The Wind, Temperature, and Surface Pressure on Pluto from a Pluto General Circulation Model

Details The Wind, Temperature, and Surface Pressure on Pluto from a Pluto General Circulation Model The Wind, Temperature, and Surface Pressure on Pluto from a Pluto General Circulation Model

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p12a..02z&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P12A-02

Other

[0343] Atmospheric Composition And Structure / Planetary Atmospheres, [3319] Atmospheric Processes / General Circulation, [5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [6270] Planetary Sciences: Solar System Objects / Pluto And Satellites

Scientific paper

A variety of methods have been used to derive Pluto's atmospheric temperature, composition, and surface pressure from spectra and stellar occultation data, while wind is less easily determined. Gravity wave dissipation has been investigated [1] in the 18 March 2007 stellar occultation dataset [2], demonstrating that wind is occurring in the form of perturbations about a mean. Rossby waves have also been proposed [2] as an explanation to the 2007 dataset; however the method was used incorrectly. General circulation models (GCMs) are a ubiquitous tool in the field of planetary atmospheres to solve for the global state of the atmosphere in a physically consistent manner, but only recently have they began to be developed for Pluto. We use a Pluto version of the Massachusetts Institute of Technology (MIT) GCM to solve for the first time for wind, temperature, and surface pressure globally in Pluto's atmosphere. The Pluto version of the MIT GCM (PGCM) uses the MIT GCM dynamical core [3] with a radiative-conductive model [4]. It includes vertical thermal conduction and non-local thermodynamic equilibrium heating and cooling by methane at 3.3 um and 7.6 um, respectively. We perform a parameter sweep with methane volume mixing ratios of 0.2, 0.6, and 1% and initial global mean surface pressures of 6-26 ubar. We ran the model from rest starting in the model year 1973. We compared the PGCM results with occultation data from the years 1988, 2002, 2006, and 2007. Model light curves were calculated from the PGCM temperature output (averaged at 90 day intervals) at the corresponding date and Pluto latitudes of each occultation. The match between data and PGCM is better than between data and the radiative-conductive equilibrium solution (i.e. no wind), but the PGCM light curves contain wave-like features while the data do not. We do not believe that this feature represents an atmospheric wave; rather, it is numerical noise known to occur in 2D GCMs. The PGCM-predicted zonal (longitudinal) winds are easterly everywhere (prograde with Pluto's rotation) and characterized by two high-speed, high-latitude jets in gradient wind balance. Meridional (latitudinal) and vertical winds do not show any large-scale structure in the latitude-height plane such as a Hadley cell or other thermally direct circulation. The lack of Hadley cells is due to the low-altitude temperature inversion (temperature increasing with height). Such a temperature structure is quite stable and inhibits vertical motions. The fractional variation of surface pressure with latitude is much less than inter-annual variations from previous studies. Pluto's atmosphere is not quiescent and contains large-scale structure. Our model, the first for Pluto, is easily adaptable to other small bodies of similar composition such as Triton and other large Kuiper Belt Objects. The model can play a critical role for NASA's New Horizons mission, which arrives at Pluto in 2015. [1] Hubbard, W. B., et al., 2009. Icarus 204, 284-289. [2] Person, M. J., et al., 2008. Astron. J. 136, 1510-1518. [3] Marshall, J., et al., 1997. J. Geophys. Res. 102, 5753-5766. [4] Yelle, R. V. and Lunine, J. I., 1989. Nature 339, 288-290.

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