Interpreting spectra

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I got a spectra I need to interpret. It is used to calculate the rotational speed of Jupiter. However, altough the computer indicates the doppler shift, I dont see anything (see photograph). I wonder if anyone could enlighten my stalled mind?

I suppose when I figure out the wavelenght change I just plug it into the doppler formula and get the velocity? (12.66 km/s from data table)

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SpaceTiger

What you're seeing is frequency in the horizontal direction and position along the slit in the vertical direction. I don't know which lines you're looking at, but based on the orientation of the slit, you'd expect them to be slanted by the rotation of the planet's atmosphere. That is, at one end of the slit you'll have a velocity away from your line of sight and, therefore, a positive Doppler shift. At the other end, the atmosphere would be moving towards you, so it would exhibit a negative shift. The net result would be a line slanted from one direction to the other. Unfortunately, the resolution of their spectrum is very small. At 12.66 km/s, you'd expect a shift of order:

[tex]\frac{\Delta \lambda}{\lambda}=v/c=4.22 \times 10^{-5}[/tex]

If your spectrum is in the visible, then you're probably looking at wavelengths of order 5000 angstroms, so you should see a shift of about 0.2 angstroms. However, if the scale they list on the image is to be believed, then each pixel represents 1 angstrom and it would be hard to see the slant of the line. How many pixels are there per resolution element?

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By looking at the picture, can you see directly that a doppler shift have taken place? I dont get why the reference line is unusually bright

SpaceTiger

No, I can't, that's what I was saying. Do you happen to know the identity of the central line? I can't comment without knowing that. By the way, here's an example of a line twisted by rotation (this time with the spectrum in the vertical direction).

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ok here is the full spectrum in the near IR range. It's some odd lines on the left hand side, do you happen to see any doppler shift?

If i understand correctly, this is sunlight reflected off Jupiter, which means that if there is ant shift, it should be relative to the solar spectrum?

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SpaceTiger

No, but I still can't tell the wavelength range.


Some of the near-IR light would be emitted from Jupiter, but most of what you're seeing is reflected, that's right. Some of the absorption may also be from Jupiter's atmosphere.