# What liquids last the longest in space?

I am trying to find something that is somewhat stable as a liquid in space. What happens to various liquid elements and compounds in a vacuum? Is there a list or table of the effects of local space on different liquids at different temperatures?

https://astronomy.stackexchange.com/questions/28531/what-liquids-can-be-found-in-the-void-space

• All liquids do have a vapor pressure depending on temperature. If ambient pressure is well below vapor pressure, the liquid can not exist for along time, it will vaporize completly. A lower temperature will decrease vapor pressure, but with a much lower temperature the liquid will turn to a solid. But such solids in vacuum may sublimate, direct transition from solid to gas. – Uwe Nov 25 '18 at 21:07
• I guess the question is what liquid has the lowest vapour pressure at its melting point. With the exception of helium, it should be easy enough to keep the liquid at that temperature by either shading it from the Sun or focusing sunlight on it. I can't find any tables of such things, but I wonder is something like a low-melting salt might be good candidate. The ionic liquid is fairly stable, but the vapour would more or less have to be plasma. – Steve Linton Nov 25 '18 at 21:17
• Found webbook.nist.gov/cgi/… from which you can read off vapour pressures. Eg sodium chloride just above its melting point is about 50Pa. – Steve Linton Nov 25 '18 at 21:23
• There was an experiment with water long ago, over 50 years, see Project high water. 86 metric tons of water was released above 100 km and turned into ice clouds. The ice then sublimed to water vapor. – Uwe Nov 26 '18 at 15:02
• @Uwe They always had catchy names for the missions back in the 60s. – Muze the good Troll. Nov 26 '18 at 16:22

While a table may exists somewhere called "handy space liquids" it may not be reliable. I think you many need to boot-strap your way there by making your own table first.

The three primary considerations here as a function of temperature and pressure:

• will it boil?
• will it freeze?
• will it evaporate too fast?

(sounds like the beginning of a song somehow)

## Pressure:

We can ignore several complicated subtleties by assuming that the total and partial pressures outside the liquid are close enough to zero to call them zero.

## Temperature:

In this answer about cooking hot-dogs in space, I've said:

In this answer I explain the equation for an estimate of the equilibrium temperature of a blackbody heated by visible light, and radiating in infrared light.

$$T \sim \left( \frac{(1-a_{vis})}{e_{ir}} \frac{I_{Sun}}{4 \sigma} \right)^{1/4}$$

where $$a_{vis}$$ is the visible light albedo, $$e_{ir}$$ is the infrared emissivity (both should really be weighted averages over the appropriate wavelength ranges), $$\sigma$$ is the Stefan–Boltzmann constant (about 5.67E-08 W m^-2 K^-4), and I is the intensity of sunlight, and for 1AU is the solar constant and about 1360 W/m^2.

You will have to treat your liquid Earth satellite as if it were a

Spherical Cow in orbit:

Most non-metal things have high infrared emissivity, but metals including eutectics can be very low as well. Materials also vary widely in visible light albedo. For ranges of 0.1 to 0.9, the equilibrium temperature at 1 AU from the sun ranges from about -100 C to +200 C:

emissivity (IR)  albedo (vis)  Equilib. T (K) / (C)
---------------  -----------   ---------------------
0.1            0.1                    482 /  208
0.1            0.9                    278 /   +5
0.9            0.1                    278 /   +5
0.9            0.9                    160 / -112


So you will need to carefully consider the optical properties of the material, and then factor in any impurities as well.

## Freezing & Boiling:

After you've considered the equilibrium temperature, you'll need to look at the freezing and boiling points of each material you consider to make sure it doesn't freeze (therefore unsuitable for your question) or boil away quickly.

## Evaporation:

Once you have a candidate, then look up the rate of evaporation. Liquids with a high evaporation rate will disappear more quickly than those that evaporate slowly. Your problem is that evaporation is caused both by thermal motion in the liquid and by incident photon flux, and you have a lot of that! The same photons of sunlight that keeps your liquid from freezing also can stimulate evaporation, even if the liquid is fairly transparent like water. Water evaporation rate is strongly affected by visible light photon flux beyond it's impact on the water's temperature.

You can get some idea about the evaporation rate in the dark from looking up the vapor pressure

The equilibrium vapor pressure is an indication of a liquid's evaporation rate

but it is not a replacement for knowing the evaporation rate in the dark, and adding to that the photon-induced evaporation rate.

Good luck!

Try it for pure water, and for gallium or some eutectic, and see what happens!