While reading this paper about the possibilities JWST will provide to study Titan, I have come across a graph that plots brightness (not sure what units it is in) against wavelength in micron. Here it is: NIRSpec IFU mode for surface features on Titan

The description of the graph says: "Titan NIR spectrum for various terrain types based on Cassini VIMS data. Shown above are the saturation thresholds for various slit (top) and IFU (bottom) observing modes of NIRSpec, showing that low spectral resolution modes will saturate the detectors around 1 μm and up to 2 μm for standard frame readout times."

From what I understand, this graph is supposed to show what the NIRSpec instrument on top of the JWST can help us see regarding the chemical composition of Titan's surface. However, when I am looking in the graph I do not understand what exactly I am seeing.

Could anyone with more knowledge about this type of graphs explain what exactly do I see in this plot? For example, I would like to know what is the meaning of the sentence "Shorter wavelengths will likely saturate NIRSpec, especially at lower resolutions" (taken from this slideshow, where it appeared next to the graph) with regard to this graph, as well as other conclusions that can be drawn from it.

  • $\begingroup$ I know spectroscopy, but I'm used to radiance units or brightness temperature. The logarithmic axis and shape reminds me of opacity, but that's unitless. $\endgroup$ – gerrit Mar 6 '18 at 14:30
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    $\begingroup$ @gerrit the Jy should be Jansky 1 Jy is 1E-26 Watts/Hz/m^2 at the measurement site. But I can't understand the rest of the legend i.stack.imgur.com/Rxvl8.png Here are some slides of the same title and the same plot is there on page 9. Perhaps this helps? Oh! A downloadable copy is available in Researchgate as well. $\endgroup$ – uhoh Mar 6 '18 at 15:16
  • $\begingroup$ It looks like Jy/arcsec^2 which would make sense if it was surface brightness for a resolved object. $\endgroup$ – uhoh Mar 6 '18 at 15:19
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    $\begingroup$ @uhoh, yes it does make sense, I have seen it in other papers in the same context, I just wasn't sure about it here. $\endgroup$ – Don_S Mar 6 '18 at 15:20
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    $\begingroup$ @uhoh Excellent. So Jy/arcsec² is just another unit of spectral radiance, then. Then why label the y-axis brightness? Astronomers! ;-) $\endgroup$ – gerrit Mar 6 '18 at 15:52

In general, a spectrum can tell us about the composition of the source of the radiation, or of any substance between the source and the detector. Each material has its own fingerprint and spectroscopy allows us to decompose this in constituent parts.

Shorter wavelengths will likely saturate NIRSpec, especially at lower resolutions

Without having studied the details, that may make sense for a detector array. The figure shows that Titans clouds are a factor 500 brighter at 1 µm than at 4 µm. The red lines show the level at which the detectors saturate. At a lower resolution, each pixel obtains photons from a larger solid angle, so it reaches its maximum number of photons per unit time (maximum brightness = saturation) at a lower intensity, compared to a higher resolution with more individual detectors. Wherever the thin lines in the figure are larger than the thick lines, the detector would be saturated and the magnitude of the peaks of the thin lines would not be detectable.


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