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By Jason Perry
February 21, 2001
In the next couple of years, Galileo will take a dive into Io's mammoth parent, Jupiter, ending the mission. With the exception of possible opportunistic observations by the Europa Orbiter in 10 years hence, no mission to Io is currently planned. However, in the last couple of years, several mission have been proposed to be sent to Io. One mission, known as Firebird, would consist of a Discovery-class flyby mission which would take 50 Gbits of data in one pass of Io. Another mission, known as Volcan, would consist of a Discovery-class spacecraft, like Firebird. Unlike Firebird, Volcan would orbit Jupiter and flyby Io four to six times for better temporal and spatial coverage. Building on that idea, John Spencer, William Smythe, Rosaly Lopes-Gautier, and Alfred McEwen today proposed a possible mission to Io at the Forum on Innovative Approaches to Outer Planetary Exploration: 2001–2020 in Houston, Texas. This proposal would orbit Jupiter like Volcan but obtain more data over a longer period of time than either Firebird or Volcan. The importance of Io as a model for the geology of early Earth and the interior of Europa was emphasized at the talk.
The speakers at the forum believe that Io is important to the study of early Earth and Europa. The volcanic processes seen on Io are similar to those seen on Earth in the Precambrian. Ionian lava are believed to be similar to komatiites, lavas that erupted 3.2 3.5 billion years ago and are found in present-day South Africa [Williams et al. 2000]. This is based on temperature measurements of Pele, Kanehekili, and particularly Pillan by the Galileo spacecraft. The eruption style of large-scale lava flows are similar to Phanerozoic flood basalts. These eruptions have never been seen by mankind and the ability to study these eruptions on Io allows us to better understand flood basalt eruptions on Earth.
In their presentation, Pillan was shown as an example of tracking a volcanic eruption through the stages of pre-eruption, eruption, and post-eruption. This eruption has allowed for better understanding of other volcanic eruptions that occurred before the Galileo era, like Zamama and Prometheus. Pillan erupted in mid-1997, creating fire fountains along a 40-km long fissure, and sending ash and SO2 snow in to the ultra-thin Ionian atmosphere creating a large dark spot surrounding the vent. This was the largest eruption detected by Galileo despite the fact that the majority of the lava was emplaced in 15 days. The authors state that due to antiquated instrumentation onboard Galileo, they will never be able tell from Galileo data: lava composition, eruption volume, time evolution of the gas and pyroclastic composition of the plume, eruption duration, and lava flow rates. The authors also point out that despite the fact that the eruption caused one of the largest changes on the surface of Io as seen by Galileo, its 3.5 µm brightness was about 4% of that seen at the August 1999 eruption northeast of Hi'iaka Patera which is believed to be the largest in the last 20 years.
Despite the recent flybys by Galileo, the authors say that the due to the limitations of the Galileo mission, many fundamental questions remain unanswered about Io. These limitations include a low data rate, aging and antiquated instrumentation, limited UV coverage, and limited spatial and temporal coverage. Galileo has allows much to be learned about Io, but due to these limitations, some important questions are left unanswered. What are the compostions of Io's lavas and volcanic gases? How old is Io's surface? Is the heat flow constant or variable? What is the range of eruption styles?
To answer these questions, the authors propose a possible mission to Io. The spacecraft would orbit Jupiter in Galileo-style eccentric orbits allows for flybys of Io about once a month and for long-term monitoring of Io's volcanoes. The spacecraft would have to be rad-hardened to protect the probe from the high-radiation environment surrounding Io. Radiation protection similar to the Europa Orbiter, whose hardness is to 4 MRads, would allow for up to 100 passes of Io. During each pass, many observations would be taken including color high resolution images of large areas, near-infrared spectroscopy with < 1 km spatial resolution, 10-20 µm thermal mapping at 10 km resolution, topographic mapping at 2 m percision, 2000–3200 Å UV spectroscopy at 20 km resolution, and in situ observations. On the cruise part of the orbit, there would data playback at 12 Kb/s, allowing for 10-30 Gbits to be returned each orbit. In addition, long distance observation would occur to monitor volcanic activity.
Compared to the Europa Orbiter, an Io mission could carry more instruments. Because it would only be required to go into orbit around Jupiter rather around Io as well, less fuel is needed and more instruments can be added. Some of the instruments proposed by the authors include high-resolution and wide-angle imagers, a near-infrared spectrometer, a thermal mapper, an Ultraviolet Spectrometer, a mass spectrometer, a laser altimeter, and some fields and particles instruments like a dust detector and a magnetometer. It might also be possible to save fuel and cost by using solar cells for power rather than RTGs but the radiation environment might be a problem.
With the Galileo mission nearing an end, a return to Io is being pondered by many groups but similarities between groups now allow a reasonable look at what a real Io mission, which could launch sometime in the second decade of this new century, will look like. Most like like it will orbit Jupiter in a less expensive version, due to lower fuel demands, of the Europa Orbiter. This mission design would allow for multiple flybys of Io from various heights with long cruises for data playback and volcano monitoring. Instrumentation would include a visible imager, a near-infrared/thermal mapper, and a magnetometer at the least though a UV spectrometer, a laser altimeter, and other instruments are possible depending on funding. A spacecraft similar to this would allow us learn quite a bit volcanic processes not seen on earth in 3.5 billion years.
For more information about the mission, read Spencer et al.'s abstract here.
