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Titan's haze from Huygens DISR: profile, albedo, and the sky from the ground

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Valutazione

Difficoltà
5/5
Tempo stimato
Più di una settimana
Idoneità per principianti
25/100
Tipo di issue
Funzionalità
Chiarezza
Abbastanza chiara
Stato di attività
Attiva
Stack tecnologico
javascript
Ambito
computer-graphics

Direzione di ricerca

Start with public/data/titan.json and the multiple-scattering section of js/scene/atmos/composition.md; compare the current haze inputs with the cited DISR measurements. The scope includes wavelength-dependent optical depth and albedo, a thick forward-scattering treatment, and optionally a detached layer. Done means the disc matches Cassini at low and moderate phase, the high-phase limb shows the layer, and ground-sky brightness is within a factor of 2 of DISR's.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

What

Fit Titan's atmosphere data (public/data/titan.json) to the haze model Huygens' DISR measured: Tomasko et al. 2008, Planet. Space Sci. 56, 669, Tables 2 and 3, and Doose et al. 2016. Use its three altitude regimes, its optical depth by wavelength and its single-scattering albedo. Then check the sky seen from the ground.

Why

Titan's sky gain is now physical (π·DISPLAY_GAIN). Its old gain of 1.0 had been standing in for the haze's absorption: with τ ≈ 8 of conservative haze, Titan was a white disc at I/F 0.59, three times Cassini's. The sky-gains PR gives the haze a single-scattering albedo of 0.95 / 0.90 / 0.80 (red, green, blue). The values are taken from DISR's absorbing, tholin-like aerosols, darkest in the blue. Measured from 5 radii at 60° phase, the disc then reads I/F 0.24 and orange.

Two things are still off:

  • The sky from the ground. With the Sun 45° up, the zenith is 0.005 of a sunlit white surface at the top of the air (0.015 before the PR, 0.07 with conservative haze at the physical gain). Huygens found the daylight about a tenth of the Sun's. The isotropic multiple-scattering sum (Hillaire) under-counts a thick, forward-scattering haze (composition.md, multiple scattering).
  • The haze's profile. It is one exponential with a 50 km scale height. Titan has a detached haze layer near 500 km, which shows as Titan's bright limb in Cassini ISS images at high phase.

Scope

  • The haze's optical depth and albedo per channel, from DISR's tables, with the values taken recorded in composition.md.
  • A second-scattering treatment that holds in a thick, forward-scattering haze: a forward-peaked multiple-scattering term or delta-M, as Mars's dust has. Without it, the ground sky can't match Huygens.
  • Optional: the detached haze layer.

Done when

  • From orbit, Titan's disc I/F and colour match Cassini ISS at low and moderate phase.
  • The limb at high phase shows the detached haze.
  • From the ground at the Huygens site, the sky's brightness is within a factor of 2 of DISR's.
Lingua principale
JavaScript
Stelle
55
Fork
4
Merge medio
10h 12m
PR unite (30g)
134

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Come iniziare

  1. Leggi tutta la issue e poi la guida ai contributi del progetto.
  2. Commenta sulla issue per dire che te ne occupi tu — evita che due persone facciano lo stesso lavoro.
  3. Fai un fork del repository e lavora su un branch.
  4. Apri una pull request che faccia riferimento al numero della issue.

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