Triton's atmosphere: energy crisis

1. Triton's atmosphere: energy crisis Leslie Young (SwRI) Glenn Stark (Wellesley) Ron Vervack (JHU/APL) Triton Atmosphere

2. Triton atmosphere overview 2706 km diameter Triton Atmosphere

3. Triton atmosphere overview 2706 km diameter Tropospheric hazes, plumes, winds Triton Atmosphere

4. Triton atmosphere overview 2706 km diameter Nitrogen frost transport Tropospheric hazes, plumes, winds Triton Atmosphere

5. Triton atmosphere overview 2706 km diameter Nitrogen frost transport UVS occultation (<80 nm) Tropospheric hazes, plumes, winds Triton Atmosphere

6. Triton atmosphere overview 2706 km diameter Nitrogen frost transport UVS occultation (>80 nm) UVS occultation (<80 nm) Tropospheric hazes, plumes, winds Triton Atmosphere

7. Triton atmosphere overview 2706 km diameter Nitrogen frost transport UVS occultation (>80 nm) UVS occultation (<80 nm) Radio occultation Tropospheric hazes, plumes, winds Triton Atmosphere

8. UVS occultation at a glance N2 electronic bands N2 ionization Other 800 1.0 0.8 600 Transmission 0.6 Altitude (km) 400 0.4 0.2 200 0.0 0 700 800 1000 600 900 Wavelength (Å) Triton Atmosphere

9. Previous Voyager-based models Triton Atmosphere

10. Everyone clamors for new N2 line data “The N2 densities can be measured to much lower altitudes by the use of the electronic bands in the 1000Å region.” -Broadfoot et al 1989 “…laboratory measurements of N2 ultraviolet absorption properties are urgently needed to permit analysis of the UVS occultation data in the 800-1000Å region, which senses the atmosphere down to ~50 km.” - Yelle et al. 1995 “The temperature profile in the 310 km gap (10-320 km) is contained in the unanalyzed N2 band absorption signatures in the UVS solar occultation data between 850 and 1000Å. Until the necessary laboratory data is available to perform this analysis, we must rely on theoretical models to infer the temperature profile connecting the tropopause to 450 km.” - Strobel et al. 1995 “Below 450 km occultation data is available in the 660-1000Å region where N2 absorbs strongly in many discrete electronic bands... We are not able to accurately model the absorption of sunlight to derive the structure of Triton’s atmosphere from the surface to 450 km because of inadequate molecular cross section data.” - Stevens et al. 1992 Triton Atmosphere

11. Limitations of old N2 line data • Line positions well known • Measured f-values (e.g. strengths) for bands, so used Hönl-London factors to derive individual line strengths. But this region is full of perturbations, so the Hönl-London factors misestimate lines strengths by factors of 2-3. • Hönl-London factors are the ratios between the strengths for individual lines and the band. For a given band, these factors are simple functions of the rotational states. • Predissociation rates (and therefore widths) are largest uncertainty. (Lines are described by Voigt profiles, with Gaussian width from temperature, Lorentz width from predissociation) Triton Atmosphere

12. New N2 line data • Collaborator Glenn Stark (Wellesley College) • Surveyed all bands, 80-100 nm, with multiple scans, multiple pressures, room temperatures (so colder Triton temperatures will not introduce hot overtones). • 17 bands available for this analysis (up from 4) • N2 photoabsorption measured by Stark and others at the Photon Factory synchrotron radiation facility at the High Energy Accelerator Research Organization in Tsukuba, Japan, 1997, 1998, 1999 • Predissociation widths from Stark’s measurements, or from Ubachs (e.g., Ubachs, W. 1997. Predissociative decay of the c4’ 1∑u+ v=0 state of N2, Chem. Phys. Lett. 268, 201.) • In most cases, measured individual line strengths and predissociation widths. (new this year) • Use cross sections for T=95 K Triton Atmosphere

13. N2 electronic bands used (new) Mean wavelength Total strength Dissociation Width band (Å) (Å cm2) (mÅ) ---- --------------- -------------- ------------------ b(8) 935.3 2.8E-18 0.500 b’(4) 938.0 1.4E-17 1.600 c3(1) 938.8 2.8E-16 0.577 b(7) 942.6 1.3E-16 0.150 b’(3) 944.8 5.6E-19 0.000 o(0) 946.3 2.0E-18 0.200 b(6) 949.4 3.1E-17 0.140 b’(2) 951.2 2.8E-19 0.000 b(5) 955.3 2.8E-17 0.240 b’(1) 958.3 1.6E-17 0.070 c4’(0) 958.6 1.1E-15 0.073 c3(0) 960.4 4.0E-16 0.670 b(4) 965.9 5.6E-16 2.900 b(3) 972.3 3.7E-16 38.727 b(2) 979.1 1.7E-16 5.165 b(1) 985.8 6.9E-17 0.050 triple 989.6 7.6E-21 5000.000 b(0) 992.0 1.9E-17 1.500 Triton Atmosphere

