# Where to get Cassini VIMS data and optical transmission spectra of Titan's atmosphere?

In the extensive 2010 paper HYDROCARBON DEPOSITS ON TITAN by Clark et al. there are two plots that show Titan's atmospheric transmission windows. These plots (Figures 1 and 5) can be used to indicate which wavelengths Titan's atmosphere absorbs light and obscures the surface and at which wavelengths the surface can be seen.

I am assuming the raw data used to plot these graphs is publicly available. If it is, where can I find it? I want to try and plot it on my own in MATLAB.

• I think it has been plotted using the ball park estimates of gas concentration of Titan. If so I wonder how they measured concentration of gases. Spectrometers only provides composition Apr 26 '18 at 15:11

This is a preliminary answer, I believe a better answer can be posted, but this will hopefully get you started.

Cassini's VIMS or Visual and Infrared Mapping Spectrometer scans a slit across a camera's field of view. At each point in the scan, the 1D slit image is dispersed by a diffraction grating producing a 2D spectrogram which is recorded by either 2D CCD for visible, or one line at a time in infrared by an InSb linear array. The slow, animated GIF below is from here

VIMS is actually 2 cameras in one: one for visible wavelengths and one for the infrared.

The visible channel (VIMS-V) is a 4.5 cm telescope that deflects its beam through slit, and then through a diffraction grating. The slit determines the field of view, allowing in only light along a line. The diffraction grating is a grooved mirror such that light reflecting from each groove interferes with the light coming from other grooves in a way that causes light to be dispersed according to wavelength, like a prism. Devices that disperse light into its component colors for analysis are called spectrometers. The light is finally focused on a CCD (Charge-Coupled Device) detector. The CCD is an array of 256x512 elements that each count the number of photons that they receive. CCDs are the detectors in digital cameras and newer digital camcorders.

[...]The infrared part of VIMS, VIMS-IR, is similar but only has a 1-dimensional InSb (Indium Antimonide, a chemical compound) detector. Thus it can only take the spectrum of one point at a time. In the amount of time it takes the optical half to make one complete exposure of the spectrum of a line of points, the IR half has to take the spectrum of 64 different points one at a time. To do this it requires a larger telescope with which to collect light, 23 cm, than did the optical assembly.

Cassini has spent many years orbiting in the Saturn system, and so it's history and dataset are huge! Data may be separated and referred to by date, or by orbit number. For example, some of the data in your paper is stated to come from Orbit 69 which was an extremely close flyby of Titan:

2008-05-28 (69) Targeted Titan flyby (1360 km)


However there have been several further flybys when Cassini was close enough to do detailed imaging and spectroscopy of Titan. You can track down events, flybys and dates starting the following links:

VIMS data appears to be catalogued as data cubes at https://pds-imaging.jpl.nasa.gov/volumes/vims.html

The page https://pds-rings.seti.org/cassini/vims/ contains the following potentially helpful tabs:

• About VIMS Data - Discussion of the organization of the archive and links to key documents.
• Finding Data - Suggestions on how to search the archive for specific data.
• Accessing Data - Links to the data volumes.
• VIMS Calibration - Discussion of, and links to, available VIMS calibration files and software.