Triton's thermosphere: another energy crisis? (a work in progress) Leslie Young (SwRI) Glenn Stark (Wellesley) Ron Vervack (JHU/APL)

Voyager (1989) models extrapolate between UVS N 2 continuum and the radio occultation, about 400 km, or about 4 decades of pressure. Most of the thermospheric heating is expected to occur near km—a region that has not been directly measured. New lab data allows us to access the region between 100 and 4500 km.

Changes in Triton’s atmosphere are expected, because of the changes in the sub-solar latitude.

Changes in Triton’s structure have been reported, but are hard to quantify without Voyager-era measurements between 50 and 400 km.

Thermospheric heating rates deduced from previous Voyager models fail to reproduce the 1997 stellar occultation temperatures.

New analysis: three elements Radio occultation UVS occultation, N 2 continuum ( Å) UVS occultation, N 2 lines ( Å)—NEW

Phase delay from Gurrola, E. M Interpretation of Radar Data from the Icy Galilean Satellites and Triton. Ph.D. Thesis, Stanford University.

N2 continuum cross sections from Heubner, W. F., J. J. Keady and S. P. Lyons 1991 Six UVS channels shortward of significant N2 line absorbtion.

b(3), b(4), c(3) and c’(4) lines from lab measurements of Stark et al. For now, using Hönl-London factors for linestrengths; should have individual linestrengths by DPS

Line of sight number density derived from N 2 continuum absorption (upper), N 2 line absorption (middle) and radio phase delay (lower).

Upper portion is consistent with an isothermal atmosphere (T = ± 0.51 K).

Line of sight number density derived from N 2 continuum absorption (upper), N 2 line absorption (middle) and radio phase delay (lower). Upper portion is consistent with an isothermal atmosphere (T = ± 0.51 K). Temperature drops between 150 and 50 km.

Line of sight number density derived from N 2 continuum absorption (upper), N 2 line absorption (middle) and radio phase delay (lower). Upper portion is consistent with an isothermal atmosphere (T = ± 0.51 K). Temperature drops between 150 and 50 km. Triton in 1997 (gray) appears to have a higher line-of-sight number density than in 1989.

Conclusions (so far…) There is no evidence for significant heating at km. The main heating seems to be below 150 km. –To reach the same thermospheric temperatures, the energy deposited in the upper atmosphere must be much greater than previously suspected. The line-of-sight density ( km) appears higher in 1997 than in –Implies higher pressure, higher temperature (or both) at km. –Stronger case for the Elliot et al. report of changes in Triton’s atmospheric structure.