A new spectrum of Triton near the time of the Voyager encounter

Abstract

We present a new spectrum of Triton from 5200 to 10,000 Å in which the 8900-Å methane ice absorption band can be clearly seen and accurately measured. The data were obtained during the summer of 1989, just before the Voyager II spacecraft encountered the Neptune system. By combining data from the spacecraft with our telescopic data, we have shown that the absorption is entirely caused by CH 4 ice on Triton's surface. We have modeled Triton's spectrum with a simple Hapke-type model which allowed us to set a 20-μm lower limit on the mean grain size of methane ice on Triton. We feel that the true grain size is probably somewhat larger, of the order 100 μm. Our model has led us to believe that methane ice is widely distributed on the surface of Triton's southern hemisphere. If CH 4 condenses together with N 2, the CH 4 must be significantly more concentrated relative to N 2 in the surface ice than it is in Triton's atmosphere.