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We have never landed on Europa, so the assumptions made on a possible ice layer on its surface are made on the data sent by various objects approaching it.

I'm sure these assumptions were accurately and carefully made, but still, I'm interested: is it possible that we're wrong and what we think ice is something else?

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The Hubble Space Telescope did an ultraviolet emission study of vapor plumes from Europa's southern hemisphere in 2012. It found that the emissions were consistent with $H_2O$.

H2O freezes (19), and any enhancement must be localized to its source. In addition, electron impact on H2O (e + H2O) yields HI 121.6 nm and OI 130.4 nm, but has a lower cross section at 135.6 nm (20). Hence, the emission surplus at HI 121.6 nm and OI 130.4 nm in an area, where no remarkable enhancement of OI 135.6 nm is found, suggests a local atmospheric H2O enhancement.

Now if liquid water is spraying from below the surface, there can be no doubt that there is water ice on the surface. This also bears significant support to the subsurface ocean theory.

Source:

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  • $\begingroup$ Is that (19) a citation on the statement "water freezes"? $\endgroup$
    – Williham Totland
    Commented Jun 15, 2016 at 16:18
  • $\begingroup$ @WillihamTotland Yes, the citation is to a paper on Europa temperatures which explains its effects on water. $\endgroup$
    – called2voyage ♦
    Commented Jun 15, 2016 at 16:20
  • $\begingroup$ Ah, that makes more sense. $\endgroup$
    – Williham Totland
    Commented Jun 15, 2016 at 16:23
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The ice layer is almost certain, in fact, it's not all that rare. Most outer solar system objects have an ice layer. The real interesting question is the potential water layer on Europa.

From Wikipedia, we get a better overall picture of what's going on. It is almost certain that there is a layer of water underneath the ice. The size of that layer is the subject of considerable debate. Ranges estimate from less than a km to a layer of around 10-30 km thick. Scientists are pretty sure that there is in fact a layer of liquid water underneath Europa, however, so it is very likely that is the case.

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  • $\begingroup$ 30 km of ice? If that's true, then I assume it's extremely unlikely that we'll reach liquid water in this century. $\endgroup$
    – Zoltán Schmidt
    Commented Jun 15, 2016 at 14:07
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    $\begingroup$ @ZoltánSchmidt "Flexing of the icy crust above an ocean could create pockets of salty impurities and partially melted areas leading to features seen in spacecraft images." solarsystem.nasa.gov/europa/water.cfm $\endgroup$
    – called2voyage ♦
    Commented Jun 15, 2016 at 14:12
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    $\begingroup$ @ZoltánSchmidt Further reading: BRUIE and VALKYRIE $\endgroup$
    – called2voyage ♦
    Commented Jun 15, 2016 at 14:22
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Europa's composition has been studied by earth based telescopes, space based telescopes and by probes from close range. Observations have been made using visible light, spectroscopes, and a variety of other instruments. All of these agree that the surface of Europa is ice, and there doesn't seem to be any scientist challenging these results.

While it is theoretically possible that all this could be wrong it's extremely unlikely.

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