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Spectro-photometry of Neptune 20149/08/22

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#1 CPellier

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Posted 06 October 2019 - 10:42 AM

Hi all,
Here are spectroscopic and photometric results on Neptune, obtained with a 305 mm Dobsonian and a Star Analyzer. This is the first run of three I have done near opposition. Results are:

1) A reflectance spectrum, calculated with a nearby G0V star
2) A colour spectrum, calculated in flux with a normalized magnitude (planet at 1 AU from Sun, and 1 AU from Earth)
3) Photometric UBVRI magnitudes, with normalized magnitudes.
I have recently modified my synthetic filters to make them more in coherence with the Bessel curves. Colour coefficients have been recalculated, and re-tested on various variable stars, and the results are more accurate than with my previous filters and coefficients.
The results obtained on Neptune look:
- In good coherence with the published ALPO/R.Schmude data for B and V, with especially the expected B-V of +0,4
- Maybe sligthly off for R and I in comparison with the same data, at least judging from the normalized magnitudes. R and I are working fine on stars though.
- Not far from published U values (+8,2/+8,3) but this is maybe just the luck!
The next two runs have been taken with an additional 3,8° prism to correct the chromatic coma of the Star Analyzer. The spectra will be more precise, but I need to re-observe stars to calculate new coefficients before making photometry again.

Regards,

Attached Thumbnails

  • neptune_20190822_reflectance_pellier.png
  • neptune_20190822_colourspectrum_pellier.png
  • neptune_20190822_photometry_pellier.png

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#2 mwr

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Posted 06 October 2019 - 12:36 PM

Very nice spectra Christophe! Can all the absorption bands be attributed to methane or are there some bands of other molecules visible?

Here is my recent raw spectrum of Neptune (uncorrected and low DSLR quality):

 

Neptune's reflection spectrum with SA100/ Molecular absorption bands of methane

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#3 CPellier

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Posted 06 October 2019 - 02:31 PM

Thanks, mwr. I suppose that most of the absorption bands are CH4, yes. Although there might be some NH3 as well. I have not found any very precise spectroscopic map of Neptune and Uranus unfortunately.



#4 Organic Astrochemist

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Posted 06 October 2019 - 02:55 PM

Great spectra Christophe,
In his great book Richard Walker says that it’s too cold on Neptune for NH3. Amazing to think how easy it is to see CH4 given that it is only about 1.5% of the outer atmosphere.

Edited by Organic Astrochemist, 06 October 2019 - 03:14 PM.

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#5 robin_astro

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Posted 07 October 2019 - 06:12 AM

Amazing to think how easy it is to see CH4 given that it is only about 1.5% of the outer atmosphere.

Indeed and it is probably even more obvious further into the IR.  The concerns over the potential effects of increased CH4 in our own atmosphere as the permafrost melts are understandable.

 

Cheers

Robin


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#6 mwr

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Posted 07 October 2019 - 10:46 AM

Thanks, mwr. I suppose that most of the absorption bands are CH4, yes. Although there might be some NH3 as well. I have not found any very precise spectroscopic map of Neptune and Uranus unfortunately.

After some literature search I did find a publication by Karkoschka on the methane spectrum:

 

"Spectrophotometry of the Jovian Planets and Titan at 300- to 1000-nm Wavelength: The Methane Spectrum"

https://www.scienced...019103584711390

 

Here are the methane bands (in Angstrom; absorption coefficients can be also found in Karkoschka's paper):

4060

4200

4410

4600

4660

4860

5100

5220

5430

5570

5760

5960

6190

6430

6670

6830

7020

7270

7840

7980

8420

8640

8870

9130

9480

9520

9710

9890

Robin has already suggested it: even more and stronger bands are present at long wavelength IR!

 

A highly interesiting paper by Lutz et al. describes the aquisition of a laboratory methane spectrum using a 33 meter (!) base path White cell with up to 64 traversals giving a total path lenght of 2.112 km (!!):

http://adsabs.harvar...ApJ...203..541L

Now it is clear to me why methane is a colorless gas at normal dimensions (short path lenght in a flask) and becomes responsible the for the blue-greenish color of Neptune and Uranus at astronomical dimensions (long path lenght of reflected light in the atmosphere of these Jovian planets)!




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