several updates away.

 

[Factabulous]

I would imagine all the planets that report as 'Thin' will be landable - around 20% of planets. Not sure if there are any WW with that atmos, none in recent finds

Edit: Oops, completely wrong - grep fail. e.g.:

{"id":229881424,"id64":504410383070999137,"bodyId":14,"name":"Flyiedgiae YH-P b11-3 5 a","type":"Planet","subType":"Water world","parents":[{"Planet":13},{"Star":0}],"distanceToArrival":116,"isLandable":false,"gravity":0.43610386926379785,"earthMasses":0.115215,"radius":3278.265,"surfaceTemperature":273,"surfacePressure":0.08148624475697014,"volcanismType":"No volcanism","atmosphereType":"Thin Water-rich","atmosphereComposition":{"Oxygen":91.39,"Water":8.49,"Sulphur dioxide":0.11},"solidComposition":{"Rock":86.11,"Metal":13.89,"Ice":0},"terraformingState":"Candidate for terraforming","orbitalPeriod":2.225576037615741,"semiMajorAxis":0.0007344878527262354,"orbitalEccentricity":0,"orbitalInclination":73.600066,"argOfPeriapsis":152.554321,"rotationalPeriod":2.2625574792361114,"rotationalPeriodTidallyLocked":false,"axialTilt":0.132226,"updateTime":"2020-06-30 17:53:53","systemId":55565348,"systemId64":7224805503585,"systemName":"Flyiedgiae YH-P b11-3"}

 

[space voyager]

I would imagine all the planets that report as 'Thin' will be landable - around 20% of planets. Not sure if there are any WW with that atmos, none in recent finds

That would be awesome - I'd spend some hours just flying over the waves... I'm afraid Sapyx is almost certainly correct in assuming that for being landable, the world must be "completely ordinary rock". Anything else would be a huge surprise.

 

[snesiscool]

I would imagine all the planets that report as 'Thin' will be landable - around 20% of planets. Not sure if there are any WW with that atmos, none in recent finds

There are actually some planets with "thin" atmospheres that actually have high surface pressures, supposedly up to the thousands of atmospheres.

Cursory glance at the "thin" atmosphere descriptor...there is something else at play other than pressure, and I'll have to do a deeper dive because at the low range, they are as expected...all the way down to right at 100 pascals, which confirms the 99.99 pascal limit for landable, however...

At the top range, "thin" goes all the way up into the thousands of atmospheres of pressure...

An example of such a planet is Flyiedge PN-R d5-55 7, which has a "Thin Nitrogen" atmosphere and a surface pressure of 9.09 atm. So the cutoff will likely be an arbitrary pressure number.

Interestingly, searching through Spansh's bodies database for high gravity bodies with "thin" atmospheres shows that the vast majority of the highest gravity planets have surface pressures of around 0.09 atm, not exceeding 0.099 atm. So it's possible that the pressure cutoff will be 0.1 atm.