JWST MIRI Mapping of Jupiter's Great Red Spot: Preparatory Data Simulations

Jupiter's extended disc and infrared brightness create a challenging target for JWST's MIRI instrument to probe. Observations with the Medium Resolution Spectrometer (MRS) integral field units are expected to saturate beyond 11 microns, and the fields of view of the MRS are so small that we can sample only regional phenomena on the Jovian disc. Also, the rapid Jovian rotation will result in an observing geometry that will vary from exposure to exposure. However, the potential to probe mid-IR spectral ranges from 5-11 µm that are partially inaccessible to ground-based observatories will enable the determination of 3D temperatures, thermal winds, atmospheric stability, gaseous composition, dynamics and aerosol distribution in unprecedented detail. The target of guaranteed-time Jovian observations (cycle 1 – GTO 1246) will be the Great Red Spot (GRS) and its environs as a test of our ability to map Jovian meteorological phenomena. The giant anticyclone will be mapped using a three-tile mosaic, which targets 10 degrees longitude East of the GRS, the centre and 10 degrees West respectively.

 

The MIRI spectral maps will sample the top half of the anticyclone above the vortex midplane, enabling probing of the low temperatures, elevated aerosols, and elevated gaseous abundances that persist within the vortex (Fletcher et al., doi: 10.1016/j.icarus.2010.01.005).  We will derive the vertical aerosol and gaseous structure throughout the 1 mbar (using 7-8 µm spectra) to 5 bar range (using 5-6 µm spectra), complementing the Juno mission investigation of the depth of the GRS at higher pressures (Bolton et al., doi: 10.1126/science.abf1015) The newly inferred properties of the GRS will allow us to better understand the dynamics and longevity of this anticyclone, as well as its interactions with the surrounding Southern Equatorial Belt (SEB). Furthermore, moist convective activity in the surrounding SEB, particularly north-west of the vortex, will also be explored using ammonia as a cloud-forming volatile and phosphine as a tracer of vertical mixing. With sufficient refinement of the atmospheric retrieval process, it may also be possible to search the spectra for signatures of the currently-unidentified red chromophore in Jupiter's belts and GRS.

 

To prepare for these observations, spectral radiative forward models of the GRS were developed using the NEMESIS suite of radiative transfer and spectral inversion software (Irwin et al., doi: 10.1016/j.jqsrt.2007.11.006).  The tool has been adapted to simulate MIRI MRS 5-28 micron spectra using temperature, composition and aerosol data generated from NEMESIS inversions of ground-based Gemini TEXES spectral maps acquired in 2017 (Fletcher et al., doi:10.1029/2020JE006399). These data were used to generate idealised synthetic spectral cubes that were then passed through the MIRISim package to simulate the distortions, transformations, efficiencies, dispersions and noise expected from the MIRI instrument. Different exposure settings and dither patterns were also tested at this stage. The resultant detector images, with spectra dispersed for each slice of the image scene, were subsequently processed using the JWST calibration pipeline. This generated 12 MIRI cubes corresponding to all 12 bands required to cover the full spectral range of MIRI MRS (although the longer-wavelength cubes were typically saturated). 

 

The simulated data were then treated as real cubes from the observatory, allowing us to develop techniques for image navigation and mapping of the mosaics and dither points.  We also tested the possibility of splitting the data into shorter integration periods through alteration of the exposure settings, allowing us to access some of the longer-wavelength regions without saturation.  Spectral retrievals were then used to assess the MIRI capability for mapping the GRS.  These GRS observations are part of a wider programme of JWST giant planet atmosphere observations, including complementary NIRSPEC (1.6-5.3 µm) mapping of the vortex at shorter wavelengths (ERS 1373) as well as global spatial and temporal context mapping by both IRTF/TEXES and VLT/VISIR. In this presentation we will: (i) summarise the science goals of GTO 1246, (ii) discuss the simulation and mapping techniques used to prepare for these observations and (iii) describe the resulting mapping and atmospheric retrieval process to be applied to the real data.