Triton's atmosphere: energy crisis

1
Triton's atmosphere energy crisis

• Leslie Young (SwRI)
• Glenn Stark (Wellesley)
• Ron Vervack (JHU/APL)

2
Triton atmosphere overview

2706 km diameter
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Triton atmosphere overview

2706 km diameter
Tropospheric hazes, plumes, winds
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Triton atmosphere overview

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

2706 km diameter
Nitrogen frost transport
UVS occultation (lt80 nm)
Tropospheric hazes, plumes, winds
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Triton atmosphere overview

2706 km diameter
Nitrogen frost transport
UVS occultation (gt80 nm)
UVS occultation (lt80 nm)
Tropospheric hazes, plumes, winds
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Triton atmosphere overview

2706 km diameter
Nitrogen frost transport
UVS occultation (gt80 nm)
UVS occultation (lt80 nm)
Radio occultation
Tropospheric hazes, plumes, winds
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UVS occultation at a glance

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)
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Previous Voyager-based models
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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 Tritons atmosphere from
  the surface to 450 km because of inadequate
  molecular cross section data. - Stevens et al.
  1992

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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)

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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 Starks measurements,
  or from Ubachs (e.g., Ubachs, W. 1997.
  Predissociative decay of the c4 1?u v0 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 T95 K

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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

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Voyager (1989) occultation data

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Voyager data details

• UVS transmission vs. altitude and wavelength
• Use Ron Vervacks 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.

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N2 cross sections

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Line-of-sight density details

• Nlos from N2 continuum transmission
• Simple Beers 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
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New line-of-sight number density
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What can we do?

• Weight transmission by solar flux
• Want a good solar spectrum with F10.7178 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 years self-consistent
  reduction...

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Old line-of-sight number density
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Nlos interpretation details

• Fit UVS and radio with (small-planet) isothermal
  models
• Upper atmosphere
• All of the SNRgt5 UVS Nlos can be fit with single
  temperature
• T104 K (c.f. Krasnopolsky et al. 1993 T1023
  K)
• Lower atmosphere
• All of the SNRgt5 radio Nlos consistent with
  single temperature
• T44 K (c.f. Gurolla 1995 T428 K)
• Comparison with 1997
• Calculate Nlos from temperature profile (Elliot
  et al. 2000)
• Nlos has increased between 1989 and 1997

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Nlos interpretation
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Temperature profile details

• Invert the spliced isothermal Nlos profile from
  above
• Use the small planet Abel transform
• Lower atmosphere (lt90 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.

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Temperature profiles
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Whats 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

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An endemic problem?

• Tritons 1989 atmosphere is warmer than models,
  as is
• Plutos 1989 atmosphere (Lellouch 1994 Strobel
  et al. 1996)
• Tritons 1997 atmosphere (Elliot et al. 2000)
• Jupiter, Uranus near 1 µbar

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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 Tritons
  atmosphere between 1989 and 1997
• So far, evidence for a major puzzle in the
  energetics of Tritons atmosphere near 100 km
  altitude (0.25 µbar)
• The solution to this puzzle may have larger
  implications in the outer solar system.