14. Voyager (1989) occultation data Triton Atmosphere

15. Voyager data details • UVS transmission vs. altitude and wavelength • Use Ron Vervack’s most recent reduction (~1992) • Use only odd channels • Account for wavelength shifts with altitude by shifting the wavelengths of channels (new; had shifted the spectra, not the wavelengths of the bins) • Estimate errors from photon count, following Yelle et al 1993 (new; had estimated errors by comparing ingress and egress). • Radio phase delay vs. altitude • Using most recent reduction: Gurrola, E. M. 1995. Interpretation of Radar Data from the Icy Galilean Satellites and Triton. Ph.D. Thesis, Stanford University, Fig 6.2, as preserved in Planetary Data System • Gurrola estimates errors at 0.1 radian; however, errors are highly correlated • Use the background estimated by Gurrola, taking into account this correlation; this is the background in Gurrola Table 8.3. Triton Atmosphere

16. N2 cross sections Triton Atmosphere

17. Line-of-sight density details - s N = transmission e los l + D l - s N ò » l 0 l transmission e d los l 0 • Nlos from N2 continuum transmission • Simple Beer’s law • , where • Nlos from N2 line transmission • Explicit averaging over wavelength • Consistent with Nlos from N2 continuum transmission • Nlos from radio phase l + D l ò s l 0 F d ¤ l l » s 0 l + D l ò l 0 F d ¤ l 0 4 pn STP = phase delay N los l N L Triton Atmosphere

18. New line-of-sight number density Triton Atmosphere

19. What can we do? • Weight transmission by solar flux • Want a good solar spectrum with F10.7=178 W/m2/Hz • Smooth spectra to match instrumental resolution • 13 Å triangular smoothing function • Use multiple channels • Use a temperature-dependent cross section • but the cross sections at 500 km should be at 95 K • For now, look at last year’s self-consistent reduction... Triton Atmosphere

20. Old line-of-sight number density Triton Atmosphere

21. Nlos interpretation details • Fit UVS and radio with (small-planet) isothermal models • Upper atmosphere • All of the SNR>5 UVS Nlos can be fit with single temperature • T=104 K (c.f. Krasnopolsky et al. 1993; T=102±3 K) • Lower atmosphere • All of the SNR>5 radio Nlos consistent with single temperature • T=44 K (c.f. Gurolla 1995; T=42±8 K) • Comparison with 1997 • Calculate Nlos from temperature profile (Elliot et al. 2000) • Nlos has increased between 1989 and 1997 Triton Atmosphere

22. Nlos interpretation Triton Atmosphere

23. Temperature profile details • Invert the “spliced isothermal” Nlos profile from above • Use the “small planet” Abel transform • Lower atmosphere (<90 km): colder in 1989 than in 1997 • As reported by Elliot et al. 2000 • Higher conductive flux will affect thermal models of the 1997 occultation, which are currently unsatisfactory. Triton Atmosphere

24. Temperature profiles Triton Atmosphere

25. What’s the energy source near 100 km? • Implied heating differs from previous models • Stevens et al. 1992; Yelle et al. 1991 • Main heating is below 150 km • c.f. Stevens et al 1992; 300-400 km • Gradients may be as high as 1.4 K/km for fluxes as high as 8x10-3 erg/cm2/s • c.f. Broadfoot et al. 1989, Yelle et al. 1991; 1x10-3 erg/cm2/s • Heating by energetic particle precipitation? • That can reach lower altitudes, but the total energy is a problem • Thermal models may not split the atmosphere into “upper” and “lower” • Page charges will be larger, author lists will be longer Triton Atmosphere

26. An endemic problem? Triton’s 1989 atmosphere is warmer than models, as is: • Pluto’s 1989 atmosphere (Lellouch 1994; Strobel et al. 1996) • Triton’s 1997 atmosphere (Elliot et al. 2000) • Jupiter, Uranus near 1 µbar Triton Atmosphere

27. Conclusions • We are close to realizing the promise of the N2 electronic bands (but more work still to be done) • Supporting evidence for changes in Triton’s atmosphere between 1989 and 1997 • So far, evidence for a major puzzle in the energetics of Triton’s atmosphere near 100 km altitude (0.25 µbar) • The solution to this puzzle may have larger implications in the outer solar system. Triton Atmosphere