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. 2019 May 29;5(5):eaav5731.
doi: 10.1126/sciadv.aav5731. eCollection 2019 May.

Detection of ammonia on Pluto's surface in a region of geologically recent tectonism

C M Dalle Ore  1   2 , D P Cruikshank  2 , S Protopapa  3 , F Scipioni  1   2 , W B McKinnon  4 , J C Cook  5 , W M Grundy  6 , B Schmitt  7 , S A Stern  3 , J M Moore  2 , A Verbiscer  8 , A H Parker  3 , K N Singer  3 , O M Umurhan  1   2 , H A Weaver  9 , C B Olkin  3 , L A Young  3 , K Ennico  2
Affiliations

Detection of ammonia on Pluto's surface in a region of geologically recent tectonism

C M Dalle Ore et al. Sci Adv. .

Abstract

We report the detection of ammonia (NH3) on Pluto's surface in spectral images obtained with the New Horizons spacecraft that show absorption bands at 1.65 and 2.2 μm. The ammonia signature is spatially coincident with a region of past extensional tectonic activity (Virgil Fossae) where the presence of H2O ice is prominent. Ammonia in liquid water profoundly depresses the freezing point of the mixture. Ammoniated ices are believed to be geologically short lived when irradiated with ultraviolet photons or charged particles. Thus, the presence of NH3 on a planetary surface is indicative of a relatively recent deposition or possibly through exposure by some geological process. In the present case, the areal distribution is more suggestive of cryovolcanic emplacement, however, adding to the evidence for ongoing geological activity on Pluto and the possible presence of liquid water at depth today.

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Figures

Fig. 1
Fig. 1. Distribution of red-tinted H2O ice exhibiting the spectral signature of NH3 in Virgil Fossae and surrounding terrain.
(A) Pluto’s encounter hemisphere as seen by New Horizons during the 14 July 2015 flyby. (B) Selected ROI in the MVIC image illustrating the uniquely bright red coloring of Virgil Fossa. (C) Geographical distribution of the 14 clusters where clusters 1 (dark blue), 2 (purple), 5 (red), 8 (orange), and 9 (yellow) show the H2O + NH3–rich clusters in a gradation from maximum to minimum (indicated by the arrow direction). Image credit: NASA, Johns Hopkins University, Southwest Research Institute.
Fig. 2
Fig. 2. Detail of the site of the putative cryovolcanic emission of NH3-bearing H2O in the main trough of Virgil Fossae.
(A) Section of Virgil Fossae from the Pluto base map. Arrows point to topographic features that are muted in form by an apparent mantling by material ejected in a cryovolcanic event within the fossa main channel. (B) Same region shown with the H2O ice distribution (17, 18) in blue overlain on the base map: Darker shade of blue indicates greater concentration based on the strength of the ice absorption bands. Image credit: NASA, Johns Hopkins University, Southwest Research Institute.
Fig. 3
Fig. 3. Spectral signature of NH3 in Virgil Fossae and surrounding terrain.
(A) Averaged spectra for the CH4-poor clusters 1, 2, 5, 8, and 9. Note that the raw spectra are similar. (B) Spectra of cluster 9 divided by clusters 1, 2, 5, and 8. The ratioed spectra reveal H2O ice absorptions at 1.5, 1.65, and 2.0 μm and a depression at 2.2 μm characteristic of NH3 products.
Fig. 4
Fig. 4. Radiative transfer model fits to the derived spectra of NH3-rich region of Virgil Fossae.
Model fits (solid line, upper section of each panel) and residuals (lower section of each panel) obtained by dividing Pluto’s ratio of cluster 9 to cluster 1 (At and Ab), Charon’s disk-averaged spectrum (Bt and Bb), and Nix’s disk-averaged spectrum (Ct and Cb) by their corresponding best fits shown as a colored solid line. Models were calculated by means of either Hapke (46) or Shkuratov (47) approaches, including H2O ice and a tholin, but not NH3. Dashed vertical lines point to the wavelengths characteristic of the NH3 spectrum. I/F, reflected intensity divided by incident solar flux.
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