The Juno MWR will be the second microwave instrument to explore the
planets since the first observations from Mariner 2 of Venus in 1962,
which confirmed that the high temperature inferred from radio measurements
indeed reflected surface and deep atmospheric conditions rather than
a hot ionosphere. The Jet Propulsion Laboratory builds the MWR.
The primary goal of the Juno Microwave Radiometer is to probe the deep atmosphere of Jupiter at radio wavelengths ranging from 1.3 cm to 50 cm using six separate radiometers to measure the planet's thermal emissions. The MWR experiment will provide answers to two key questions: How did Jupiter form? and How deep is the atmospheric circulation that was measured from the Galileo Probe down to 22 bars of pressure, and at the cloud top level from imaging data returned by other missions?
The first question will be addressed by the determination of the water abundance in the deep atmosphere. The MWR will obtain measurements of ammonia and water in the Jupiter atmosphere, which are the principle absorbers in the microwave region, by scanning Jupiter along the orbital track as the spacecraft spins. These observations will allow scientists to determine whether the water abundance on Jupiter is three times that of the sun or nine times that of the sun.
The Juno MWR avoids the synchrotron emission from Jupiter's magnetosphere
by using shorter wavelengths and achieves high accuracy to measure water
abundance in the deep atmosphere by using "relative limb darkening",
a parameter that depends on the emission angle of the radiation. The
vertical profile of water abundance is obtained by using multiple frequencies,
much like the retrieval of temperature profiles on earth with multi-spectral
infrared measurements from orbiting weather satellites.
The MWR uses three antennae mounted on the spacecraft body, which sweep across the planet as the spacecraft spins to measure the radiation at six different wavelengths along the orbital track. Successive orbits will map the planet longitudinally. The six different wavelengths observed by the MWR, combined with the emission angle dependence will provide a good idea of the atmospheric temperature profile to ~ the 200 bar pressure level on Jupiter. The latitudinal dependence of the temperature profile and depth will enable inference of the circulation of Jupiter's deep atmosphere to a much greater depth than that obtained by the Galileo probe.
Different wavelengths sample different depths of Jupiter atmosphere and yield composition and thermal structure
The weighting function prescribes how much of the radiation at that wavelength comes from different depths in the atmosphere
The multiwavelength measurements of brightness temperature with latitude provides estimates of the strength of the zonal circulation with depth
Dr. Mike Janssen (JPL) is the Lead Co-Investigator for the MWR Instrument Team.