Europa & Titan

For the three large icy Galilean satellites, Callisto, Ganymede, and Europa, the primary NIMS science objectives are to map the various surface compositional units and to identify their elemental and mineralogical composition.

A primary question concerning these units is the composition of the dark components that are mixed with the dominant water-ice crusts. Are these materials silicates or organic-rich materials derived from primitive objects such as comet nuclei? If silicates, NIMS may detect bands due to olivines, pyroxenes, or a range of iron-bearing minerals. C-H features might be present if the satellite surfaces contain dark organic components such as those found on some asteroids. Magnetospheric sources of implanted material (e.g. sulfur) may be an important process, providing chemically reactive species which can modify the surfaces (e.g. generating S-O from the S implanted in H2O).

NIMS Instrument on Galileo

View of a small region of the thin, disrupted, ice crust in the Conamara region of Jupiter's moon Europa showing the interplay of surface color with ice structures. The white and blue colors outline areas that have been blanketed by a fine dust of ice particles ejected at the time of formation of the large (26 kilometer in diameter) crater Pwyll some 1000 kilometers to the south. A few small craters of less than 500 meters or 547 yards in diameter can be seen associated with these regions. These were probably formed, at the same time as the blanketing occurred, by large, intact, blocks of ice thrown up in the impact explosion that formed Pwyll.

A color-enhanced close-up of Europa's surface. Areas of blue are thought to be pure water ice. The "cryo-volcanic" ridges have a brown color showing where mineral-laden water from underground appears to have percolated to the surface through cracks in the crust.

Thera and Thrace are two dark, reddish regions of enigmatic terrain that disrupt the older icy ridged plains on Europa.

One model for the formation of these and other chaos regions on Europa is complete melt-through of Europa's icy shell from an ocean below. Another model is that warm ice welled up from below and caused partial melting and disruption of the surface.

Galileo’s MagnetometerThe Jovian magnetosphere envelopes the planet in a vast cocoon that extends more than seven million kilometers from the planet in all directions and deflects the solar wind in its outward flow from the Sun. The gargantuan scale of the system is readily understood as arising from a combination of factors: the low dynamic pressure of the solar wind at the orbit of Jupiter, down by a factor of more than 25 from its level at 1 AU; the large planetary radius, 11 times that of the Earth; the strong dipole magnetic moment of the planet, more than four orders of magnitude larger than that of the Earth; and the existence of a relatively dense, centrifugally accelerated plasma that acts to inflate the magnetosphere from within.

From Khurana et al. 1998

Initial results from the spacecraft's magnetometer1,2 have indicated that neither Europa nor Callisto have an appreciable internal magnetic field, in contrast to Ganymede3 and possibly Io4. Here we report perturbations of the external magnetic fields (associated with Jupiter's inner magnetosphere) in the vicinity of both Europa and Callisto. We interpret these perturbations as arising from induced magnetic fields, generated by the moons in response to the periodically varying plasma environment. Electromagnetic induction requires eddy currents to flow within the moons, and our calculations show that the most probable explanation is that there are layers of significant electrical conductivity just beneath the surfaces of both moons. We argue that these conducting layers may best be explained by the presence of salty liquid-water oceans, for which there is already indirect geological evidence5,6 in the case of Europa.

These artist's drawings depict two proposed models of the subsurface structure of Europa. Geologic features on the surface, imaged by the Solid State Imaging (SSI) system on NASA's Galileo spacecraft might be explained either by the existence of a warm, convecting ice layer, located several kilometers below a cold, brittle surface ice crust (top model), or by a layer of liquid water with a possible depth of more than 100 kilometers(bottom model). If a 100 kilometer (60 mile) deep ocean existed below a 15 kilometer (10 mile) thick Europan ice crust, it would be 10 times deeper than any ocean on Earth and would contain twice as much water as Earth's oceans and rivers combined. Unlike the Earth, magnesium sulfate might be a major salt component of Europa's water or ice, while the Earth's oceans are salty due to sodium chloride (common salt).

Analog sites on Earth

In 1974, a team of scientists conducting airborne research passed over the Soviet research station Vostok in Antarctica. Their sounding instruments detected an expanse of water beneath the ice roughly the size of Lake Ontario. Although Antarctica records some of the coldest temperatures on Earth, Lake Vostok is buried under four kilometers of ice. The ice sheet acts as a blanket, shielding the lake from cold temperatures on the surface. It is also thought that geothermal heat helps keep the water liquid.

Life beneath the ice

•Ice cores from Lake Vostok reveal traces of microbial life

•Included are cyanobacteria, bacteria, fungi, spores, pollen grains, diatoms, others

Future Europa Explorers

Titan

Determine Landing Site

Volcanism

Huygens Probe

A bright linear feature suggests an area where water ice may have been extruded onto the surface. Also visible are short, stubby dark channels that may have been formed by 'springs' of liquid methane rather than methane 'rain.'

The left-hand side shows a boundary between light and dark areas. The white streaks seen near this boundary could be ground 'fog,' as they were not immediately visible from higher altitudes.

The surface is darker than originally expected, consisting of a mixture of water and hydrocarbon ice. There is also evidence of erosion at the base of these

objects, indicating possible fluvial activity.

Titan’s Atmosphere

This graph shows data acquired by Cassini as it flew by Titan at an altitude of 1,200 kilometers (745 miles) on Oct. 26, 2004 - its closet approach yet to the hazy moon. The data is from Cassini's ion and neutral mass spectrometer, which detects charged and neutral particles in the atmosphere. The graph reveals a diversity of hydrocarbons in the high atmosphere above Titan, including benzene and diacetylene.

•1.5 bars

•N2 and Methane

•Methane photodissociates in 107 years once in the atmosphere. Something must replenish it!

Methane

In this false color rendition, green light is the fluorescent emission of methane gas powered by sunlight, at a wavelength of 3.3 microns…The glow extends over 700 kilometers (435 miles) above the surface, revealing the unusual thickness of the moon's atmosphere, which nearly doubles Titan's volume compared to the volume of the solid sphere, indicated by the solid line.

On the nightside (right side), the moon glows red out for over 200 kilometers (125 miles) altitude, indicating carbon-monoxide emission at 4.7 micron wavelength produced in Titan's relatively warm stratosphere.

What is the source of the Methane?

• Methane ocean?

• Cryovolcanism?

• Methanogens?