The 9th ray play an important part in the plot of the John Carter 2012 movie, but what are the 8 others ray? What could they do? Why the 9th is more important/powerful?
The first 7 rays are the colors of the rainbow, red, orange, yellow, green, blue, indigo, and violet. In the books, the 8th and 9th rays were colors not seen on Earth and one place John Carter sees them is shining from a jewel on the man who runs the atmosphere plant that generates the air so everyone can live on Barsoom.
When he sees this man, wearing a diadem, the diadem scintillates in all 9 colors of light, including 2 we've never seen on Earth, the 8th and 9th rays. Again, this is in the book. The 8th ray is what makes light move -- it's a propulsion ray that propels light away and is what they capture and store in the bladders of their fliers so they can fly. (When a bladder bursts, this ray leaks out and the flier falls.) The 9th ray is never explained in detail, but it's critical for them to use in the atmosphere plant to provide enough oxygen for everything on Barsoom to live.