Sunday, February 15, 2009

Io Science with the Jupiter Ganymede Orbiter

In my previous articles on the Europa/Jupiter System Mission, I have focused on the kinds of science that the Jupiter Europa Orbiter (JEO) can gather at Io, and rightly so. Of the two components of the mission, JEO will be the probe that will flyby Io, perform high-resolution science observations of that satellite. In addition, only the Jupiter Europa Orbiter has a narrow-angle camera capable of studying Io's geology and surface changes with spatial resolutions of better than 10 kilometers per pixel during most orbits during the Jupiter tour segment for that spacecraft, and while in Europa orbit. But can the ESA-supplied Jupiter Ganymede Orbiter still provide useful science at Io, even though it never comes within 650,000 km of Io? Let's delve a bit into the ESA Assessment Report for JGO to find out.

The Jupiter Ganymede Orbiter (JGO) is a solar-powered space probe designed to study Jupiter's two outer Galilean satellites, Ganymede and Callisto, extensively. To reduce the radiation dosage that could harm the solar panels, JGO never approaches Jupiter from closer than Ganymede's orbit. So, during the Jupiter tour phase of that probe's mission, from JOI on February 4, 2026 till GOI on May 22, 2028, spends its time flying by Ganymede and Callisto repeatedly. To support Callisto science, JGO will spend a year from February 2027 to February 2028 in a resonant orbit with Callisto, which allows the probe to encounter the moon 19 times. In May 2028, JGO goes into orbit around Ganymede, first in an elliptical 200x6000 km orbit, then in a circular, near-polar, 200-km altitude orbit. JGO would finally be crashed into Ganymede on February 6, 2029.

To support the science goals at these two icy satellites, JGO will carry a similar complement of instruments as the Jupiter Europa Orbiter, with a few exceptions. Like JEO, the Jupiter Ganymede Orbiter will carry an ice-penetrating radar, wide- and medium-angle cameras, a mid-infrared thermal mapper, a visible/near infrared imaging spectrometer, a laser altimeter, and an ultraviolet imaging spectrometer. Unlike JEO, JGO is not currently outfitted with a narrow-angle camera or an ion and neutral mass spectrometer, though the NAC is at the top of the JGO team's wish list. The NAC was dropped from the initial spacecraft baseline payload so the pointing accuracy needed for the spacecraft could be reduced, which helped to reduce JGO's cost. The Jupiter Ganymede Orbiter would carry a sub-millimeter wave sounder which would be used to better understand the dynamics of Jupiter's stratosphere and to measure local wind speeds and temperatures on Jupiter. This helps fill in gaps in our knowledge of Jupiter's atmosphere left behind by Juno.

Lacking a narrow-angle camera and with JGO never coming closer than 650,000 km of Io during its mission, what kinds of science could we possibly expect from the Jupiter Ganymede Orbiter? In terms of direct imaging of Io's surface, the best instrument might be the visible and near-infrared imaging spectrometer (VIRHIS). From Ganymede's orbit, the VIRHIS instrument would have a spatial resolution (using its high-spatial resolution mode at wavelengths between 400-2200 nm) of 81-185 km/pixel. This would allow JGO to monitor Io's volcanic activity at spatial scales comparable to the NIMS instrument during Galileo's primary mission of 1996-1997. VIRHIS could also be used to investigate the distribution of SO2 across Io's surface. In combination with a similar instrument on JEO, during the Jupiter Tour phase, this would allow more continuous coverage of Io's volcanic activity as JGO could observe Io while JEO was near apojove, and vice versa. JGO could also observe Io when JEO is more intensively observing Europa during the satellite orbital stage. The medium-angle camera on JGO could observe Io at pixel scales of 165-375 km/pixel, which is comparable to Hubble. These pixel scales are not as useful for surface science (not at visible wavelengths anyway), and might be more useful for "Kodak Moment™" shots in conjunction with Jupiter or the other Galileans. The Thermal Mapping can also observe Io's thermal emission at longer wavelengths, but at a peak resolution from Ganymede orbit of 325 km/pixel.

Both the JEO Final Report and the JGO Assessment Report discuss the potential for synergistic science between the two orbiters, such as at Io. For example, while JEO is encountering Io, JGO could observe Io from a distance, focusing on the state of the Io Plasma Torus (observing the structure using JGO's ultraviolet imaging spectrometer) and the state of Io's atmosphere. These observations would help to put the Europa Orbiter's in-situ data in context. In another example, while JEO monitors a major volcanic eruption on Io and its effects on the plasma torus, JGO could look for effects on the Jovian magnetosphere as a whole. While Io is in eclipse, the two spacecraft could observe Io's plasma interactions in three-dimensions. Low-spatial resolution observations of Io from JGO could also help to increase phase angle and longitude coverage of bolometric albedo measurements, which can help to constrain Io's thermal inertia and from there heat flow. Finally, JEO and JGO could link up to acquire radio occultations of satellite atmospheres, such as Io's. This would allow scientists to probe Io's night-side atmosphere for example, which would be impossible simply using a direct-to-Earth link for a radio occultation except for near dawn and dusk. The JEO Final Report suggests that the two spacecraft could also be used to acquire stereo coverage of Io's plumes, but without a narrow-angle camera, such observations may not be possible (or as useful).

While the Jupiter Ganymede Orbiter, the European Space Agency's contribution to the EJSM, would not come very close to Io because of the high-radiation environment, the instruments onboard JGO would provide some useful complimentary science to what could be acquired by JEO. JGO could use its Visual and Near-Infrared Imaging Spectrometer to monitor Io's volcanic activity at spatial scales comaprable to Galileo's during that spacecraft's primary mission. These observations would improve upon similar observations by JEO by increasing temporal coverage. JGO could also provide synergistic science by observe magnetospheric and atmospheric structures at Io while JEO is flying by the moon.

Link: Jupiter Ganymede Orbiter ESA Assessment Report []

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