Authors

Citation

Va. Krasnopolsky et al., TEMPERATURE, N2, AND N DENSITY PROFILES OF TRITONS ATMOSPHERE - OBSERVATIONS AND MODEL, J GEO R-PLA, 98(E2), 1993, pp. 3065-3078

Citations number

52

Language

INGLESE

art.tipo

Article

Journal title

JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS

ISSN journal

2169-9097
→ ACNP

Volume

98

Issue

E2

Year of publication

1993

Pages

3065 - 3078

Database

ISI

SICI code

2169-9097(1993)98:E2<3065:TNANDP>2.0.ZU;2-Q

Abstract

Improved analysis of the Voyager Ultraviolet Spectrometer (UVS) observ
ations of the solar occultation by Triton yields the isothermal temper
ature and N2 number densities in the altitude range 475-675 km: T = 10
2 +/- 3 K, [N2]= (4 +/- 0.4) x 10(8) cm-3 at 575 km. A distinct step-l
ike absorption feature at 850 angstrom is due to the atomic nitrogen i
onization continuum. It allows a measurement of N number densities in
the range from 170 to 570 km. which correspond to diffusive equilibriu
m above 300 km with [N] = (1 +/- 0.25) x 10(8) cm-3 at 400 km and T =
100 +/- 7 K. Deviations from diffusive equilibrium become important be
low 300 km, and [N] = (5 +/- 2.5) x 10(8) cm-3 at 200 km. The exobase
altitude is 870 km, and the total escape rate of atomic nitrogen is (1
+/- 0.3) x 10(25) s-1. The main condensible product of methane chemis
try is ethylene, C2H4, with a peak number density of 6 x 10(6) cm-3 ne
ar 25 km. The striking similarity of the thermospheric properties at b
oth occultation sites despite substantial differences in latitudes, se
asons, local time, and incoming flux of magnetospheric electrons impli
es very effective winds and suggests that one-dimensional modeling is
applicable. Temporal variations of the temperature profile should be r
ather small in spite of strong variations of the electron flux. The pr
esence of CO in the atmosphere as suggested by recent measurements of
the CO absorption bands in the surface ice results in cooling by the r
otational lines, deactivation and radiation of vibrational excitation
of N2, and heating by quenching of N(2D). Heating efficiencies of elec
tron precipitation and solar EUV radiation in the ranges of the N2 con
tinuum and bands are calculated equal to 0.2-0.29), 0.24, and 0.19, re
spectively. The column integrated cooling by the CO lines is weaker th
an the thermospheric heating and cannot form a mesosphere. A CO mixing
ratio of up to 10(-2) is consistent with thermal balance calculations
. Sixteen optimized versions of the model are considered with profiles
of T and [N2] which agree with the measurements. Profiles of N, H-2,
and H densities are calculated. The calculated profile of atomic nitro
gen and its escape rate are in excellent agreement with the measured v
alues. Mixing ratios of H-2 and H at 400 km are equal to 240 and 23 pp
m, respectively. The total escape rate of hydrogen atoms (H + 2H-2) is
determined by photolysis of methane by H Lyman alpha radiation, does
not depend on the ionospheric processes which transform H-2 to H, and
is equal to 2.3 x 10(26) s-1. The ratio of H to N escape rates on Trit
on, 20, does not correspond to the measured ratio H+/N+ is similar to
2-3 in Neptune's magnetosphere and should be taken into account in its
modeling. The recommended model for Triton's atmosphere is based on t
he temperature profile calculated for a CO mixing ratio of 10(-3), a r
atio of the average to the maximum electron fluxes of 0.162, and inclu
des data on temperature, N2, N, CH4, H-2, and H number densities,