Does global warming make Triton blush?

Abstract

Neptune's largest moon, Triton, is one of two satellites in the Solar System that are currently geologically active. At least two geyser-like plumes were observed by the Voyager 2 spacecraft in 1989, and dozens of streaky deposits hint at the existence of many more¹. Triton also exhibits complex seasonal changes in its 165-year journey about the Sun². Because Triton's atmosphere transports volatiles (primarily nitrogen and methane) during this seasonal cycle, its atmospheric pressure may fluctuate by up to an order of magnitude over decades³. Photometric measurements of its albedo and colour over half a century show that seasonal volatile transport has occurred⁴. There have also been indications that more extreme, short-lived changes, perhaps due to geological events, have occurred on Triton. An anomalously red spectrum was reported for Triton in 1977 (56), and global warming has now been observed⁷.

Main

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.

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.