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The Atmosphere of Pluto As Observed by New Horizons

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The Atmosphere of Pluto As Observed by New Horizons RESEARCH ◥ has a bluish color, suggesting a composition of RESEARCHARTICLESUMMARY very small particles. The observed scattering prop- erties of the haze are consistent with a tholin-like composition. Buoyancy waves generated by winds PLANETARY SCIENCE flowing over orography can produce vertically propagating compression and rarefaction waves that may be related to the narrow haze layers. The atmosphere of Pluto as Pluto’s cold upper atmosphere means atmo- spheric escape must occur via slow thermal observed by New Horizons Jeans’ escape. The inferred escape rate of nitro- gen is ~10,000 times slower than predicted, G. Randall Gladstone,* S. Alan Stern, Kimberly Ennico, Catherine B. Olkin, whereas that of methane is about the same Harold A. Weaver, Leslie A. Young, Michael E. Summers, Darrell F. Strobel, as predicted. The low nitrogen loss rate is con- David P. Hinson, Joshua A. Kammer, Alex H. Parker, Andrew J. Steffl, Ivan R. Linscott, sistent with an undetected Charon atmosphere Joel Wm. Parker, Andrew F. Cheng, David C. Slater,† Maarten H. Versteeg, but possibly inconsistent with sublimation/ Thomas K. Greathouse, Kurt D. Retherford, Henry Throop, Nathaniel J. Cunningham, erosional features seen on Pluto’s surface, so William W. Woods, Kelsi N. Singer, Constantine C. C. Tsang, Eric Schindhelm, that past escape rates may have been much Carey M. Lisse, Michael L. Wong, Yuk L. Yung, Xun Zhu, larger at times. Capture of escaping methane Werner Curdt, Panayotis Lavvas, Eliot F. Young, G. Leonard Tyler, and photochemical products by Charon, and the New Horizons Science Team subsequent surface chemical reactions, may con- tribute to the reddish color of its north pole. INTRODUCTION: For several decades, tele- and egress, as expected from sublimation driven scopic observations have shown that Pluto has dynamics, so horizontal winds are expected CONCLUSION: New Horizons observations a complex and intriguing atmosphere. But too to be weak. The shallow near-surface boundary have revolutionized our understanding of Pluto’s little has been known to layer observed at ingress may arise directly atmosphere. The observations revealed major ◥ ON OUR WEB SITE allow a complete under- from sublimation. surprises, such as the unexpectedly cold upper standing of its global struc- The Alice solar occultation showed absorption Read the full article atmosphere and the globally extensive haze at http://dx.doi. ture and evolution. Major by methane and nitrogen and revealed the pres- layers. The cold upper atmosphere implies much org/10.1126/ goals of the New Horizons ence of the photochemical products acetylene lower escape rates of volatiles from Pluto than science.aad8866 mission included the char- and ethylene. The observed nitrogen opacity at predicted and so has important implications on March 17, 2016 .................................................. acterization of the struc- high altitudes was lower than expected, which for the volatile recycling and the long-term evo- ture and composition of Pluto’s atmosphere, is consistent with a cold upper atmosphere. lution of Pluto’s atmosphere. as well as its escape rate, and to determine Such low temperatures imply an additional, ▪ whether Charon has a measurable atmosphere. but as yet unidentified, cooling agent. The list of author affiliations is available in the full article online. A globally extensive haze extending to high al- *Corresponding author. E-mail: [email protected] †Deceased. RATIONALE: The New Horizons spacecraft in- titudes, and with numerous embedded thin lay- Cite this article as G. R. Gladstone et al., Science 351, cluded several instruments that observed Pluto’s ers, is seen in the New Horizons images. The haze aad8866 (2016). DOI: 10.1126/science.aad8866 atmosphere, primarily (i) the Radio Downloaded from on March 17, 2016 Experiment (REX) instrument, which produced near-surface pres- sure and temperature profiles; (ii) the Alice ultraviolet spectro- graph, which gave information on atmospheric composition; and (iii) the Long Range Reconnaissance Imager (LORRI) and Multispectral Visible Imaging Camera (MVIC), which provided images of Pluto’s hazes. Together,these instruments have provided data that allow an understanding of the current state of Pluto’s atmosphere and its evolution. RESULTS: The REX radio occul- tation determined Pluto’s sur- face pressure and found a strong temperature inversion, both of which are generally consistent with atmospheric profiles retrieved from Earth-based stellar occul- MVIC image of haze layers above Pluto’s limb. About 20 haze layers are seen from a phase angle of 147°. tation measurements. The REX The layers typically extend horizontally over hundreds of kilometers but are not exactly horizontal. For ex- data showed near-symmetry be- ample, white arrows on the left indicate a layer ~5 km above the surface, which has descended to the surface tween the structure at ingress at the right. 