Triton's distorted atmosphere

Science

Volumn 278, Issue 5337, 1997, Pages 436-439

  Elliot, J L ^a,b   Stansberry, J A ^b   Olkin, C B ^b   Agner, M A ^a   Davies, M E ^c  

^a MASSACHUSETTS INSTITUTE OF TECHNOLOGY   (United States)

^b LOWELL OBSERVATORY   (United States)

^c RAND CORPORATION   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ARTICLE; ASTRONOMY; ATMOSPHERE; MODEL; PHYSICS; PRIORITY JOURNAL;

ATMOSPHERE; EXTRATERRESTRIAL ENVIRONMENT; GRAVITATION; NEPTUNE; OPTICS; TEMPERATURE;

EID: 0030696762     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.278.5337.436     Document Type: Article

References (35)

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20. W. B. Hubbard et al., Astron. Astrophys. 269, 541 (1993).
21. 0 and b is the power-law exponent. The refractivity model used here is an elliptical generalization of this model.
22. The atmospheric parameters apply to the subimmersion (longitude 81.9°, latitude -6.0°) and subemersion (longitude 270.5°, latitude 13.5°) half-light points on Triton (8). All the fitted global limb shapes give nearly the same temperature (Table 1) for a radius of 1400 km: 43.5 ± 2.5 K for the oblate solution. This agrees within the error with the 44.5 ± 1.8 K temperature derived from models excluding the central flash (8), which are about 5 K colder than models based on Voyager data (6). The pressure at 1400 km is somewhat dependent on the assumption of limb shape (Table 1), but all pressures are well above the predictions of models based on Voyager data (6). Our higher pressure may be due to an inadequacy of the models or to a change in Triton's surface pressure between 1989 and 1995 (5).
23. M. E. Davies, P. G. Rogers, T. R. Colvin, J. Geophys. Res. 96, 15 (1991).
24. Triton's atmospheric figure at the half-light level from the occultation chords is ε = -0.029, PA = -20 ± 10 (Table 1), minimum center distance = 191 ± 36 km (8). This figure applies to a radius of about 1451 km and applies predominantly to the eastern and western limbs, whereas the light-curve figure is dominated by light from the northern or southern limb that passes through the atmosphere at a smaller radius. Hence the shape of Triton's atmosphere may be changing with altitude (a property of some of the global circulation models we investigated).
25. χ or larger.
26. -1 value.
27. Winds with speeds exceeding the tangential rotational velocity of the surface have dynamics dictated by the cyclostrophic balance between the force of gravity, the centrifugal force due to the zonal wind circulation, and the pressure gradient force [see J. R. Holton, An Introduction to Dynamic Meteorology (Academic Press, New York, 1979)].
28. The altitude of the transition between prolate and oblate isopycs depends somewhat on atmospheric temperature, which was set to 40 K in these models.
29. Altitudes are relative to a spherical surface, assumed to be at the base of the model region, where the pressure is constant. This reference surface need not coincide with Triton's physical surface, so the altitudes of the prolate and oblate regions are adjustable, and the altitude of the reference surface could be different in the northern and southern hemispheres.
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32. P. C. Thomas, personal communication.
33. The overlying circular model was derived by decreasing the closest approach distance to create a central flash that was everywhere larger than the oblate model. Using this closest approach distance, we fit a circular model to the oblate model with the central 40% removed (8) to match the main drop and recovery of the data. For the IR model, the atmosphere was assumed to be spherically symmetric, with the same atmospheric parameters and closest approach as the overlying circular model. The only source of asymmetry in this model is extinction.
34. J. Hillier and J. Veverka, Icarus 109, 284 (1994).
35. We thank W. McKinnon and P. Thomas for helpful discussions and P. Nicholson and W. Hubbard for comments and criticisms. This work was supported in part by NASA grant NAGW-1494 and by the Friends of Lowell Observatory.