Research Article

The atmosphere of Pluto as observed by New Horizons

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Science  18 Mar 2016:
Vol. 351, Issue 6279, aad8866
DOI: 10.1126/science.aad8866

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  1. MVIC image of haze layers above Pluto’s limb.

    About 20 haze layers are seen from a phase angle of 147°. The layers typically extend horizontally over hundreds of kilometers but are not exactly horizontal. For example, white arrows on the left indicate a layer ~5 km above the surface, which has descended to the surface at the right.

  2. Fig. 1 Pressure and temperature in Pluto’s lower atmosphere.

    (Left) Pressure. (Right) Temperature. These profiles were retrieved from radio occultation data recorded by the REX instrument onboard New Horizons. Diffraction effects were removed from the data (53), which greatly improves the accuracy of the results, and the conventional “Abel-transform” retrieval algorithm (2, 54, 55) was applied to the diffraction-corrected phase measurements. Each graph shows results at both entry (red line with circles) and exit (blue line with triangles), situated on opposite sides of Pluto. The profiles are most accurate at the surface, where the uncertainties in pressure and temperature are ~1 μbar and 3 K, respectively. Temperature fluctuations at altitudes of >20 km are caused by noise; no gravity waves were detected at the sensitivity of these measurements. The dashed line indicates the saturation temperature of N2 (29).

  3. Fig. 2 Ultraviolet transmission of Pluto’s atmosphere.

    (A) Line-of-sight (LOS) transmission as a function of ultraviolet wavelength and tangent altitude for the M2 model Pluto atmosphere (37), with the τ =1 line indicated along with the regions where N2, CH4, C2H2, C2H4, and C2H6 contribute to the opacity. N2 absorbs in discrete bands for wavelengths 80 to 100 nm, with bands and an underlying continuum at wavelengths 65 to 80 nm and an ionization continuum at wavelengths of <65 nm. CH4 dominates the opacity at wavelengths of <140 nm. C2H6 has a similar cross section to CH4 but absorbs to 145 nm, where it contributes to the opacity. C2H2 has strong absorption bands at 144, 148, and 152 nm. C2H4 dominates the opacity at 155 to 175 nm. The model also contains C4H2, which accounts for much of the opacity at wavelengths 155 to 165 nm. (B) LOS transmission of Pluto’s atmosphere determined from the Alice solar occultation data. The Alice data are normalized (at each ultraviolet wavelength) to unabsorbed levels at high altitude. In comparison with the model transmission, N2 opacity begins at much lower altitudes (~500 km lower), whereas CH4 opacity begins ~100 km higher than in the model. Pluto’s atmosphere has somewhat less C2H2 and C2H4 than the model. Continuum absorption by Pluto’s haze (not included in the model) is important at wavelengths >175 nm. (C) LOS column density profiles retrieved from the observed transmission data of (B) using known absorption cross sections for the indicated species. The quality of the data can be judged by the overlap of ingress and egress profiles (because the atmosphere is expected to be nearly spherically symmetric away from the surface) and by the amount of scatter in the data points. The dashed lines are LOS column densities computed by using the N2 and CH4 number density profiles in Fig. 3.

  4. Fig. 3 Pluto’s atmospheric composition and structure.

    Model profiles of temperatures, densities, and other relevant quantities (such as gravity g, mean mass Embedded Image, and N2 density scale height H—plotted as H/2 in order to facilitate a common x axis range) in the atmosphere of Pluto are shown, which are consistent with the transmission results of Fig. 2. Methane, acetylene, and ethylene densities retrieved from the solar occultation data are indicated (diamonds). Pre-encounter model values (37) are given by dashed lines. Pluto’s upper atmosphere is very cold (T ~ 70 K), resulting in a very low escape rate.

  5. Fig. 4 Pluto hazes.

    LORRI two-image stack at 0.95 km pixel−1 resolution, showing many haze layers up to an altitude of ~200 km, as well as night-side surface illumination. Acquired on 14 July 2015 starting at 15:45:43 UTC (observation 5 of P_MULTI_DEP_LONG_1 at MET 299194661-299194671; 0.3 s total exposure time), at a range from Pluto of 196,586 km and a phase angle of 169°. The raw images have been background-subtracted and sharpened and have a square root stretch. (Inset) The orientation of the image, with Pluto’s south pole (SP) indicated, along with the direction to the Sun (11° from Pluto), and the latitude and longitude of the sub–anti-Sun (AS) position.

  6. Fig. 5 Haze layer production.

    Haze particles undergo vertical displacements ζ by vertical gravity wave parcel velocity w′, which at saturation equals wg, the vertical group velocity. Because w′ is much larger than the sedimentation velocity, compression and rarefaction of haze particle densities are associated with gravity wave displacements. The quantity η = (½λZ + 2ζ)/(½λZ – 2ζ) is a measure of compaction and layering.

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