Triton: Atmosphere and Surface Observed with ALMA and Comparison with Pluto
Abstract
Our Atacama Large Millimeter/Submillimeter Array (ALMA) observations of the Pluto-Charon system confirmed the low Pluto mm/submm surface brightness temperature (TB=33 K, [1]), measured atmospheric thermal structure to 400 km altitude, measured the CO abundance, and detected for the first time HCN, in extreme supersaturation in the upper atmosphere ([2], [3], [4]). They led to higher resolution ALMA studies of Pluto's atmosphere [5], and surface [6], and also motivated this new study of the atmosphere and surface of Triton using the same techniques. Observations of Triton were obtained October 25 and 26, 2016 using ALMA for a total of 150 minutes on-source. At the time 40 ALMA 12-m antennas were available, providing baselines ranging from 19 m to 1.4 km. During the observations Triton ranged between 13.6" and 16.6" from Neptune, and was just over 127 mas in diameter. The spectral coverage was nearly identical to the 2015 observations of Pluto, allowing 60 kHz spectral resolution covering CO(3-2), HCN(4-3) and two HC3N transitions, coarser observations of HCN isotopologues, and a thermal continuum band at 355.6 GHz. The FWHM of the ALMA primary beam is about 18" at these frequencies; thus Neptune ( 1000 times the flux density of Triton) was not fully excluded, and significant sidelobes affected the imaging. Using a spatial high-pass visibility filter (using spatial frequencies >400 kλ), we effectively eliminated confusion from Neptune, providing high dynamic range imaging of Triton at 155 mas x 122 mas resolution. The thermal continuum of Triton was 28.1±0.2 mJy at 355.6 GHz, equivalent to a mean TB of 32.0+/-0.2 K (formal error, with absolute flux scale error 5%, or 1.6 K). This is within the error of the TB of Pluto measured in 2015 ([1],[6]). We also report the first mm/submm detection of CO in Triton's atmosphere. We will present a full analysis of the CO line emission, as well as limits on HCN and HC3N, and compare and contrast to the similar observations of Pluto from 2015. [1] Butler et al 2015, DPS #47, id.210.04. [2] Lellouch et al 2017, Icarus 286, 289 [3] Gurwell et al 2015, DPS #47 id.105.06 [4] Lellouch et al 2015, DPS #47, id.105.07, [5] Lellouch et al, this meeting, [6] Butler et al, this meeting
- Publication:
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AAS/Division for Planetary Sciences Meeting Abstracts #50
- Pub Date:
- October 2018
- Bibcode:
- 2018DPS....5050207G