by neufer » Mon May 06, 2019 1:41 pm
Bugui wrote: ↑Mon May 06, 2019 7:36 am
Why do we see so big differences in Titan's atmosphere thickness?
Is it an optical effect? Or is really like that? Thank you.
Space stations on our own Moon observe a big difference in the solar illumination of the Earth's atmosphere during a
penumbra lunar eclipse due to the
asymmetric back-lighting from the Sun:
https://apod.nasa.gov/apod/ap140407.html
Atmosphere free moons (such as Enceladus in todays APOD) produce a thin crescent.
A thick hazy moon like Titan can extend that crescent into
a broad asymmetric ring.
Bugui wrote: ↑Mon May 06, 2019 7:36 am
Why do we see so big differences in Titan's atmosphere thickness?
Is it an optical effect? Or is really like that? Thank you.
https://solarsystem.nasa.gov/resources/13189/amazing-hazes/ wrote:
Saturn's atmosphere
Photojournal: PIA08214
Published: July 5, 2006
<<Sunlight streams through the high-altitude haze layer that extends completely around the giant moon, Titan, in this view of the moon taken by the Cassini spacecraft. Some fine structure can be spotted in the ever-shifting hazes in Titan's northern polar reaches to the top.
The distant sky beyond Titan (5,150 kilometers across) is not empty, but instead is filled in the lower half by the barely visible, immense bulk of Saturn 1.3 million kilometers (800,000 miles) beyond. The view is toward the night side of both worlds.
Titan's image is saturated, or over exposed, near the five o'clock position, obscuring the details in the atmosphere.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on June 2, 2006 at a distance of approximately 2.3 million kilometers (1.4 million miles) from Titan and at a Sun-Titan-spacecraft, or phase, angle of 163 degrees. Image scale is 14 kilometers per pixel on Titan.
https://www.nature.com/articles/s41550-017-0114 wrote:
Titan brighter at twilight than in daylight
by A. García Muñoz, P. Lavvas & R. A. West
Nature Astronomy volume 1, Article number: 0114 (2017)
Letter | Published: 24 April 2017
Abstract: Investigating the overall brightness of planets (and moons) provides insights into their envelopes and energy budgets 1,2,3,4 . Phase curves (a representation of the overall brightness versus the Sun–object–observer phase angle) for Titan have been published over a limited range of phase angles and spectral passbands. Such information has been key to the study of the stratification, microphysics and aggregate nature of Titan’s atmospheric haze and has complemented the spatially resolved observations showing that the haze scatters efficiently in the forward direction. Here, we present Cassini Imaging Science Subsystem whole-disk brightness measurements of Titan from ultraviolet to near-infrared wavelengths. The observations show that Titan’s twilight (loosely defined as the view at phase angles ≳150°) outshines its daylight at various wavelengths. From the match between measurements and models, we show that at even larger phase angles, the back-illuminated moon will appear much brighter than when fully illuminated. This behaviour is unique in our Solar System to Titan and is caused by its extended atmosphere and the efficient forward scattering of sunlight by its atmospheric haze. We infer a solar energy deposition rate (for a solar constant of 14.9 W m
−2) of (2.84 ± 0.11) × 10
14 W, consistent to within one to two standard deviations with Titan’s time-varying thermal emission from 2007 to 2013. We propose that a forward scattering signature may also occur at large phase angles in the brightness of exoplanets with extended hazy atmospheres and that this signature has a valuable diagnostic potential for atmospheric characterization.
[quote=Bugui post_id=291995 time=1557128177]
Why do we see so big differences in Titan's atmosphere thickness?
Is it an optical effect? Or is really like that? Thank you.[/quote]
Space stations on our own Moon observe a big difference in the solar illumination of the Earth's atmosphere during a [b][u][color=#FF0000]penumbra[/color][/u][/b] lunar eclipse due to the [b][u]asymmetric back-lighting[/u][/b] from the Sun: https://apod.nasa.gov/apod/ap140407.html
[b][u]Atmosphere free[/u][/b] moons (such as Enceladus in todays APOD) produce a thin crescent.
[b][u][color=#FF4000]A thick hazy moon[/color][/u][/b] like Titan can extend that crescent into [b][u]a broad asymmetric ring[/u][/b].
[quote=Bugui post_id=291995 time=1557128177]
Why do we see so big differences in Titan's atmosphere thickness?
Is it an optical effect? Or is really like that? Thank you.[/quote]
[quote="https://solarsystem.nasa.gov/resources/13189/amazing-hazes/"]
[float=left][img3=""]https://solarsystem.nasa.gov/system/resources/detail_files/13189_PIA08214.jpg[/img3][/float][size=150]Saturn's atmosphere[/size]
Photojournal: PIA08214
Published: July 5, 2006
<<Sunlight streams through the high-altitude haze layer that extends completely around the giant moon, Titan, in this view of the moon taken by the Cassini spacecraft. Some fine structure can be spotted in the ever-shifting hazes in Titan's northern polar reaches to the top.
The distant sky beyond Titan (5,150 kilometers across) is not empty, but instead is filled in the lower half by the barely visible, immense bulk of Saturn 1.3 million kilometers (800,000 miles) beyond. The view is toward the night side of both worlds.
Titan's image is saturated, or over exposed, near the five o'clock position, obscuring the details in the atmosphere.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on June 2, 2006 at a distance of approximately 2.3 million kilometers (1.4 million miles) from Titan and at a Sun-Titan-spacecraft, or phase, angle of 163 degrees. Image scale is 14 kilometers per pixel on Titan.[/quote][quote="https://www.nature.com/articles/s41550-017-0114"]
[float=left][img3="Titan's atmosphere backlit by the Sun, with Saturn's rings behind. An outer haze layer merges at top with the northern polar hood."]https://upload.wikimedia.org/wikipedia/commons/2/26/PIA_08211_Titan_backlit.jpg[/img3][/float][size=150]Titan brighter at twilight than in daylight[/size]
by A. García Muñoz, P. Lavvas & R. A. West
Nature Astronomy volume 1, Article number: 0114 (2017)
Letter | Published: 24 April 2017
Abstract: Investigating the overall brightness of planets (and moons) provides insights into their envelopes and energy budgets 1,2,3,4 . Phase curves (a representation of the overall brightness versus the Sun–object–observer phase angle) for Titan have been published over a limited range of phase angles and spectral passbands. Such information has been key to the study of the stratification, microphysics and aggregate nature of Titan’s atmospheric haze and has complemented the spatially resolved observations showing that the haze scatters efficiently in the forward direction. Here, we present Cassini Imaging Science Subsystem whole-disk brightness measurements of Titan from ultraviolet to near-infrared wavelengths. The observations show that Titan’s twilight (loosely defined as the view at phase angles ≳150°) outshines its daylight at various wavelengths. From the match between measurements and models, we show that at even larger phase angles, the back-illuminated moon will appear much brighter than when fully illuminated. This behaviour is unique in our Solar System to Titan and is caused by its extended atmosphere and the efficient forward scattering of sunlight by its atmospheric haze. We infer a solar energy deposition rate (for a solar constant of 14.9 W m[sup]−2[/sup]) of (2.84 ± 0.11) × 10[sup]14[/sup] W, consistent to within one to two standard deviations with Titan’s time-varying thermal emission from 2007 to 2013. We propose that a forward scattering signature may also occur at large phase angles in the brightness of exoplanets with extended hazy atmospheres and that this signature has a valuable diagnostic potential for atmospheric characterization.[/quote]