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I read this question on Worldbuilding.SE, and figured that the astronomy site would have answers too, particularly for the specific example of Europa. The idea is that Earth's oceans are salty because rain falls on continents, and while the rain makes its way to the sea it absorbs minerals and salts from the land. That piles up in the oceans, and that's why you can drink river water but not sea water.

Europa has no continents; as far as we know it's an ice cap of a couple tens of km, then roughly 100 km of liquid water, and only then something rocky that might contain salts.

Does that mean that the mentioned liquid water is likely to be pure? Or at least pure compared to the Atlantic Ocean? Or do we not know?

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According to this 2007 paper, the current research as of the time of their own research had a huge range in possible concentrations of $\text{MgSO}_4$, magnesium-sulfate, with over four orders magnitude (approximately times $30,\!000$) differences between the extreme ends of the predictions. It conducts its own analyses and near the end of the paper makes some analysis on habitability. They say (with slight formatting modifications for units by me):

If the ice and liquid water layers on Europa fall within the limits of Fig. 2 (A = 0.7) then, by standard definitions of “freshwater” environments on Earth [broadly meaning $<3$ g salt per kg H${}_2$O (Barlow, 2003)], Europa’s ocean would be a freshwater ocean, though admittedly more salty than most terrestrial lakes. Indeed, in this case, the putative global ocean of Europa could be more like the mildly saline environment of Pyramid Lake, Nevada than like the Earth’s ocean. While the drinking water regulations of the U.S. Environmental Protection Agency recommend no more than 0.25 g of sulfate per kilogram of water, adult humans can acclimatize to drinking water with nearly 2g MgSO${}_4$ per kg H2O without much discomfort (EPA, 2004; CDC-EPA, 1999). Animal toxicity (the lethal dose for 50% of the population) is in the range of 6 g MgSO4 per kg H2O (CDC-EPA, 1999), but most livestock are satisfied provided the total salt concentration is less than 5 grams per kilogram of water (ESB-NAS, 1972). If we assume the low amplitude regime for our solution (A < 0.8) then it is possible that human or beast could drink the water of Europa.

However the best estimates for that parameter $A$ they had were from magnetic field observations, which put $A=.97\pm .02$. In this case the article concludes that the subsurface ocean would then be very salty. The most salt tolerant organisms we know of could potentially survive in the environment. However, such organisms evolved into such salty niches from less salty ones, rather than having evolved directly within them. Current evidence suggests that life as we know it is unlikely to be able to arise in such a salty medium.

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  • $\begingroup$ A well supported answer, thank you. The paper was very interesting too. I do wonder how the salt ended up in the Europan sea in the first place; that would imply very active water circulation, would it not? Else I imagine all the impurities would have sank to the bottom over the billions of years. $\endgroup$
    – KeizerHarm
    Dec 6 '20 at 0:01
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    $\begingroup$ @KeizerHarm There are plenty of factors to circulate the water; pretty much the same ones on Earth. Europa experiences enough tidal heating to retain a warm interior and active volcanism, which continually pumps materials into the ocean. Convective currents will then arise to transport heat to the surface. And there's latitudinal variations from rotation and the Coriolis effect. This article talks about research suggesting that Juptier's magnetic field induces an equatorial jet stream in Europa's ocean, even. $\endgroup$ Dec 6 '20 at 17:45
  • $\begingroup$ Even if drinkable, the resulting metabolic contributions to a closed atmosphere would result in some "salty" conversations. $\endgroup$ Dec 8 '20 at 12:57
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Since water on all planets is in contact with impurities, I would think that the default for water is salty, and a small percentage on Earth (2.5%) gets desalinated by going through the evaporation/condensation cycle. Since no similar cycle is operating on Europa, I imagine its ocean is salty.

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  • $\begingroup$ Where does your link say that the "default for water is salty"? Or what that means? I'm an adult, I understand that most of Earth water is saline, but that does not need to be the case for other planets, especially those that lack a water cycle (which is the primary reason our water got salty in the first place, by my understanding). $\endgroup$
    – KeizerHarm
    Dec 5 '20 at 21:28
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    $\begingroup$ @KeizerHarm: I think terrestrial water cycles are the only reason that the Earth has a significant quantity of water that isn't salty. $\endgroup$
    – supercat
    Dec 6 '20 at 23:03
  • $\begingroup$ @supercat I may have to draw in Earth Sciences SE to settle this xD $\endgroup$
    – KeizerHarm
    Dec 6 '20 at 23:49
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The most popular hypothesis for explaining the various zigzagged lines (lineae) that cover Europa’s surface is that they are caused by cracks appearing in the ice shell because of tidal deformations, and that these cracks are then filled by water oozing out from the interior. The dark, reddish color of these lineae is thought to be due to magnesium sulfate, among other salts, contained in that water.

If these hypotheses are true, and even though some mineral content is still acceptable for water to be considered potable (think e.g. mineral water, or even some aqueduct systems that deliver “hard” water [such as at my place, btw!]), water from Europa’s interior ocean would most likely not be considered “potable.”

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    $\begingroup$ N.B. almost all "drinking water" on Earth is not pure. Bottles of "mineral water" are something of a scam, because the water just coming out of your tap is already "mineral water". Next to nobody drinks pure water and it wouldn't be even nearly as beneficial to do so. You seem to imply this is a rare thing whereas in fact the opposite is true. I agree though that Europa's water would seem to contain the "wrong" minerals ;) $\endgroup$ Dec 6 '20 at 19:40
  • $\begingroup$ @AsteroidsWithWings I fully agree with you. I should have been more specific. Indeed, some health gurus recommend NOT drinking pure water, or at least not a significant proportion of the water one drinks. $\endgroup$ Dec 7 '20 at 4:02
  • $\begingroup$ @PierrePaquette Well, aqua dest. would certainly not good $\endgroup$ Dec 7 '20 at 15:35

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