Comparative planetary nitrogen atmospheres: Density and thermal structures of Pluto and Triton

• Published in: Icarus (New York, N.Y. 1962) Vol. 291; pp. 55 - 64
• Main Authors: Strobel, Darrell F., Zhu, Xun
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 15.07.2017
• Subjects: Atmosphere; Atmospheres; Pluto; Radiative transfer; Satellites; Structure; Triton; Satellites; Atmospheres; Atmosphere; Triton; Radiative transfer; Structure; Pluto
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2017.03.013

•1D Pluto atmosphere model with enhanced radiation code applied to Triton's atmosphere.•Confirm magnetospheric heating is necessary to explain Voyager Triton occultation data.•Can explain isothermal Triton stratosphere between 25 and 50km.•CH4 column density differences explain Pluto, Triton different T(z) profiles below 50km.•Hypothesize that H2O, delivered by dust grains, cools Pluto's thermosphere to 70K. Both atmospheres of Pluto and Neptune's largest satellite Triton have cold surfaces with surface gravitational accelerations and atmospheric surface pressures of comparable magnitude. To study their atmospheres we have updated Zhu et al. (2014) model for Pluto's atmosphere by adopting Voigt line profiles in the radiation module with the latest spectral database and extended the model to Triton's atmosphere by including additional parameterized heating due to the magnetospheric electron transport and energy deposition. The CH4 mixing ratio profiles play central roles in differentiating the atmospheres of Pluto and Triton. On Pluto the surface CH4 mole fraction is in the range of 0.3-0.8%, sufficiently high to ensure that it is well mixed in the lower atmosphere and not subject to photochemical destruction. Near the exobase CH4 attains comparable density to N2 due to gravitational diffusive separation and escapes at 500 times the N2 rate (= 1 × 1023 N2s−1). In Triton's atmosphere, the surface CH4 mole fraction is on the order of 0.015%, sufficiently low to ensure that it is photochemically destroyed irreversibly in the lower atmosphere and that N2 remains the major species, even at the exobase. With solar EUV power only, Triton's upper thermosphere is too cold and magnetospheric heating, approximately comparable to the solar EUV power, is needed to bring the N2 tangential column number density in the 500–800km range up to values derived from the Voyager 2 UVS observations (Broadfoot et al., 1989). Due to their cold exobase temperatures relative to the gravitational potential energy wells that N2 resides in, atmospheric escape from Triton and Pluto is not dominated by N2 Jeans escape but by CH4 from Pluto and H, C, N and H2 from Triton. The atmospheric thermal structure near the exobase is sensitive to the atmospheric escape rate only when it is significantly greater than 2×1027amus−1, above which enhanced Jeans escape and larger radial velocity adiabatically cools the atmosphere to a lower temperature. Finally we suggest that Pluto's thermosphere is a cold ∼ 70K due to ablation of H2O molecules from the influx of dust grains detected by New Horizons Student Dust Counter.