Studying a Giant Planet
For generations, astronomers have argued over how the planets in our solar system were formed. Today, most theories assume that planets were formed in a nebula of gas and dust that condensed around what eventually became our sun, but there is still great disagreement over details, particularly for gas giant planets like Jupiter: Did a small core form first around which each planet condensed, or did instability in the nebula cause pockets to collapse directly into planets?
Eight years from now, if all goes as planned, a spacecraft will enter orbit around Jupiter that should provide insight into planet formation. Juno, the first solar-powered mission to the outer planets (it will carry no nuclear materials), will be inserted into a polar orbit that will approach the gas giant world much more closely than any previous mission. It will carry several instruments intended to determine the structure of Jupiter’s atmosphere.
Among these is the Jovian Infra-Red Auroral Mapper (JIRAM), which will provide both images and spectra in the near infra-red of hot spots that are believed to provide a window into Jupiter’s lower atmosphere. Working in conjunction with a microwave sounding instrument, JIRAM should help determine the quantity of water in the lower atmosphere.
Together with accurate maps of Jupiter’s gravity field, which will be developed by observing how Juno’s orbit changes over time, this should settle once and for all whether the giant planet has a solid core, and how large it is. That will provide a direct test for one of two currently popular theories of planet formation, which predicts that Jupiter should have a substantial core of many times the mass of the Earth.
JIRAM also will provide images of Jupiter’s aurora, which is similar to, but much more powerful than, Earth’s familiar Northern Lights. The aurora forms when gas in the upper atmosphere is ionized by streams of charged particles trapped by a planetary magnetic field. Jupiter has the most powerful magnetic field of any planet in our solar system, and its auroral displays are so bright they have been seen using the Hubble Space Telescope. JIRAM will provide a close-up look at Jupiter’s aurora, which according to a recent paper published in the journal Astrobiology, “…provides a model system for potentially observable phenomena associated with Jupiter-mass and super-Jupiter-mass bodies around nearby stars.”
Jonathan Lunine, a professor at the University of Arizona’s Lunar and Planetary Laboratory, is a member of the JIRAM research team, and has high hopes for the results it will offer in conjunction with Juno’s other instruments: “We will obtain a detailed map of the Jovian gravity field, establishing once and for all whether there’s a core. Also, the oxygen and nitrogen abundances, once accurately established, provide a good test for determining the composition of the icy bodies that created Jupiter’s supersolar abundance–not directly tied to the issue of whether core formation occurred but it will help us determine conditions where Jupiter formed.”
Lunine also believes 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 can test theories of auroral formation under conditions very different from on Earth.”
To meet these goals, a team of scientists and engineers, who also developed instruments for NASA’s Cassini and Dawn missions and ESA’s Rosetta and Venus Express missions, built what amounts to two instruments in one: A two-dimensional imaging detector that works like a digital camera, and a separate grating spectrometer, which functions like a prism, breaking light up into a spectrum. Both the imager and spectrometer share a common focal plane, looking through a single telescope. The designers faced a unique challenge because Juno is designed to spin, which could smear images. In JIRAM, a compensating mirror will be used that rotates in the opposite direction to the spacecraft, giving the imager a stable picture for at least part of each rotation cycle.
Developing JIRAM was complicated by U.S. International Traffic in Arms (ITAR) regulations, which require an export license from the Department of State before foreign nationals can receive technical data about U.S. launch vehicles. Dr. Alberto Adriani of the Istituto di Fisica dello Spazio Interplanetario in Rome, Italy said: “Not being Americans, we JIRAMs had to work in quite difficult conditions where the flow of information necessary for the instrument development was very slow and sometime limited… In particular the conditions in which JIRAM has to work were not well known: the spacecraft thermal environment, expected radiation environment around Jupiter, and vibrations during launch — key elements for proper structural design — were all unknown to us in the beginning.” To make progress while waiting for information to come from the U.S., Adriani and his colleagues assumed these factors would be similar to those on previous missions—then modified their design when Juno-specific data became available.
Juno is scheduled for launch in August 2011, and should arrive at Jupiter 61 months later.