The Jovian Moons

The spacecraft returned spectacular photographs of the four satellites and of Jupiter itself as well as a plethora of new data about their structure and composition. 

Scientists were also quite surprised by the surface features Callisto displayed. Images taken from only 86 miles (138 km) about the moon s surface showed strong evidence of erosion. Among the most prominent features were strings of jagged hills, made of ice and rock, surrounded by dark patches of dust. Researchers hypothesize that the hills were formed when bodies from interplanetary space collided with the moon, throwing crustal material upward. Over time, ice contained within the crustal material may have evaporated away, leaving behind the solid dusty material that forms the dark patches around the hills. 

Ganymede s surface indicates that a great deal of tectonic activity is taking place on the moon. It is covered with folds, faults, and fractures, similar to those of mountainous regions on Earth, where land movements are frequent and large. Although craters are visible on the moon s surface, Ganymede s wrinkled tectonic shapes are most characteristic. 

Like Callisto and Ganymede, Jupiter s moon Europa is thought to have a saltwater ocean buried beneath its crust. Evidence for this hypothesis includes not only the changing electrical fields in the moon s atmosphere, but also the fact that its north pole changes direction every 5V2 hours, a phenomenon that can best be explained by the presence of a conductive layer (such as saltwater) beneath the moon s surface. Some scientists believe that an underground salt-water ocean of this kind might be able to support primitive forms of life. 

Images of Europa relayed by Galileo also showed features that had quite obviously been separated from each other at some time in the past, like pieces of a jigsaw puzzle. Scientists hypothesize that 

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Io is one of the most interesting objects in planetary research. However, it is completely irrelevant to the biogenesis problem, in complete contrast to the Jovian moon Europa. 

Fig. 3.2 The three possible models for the inner structure of the Jovian moon Europa model 1 has a thin layer of ice at the surface, model 2 is the ice-water model and model 3 involves a thick ice layer...

The purine base guanine is also formed in concentrated solutions of ammonium cyanide, i.e., the same substance which became known from Or6 s adenine synthesis. Or6, as well as Stanley Miller, was involved in a new series of experiments (Levi et al., 1999). The yield of guanine is, however, 10 10 times lower than that of adenine surprisingly, the synthesis is just as effective at 253 K as at 353 K. Low temperatures seem conceivable in certain parts of Earth as well as on the Jovian moon Europa (see Sect. 3.1.5) or in the Murchison meteorite. 

The current models of the Sun suggest that its luminosity would have been some 20-30 per cent lower than its present value during the early part of the formation of the Earth. After the enormous temperatures of the Hadean period, the early precambrian may have been cooler, requiring prebiotic chemistry to occur below a layer of ice, perhaps heated by volcanic activity such as that found in geothermal vents. A layer of ice several hundreds of kilometres thick may have formed over the entire surface of the early Earth, providing protection from UV radiation and some global warming - conditions such as these may exist on the Jovian moon Europa. 

Figure 6.1. The Jovian moon lo deep ultraviolet (UV) photolysis of its methane atmosphere proceeds with electron ejection, generating the molecular ion of methane (see color insert). NASA JPL Galileo program image from Voyager 1, http //www.jpl.nasa.gov/galileo/io/...

The Jovian moon, lo, shows an orange hue (Fig. 6.1), which may be due to long-chain alkane radical cations. The atmosphere of lo consists mostly of methane deep UV photolysis proceeds with electron ejection thus, the molecular ion of methane was perhaps the earliest organic radical cation, generated by solar irradiation aeons ago. 

Can one be so sure about this invariance concept only because we can never test this prediction Not really. The Jovian moons are within our reach and Mars is still in contention as a life-supporting rock, so one must consider the idea in principle testable. Life will be the same and everybody who is searching for life elsewhere knows it intuitively, because they are looking with the methods it takes to detect our kind of life. They are all unwitting supporters of the invariance concept of the Genomic Potential Hypothesis. 

Sulfur can form halogen compounds and nitrides. The nitrogen compounds are found as interstellar molecules and chlorides postulated on the Jovian moon, lo, but these compounds do not appear to be a significant part of biogeochemical cycles on the Earth. 

The active volcanoes of the Jovian moon lo release large quantities of sulfur and other materials (Spencer and Schneider, 1996) that recover the surface at a rapid rate and maintain a tenuous atmosphere. This sulfur is largely as sulfur dioxide, which is also found as condensate that covers some three-quarters of the surface. However, sulfur is also found as elemental sulfur, with perhaps traces of hydrogen sulfide (Russell and Kivelson, 2001 Zolotov and Fegley, 1998). Low pressures in the atmosphere of lo mean that sulfur can remain in seemingly exotic forms such as sulfur monoxide (SO), which has been calculated to have an SO/SO2 ratio of 3-10% (Zolotov and Fegley, 1998). Others suggest that OSOSO, and its cation, are likely present in the lo s atmosphere (Cacace et al., 2001). 

Figure 21 Radiolytic sulfur cycling on the Jovian moon Europa with reaction rates marked numerically as reaction efficiencies. The transient sulfinic acid is circled as an intermediate (source Carlson et aL, 2002).

The results we obtained at 77 K, a temperature more appropriate to the jovian moons, are qualitatively and quantitatively the same as those at 16 K. Moreover, after implantation, we warmed the samples and obtained spectra at progressively higher temperatures Carbon dioxide is still there at 140 K when water ice completed the crystallization process, and it remains trapped until water is lost by sublimation. This gives us confidence that our findings can be extrapolated to environments with a large range of temperatures. " ... 

Io is the innermost of the four Galilean satellites and the most dense of the Jovian moons. Its density is estimated at 3.5 g/cm3. In some ways, its size, structure, and chemical composition are similar to those of our Earth s Moon. As the illustration shows, Io probably has the largest core for its size of any of the four moons. Io s surface is unusually smooth, essentially lacking in any impact craters. This evidence suggests that the moon s surface is fairly new, probably no more than about a million years old. 

Electric power output of the reactor module Is one of the most important requirements because it ultimately drives system mass and volume. While thrusting, the majority of electric power is used to drive the ion propulsion thrusters. Minimum acceptable thrust (and thus power) was determined to be driven by the complex gravity fields around the Jovian moons, where a minimum thrust level is required to achieve stable orbit and de-orbit of the moons. Due to the high mass of the JIMO vehicle, driven in part by the systems required to support the high voltage ion propulsion thrusters, the xenon propellant, the 1500 kg science payload, and the reactor module, an output power of 130 kWe for propulsion is required. The 200 kWe reactor module power output listed in Table 1-1 results after accounting for propulsion unit efficiency of 72%, conversion losses, transmission losses, and other vehicle power requirements. Explicitly stated, the required reactor output is estimated as follows [(130 kWe thruster output / 0.72 Propulsion Power Units Efficiency) + 5 kWe Vehicle Operation] / 0.95 Power Conversion and Distribution (PCAD) efficiency = 195 kWe ( 200 kWe). 

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