Follow the Water

If liquid water were to exist on these moons, it would not be unreasonable to speculate on the existence of life there, perhaps forming near undersea volcanic vents. Life on Earth has been discovered at great ocean depths, beyond the penetration of sunlight, thriving on upwelling chemical nutrients from the interior of the planet."We don't know if life is there. But this mission will allow to ask that question with some pretty sound tools," said Christopher McKay of NASA's Ames Research Center. Jimo won't launch until at least 2011. At the December 8th meeting of the American Geophysical Union, scientists prepared a briefing on the mission's progress.

The unmanned craft, far larger and more powerful than any other sent to explore the outer solar system, would spend years studying the moons' makeup, geologic history and potential for sustaining life, as well as Jupiter itself. The spacecraft is envisioned as being 60 to 100 feet in length. Early conceptions place its nuclear reactor at the end of a boom to shield the scientific instruments from radiation. Jimo also would bristle with fins to dissipate the intense heat from its reactor.

Inspired Discoveries

The icy moons are part of the original Galilean system, named after their discoverer, Galileo. Their presence has been a kind of benchmark for planetary science, since they helped solidy the Copernican view of the universe, that the Earth is not at the center of the known universe. The mini solar system around Jupiter instead has its own unique orbital mechanics, governed by the powerful magnetism and gravity of its parent planet. The first images of these moons sent back by Pioneer and Voyager stunned planetary scientists in their richness, eventually culminating in the 1995 discovery of evidence for subsurface, briny oceans. Because of its enormous volcanoes, Io is the only place beyond Earth where we can watch geological processes in action. But other than a predictable sulfur dioxide content from its volcanoes, Io's surface chemistry is still largely unknown. To fully understand geological processes, the subsurface (> 1 meter) heterogeneity should be mapped at better than 100 meter horizontal resolution over at least 50% of the surface of the three moons.

What's On Tap

The Jupiter Icy Moons Orbiter mission thus has three major science goals:

Potential for Life
The mission would scout the potential for sustaining life on Callisto, Ganymede and Europa. This includes:

1) Determining whether the moons do indeed have subsurface oceans.

2) Mapping where organic compounds and other chemicals of biological interest lie on the surface.

3) Determining the thickness of icy layers, with emphasis on locating potential future landing sites.

Origins and Evolution
Another main science objective would be to investigate the origin and evolution of these moons. This includes:

1) Determining their interior structures, surface features and surface compositions in order to interpret their evolutionary histories (geology, geochemistry, geophysics) and how this contributes to the understanding of the origin and evolution of Earth.

Radiation Environments
The mission would also determine the radiation environments around these moons and the rates at which the moons are weathered by material hitting their surfaces. Callisto, Ganymede and Europa all orbit within the powerful magnetic environment that surrounds Jupiter. They display varying effects from the natural radiation, charged particles and dust within this environment. Understanding this environment has implications for understanding whether life could have arisen on these distant moons.

Fission Fuel Far from the Sun
The spacecraft would be the first in a series of robotic NASA probes that rely on uranium-fueled fission reactors to generate large amounts of electricity. While probes such as Galileo and Cassini have made do with hundreds of watts of electricity, Jimo might have thousands of watts to power its thrusters and instruments, said Torrence Johnson of NASA's Jet Propulsion Laboratory.

The reactor conceivably could produce enough electricity to power several U.S. homes. That could provide Jimo a hundredfold boost over previous missions in the amount of data it would be able to beam back to Earth. Jimo would carry high-resolution cameras and other instruments, including radar and lasers to map the thickness and elevation of the ice that envelops each moon.

McKay's presentation posed the question: "How could a search for life be accomplished on a near-term mission given the thick ice cover? One answer may lie in the surface materials. If Europa has an ocean, and if that ocean contains life, and if water from the ocean is carried up to the surface, then signs of life may be contained in organic material on the surface. Organics that derive from biological processes (dead organisms) are distinct from organics derived from non-biological processes in several aspects. First, biology is selective and specific in its use of molecules.

