Physics of Solid N_2: a Possible Solution for Pluto's and Triton's Albedo Conundrums

Details Physics of Solid N_2: a Possible Solution for Pluto's and Triton's Albedo Conundrums Physics of Solid N_2: a Possible Solution for Pluto's and Triton's Albedo Conundrums

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0606d&link_type=abstract

American Astronomical Society, DPS meeting #28, #06.06; Bulletin of the American Astronomical Society, Vol. 28, p.1080

Physics

Scientific paper

According to the albedo maps based on mutual event lightcurves Pluto's northern hemisphere is much darker than the southern. (We use Pluto's angular momentum vector to define North). Currently it is the end of the southern summer on Pluto, with the subsolar point being ~ 12 deg S. Thus, it was surprising to find a large bright feature near the south pole. Even more puzzling was why the visible north polar regions appeared much darker than the south polar regions. Though the precessional cycle can explain the current large south cap (Binzel 1992), it does not account for the lack of the bright fresh frost in the northern hemisphere at the end of northern winter and does not explain the brightness of the south cap. We propose a solution to these puzzles based upon the physics of solid N_2. We also estimated an initial N_2 grain diameter on the surfaces of Pluto and Triton to be <0.3 mu m. First, from the radiative balance at Pluto's icy surface we calculate the current highest condensation rate to be ~ 4.57*10(-8) cm/s (or ~ 1.65 microns /hr). From laboratory experiments we found that at about 3. microns /hr or lower, N_2 condenses as a transparent layer. We conclude that what we see in the north is probably a relatively dark substrate under a clear layer of N_2 ice. Our result moreover holds for Triton. This explains the relative darkness of the northern regions up to 45 deg N seen in the Voyager 2 images. Concerning bright regions adjacent to the large, dark areas north of Triton's equator, we conclude that they may be the regions where N_2 condensed earlier than the clear N_2 did. At that time probably Tice>40 K and hence the condensation rate was higher than the rate sufficient for the deposition of an initially transparent layer. Thus, the frost condensed as a bright layer. The same explanation is applicable to Pluto's brighter regions in mostly darker northern hemisphere. Finally, the crystalline phase transition and subsequent shattering of N_2 ice (Duxbury and Brown 1993) can explain the brightness of the south perennial polar cap.

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