This unaltered story was originally published by U.S. NASA Space Agency.
URL:
https://www.nasa.gov
Content is in public domain.
------------
SOFIA Observations of Variability in Jupiter's Para-H2 Distribution and Subsurface Emission Characteristics of the Galilean Satellites
['Imke De Pater', 'Department Of Astronomy', 'University Of California', 'Campbell Hall', 'Berkeley', 'Ca', 'Http', 'Leigh N. Fletcher', 'School Of Physics', 'Astronomy']
Date: 2022-06
We observed Jupiter at infrared wavelengths (8–37 μm) on 2018 August 22 and 29 (θ s = −3 5, L s = 279°) and on 2019 July 11 (θ s = −2 9, L s = 304°) with FORCAST (Adams et al. 2010; Herter et al. 2012) on the SOFIA airborne observatory (Gehrz et al. 2009; Young et al. 2012). Both sets of observations were acquired after southern summer solstice (L s = 270°, 2018 May), meaning that despite Jupiter's small 3° axial tilt, they afford our best views of the southern high latitudes. Both images (8–37 μm) and spectral data cubes (17–37 μm) were obtained. We compare these new data with previous SOFIA/FORCAST data from 2014 May (L s = 158°, Fletcher et al. 2017), which occurred shortly after northern summer solstice (L s = 90°, 2012 March) and therefore provided better views of the northern high latitudes. The new "southern summer" epochs will be contrasted with the "northern summer" epoch to search for any variations in atmospheric properties.
Calibration of the spectroscopic data is best performed using images in overlapping filters, as done by Fletcher et al. (2017). We therefore acquired images in 7 different filters, as tabulated in Table 1. Ideally, images and spectra are taken during the same flight. Although a full set of images was obtained on 2018 August 22, no spectra were taken that night due to telescope problems. In addition to the F111 acquisition images (Table 1), images were taken in only two additional filters on 2018 August 29, and none were obtained on 2019 July 11. The images were reduced and calibrated by the SOFIA Science Center using REDUX v1.3.1. 7 SOFIA/FORCAST is approximately diffraction limited at 24 μm and longer wavelengths, with a FWHM or the width of the point-spread function (PSF) in arcsec equal to a tenth of the wavelength in microns, while at the shorter wavelengths observed in this project the FWHM is 2 4. 8 The pixel spacing is 0 768 which oversamples the PSF for accurate imaging.
Table 1. SOFIA/FORCAST Images of Jupiter UT Date Filter Wavelength Wavenumber Bandwidth Diameter CML a Latitude Flux Density b T B c yr-month-date hr:m:s Name (μm) (cm−1) (μm) ('') Sys.III (°) Planetographic (°) Total (kJy) (K) 2018-08-22 04:28:10 F088 8.800 1136.364 0.41 35.69 226 −3.36 12.2 ± 0.4 (0.23) 141.4 ± 0.4 2018-08-22 04:36 F111 11.089 901.795 0.954 35.69 231 −3.36 24.0 ± 0.7 (0.26) 127.3 ± 0.4 2018-08-22 04:20:40 F197 19.712 507.305 5.506 35.69 222 −3.36 245 ± 7 (4.1) 118.7 ± 0.6 2018-08-22 04:24:40 F253 25.248 396.071 1.807 35.69 225 −3.36 464 ± 15 (10) 119.3 ± 0.8 2018-08-22 04:16:40 F315 31.457 317.894 5.655 35.69 219 −3.36 649 ± 23 (16) 120.7 ± 1.2 2018-08-22 04:20:40 F348 34.807 287.299 3.759 35.69 222 −3.36 814 ± 28 (19) 126.2 ± 1.3 2018-08-22 04:24:40 F371 37.144 269.222 3.284 35.69 225 −3.36 976 ± 32 (20) 132.8 ± 1.5 2018-08-29 03:42 F111 11.089 901.795 0.954 35.04 172 −3.35 22.6 ± 0.6 (0.25) 126.9 ± 0.3 2018-08-29 03:27:40 F253 25.248 396.071 1.807 35.04 162 –3.35 468 ± 16 10) 120.5 ± 0.9 2018-08-29 03:27:40 F348 34.807 287.299 3.759 35.04 162 –3.35 817 ± 28 (20) 127.7 ± 1.3 2019-07-11 07:01:00 F111 11.089 901.795 0.954 44.75 317 –3.09 45.8 ± 1.5 (0.9) 129.7 ± 0.4 Notes. a Central Meridian Longitude, measured in System III, West longitude. b The error includes a 2.5% uncertainty in the determination of the flux density from the images, as based on 8–12 images in F111 taken on 2018 August 22 and 29, augmented by the absolute calibration uncertainty as provided by the SOFIA analysts; the latter quantity (typically between 1 and <2.5%) is provided in brackets. c Disk-averaged brightness temperature. Central Meridian Longitude, measured in System III, West longitude. The error includes a 2.5% uncertainty in the determination of the flux density from the images, as based on 8–12 images in F111 taken on 2018 August 22 and 29, augmented by the absolute calibration uncertainty as provided by the SOFIA analysts; the latter quantity (typically between 1 and <2.5%) is provided in brackets. Disk-averaged brightness temperature. Download table as: ASCIITypeset image
