Note that the map they produce will be of surface properties (like mineral composition), not methane.
The surface properties are the main intermediate product on the way to the climate-related overall goal (https://earth.jpl.nasa.gov/emit/science/objectives/). Namely, does the dust lofted from these deserts heat or cool the Earth?
I posted the original NASA article a few days ago, which has better info than this Smithsonian article: https://news.ycombinator.com/item?id=33349999