Europa
Europa is a small ice covered moon that orbits Jupiter at a distance that places it squarely in the path of high energy charged particles captured by Jupiter's huge magnetosphere. It's low density, anomalous rotation properties, and highly fractured and faulted surface are indicative of some past glacial and cryovolcanic activity. Consequently, this strongly suggests the existence of a liquid or slushy fluid mantle beneath the icy crust. Just as the composition of Earth's oceans are important geological parameters that provide information about the formation and early history of the Earth, so would knowledge of that of Europa's subsurface ocean. Remote sensing measurements by various Earth and space-based telescopes and spectrometers, including the recent Galileo probe sent to orbit Jupiter have recorded pieces of evidence for the composition of the icy surface. Our lab has prepared a homologous series of potential Europa surface analogs and studied their spectral and chemical properties in search of the most consistent explanation for each of the observations.
Jupiter is the largest planet in the solar system and an enormous planet by any comparison; it is only 1:16th the size of a small star. Its four largest moons are known as the Galilean satellites after their discovery by Galileo using the first telescope. The moons experience a strong tidal force that causes internal heating. The innermost moon, Io, shows active volcanic eruptions. Further away, the moons progressively get colder. Europa and Ganymede have large quantities of water ice on their surfaces. A matter of open debate is whether the tidal heating at Europa is strong enough to maintain a liquid ocean beneath its frozen crust. Liquid water is believed to be the first requirement for the existence of life on a planetary body.
The recent Galileo space probe sent to Jupiter acquired spectroscopic measurements and high resolution images of Europa's surface. These spectra show regions with strange distortions of the ice bands in the infrared region. An ocean would be salty, since water dissolves the minerals that make up the rocky material from which Europa formed. The question is: are the infrared bands indicative of the endogenous composition of a salty ocean, or is there a source of surface contamination from Europa's sister moons?
The Near Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft acquired spectral images over the surface of Europa, Ganymede and Callisto. Areas of large flat plains exhibit normal ice spectra, however the cracked fault lines show a distinctly different spectrum. The peaks still correspond to water ice though distorted and asymmetrical. Ganymede also shows distorted bands, but those can still be explained by water ice mixed with rocky surface material (not distorted as much as simply attenuated).
The infrared spectra of ice corresponds to vibrational frequencies characteristic of the bonds within the water molecules. The bands in the near-IR are overtones, normally forbidden transitions that are switched on by the broken symmetry of the crystal. Distortions in these bands are not themselves evidence of a contaminant, merely some strong perturbation of the ice itself.
As chemists, we know that nothing perturbs water like the solvation of ions, such as salts. The strong dielectric properties of ice allow it to strongly stabilize ions by re-orienting the water dipole moment toward the ion. This would represent a very strong geometrical distortion, breaking the normal pattern of repeating hexagonal lattice sites. The degree of solvation is heavily dependent on the charge (+1, +2, -1, -2, etc.) of the ion, as well as the ionic radius (Coulomb force varies as r-2). The hypothesis is that a subsurface ocean of salt water could have percolated through the creviced ice crust and leave a telltale indication in the near infrared spectra.
We can simulate the vacuum of space, the super low temperatures and acquire the infrared spectra of ice mixtures in the laboratory, using a stainless steel chamber equipped with vacuum pumps, liquid nitrogen-cooled sample plate and a commercial Fourier-Transform Infrared Spectrometer with an external MCT detector.
Salt, acid and base solutions in water can be prepared and injected into the system while under vacuum. This actually helps form a frosty surface that provides the required granularity and diffuse reflection needed to properly simulate the Europa observations.
Mixtures of salts, acids and bases can influence each other greatly. Simply superimposing spectra of each component individually cannot capture the non-additive effects that ions have in solution. In particular, the degree to which water can be amphoteric (act as a proton donor or acceptor) depends on the ionic strength of the solution and the temperature. When frozen, ice can still undergo hydrolysis, but with a sharply different activity than liquid water above freezing. As a consequence, some salts that are mildly basic undre normal conditions can become mildly acidic when frozen. Thus, even relatively neutral salts can generate protons in ice which radically affects its structure and spectra.
Whether protons in ice are responsible for the spectral distortions, and if these protons are from an endogenous salt source or from ion implantation in the plasma torus are the main problems we seek to address.
We have found that there is no way currently to uniquely assign the NIMS spectra to a mixture of components. It cannot be ruled out that the signal could be due to radiation processing since this hypothesis also produces an exact match (pure sulfuric acid). However, when taken into consideration other observations, such as the emission of Na+ and K+ from the moons surface, the correlation of the distorted peaks with the fractures on the surface, and geochemical models that predict significant hydrothermal processing of the chondritic meteorites that originally formed Europa, it is highly unlikely that the Europan surface is composed of pure water ice. The most consistent assignment is that radiation processing is having an effect, but it is less than or equal to the effect of endogenous salts from the mantle. The question will require more advanced missions to be sent to Europa.
References
- McCord, T.B., Orlando, T.M. et al. ."Thermal and radiation stability of the hydrated salt minerals epsomite, mirabilite, and natron under Europa environmental conditions" JGR. 2001. 106 E2, 3311.
- Orlando, T.M.; McCord, T.B.; Grieves, G.A. "The Chemical Nature of Europa Surface Material and the Relation to a Subsurface Ocean" Icarus. 2005. 177, 528-533.
- McCord, T.B.; Teeter, G.; Hansen, G.B.; Sieger, M.T.; Orlando, T.M. "Brines exposed to Europa surface conditions" JGR. 2002. 107, 5004.
Members on Project
Gregory Grieves
Funding
NASA