The best workaround I've found so far is to add a Weather Report applet to one of the GNOME panels (right-click top or bottom panel > Add to Panel > Weather Report), for Location, select any available city on Earth (just as a placeholder, though I use MARSeille :-) and then in Preferences > General, Enable Radar Map and Use custom address for radar map, with the Address of a graphical version of the Mars weather report, e.g. http://150.135.111.162/weather/images/weather_image.png (image source page: http://www.theweathernetwork.com/marsweather/ ).
It would be great to have a more elegant and nicer-looking solution.
Perhaps add Mars > Phoenix Lander as a legitimate Location in the Weather Report applet? Just one option.