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Title: Haze in Pluto's Atmosphere
Author: Andrew F. Cheng, Michael E. Summers, G. Randall Gladstone, Darrell F. Strobel, Leslie A. Young, Panayotis Lavvas, Joshua A. Kammer, Carey M. Lisse, Alexander H. Parker, Eliot F. Young, S. Alan Stern, Harold A. Weaver, Cathy B. Olkin, Kimberley Ennico

Haze in Pluto's atmosphere was detected in images by both the Long Range Reconnaissance Imager (LORRI) and the Multispectral Visible Imaging Camera (MVIC) on New Horizons. LORRI observed haze up to altitudes of at least 200 km above Pluto's surface at solar phase angles from ~20░ to ~169░. The haze is structured with about ~20 layers, and the extinction due to haze is greater in the northern hemisphere than at equatorial or southern latitudes. However, more haze layers are discerned at equatorial latitudes. A search for temporal variations found no evidence for motions of haze layers (temporal changes in layer altitudes) on time scales of 2 to 5 hours, but did find evidence of changes in haze scale height above 100 km altitude. An ultraviolet extinction attributable to the atmospheric haze was also detected by the ALICE ultraviolet spectrograph on New Horizons. The haze particles are strongly forward-scattering in the visible, and a microphysical model of haze is presented which reproduces the visible phase function just above the surface with 0.5 Ám spherical particles, but also invokes fractal aggregate particles to fit the visible phase function at 45 km altitude and account for UV extinction. A model of haze layer generation by orographic excitation of gravity waves is presented. This model accounts for the observed layer thickness and distribution with altitude. Haze particles settle out of the atmosphere and onto Pluto's surface, at a rate sufficient to alter surface optical properties on seasonal time scales. Pluto's regional scale albedo contrasts may be preserved in the face of the haze deposition by atmospheric collapse.

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New Horizons discovery raises solar wind riddle around Pluto

When Carey Lisse at Johns Hopkins University, Maryland, and colleagues viewed Pluto using the Chandra X-ray Observatory in 2013, they saw far fewer X-rays than anticipated.
During last year's visit, New Horizons solved half the mystery: it found that Pluto is losing its atmosphere to space 100 times more slowly than predicted and so has a compact rather than tenuous atmosphere. That means it is less exposed to the solar wind.
But the probe also found that the stream of particles from the sun does not spike in response to solar flares. Instead, the solar wind is surprisingly steady and placid at the edge of the solar system, and no one knows why.

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Title: On the Provenance of Pluto's Nitrogen (N2)
Author: Kelsi N. Singer, S. Alan Stern

N2 is abundant in Pluto's atmosphere and on its surface, but the supply is depleted by prodigious atmospheric escape. We demonstrate that cometary impacts could not have delivered enough N2 mass to resupply Pluto's atmospheric escape over time; thus Pluto's N2 is likely endogenous, and therefore was either acquired early in its history or created by chemistry inside/on Pluto. We find that cratering could excavate a considerable amount of N2 to resupply the atmosphere against escape if the near-surface N2 reservoir is deep. However, we find that this process likely falls short of that necessary to resupply the atmosphere against escape by at least an order of magnitude. We conclude that either the escape of N2 from Pluto's atmosphere was on average much lower than the predictions for the current epoch, or that internal activity could be necessary to bring N2 to the surface and resupply escape losses. Observations made by the New Horizons spacecraft in mid-2015 will yield further constraints on the provenance and evolution of Pluto's surface and atmospheric N2, and could reveal evidence for past or present internal activity.

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Title: On the possible noble gas deficiency of Pluto's atmosphere
Authors: Olivier Mousis, Jonathan I. Lunine, Kathleen E. Mandt, Eric Schindhelm, Harold A. Weaver, S. Alan Stern, J. Hunter Waite, Randy Gladstone, Audrey Moudens

We use a statistical-thermodynamic model to investigate the formation and composition of noble-gas-rich clathrates on Pluto's surface. By considering an atmospheric composition close to that of today's Pluto and a broad range of surface pressures, we find that Ar, Kr and Xe can be efficiently trapped in clathrates if they formed at the surface, in a way similar to what has been proposed for Titan. The formation on Pluto of clathrates rich in noble gases could then induce a strong decrease in their atmospheric abundances relative to their initial values. A clathrate thickness of order of a few centimeters globally averaged on the planet is enough to trap all Ar, Kr and Xe if these noble gases were in protosolar proportions in Pluto's early atmosphere. Because atmospheric escape over an extended period of time (millions of years) should lead to a noble gas abundance that either remains constant or increases with time, we find that a potential depletion of Ar, Kr and Xe in the atmosphere would best be explained by their trapping in clathrates. A key observational test is the measurement of Ar since the Alice UV spectrometer aboard the New Horizons spacecraft will be sensitive enough to detect its abundance $\sim$10 times smaller than in the case considered here.

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