For example, Earth life uses 20 left-handed amino acids. Second, biology can leave characteristic isotopic patterns. Third, biology often produces large complex molecules in high concentrations, for example lipids. Evidence of life in the ocean may be found on the surface of Europa if regions of the surface contained relatively recent material carried up from the ocean through cracks in the icy lithosphere. But organic material that has been on the surface of Europa for long periods of time would be reprocessed by the strong radiation field probably erasing any signature of biological origin".

Uniqueness

Other presentations described mission plans for science where JIMO might differ from the previous probes ranging from the recent Galileo, Cassini, Voyager series back to Pioneer. For instance, JIMO offers a potential breakthrough in remote sensing: The 1-3 Mbps [megabits per second] data rate is 2 orders of magnitude (100 times) greater than that of previous missions. The circular orbit offers continuous planet viewing during the 3 months between satellite encounters. The 10-30 kilowatts (kW) of power offers advantages for radio occultations and other active sensors. In addition, JIMO can carry a probe, which can determine the water abundance, deep winds, and thermal structure to 100 bars. Dust measurements have shown that the Galilean moons are surrounded by tenuous dust clouds formed by collisional ejecta from their icy surfaces, kicked up by impacts of interplanetary micrometeoroids. Even a flyby mission, without landing, thus may offer a chance to sample chemistry from the surface of these moons. On January 4th, a similar mission profile of flying through a dust cloud is planned for the comet sampling and return mission called Stardust. The economics of a flyby are considerably less complicated than landing, particularly for sample returns.

The Biological Triad

One of the most startling aspects of testing for life so far from the Sun, and potentially under 8 to 30 miles of ice, would be its energy source. As these inner Jovian moons orbit the powerful gravitational and magnetic fields of Jupiter, their surfaces flex and this tidal friction generates heat internally. Tidal dissipation in Jupiter is the ultimate source of the energy that powers Io's volcanism and may also be an important cause of heating in Europa and Ganymede. However, the mechanism of jovian tidal dissipation is still unknown.

It takes at least three elements to harbor life as we know it: water, energy and an atmosphere. Among Mars and the moons around both Jupiter and Saturn (mainly its largest one, called Titan), there is evidence of one or two of these three elements, but less is known if a complete set is available. For instance, the icy Galilean satellites are known to have tenuous atmospheres of hydrogen (H), oxygen (O, O2) and carbon dioxide (CO2), among other species. A hydrogen corona has been detected around Ganymede and Callisto. An oxygen (O2) atmosphere has been inferred at Europa as a result of Hubble Telescope (HST) measurements of oxygen emission features. But only Saturn's moon, Titan, has an atmosphere comparable to Earth's in pressure, and is much thicker than the martian one (1% of earth's sea level pressure). A day on Mars is like continuously flying at 70,000 feet on Earth, as far as available air pressure. The icy satellites of Jupiter are embedded within the magnetosphere and as such, are constantly bombarded by intense radiation and charged particles. Salt plumes of sodium chloride have been observed by Hubble on the volcanic moon, Io. Ozone (O3) has been detected on Ganymede primarily in the polar regions, suggesting that the source is bombardment by electrons travelling along the field lines and impacting the polar ice.

One reason planetary scientists want to update future imagery of these moons is to see if they change. Studying images of Ganymede and Callisto and Europa, especially at higher resolutions, will reveal if any changes have taken place since previous probes like Galileo or Voyager. Visible and infrared imaging may be able to detect thermal anomalies due to intrusions of warm water into ice for hundreds of years on Europa, and larger-scale thermal plumes could leave areas of thinner crust for up to a million years. These plumes might be detected by radar sounding from orbit. During its numerous flyby missions of the moons, Galileo provided spatial resolution down to a few meters but temporal resolution no better than a few months, and Earth-based techniques provide temporal resolution down to hours or days.

Such changes over a short time period may suggest activity that serves as another remote signature for the partial triad of life: water, energy and a significant atmosphere.

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