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.

The explosive BN/KL outflow emerging from OMC1 behind the Orion Nebula may have been powered by the dynamical decay of a non-hierarchical multiple system ∼500 years ago that ejected the massive stars I, BN, and source n, with velocities of about 10 to 30 km s −1 . New proper motion measurements of H 2 features show that within the errors of measurement, the outflow originated from the site of stellar ejection. Combined with published data, these measurements indicate an outflow age of ∼500 years, similar to the time since stellar ejection. The total kinetic energy of the ejected stars and the outflow is about 2 to 6 × 10 47 ergs. It is proposed that the gravitational potential energy released by the formation of a short-period binary, most likely source I, resulted in stellar ejection and powered the outflow. A scenario is presented for the formation of a compact, non-hierarchical multiple star system, its decay into an ejected binary and two high-velocity stars, and launch of the outflow. Three mechanisms may have contributed to the explosion in the gas: (i) Unbinding of the circumcluster envelope following stellar ejection, (ii) disruption of circumstellar disks and high-speed expulsion of the resulting debris during the final stellar encounter, and (iii) the release of stored magnetic energy. Plausible proto-stellar disk end envelope properties can produce the observed outflow mass, velocity, and kinetic energy distributions. The ejected stars may have acquired new disks by fallback or Bondi-Hoyle accretion with axes roughly orthogonal to their velocities. The expulsion of gas and stars from OMC1 may have been driven by stellar interactions.

Ghebregziabher, I.; Maharjan, C.; Liu, Cheng; Golovin, Grigory V.; Banerjee, Sudeep; Zhang, J.; Cunningham, N.; Moorti, A.; Clarke, S.; and Pozzi, Sara, "MeV-Energy X Rays from Inverse Compton Scattering with Laser-Wakefield Accelerated Electrons" (2013). Donald Umstadter Publications. 87.

The European Space Agency's Rosetta spacecraft, en route to a 2014 encounter with comet 67P/Churyumov-Gerasimenko, made a gravity assist swingby of Mars on 25 February 2007, closest approach being at 01:54 UT. The Alice instrument on board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for in situ cometary spectroscopy in the 750-2000Å spectral band, was used to study the daytime Mars upper atmosphere including emissions from exospheric hydrogen and oxygen. Offset pointing, obtained five hours before closest approach, enabled us to detect and map the HI Lyman-α and Lyman-β emissions from exospheric hydrogen out beyond 30,000 km from the planet's center. These data are fit with a Chamberlain exospheric model from which we derive the hydrogen density at the 200 km exobase and the H escape flux. The results are comparable to those found from the the Ultraviolet Spectrometer experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen emission at 1304Å is detected at altitudes of 400 to 1000 km above the limb during limb scans shortly after closest approach. However, the derived oxygen scale height is not consistent with recent models of oxygen escape based on the production of suprathermal 1 oxygen atoms by the dissociative recombination of O + 2 .

The Alice instrument on NASA's New Horizons spacecraft observed an ultraviolet solar occultation by Pluto's atmosphere on 2015 July 14. The transmission vs. altitude was sensitive to the presence of N 2 , CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , and haze. We derived line-of-sight abundances and local number densities for the 5 molecular species, and line-of-sight optical depth and extinction coefficients for the haze. We found the following major conclusions: (1) We confirmed temperatures in Pluto's upper atmosphere that were colder than expected before the New Horizons flyby, with upper atmospheric temperatures near 65-68 K. The inferred enhanced Jeans escape rates were (3-7) x 10 22 N 2 s -1 and (4-8) x 10 25 CH 4 s -1 at the exobase (at a radius of ~ 2900 km, or an altitude of ~1710 km). (2) We measured CH 4 abundances from 80 to 1200 km above the surface. A joint analysis of the Alice CH 4 and Alice and REX N 2 measurements implied a very stable lower atmosphere with a small eddy diffusion coefficient, most likely between 550 and 4000 cm 2 s -1 . Such a small eddy diffusion coefficient placed the homopause within 12 km of the surface, giving Pluto a small planetary boundary layer. The inferred CH 4 surface mixing ratio was ~ 0.28-0.35%. (3) The abundance profiles of the "C 2 H x hydrocarbons" (C 2 H 2 , C 2 H 4 , C 2 H 6 ) were not simply exponential with altitude. We detected local maxima in line-of-sight abundance near 410 km altitude for C 2 H 4 , near 320 km for C 2 H 2 , and an inflection point or the suggestion of a local maximum at 260 km for C 2 H 6 . We also detected local minima near 200 km altitude for C 2 H 4 , near 170 km for C 2 H 2 , and an inflection point or minimum near 170-200 km for C 2 H 6 . These compared favorably with models for hydrocarbon production near 300-400 km and haze condensation near 200 km, especially for C 2 H 2 and C 2 H 4 (Wong et al. 2017). (4) We found haze that had an extinction coefficient approximately proportional to N 2 density. This paper extends the analysis of Gladstone et al. (2016) in the following ways: (i) it uses an improved reduction of the raw observations, and includes more details about the observation and reduction process, (ii) it presents error analysis, including correlations between the measurements of various species, (iii) it includes analysis of extinction by haze at the long-wavelength end of the Alice range, (iv) it improves or extends the density retrievals of N 2 , CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 and haze, and (v) it includes a joint analysis with new results from the New Horizons radio occultation (Hinson et al. 2017). Observations and ReductionWe recap here the salient features of the Alice ultraviolet spectrograph on the New Horizons spacecraft and its observation of Pluto's atmosphere during the solar occultation. Alice (which is a name, not an acronym) is described in more detail in Stern et al. (2008), FIGURE 1. The Alice Solar Occultation Channel (SOCC), Pluto, and the Sun at the time of solar ingress at 2015 Jul 14 12:44 UT (left) an...