Exploring Titan at UCF

Initial progression

Update

Below this update is some information of the project I am currently working on. As part of continued documentation, feel free to read, however, in this update, I am providing the latest report on my research having completed the light curves, a comprehensive explanation of my code and goals moving forward. Here is my most recent report:

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Background

While previous research of mine was more astronomy based, in general I gravitated towards anything space related, especially as part of a physics program that specializes in computational and matter based physics. I found Dr. Theodora Karalidi's research in planetary science quite compelling as an attempt to bridge what is both beyond us in space yet so close to us in life.

My research at UCF begins with image processing. While I am experienced in coding, I have rarely dealt with images. You can find the images in question here:

OPUS - Data Search for Outer Planets NASA Mission Data - NASA PDS Ring-Moon Systems Node

Image Processing

The primary object of my work with Dr. Karalidi is to make use of UCF's in house atmospheric models and apply it to Titan.

My first step towards this was processing the images. In particular, OPUS provides calibrated images from the Cassini mission and along with each image comes a variety of parameters within .csv files. One such parameter was the phase. While the phase angle of Titan for each image is contained within the header, I wanted to create a code such that for any arbitrary image, the phase could be determined instead of searching through the admittedly messy files. This involved:

1. Downloading images from URLs for ease.

2. Selecting only images where the whole disk of Titan can be seen in detection of the disk.

3. Running a detection algorithm and encircling the disk of Titan through a radius vs. apparent brightness plot.

4. Masking the disk of Titan and calculating the brightness and running it through an equation such that a fully bright Titan produces a 0 degree phase angle while a fully dark Titan would produce 180 degree phase.

Additionally, for further validation, I ran this code on 4 control images with known phase angles and the set of images for which the code was ran would have a specific filter such that the proceeding light curves take into account variation from different wavelengths.

The key parameters are the degree of polarization and the normalized brightness against the phase angles to produce light curves.

Updated Code

After discussions with Bob West, a star in the field of polarization, I made some much needed updates to my code. Before, I was dealing with raw images, however, OPUS gives access to calibrated versions of the images, where each pixel is calibrated to I/F and the measurement is reflectivity.

The calibrated images are in a .IMG extension, hence, I used rms-vicar to read the manipulate the image. Most of the light off the limb of Titan is light scattered from Titan within the camera, so instead of just summing the disk of Titan, to get an accurate measure of the integrated reflectivity, I sum all the pixels in the image, not just Titan. This meant deconvolving the image with the camera filter PSF (point spread function).

On getting geometric albedo times phase law from a whole-image sum of I/F, geometric albedo is defined to be the ratio of light scattered in the direct back-scatter direction (zero phase) to the light that would be directly back-scattered by a Lambertian disk of the same angular diameter as the planet disk. A Lambert surface at zero phase has I/F = 1.0. So if you have an image of Titan at zero phase angle, sum I/F and divide by the number of pixels (on the disk) to get a geometric albedo. At other phase angles the whole-disk reflectivity can be expressed by geometric albedo times a phase law.
That is calculated by summing I/F for all the pixels on Titan, including those on the night side, and dividing by the number of pixels subtended by the whole body. This implies to sum all the pixels on the disk of Titan instead of the whole image, then do the normalization.

Caveat: Titan's disk does not have a sharp edge due to the way the atmosphere thins out as altitude increases. One can define an 'effective' radius for the disk. How I went about this was to use my old code. So as to not mess with the image data, I created a copy of the image, ran my circle detection to find the center of the disk and an effective radius, than assigned those values to the original copy.

This is an example of my output on the CL1_CB2 filter.

Once the code it finalized, I will paste it here with a procedure. These graphs can be produced in minutes with my code.

Further, another parameter of interest is degree of linear polarization. Assessing the direction and intensity of light polarization is a vital aspect of this study. When spread across a stellar disk, light from solar-type stars tends to be nearly unpolarized. In contrast, light that has bounced off an object usually shows polarization, thanks to scattering and reflection in the planet's atmosphere and surface interactions.

Bob West has generously provided a code that runs on the IDL Virtual Machine to create polarized images. While it is a bit of a tedious process that require matching 2 to 3 images in a folder, over 30 times, and running the code on each folder it has dramatically saved time. While this section is still in progress, the first results of my analysis has produced, which will only be enhanced in the coming weeks.

This was done on the CL1_GRN filter.

Theoretical Comparisons
To bring meaning to the observational data, the next phase of this project involves learning Fortran in order to implement the theoretical atmospheric models into my code. Ultimately, this would allow us to understand the composition and dynamics of Titan's atmosphere, however, that phase is TBD.