The discovery of water vapor plumes on Europa, one of Jupiter's moons, indicates that material may be released from the subsurface ocean. Recent research shows that it could be feasible with the ESA JUICE (JUpiter ICy moon Explorer) mission to take in-situ samples of these plumes, and thereby study the contents of Europa's ocean, without having to penetrate through the icy surface.
Figure 1 illustrates why Europa's ocean is thought to be interesting as a potential habitat. Four basic criteria can be identified for (Earth-like) life: liquid water, a stable environment (this means that liquid water is present for an extensive period of time), essential elements (the basic elements needed for life: S, P, O, N, C and H) and chemical energy (some source of energy). When these criteria are evaluated for several habitats in the solar system, it can be seen that Europa (and Earth) really stands out. The evidence pointing to the presence of liquid water on Europa has already been outlined previously. Furthermore, it is thought that this ocean has been liquid for a large part of this moon's history. The basic elements have been observed at the Galilean moons or are thought to be present from modeling of the formation of the moons. Photosynthesis is not a likely source of energy in Europa's ocean, due to the lack of sunlight in the ocean. However,
Figure 1 - Hydrogen and oxygen emissions in the Europa aurora. The plume, a surplus of oxygen and hydrogen is visible at the south pole. some form of chemosynthetic metabolism is more probable. Unlike the other Galilean moons, Europa's ocean is thought to have a direct interface with the rocky crust beneath. This implies that geothermal vents at the bottom of the ocean could release reducing substances into the ocean that could fuel chemosynthetic metabolism (comparable to the black smokers on Earth's ocean floor). Oxidizing substances that are formed at the surface of Europa (created by the charged particle bombardment) and transported back into the ocean could also be of importance for chemosynthetic metabolism.
EUROPA'S WATER VAPOR PLUMES In December 2012, a large water vapor plume was observed at Europa with the Hubble Space Telescope (Roth et al., 2014a). The scientists conducting these observations were studying Europa's aurora. Europa is known to have a tenuous atmosphere, or exosphere. Particles in this exosphere are bombarded by high-energy electrons from
SURFACE HABITATS SHALLOW WATER Earth
Jupiter's magnetosphere that cause auroral emissions. These auroral emissions are studied in the ultraviolet spectrum, but not in the visible spectrum, because they are too weak compared to other visible light sources in the environment. During the observations made in December 2012, a large surplus in H and O emissions indicative of H2O, was observed (see Figure 2). It is suggested that this is caused by the presence of two water vapor plumes, which are approximately ~200km high and persisted for seven hours (the Hubble observation window). To reach such altitudes, supersonic velocities of 700m/s are needed (still lower than the escape velocity of approximately 2000km/s). From the observations, an enormous mass flux of 7000kg/s is estimated. No plume signatures have been observed in repetitions of the original observation in January and February of 2014 (Roth et al., 2014b), when Europa was at a similar orbital position. Saturn's moon Enceladus has regular plume activity that is linked to the position of Enceladus in
DEEP HABITATS TRAPPED OCEANS Ganymede
HABITABILITY OF EUROPA'S OCEAN
EUROPA'S OCEAN Though no mission has ever penetrated Europa's ice layer or even landed on the surface, Europa is thought to have a liquid ocean because of several reasons. Firstly, the presence of certain surface features on Europa can be explained by, or require the presence of a liquid layer. Secondly, measurements of Europa's gravity field indicate that a liquid layer could be present. However, the strongest argument is the presence of Europa's induced magnetic field. While moving through Jupiter's strong magnetic field, a magnetic field is induced in Europa. This requires the presence of a conductive layer inside of Europa - the most likely candidate being a salty ocean. Estimates of the thickness of the ice-ocean layer range from 80-150km, of which the outer 7-15km could be ice.
TOP OCEANS Titan
Liquid water Stable environment Essential elements Chemical energy Table 1 - Present state of the existing and past habitable worlds in the solar system. For each object the status of the four pre-requisites for a habitable environment is ranked from red (not possible), to yellow (likely but not yet demonstrated) and to green (demonstrated or very likely). LEONARDO TIMES N째2 2016