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Combined CASSINI VIMS 3.5–5 um and CIRS 7–16 um Spectra of Enceladus Thermal Emission along Damascus Sulcus

Presentation #521.01 in the session Dark Sea: Icy ocean worlds and astrobiology (iPosters).

Published onOct 20, 2022
Combined CASSINI VIMS 3.5–5 um and CIRS 7–16 um Spectra of Enceladus Thermal Emission along Damascus Sulcus

The south pole ‘tiger stripe’ fissures on Enceladus emit plumes of vapor and particles that supply the material forming the Enceladus ring. The fissure walls and surrounding area are heated to T ~ 200 K by this endogenic activity and IR spectra of the thermal radiation emitted by these warmest fissure-adjacent areas are a valuable tool for characterizing the T distribution over area that has implications for the physical mechanisms of heat transfer. During CASSINI’s 228th orbit, in a 9 minute interval on 12/19/2015 begining at 17:44:02.8 UTC, CASSINI’s two infrared spectrometers VIMS and CIRS surveyed the Damascus Sulcus fissure from a distance of approximately 5100 km. This distance was selected as the optimal compromise between the pixel spatial resolution at Enceladus’ surface while keeping smear <0.5 CIRS pixel between exposures as the spectrometers scanned the length of the fissure. Enceladus south pole was deep in winter night with surface temperatures <60 K on this date (Howett et al. EPSC 2022) and passive background thermal emission from pixels not on the fissure was negligible even at the longest 16 um wavelengths used here. This observation sequence is unique for its combination of broad wavelength and fissure length coverage of one of the most active fissures on Enceladus. Spencer et al. (EPSC-DPS2011-1630, 2011) analyzed previous CIRS spectra of Damascus and found that isothermal fits to 7-9 um spectra yield higher T and smaller source area than 9-11 um spectra of the same source which they interpreted as evidence of a range of T within the 0.8 km scale pixel. Here we show that this trend persists for more than 50 of the brightest pixel spectra along Damascus and define more details of this trend by extending it to the 3.5-5 um VIMS wavelengths (fig. 1). In this talk we map the pixel footprints, reconstruct the 3.5-16 um spectra of the radiation emitted by warm sources along the fissure, and present physical heating models capable of reproducing these spectra.

Best fit isothermal temperatures and fill-factors for the brightest fissure pixels data subset. VIMS[3.5–5.2 um], FP4[7–9 um] and FP3[9–11 um] pixels are shown as blue, orange and red symbols, respectively. The larger square symbols are for the bright source at the Y-junction on Damascus Sulcus. A fill-factor of 0.1 corresponds to a square area on Enceladus approximately 0.5 km on a side.

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