Contrast-enhanced images of Jupiter are shown in Figure 1, and images showing the various satellites are shown in Figure 2. Images are background subtracted and a limb-darkening correction was applied. The background was modeled using the SExtractor program suite (Bertin & Arnouts 1996). SExtractor models the background by dividing the image into a grid specified by the user, then uses sigma-filtering to mask pixels containing a signal from sources and artefacts in the image. The remaining unmasked data is used to calculate a smooth background with a user-specified granularity. Successive iterations of SExtractor were performed to mask, model, and subtract the non-uniform background signal and image artefacts, as well as identify the location and extent of Jupiter and its satellites. An example for the 34.8 μm image is shown in Figure 3. Limb-darkening was corrected via a custom algorithm designed to enhance the contrast of Jupiter's features. No correction was performed for the 8.8 and 25.3 μm images, as we determined their natural contrast sufficient to see Jupiter's structure. The FORCAST contribution function probes different altitudes as a function of wavelength, and a guide to the wavelength dependence of the sensitivity is given in Figure 6 of Fletcher et al. (2017). Images in Figure 1 mostly sample the collision-induced H 2 –H 2 and H 2 –He continuum (longward of 17 μm), sensing temperature and para-H 2 in a range between 300 and 600 mbar in nadir viewing. Subdued contrast at the longest wavelengths is a consequence of the diffraction-limited PSF. These images are complemented by images at 8.8 and 11.1 μm, which both sense a combination of temperatures, ammonia gas, and aerosol absorption in the 400–600 mbar region.
Figure 1. SOFIA images of Jupiter at all wavelengths and days during the flights, as indicated. Although several images were taken at 11.1 μm, we only show the stacked images here. The images are contrast enhanced sufficiently to demonstrate variations in brightness temperature with latitude and longitude. Jupiter north is up in all images. The GRS and Ganymede are indicated. Download figure: Standard image High-resolution image
Figure 2. SOFIA images from Figure 1 which show the nearby satellites. Jupiter itself is saturated in the image display scale to bring out the fainter satellites. Beyond the disk of Jupiter is a halo of scattered light from the telescope, plus diagonal striations due to the detector and electronics at long wavelengths. These images show the challenge of determining accurate flux densities near Jupiter. Jupiter north is up in all images. Download figure: Standard image High-resolution image
Figure 3. Example of the background subtraction applied to the images. (a) Original 34.8 μm image, after subtraction of the average frames taken to the north and south of Jupiter. The white circles show the approximate outline of Jupiter in the negative frames due to the symmetric chop-nod observing sequence on the telescope. (b) Background is determined as described in the text. (c) Final background-subtracted image. The colorscale was chosen such as to highlight the background. Min/max values (Jy pixel−1) in panel (a) are −17/+34.6. Download figure: Standard image High-resolution image
The belt-zone structure on the planet is clearly recognized at low latitudes in all images, with warm belts and colder zones, caused by latitudinal temperature variations near the ∼500 mbar level (e.g., Fletcher et al. 2016a). Similarly, the longitudinal variations in brightness can also be explained as variations in temperature at these levels. On 2018 August 29 and 2019 July 11 the Great Red Spot (GRS) is visible as a cold anomaly at 11 μm as indicated. The GRS is cold due to adiabatic cooling of the rising gas, while we see hints of warmer temperatures at its southern edge, as shown at a higher spatial resolution by e.g., Fletcher et al. (2016a). A relatively warm spot is visible at 11.1–32 μm in the far north on 2018 August 22, near 220–230°W longitude in System III; this is the satellite Ganymede, transiting Jupiter during the observing run. Bright south polar auroral emission is observed in the 11.1 μm image on July 11, from the contribution of ethylene near 10.5 μm (e.g., Sinclair et al. 2019).
