Surface Ices and the Atmospheric Composition of Pluto

NASA, Вашингтон, West Virginia, United States
Science (Impact Factor: 33.61). 09/1993; 261(5122):745-8. DOI: 10.1126/science.261.5122.745
Source: PubMed


Observations of the 1.4- to 2.4-micrometer spectrum of Pluto reveal absorptions of carbon monoxide and nitrogen ices and confirm the presence of solid methane. Frozen nitrogen is more abundant than the other two ices by a factor of about 50; gaseous nitrogen must therefore be the major atmospheric constituent. The absence of carbon dioxide absorptions is one of several differences between the spectra of Pluto and Triton in this region. Both worlds carry information about the composition of the solar nebula and the processes by which icy planetesimals formed.

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    • "However, while viscous crater relaxation on Charon is likely to be negligible currently unless there exist unseen sources of local or global internal heat, Pluto may have experienced much more significant crater relaxation because its surface is dominated by N2 ice (e.g. Owen et al. 1993), which is considerably weaker (less viscous) than H2O ice. If this N2 is present only as a thin coating on top of underling cold H2O ice, one would not expect craters on Pluto to be relaxed, for reasons similar to those outlined above for Charon. "
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    ABSTRACT: Pluto and its satellites will be the most distant objects ever reconnoitered when NASA's New Horizons spacecraft conducts its intensive flyby of this system in 2015. The size-frequency distribution (SFD) of craters on the surfaces in the Pluto system have long been expected to provide a useful measure of the size distribution of Kuiper Belt Objects (KBOs) down to much smaller size scales than presently observed. However, currently predicted escape rates of Pluto's atmosphere suggest that of order one-half to several kilometers of nitrogen ice has been removed from Pluto's surface over geologic time. Because this range of depths is comparable to or greater than most expected crater depths on Pluto, one might expect that many craters on Pluto's surface may have been removed or degraded by this process, biasing the observed crater SFD relative to the production-function crater SFD. Further, if Pluto's surface volatile layer is comparable to or deeper than crater depths, and if the viscosity of this layer surface ice is low like the viscosity of pure N2 ice at Pluto's measured 35 K surface temperature (or as low as the viscosity of CH4 ice at warmer but plausible temperatures on isolated pure-CH4 surfaces on Pluto), then craters on Pluto may also have significantly viscously relaxed, also potentially biasing the observed crater SFD and surface crater retention age. Here we make a first exploration of how these processes can affect the displayed cratering record on Pluto. We find that Pluto's surface may appear to be younger owing to these effects than it actually is. We also find that by comparing Pluto's cratering record to Charon's, it may be possible to estimate the total loss depth of material from Pluto's surface over geologic time, and therefore to estimate Pluto's time-averaged escape rate.
    Preview · Article · Dec 2014 · Icarus
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    • "Methane was the first volatile species discovered on Pluto as a surface species (Cruikshank et al. 1976) but its identification as a minor species had to await the discovery of N 2 ice on the surface in much larger amounts (Owen et al. 1993). Since then, detailed studies of the surface visible and near-IR spectra revealed the complexity of the methane surface distribution, exhibiting large lateral and vertical variability (Douté et al. 1999, Olkin et al. 2007, Merlin et al. 2010) as well as temporal evolution (Grundy and Buie 2001, Grundy et al. 2013). "
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    ABSTRACT: High-resolution spectra of Pluto in the 1.66 um region, recorded with the VLT/CRIRES instrument in 2008 (2 spectra) and 2012 (5 spectra), are analyzed to constrain the spatial and vertical distribution of methane in Pluto's atmosphere and to search for mid-term (4 year) variability. A sensitivity study to model assumptions (temperature structure, surface pressure, Pluto's radius) is performed. Results indicate that (i) no variation of the CH4 atmospheric content (column-density or mixing ratio) with Pluto rotational phase is present in excess of 20 % (ii) CH4 column densities show at most marginal variations between 2008 and 2012, with a best guess estimate of a ~20 % decrease over this time frame. As stellar occultations indicate that Pluto's surface pressure has continued to increase over this period, this implies a concomitant decrease of the methane mixing ratio (iii) the data do not show evidence for an altitude-varying methane distribution; in particular, they imply a roughly uniform mixing ratio in at least the first 22-27 km of the atmosphere, and high concentrations of low-temperature methane near the surface can be ruled out. Our results are also best consistent with a relatively large (> 1180 km) Pluto radius. Comparison with predictions from a recently developed global climate model GCM indicates that these features are best explained if the source of methane occurs in regional-scale CH4 ice deposits, including both low latitudes and high Northern latitudes, evidence for which is present from the rotational and secular evolution of the near-IR features due to CH4 ice. Our "best guess" predictions for the New Horizons encounter in 2015 are: a 1184 km radius, a 17 ubar surface pressure, and a 0.44 % CH4 mixing ratio with negligible longitudinal variations.
    Preview · Article · Mar 2014 · Icarus
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    • "Pluto receives nearly three times less sunlight at aphelion than perihelion, prompting early modelers to predict that Pluto's atmosphere would expand and collapse over its orbit (Stern and Trafton, 1984). More sophisticated models were made in the 1990s (Hansen and Paige, 1996), after the definitive detection of Pluto's atmosphere in 1988 (Millis et al., 1993) and the discovery of N 2 as the dominant volatile in the atmosphere and on the surface (Owen et al., 1993). Similar models were run recently (Young, 2013), systematically exploring a range of parameter space. "
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    ABSTRACT: Combining stellar occultation observations probing Pluto's atmosphere from 1988 to 2013, and models of energy balance between Pluto's surface and atmosphere, we find the preferred models are consistent with Pluto retaining a collisional atmosphere throughout its 248-year orbit. The occultation results show an increasing atmospheric pressure with time in the current epoch, a trend present only in models with a high thermal inertia and a permanent N-2 ice cap at Pluto's north rotational pole. (C) 2014 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license.
    Full-text · Article · Mar 2014 · Icarus
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