2.2 Ice and salt
As noted in Section 1.5, Europa's near-infrared reflectance spectrum was used as long ago as the 1950s to demonstrate that its surface is mostly water-ice. More recently, spectroscopic observations by the Hubble Space Telescope and Galileo have revealed some regions where the ice appears to be salty (see below) and have also detected traces of molecular oxygen (O2) and smaller amounts of ozone (O3). The oxygen and ozone almost certainly result from the breakdown of water molecules in the ice brought about by exposure to charged particles (this process is known as radiolysis) that are channelled onto Europa by Jupiter's magnetic field, and by solar ultraviolet radiation (a process called photodissociation or photolysis. Most of the oxygen and ozone is probably held within the ice (as isolated molecules trapped within ice crystals), but some may constitute an extremely tenuous atmosphere.
Apart from various forms of oxygen, what else would you expect to be produced when water molecules are broken down by radiation?
Given that the formula for water is H2O, hydrogen should also be produced.
Box 4 shows a series of reactions that could produce oxygen and hydrogen in Europa's ice.
Box 4: Radiolytic and photolytic breakdown of water molecules in ice
The reactions that occur to generate oxygen and hydrogen within the surface ice of an icy satellite can be summarised, in simplified form, as:
Europa's ozone is likely to be the product of a chain of reactions involving radiolytic and photolytic breakdown and recombination of oxygen molecules, similar to the photolytically driven reactions that generate ozone from oxygen in the Earth's stratosphere.
Hydrogen has not yet been detected on Europa, but on Ganymede, where similar 'space weathering' of exposed ice occurs, hydrogen has been found leaking away into space.
Suggest a simple explanation to explain why there is a lot less free hydrogen than oxygen in or above Europa's surface.
Hydrogen is a much smaller and lighter atom therefore it is easier for hydrogen to escape from within the ice. Once liberated, it is so loosely bound by Europa's weak gravity that it would be lost to space much faster than oxygen or ozone.
Hydrogen peroxide (H2O2), which is an intermediate product of the sequence of reactions in Box 4, has been identified as a trace component of the ice in reflectance spectra obtained using Galileo's near-infrared imaging spectrometer. The same instrument has also revealed distortion of the absorption bands associated with water. This indicates that, in addition to forming ice crystals, some of the water molecules are bound within hydrated salt crystals. The best match to the spectra is from a mixture of magnesium and sodium salts such as magnesium sulfate hexahydrate (MgSO4.6H2O), epsomite (MgSO4.7H2O), bloedite (MgSO4.Na2SO4.4H2O) and natron (Na2CO3.10H2O). The occurrence of sulfates is supported by Galileo ultraviolet spectroscopic data that indicate the presence of compounds containing a sulfur-oxygen bond.
Although carbonates and sulfates are fairly common salts on Earth, they are not the most abundant. What kind of salt appears to be missing on Europa, compared with the Earth?
No chlorides are in the above list - note that sodium chloride (NaCl), which is the most abundant salt dissolved in the Earth's oceans, is absent.
Actually, chlorides produce no spectral features in the available part of the spectrum, so direct observational data cannot tell us whether any chlorine salts occur on Europa's surface. What the spectral mapping by Galileo did achieve, however, was to show that the distribution of salts across Europa's surface is highly non-uniform. Large expanses are relatively salt-free, but in places where the surface has been most recently and most greatly disrupted from below, the surface salt concentration reaches 99 per cent. You will see what these areas look like shortly.
The salts occurring on Europa's surface are unlikely to be a straightforward representation of those dissolved in any ocean beneath Europa's ice - calculations have shown that the freezing process would tend to concentrate sulfates of magnesium and sodium into the ice. This is consistent with the observed preponderance of these salts at the surface. However, the concentrations of elements dissolved in Europa's ocean are largely a matter of speculation. Two of the factors that have to be considered are the composition of Europa's rocky component, and the efficiency with which each element becomes dissolved from it into the ocean. Neither of these factors is known. Although, on average, Europa's rock is likely to be similar to carbonaceous chondrites, geochemical differentiation could mean that the rock nearest to the ice-rock interface might well be very different (as is probably the case in Io's crust, for example). The efficiency with which elements become dissolved (or sometimes reprecipitated) depends upon the temperature at which it occurs, as well as on the overall chemistry of the solution. Despite the uncertainties, attempts have been made to model the likely concentrations of dissolved elements in Europa's ocean. The results of one such model are shown in Figure 15.
According to Figure 15, how many more times greater is the concentration of chloride (Cl−) in terrestrial seawater than in Europa's ocean?
This is an exercise in reading values of a logarithmic scale. The concentration of Cl− in terrestrial seawater is shown as 0.6 moles per litre. The concentration of Cl− in Europa's ocean is shown as 0.02 moles per litre. The ratio between the two is 0.6/0.02 = 30. Thus the concentration of Cl− in terrestrial seawater is thirty times that in Europa's ocean.