The atmosphere of Pluto as observed by New Horizons

Abstract

Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Pluto's atmosphere. While the lower atmosphere (at altitudes <200 km) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N 2 ) dominates the atmosphere (at altitudes <1800 km or so), while methane (CH 4 ), acetylene (C 2 H 2 ), ethylene (C 2 H 4 ), and ethane (C 2 H 6 ) are abundant minor species, and likely feed the production of an extensive haze which encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Pluto's atmosphere to space. It is unclear whether the current state of Pluto's atmosphere is representative of its average state-over seasonal or geologic time scales.

Pluto's interaction with the solar wind

“…That location was past closest approach along Pluto's dawn flank, and these authors further estimated that this amount of slowing might occur~6 R P directly upstream from Pluto. This indicates a smaller obstacle than anticipated, consistent with the lower than previously expected atmospheric escape rates, which are currently estimated to be~1 × 10 23 N 2 s À1 and~5 × 10 25 CH 4 s À1 from NH occultation measurements [Gladstone et al, 2016]. We note that these values are still highly uncertain with estimated uncertainties of about an order of magnitude for N 2 and a factor of 3-4 for CH 4 .…”

“…Ultimately, we hope to be able to use SWAP data to differentiate between CH 4 and N 2 , which are expected to have the highest escape rates. Here we calculate the heavy ion moments ( Figure 6, thick lines) assuming that all ions are CH 4 , which are currently expected to dominate N 2 [Gladstone et al, 2016] and numerically integrate the observed distributions. We assume that the peak of a symmetric distribution is in the SWAP FOV and the 13.3 km s À1 tailward motion of NH is accounted for.…”

“…If so, the heavy ion ratios and densities would be larger by the inverse of whatever fraction of their momentum was already effectively coupled to the flow. We also note that the heavy/light ion ratios and heavy ion densities are based on the assumption that the escaping ions are CH 4 + as indicated by recent atmospheric escape models [Gladstone et al, 2016]; if it turns out that N 2 + or some other species dominates, the numbers would need to be adjusted by the ions' relative mass ratio. The SWAP observations provide very strong constraints on the solar wind's interaction with Pluto, and any models seeking to emulate this interaction will need to quantitatively match the slowing and exclusion at all of the points given in Table 1.…”

“…For the upstream solar wind at the time of the NH flyby (403 km s À1 and 0.025 cm À3 ) the dynamic pressure is~6 pPa. Postencounter estimates of the ionosphere of Pluto by coauthor D. F. S. based on atmospheric model in Gladstone et al [2016] and considerations in Cravens and Strobel [2015] indicate a maximum density of~1500 cm À3 at about 700 km altitude or 1.6 R P from the center of the Pluto. These numbers are also consistent with atmosphere/ionosphere models [Lara et al, 1997;Ip et al, 2000].…”

Impacts of Cosmic Dust on Planetary Atmospheres and Surfaces

“…Both atmospheres also contain other minor species, such as CH 4 , CO, and various hydrocarbons Gladstone et al 2016), and both atmospheres are observed to have clouds and/or haze layers (Yelle et al 1991(Yelle et al , 1995. At Triton, observations of ion layers below the main ionospheric peak could be evidence of the presence of metallic ions introduced from meteoric ablation (cf.…”