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EDG
2013-Mar-17, 01:43 AM
So, this is something we don't get on Earth, or even in our own solar system... For a SF background i'm writing, I'm trying to figure out what happens if you get mafic (i.e. low silica) lava erupting onto an airless surface with cryogenic temperatures (around 20-30 kelvin). Answers for volatile-rich and volatile-poor lava would be appreciated!

Essentially, think of a volcanically-active rocky planet, between the size of Earth and Mars, orbiting a white dwarf at a great distance. Any previously-existing atmosphere was blasted off during the red giant phase and transition to white dwarf, but the planet is young enough that volcanism is still continuing. However, now it's orbiting a white dwarf at a distance, and the star's very low luminosity means that the daylight temperature at the planet is about 30K - the blackbody temperature there is actually more like 20K, but the heat radiated by the planet is enough to increase its surface temperature by a few kelvin. With very little solar heating, the planet has no atmosphere at all because all of its atmospheric gases freeze out. (there's a question - does molecular hydrogen tend to erupt from volcanoes at all? That might actually rain out as liquid in this kind of environment).

So I'm figuring it'll be similar to Io (but without the sulphur), and that all the volatiles would end up falling out of the eruption plume or being deposited by it as ice. But in those conditions, if lava erupts from fissures (as I suspect they would in the volatile-poor case) would the lava cool to become basalt, or would it become glass because it would cool so suddenly? I guess in the long term any glass would turn into basalt anyway as it crystallises over time, but would that still happen in this kind of environment?

Any help thinking this through would be appreciated! (I've got a (slightly rusty) geology degree, so don't pull any punches in terms of detail ;) ). Also, not that I am NOT asking about what are usually referred to as 'cryovolcanoes' - those are found on icy satellites and are 'molten ice' erupting through solid ice. What I'm asking about here is molten rock erupting through and onto solid rock, but in temperatures not far off absolute zero!

neilzero
2013-Mar-17, 04:10 AM
On Earth's surface lava cools rather quickly partly because the wind blows cool air across the surface. With no air other than solid volitiles only radiation of infrared photons can cool the lava and conduction to the very dry surface under the lava. The very dry make the surface a better thermal insulator, so my guess is average cooling rate is about the same inspite of the average ambient temperature being about 260 c cooler. Under water lava on Earth will cool quicker than than on the solid only world. Neil

EDG
2013-Mar-17, 06:36 PM
Hm, that does make sense... so it's not really the ambient temperature that matters, it's how quickly the heat from the lava is transferred away (if it's quick enough, as in water, it "quenches" and doesn't get a chance to crystallise). So I guess the lava would cool a little quicker through radiation than it would say on the moon (because it's a few hundred degrees colder) but that's about it?

cjameshuff
2013-Mar-17, 07:49 PM
Hm, that does make sense... so it's not really the ambient temperature that matters, it's how quickly the heat from the lava is transferred away (if it's quick enough, as in water, it "quenches" and doesn't get a chance to crystallise). So I guess the lava would cool a little quicker through radiation than it would say on the moon (because it's a few hundred degrees colder) but that's about it?

With an airless surface, there is no "ambient temperature". There's just the surface temperature. Lava will lose heat via radiation exactly as quickly as it does on any other airless body, the sky's the same 2.7 K.

The surface starts off a couple hundred K lower, which isn't all that much when the lava temperature is around 1200 K. Sunlight heating similarly doesn't make that much of a difference...it might be significant at equatorial noon on Mercury, but for the most part the bulk of the lava would behave pretty much like it does on any other airless body. For cold bodies, the volcanism, falling ash, etc would likely warm the nearby surface a fair bit, further reducing the differences. The differences would be spatters that hit parts of surface that haven't been warmed up by the volcanic activity which might cool too quickly to crystallize, the freezing out of volatiles that reach cold parts of the surface, and effects from lava landing on such volatile-covered parts of the surface or meeting volatiles trapped underground.

Ara Pacis
2013-Mar-17, 11:30 PM
With an airless surface, there is no "ambient temperature". There's just the surface temperature. Lava will lose heat via radiation exactly as quickly as it does on any other airless body, the sky's the same 2.7 K.

The surface starts off a couple hundred K lower, which isn't all that much when the lava temperature is around 1200 K. Sunlight heating similarly doesn't make that much of a difference...it might be significant at equatorial noon on Mercury, but for the most part the bulk of the lava would behave pretty much like it does on any other airless body. For cold bodies, the volcanism, falling ash, etc would likely warm the nearby surface a fair bit, further reducing the differences. The differences would be spatters that hit parts of surface that haven't been warmed up by the volcanic activity which might cool too quickly to crystallize, the freezing out of volatiles that reach cold parts of the surface, and effects from lava landing on such volatile-covered parts of the surface or meeting volatiles trapped underground.

Are we assuming there's a fair amount of dissolved gasses in the lava, or does mafic lava tend not to have dissolved gasses, I forget. With gasses, I'd expect lots of expansion creating foamy rocks that would be good insulators. If so, then the ejecta won't cool so fast by conduction when it lands.

Also, OP, it might be useful to know the orbit and tides of the planet. Did it have a circular orbit and would it still after a solar expansion event, or would the gas and dust blown off cause it to move inward and would that result in a circular orbit or a more elliptical orbit. I ask, because I was wondering if tides were powering the volcanism. Also, it the linger time of the sun might change the insolation, and this might affect other variables. Or not, I dunno.

EDG
2013-Mar-18, 12:25 AM
I was asking about scenarios where there are volatiles in the lava, and scenarios where the lava is pretty depleted in volatiles. So both options can be considered!

Tides aren't powering the volcanism - the orbit is a little eccentric, but not hugely so - the orbits expanded outwards when the star shed its mass to form a white dwarf. The volcanism is powered simply by the normal radiogenic heating inside the planet, since it's reasonably large and only about 3 billion years old. It's not like Io where it's erupting all over the place all of the time, it's more like Venus' volcanism (which I presume is ongoing - big rift volcanoes, shield fields and so on).

icefoxen
2013-Mar-28, 04:14 AM
The surface starts off a couple hundred K lower, which isn't all that much when the lava temperature is around 1200 K.

Ach, mein freund, you mean 1200 C. But, essentially correct, yes.

Dissolved gases exolve... but if it's really mafic lava it's going to be low-viscosity, making it relatively easy for the bubbles to escape. A real hot ultramafic lava (komatiite) is probably about as viscous as water.

Honestly, you can take a look at Io and (now quiescent) Mars to get an idea of what this might look like... not all that different from Earth, in terms of physics. The real excitement is probably going to occur if your lava flows across or erupts under ice, whether made of water or what was once the atmosphere (if any of it remains after the star became a white dwarf). Google lahar and jökulhlaup for an idea of what might happen.