We determined the disk-integrated flux densities from each image by first carefully modeling the background over the entire image, as described previously, so after subtraction the background was essentially zero. We used SExtractor to determine which pixels contained emission from Jupiter or the Galilean satellites. As much of the background flux is caused by scattered light from Jupiter, we determined the Jovian flux densities from images before background subtraction (e.g., as in Figure 3(a)), and subtracted a background determined a large distance away from the planet. The flux density determined this way was of order 1%–2.5% (depending on wavelength) larger than the numbers determined directly from the background-subtracted images (Figure 3(c)). The results for Jupiter are listed in Table 1, where we show both the total flux densities and the derived disk-averaged brightness temperatures. We derived the latter ones from the flux densities using Planck's radiation law for an oblate planet, using an effective radius of 69,134 km (i.e., ). As we had a dozen images in the F111 filter on 2018 August 22 and another eight images on August 29 (only 2 images in 2019), we were able to estimate an uncertainty of ∼2.5% from the dispersion in the measured flux densities for Jupiter across these images. We assumed the same error in all other filters, and added the absolute calibration error in quadrature to obtain the uncertainties as listed in Table 1. The absolute calibration error was provided by the SOFIA Science Center, using the dispersion in the ratio of observed-to-reference fluxes of standard stars, where the reference fluxes are stellar atmosphere models scaled to match observed fluxes (Dehaes et al. 2011). This uncertainty was typically between 1% and <2.5%, depending on the filter used.
Several of the Galilean satellites are visible in the images, as shown. We were able to determine the flux densities for all four satellites in a few filters, as tabulated in Table 2 together with the derived disk-averaged brightness temperatures. The uncertainty in the flux densities were estimated from a combination of the dispersion in the F111 values and the estimated accuracy of the background subtraction. We typically estimated the uncertainty at 10%. As shown in Figure 2, at the long (≳30 μm) wavelengths the diagonal striations sometimes interfere with the satellites, which makes extraction of accurate flux densities quite challenging. In these cases, as well as for the relatively faint Europa, we adopted an uncertainty of 25%. The brightness temperature values are shown in Figure 4 together with previous data to show the SOFIA values in context with the satellites' full thermal emission spectra. The visible-light temperature (open blue squares) are maximum temperatures calculated based upon the Bond albedo and phase integral of the satellites (Morrison & Morrison 1977; Johnson 1978). We note that the measured brightness temperatures, in particular at the shortest wavelengths, may differ some from one another due to variations in heliocentric distance, and there may also be hemispheric (such as leading versus trailing) differences. However, given these potential differences, our calibration is essentially validated by the agreement between the shortest-wavelength SOFIA measured brightness temperatures and previous measurements.
Figure 4. Brightness temperature plots for the four Galilean satellites. The red data points are the SOFIA data from Table 2, and which are shown in Figure 2. The cyan data points are from ALMA (Io: de Pater et al. 2020a), (Ganymede: de Kleer et al. 2021), and the black data points are older measurements (taken from: Berge & Muhleman 1975; Ulich & Conklin 1976; Morrison & Morrison 1977; Pauliny-Toth et al. 1977; Ulich 1981; de Pater et al. 1982, 1984, 1989; Ulich et al. 1984; Muhleman et al. 1986; Muhleman & Berge 1991; Moreno 2007; Butler 2012; Müller et al. 2016). The blue open squares are the expected maximum surface temperatures at visible wavelengths (from: Morrison & Morrison 1977; Johnson 1978). Download figure: Standard image High-resolution image
[END]
[1] Url:
https://iopscience.iop.org/article/10.3847/PSJ/ac2d24
Published by NASA.Gov
via Magical.Fish Gopher News Feeds:
gopher://magical.fish/1/feeds/news/nasa/
