Forgenerations, astronomers have argued over how the planets in our solar systemwere formed. Today, most theories assume that planets were formed in a nebulaof gas and dust that condensed around what eventually became our sun, but thereis still great disagreement over details, particularly for gas giant planetslike Jupiter: Did a small core form first around which each planet condensed,or did instability in the nebula cause pockets to collapse directly intoplanets?
Eight yearsfrom now, if all goes as planned, a spacecraft will enter orbit around Jupiter that should provide insightinto planet formation.
Juno,the first solar-powered mission to the outer planets (it will carry no nuclearmaterials), will be inserted into a polar orbit that will approach the gasgiant world much more closely than any previous mission. It will carry severalinstruments intended to determine the structure of Jupiter's atmosphere.
Among theseis the Jovian Infra-Red Auroral Mapper (JIRAM), which will provide both imagesand spectra in the near infra-red of hot spots that are believed to provide awindow into Jupiter's lower atmosphere. Working in conjunction with a microwavesounding instrument, JIRAM should help determine the quantity of water in thelower atmosphere.
Togetherwith accurate maps of Jupiter'sgravity field, which will be developed by observing how Juno's orbit changesover time, this should settle once and for all whether the giant planet has asolid core, and how large it is. That will provide a direct test for one of twocurrently popular theories of planet formation, which predicts that Jupitershould have a substantial core of many times the mass of the Earth.
JIRAM alsowill provide images of Jupiter's aurora, which is similar to, but much morepowerful than, Earth's familiar Northern Lights. The auroraforms when gas in the upper atmosphere is ionized by streams of chargedparticles trapped by a planetary magnetic field. Jupiter has the most powerfulmagnetic field of any planet in our solar system, and its auroral displays areso bright they have been seen using the Hubble Space Telescope. JIRAM willprovide a close-up look at Jupiter's aurora, which according to a recent paperpublished in the journal Astrobiology, " ... provides a model systemfor potentially observable phenomena associated with Jupiter-mass andsuper-Jupiter-mass bodies around nearby stars."
JonathanLunine, a professor at the University of Arizona's Lunar and PlanetaryLaboratory, is a member of the JIRAM research team, and has high hopes for theresults it will offer in conjunction with Juno's other instruments: "Wewill obtain a detailed map of the Jovian gravity field, establishing once andfor all whether there's a core. Also, the oxygen and nitrogen abundances, onceaccurately established, provide a good test for determining the composition ofthe icy bodies that created Jupiter's supersolar abundance — not directly tiedto the issue of whether core formation occurred but it will help us determineconditions where Jupiter formed."
Lunine alsobelieves JIRAM will help scientists understand Earth's Northern Lights:"Looking at aurora formed in a hydrogen vs. nitrogen-oxygen atmosphere,and with different particle sources (from the Jovian magnetosphere), we cantest theories of auroral formation under conditions very different from onEarth."
To meetthese goals, a team of scientists and engineers, who developed instruments forNASA's Cassini and Dawn missions and ESA's Rosettaand Venus Express missions, built what amounts to two instruments in one: atwo-dimensional imaging detector that works like a digital camera, and aseparate grating spectrometer, which functions like a prism, breaking light upinto a spectrum. Both the imager and spectrometer share a common focal plane,looking through a single telescope. The designers faced a unique challengebecause Juno is designed to spin, which could smear images. In JIRAM, acompensating mirror will be used that rotates in the opposite direction to thespacecraft, giving the imager a stable picture for at least part of eachrotation cycle.
DevelopingJIRAM was complicated by U.S. International Traffic in Arms (ITAR) regulations,which require an export license from the Department of State before foreignnationals can receive technical data about U.S. launch vehicles. Dr. AlbertoAdriani of the Istituto di Fisica dello Spazio Interplanetario in Rome, Italy said: "Not being Americans, we JIRAMs had to work in quite difficultconditions where the flow of information necessary for the instrumentdevelopment was very slow and sometime limited.... In particular the conditionsin which JIRAM has to work were not well known: The spacecraft thermalenvironment, expected radiation environment around Jupiter, and vibrationsduring launch — key elements for proper structural design — were all unknownto us in the beginning." To make progress while waiting for information tocome from the U.S., Adriani and his colleagues assumed these factors would besimilar to those on previous missions — then modified their design whenJuno-specific data became available.
Juno isscheduled for launch in August 2011, and should arrive at Jupiter 61 monthslater.
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