Photometric band ratios for various surface pressures as a function of oxygen fugacity at a_c = 10⁻⁶. We overplot the extreme values of graphite activity, a_c, 10⁻⁷ and 10⁻³ for 100bar surface pressure. Top: ratio of the 12.8 μm to 15 μm spectral bands. Middle: ratio of the 15 μm to 21 μm. Bottom: ratio of the 10 μm to 15 μm. Variation in surface pressure is indicated with different colored shapes.