Triton's atmosphere: energy crisis

SwRI 2003, Dec 16 Triton Atmosphere

Triton's atmosphere: energy crisis

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


Triton atmosphere overview

2706 km diameter



2706 km diameter

Tropospheric hazes, plumes, w

inds



2706 km diameter

Nitrogen frost transport


inds



2706 km diameter



inds

UVS occultation (<

80 nm)



2706 km diameter



inds

UVS occultation (>

80 nm)

UVS occultation (<

80 nm)



2706 km diameter



inds

UVS occultation (>

80 nm)

UVS occultation (<

80 nm)

Radio occultation


UVS occultation at a glance

600Wavelength (Å)

700 800 900 10000

200

400

600

800

Alti

tude

(km

)

0.0

0.2

0.4

0.6

0.8

1.0

Tra

nsm

issi

on

N2 electronic bands

N2 ionization Other


Previous Voyager-based models

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700 800

to lower atmosphereFlux of thermospheric energy

Altitude (km)

Thermospheric heating peaks at 350-400 km•Solar EUV and UV heating•Precipitation of electrons from Neptune's magnetosphere•Chemical heating

36

38

40

42

44

0 10 20 30 40 50

Altitude (km)

to lower atmosphereFlux of thermospheric energy

•Turbulent heating & cooling•Negligible radiation and condensation

Tropospheric cooling peaks at 8-15 km


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


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)


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


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


Voyager (1989) occultation data

0

0.2

0.4

0.6

0.8

1

0

0.4

0.8

1.2

1.6

2

0 100 200 300 400 500 600 700 800 altitude (km)

UVS Channel 9(576.48 to 585.74 Å)

UVS Channel 49(945.63 to 954.89 Å)

Radio egress

0

0.2

0.4

0.6

0.8

1

0

0.4

0.8

1.2

1.6

2

0 20 40 60 80 100 altitude (km)

Radio egressUVS Channel 49(945.63 to 954.89 Å)

UVS Channel 9(576.48 to 585.74 Å)


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.


N2 cross sections

10-21

10-20

10-19

10-18

10-17

10-16

10-15

10-14

940 945 950 955 960

Cross Section (cm

2)

wavelength (A)

Channel 49


Line-of-sight density details• 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

–

transmission eNlos

F d 0

0

F d0

0

transmission e Nlos

d 0

0

phase delay 4STPNL

Nlos


New line-of-sight number density

1015

1016

1017

1018

1019

1020

1021

1022

1023

0 100 200 300 400 500 600 700 800altitude (km)

Radio egress

UVS Channel 49

UVS Channel 9

1021

1022

1023

0 20 40 60 80 100altitude (km)

Radio egress


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...


10 15

10 16

10 17

10 18

10 19

10 20

10 21

10 22

10 23

0 100 200 300 400 500 600 700 800

altitude (km)

Radio egress

UVS Channel 49(957.90 to 967.16 Å)

UVS Channel 9(587.53 to 596.79 Å)

10 21

10 22

10 23

0 20 40 60 80 100

altitude (km)

Radio egress

Old line-of-sight number density


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

Nlos =Nlos(ro)rr0

⎛

⎝ ⎜

⎞

⎠ ⎟

3/ 2

exprH

−r0H0

⎡

⎣ ⎢

⎤

⎦ ⎥ 1+

98

Hr

−H0r0

⎛

⎝ ⎜

⎞

⎠ ⎟

⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥


Nlos interpretation

10 15

10 16

10 17

10 18

10 19

10 20

10 21

10 22

10 23

0 100 200 300 400 500 600 700 800

1989 UVS N2 lines

1989isothermalloweratmosphere(43 K) 1989 isothermal

upper atmosphere (104 K)

altitude (km)

1989 Radio

1989 UVS N2 continuum

1997 Stellar Occultation

10 15

10 16

10 17

10 18

10 19

10 20

10 21

10 22

10 23

0 50 100 150 200

1989 UVS N2 lines1989

isothermalloweratmosphere(43 K) 1989 isothermal

upper atmosphere (104 K)

altitude (km)

1989 Radio1997 Stellar Occultation


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.

n(r) =−1

πr2dsNlos(s)

dssds

s2 −r20

∞∫


Temperature profiles

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700 800

Altitude (km)

1997stellar occ

1989 UVS and radio occs, new temperature profile (example)

1989 UVS and radio occs, previous temperature profiles

40

50

60

70

80

90

100

110

0 50 100 150 200

Altitude (km)

1997stellar occ

1989 occs, new profile

1989 occs, previous profiles


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


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


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.

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Triton's atmosphere: energy crisis