We obtained four spectra of Triton between 0.35 and 0.95 μm using the 200-inch Hale telescope and the double spectrograph at Palomar Mountain Observatory on 23 October 1997. The spectral resolution was 10 Å between 0.35 and 0.55 μm, and 5 Å between 0.55 and 0.95 μm. We analysed the data according to standard procedures, and the 4-arcsecond slit was aligned to minimize the possibility of differential refraction. Our reduced spectrum for Triton is compared with previous observations in Fig. 1 : it is significantly different from that normally observed, but is similar to the anomalous 1977 spectrum5,6. The upper limit for the timescale for our observed spectrum is seven years (the period between the last published spectrum in the range 0.35-0.55 μm (ref. 4) and October 1997).

Figure 1: Spectral reflectivity of Triton.
figure 1

Our data were obtained in October 1997 at the Palomar Mountain Observatory (dots). The spectrum from the Voyager Imaging Science Subsystem and Photopolarimeter Subsystem (diamonds) is representative of Triton's colour in 1990 and just before, whereas the 1977 data5,6 (triangles) represent a previous observation of a reddening of Triton's spectrum.

A global temperature increase of nearly 2 K was observed on Triton between 1989 and November 1997 (ref. 7). Our observations indicate that this change may be due to deposition of material from a geological process on Triton. Both the spectral changes and the global warming may have been caused by a triggering event, such as massive venting, causing the deposition of dark, red material onto the surface of Triton. The spectrum of the streaks that appear to be plume deposits are indeed darker and redder than Triton as a whole8. The most volatile, fresh, blue nitrogen ice may initially sublimate to expose a lower layer of red photolysed methane ice9, perhaps causing an additional temperature rise. Of the mapped geological regions, our spectrum is most like that of a region that is probably rich in irradiated methane clathrate8. Our spectrum has a more pronounced methane absorption band at about 0.73 μm than a spectrum obtained in 1989 (ref. 10).

Whatever the triggering mechanism(s), Triton's surface will warm significantly as its visual and near-ultraviolet albedo decrease. Triton's Bond albedo (A=pq, where p is the geometric albedo and q is the phase integral) is among the highest of any body in the Solar System. Its energy balance is therefore particularly sensitive to small variations in its spectral albedo. At the time of the Voyager 2 encounter, Awas 0.89 in the visual region of the spectrum, where most of the solar flux occurs1. Because T ≈ (1−A)1/4, where T is the temperature, a decrease of only about 3% in the Bond albedo would account for the observed temperature increase7. Increases in the absolute (geometric) albedo, which our observations were not designed to measure, may mitigate some of the decrease in the Bond albedo.

Models for global seasonal change on Triton that include only the effects of solar insolation could underestimate global temperature changes and the atmospheric loading of volatiles.