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 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.
advertisement
