Deep Atmosphere Microwave Radiometer for Ice-Giant Exploration

The Juno microwave radiometer (MWR) demonstrated new type of instrument that provided a major breakthrough and paradigm shift in our understanding of giant planet atmospheres. MWR was originally designed based on theories of Jupiter’s atmospheric structure that assumed homogeneity at depth. An MWR uniquely interrogates atmospheric composition and structure (down to >1000 bars pressure). Results revealed that Jupiter’s deep atmosphere is not globally uniform below the meteorological layer (see Figure 1-1). This discovery is likely a fundamental characteristic of giant planets atmospheres. Based on Juno results we identified two key technologies to develop for advancing the next generation MWR to address the expected deep variability in an ice giant’s deep atmosphere. The technologies proposed would be equally important to an investigation of Saturn or even a return to Jupiter. As described below, the questions that an MWR can address at ice giants align with the fundamental questions regarding ice giant deep atmospheres and planetary formation reflected in the last National Academy’s Decadal Report (V&V, 2011). Ice giants are an essential part of understanding planetary origin and evolution within and outside of our solar system. Ice giants present the most serious gap in understanding planetary atmospheres (p 81, Visions and Voyages). An MWR can provide insights into the dynamics, structure and variability of NH3, H2S and H2O. These constrain heat flux in a giant planet connecting the interior to the atmosphere. Juno demonstrated that its high spatial resolution, signal to noise and a simultaneous set of observations covering a wide range of frequencies (corresponding to different atmospheric depths) are essential to understanding giant planet deep atmospheres. Measurements of the ice giants at sufficient spatial resolution are not possible from ground based telescopes (i.e. ALMA or VLA).

The Juno MWR comprises six independent radiometer channels from 0.6 to 22 GHz. Each radiometer has a separate antenna that produces a single beam. It takes advantage of spacecraft rotation to acquire observations of the atmosphere at range of emission angles along the sub-spacecraft track to probe the depth of processes and composition. Observing the emission angle dependence is a powerful approach to separate temperature and opacity information contained in the microwave spectrum (Janssen et al., 2005). Juno also demonstrated a second powerful observation mode enabled by spinning orthogonal to the trajectory. This mode scans the antenna beams longitudinally mapping the deep atmosphere in three dimensions, but gives up the limb darkening information since each observation is at a single incidence angle.

The goal of this proposal is to mature the technology for a next generation MWR instrument with the following innovations: • Combine multiple channels into a single wideband, flexible, spectrometer instrument, and increasing the number of observed frequencies channels • Electronically form multiple antenna beams in two principal planes to replicate the along-track and cross-track scanning modes without the need to spin the spacecraft

Achieving these innovations requires development of a broadband array antenna capable of covering multiple MWR channels and a digital processor that separates the broadband single into multiple spectral bands and digitally forms simultaneous beams. This proposal builds upon the immensely successful Juno MWR instrument with two key contributions, (1) The proposed wide-band antenna will be designed to match the performance of the Juno MWR A1 and A2 receivers (600MHz and 1.2GHz) with the added capability of measuring the full spectra from 500 MHz to 1.5 GHz, and (2) the ability to electronically form multiple along-track and across-track beams from a single stationary antenna. More »