1280 18 MARCH 2016• VOL 351 ISSUE 6279 sciencemag.org SCIENCE RESEARCH ◥ properties of Pluto’s hazes are derived from RESEARCHARTICLE LORRI and MVIC images. This Research Article provides an overview of atmosphere science results. A suggested atmosphere around Pluto (6–11) PLANETARY SCIENCE was confirmed by means of ground-based stellar occultation in 1988 (12, 13) and subsequently studied with later occultations (14–16) and spectra The atmosphere of Pluto as observed at near-infrared and microwave wavelengths (17, 18) and with models of increasing sophisti- by New Horizons cation. These results revealed a primarily N2 at- mosphere with trace amounts of CH4, CO, and HCN with complex surface interaction, an un- G. Randall Gladstone,1,2* S. Alan Stern,3 Kimberly Ennico,4 Catherine B. Olkin,3 5 3 6 7 certain surface pressure of ~3 to 60mbar, and a Harold A. Weaver, Leslie A. Young, Michael E. Summers, Darrell F. Strobel, warm stratosphere at ~100 K above a much 8 3 3 3 David P. Hinson, Joshua A. Kammer, Alex H. Parker, Andrew J. Steffl, colder surface (38 to 55 K). On the eve of the 9 3 5 1 Ivan R. Linscott, Joel Wm. Parker, Andrew F. Cheng, David C. Slater, † New Horizons flyby, critical questions remained 1 1 1,2 7 Maarten H. Versteeg, Thomas K. Greathouse, Kurt D. Retherford, Henry Throop, about the atmospheric temperature and pressure Nathaniel J. Cunningham,10 William W. Woods,9 Kelsi N. Singer,3 profiles, dynamics, the presence and nature of pos- Constantine C. C. Tsang,3 Eric Schindhelm,3 Carey M. Lisse,5 Michael L. Wong,11 sible clouds or hazes, the escape of Pluto’s atmo- Yuk L. Yung,11 Xun Zhu,5 Werner Curdt,12 Panayotis Lavvas,13 Eliot F. Young,3 sphere, and possible interactions with its large G. Leonard Tyler,9 the New Horizons Science Team‡ moon, Charon. The New Horizons flyby (19) en- abled us to address these questions using radio Observations made during the New Horizons flyby provide a detailed snapshot of the occultations, ultraviolet occultations, and imag- current state of Pluto’s atmosphere. Whereas the lower atmosphere (at altitudes of less ing at several phase angles between 15° and 165°. than 200 kilometers) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Pressure and temperature Molecular nitrogen (N2) dominates the atmosphere (at altitudes of less than 1800 kilometers The New Horizons trajectory was designed to or so), whereas methane (CH4), acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) permit nearly simultaneous radio and solar occul- are abundant minor species and likely feed the production of an extensive haze that tations (20). The radio occultation was implemented encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, in an uplink configuration by using 4.2-cm- hydrodynamic-like escape of Pluto’s atmosphere to space. It is unclear whether the current wavelength signals transmitted by antennas of state of Pluto’s atmosphere is representative of its average state—over seasonal or geologic the NASA Deep Space Network and received by time scales. the REX instrument onboard New Horizons (2). The spacecraft passed almost diametrically be- ajor goals of the New Horizons mission New Horizons to Pluto at 11:48 UT on 14 July hind Pluto as viewed from Earth, with ingress at were to explore and characterize the 2015. Pressure and temperature profiles of the sunset near the center of the anti-Charon hemi- structure and composition of Pluto’s lower atmosphere are derived from the REX data, sphere and egress at sunrise near the center of atmosphere and to determine whether the composition and structure of the extended the Charon-facing hemisphere. Other character- M Charon has a measurable atmosphere atmosphere are derived from the Alice data (sup- istics of the REX observation are listed in table S1. of its own (1). Several instruments contribute ported by approach observations of reflected The location of Pluto’s surface is indicated by to these goals, primarily (i) the Radio Experiment ultraviolet sunlight), and the distribution and a characteristic diffraction pattern in the REX (REX) instrument (2), through uplink X-band radio occultations; (ii) the Alice instrument (3), 50 through extreme- and far-ultraviolet solar occul- tations; and (iii) the Long Range Reconnaissance Imager (LORRI) and Multispectral Visible Imaging 40 Camera (MVIC) (4, 5), through high-phase-angle imaging. The associated data sets were obtained 30 within a few hours of the closest approach of 20 1Southwest Research Institute, San Antonio, TX 78238, USA. Altitude (km) 2University of Texas at San Antonio, San Antonio, TX 78249, USA. 3Southwest Research Institute, Boulder, CO 80302, 10 USA. 4National Aeronautics and Space Administration, Ames Research Center, Space Science Division, Moffett Field, CA 5 94035, USA. The Johns Hopkins University Applied Physics 0 6 Laboratory, Laurel, MD 20723, USA. George Mason 3 5 7 9 11 40 60 80 100 120 7 University, Fairfax, VA 22030, USA. The Johns Hopkins Pressure (microbar) Temperature (K) University, Baltimore, MD 21218, USA.
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