Triton: Background and Science

Triton's History

Neptune was discovered on September 23rd, 1846. Less than a month later, on October 10th, William Lassell discovered the first satellite around Neptune. This satellite was named Triton (the son of Neptune/Poseidon in Greek mythology).

Triton is the largest moon of Neptune, with a diameter of 2,700 kilometers (1,680 miles). It also has a density of about 2.050 grams per cubic centimeter (the density of water is 1.0 gram per cubic centimeter). This is a higher density than that measured for almost any other satellite of an outer planet (Europa and Io have higher densities). This implies that Triton contains more rock in its interior than the icy satellites of Saturn and Uranus do.

Triton is the only large satellite in the solar system to circle a planet in a retrograde direction -- in a direction opposite to the rotation of the planet. This retrograde orbit and Triton's relatively high density has led some scientists to suggest that Triton may have been captured by Neptune as it traveled through space several billion years ago. If that is the case, tidal heating could have melted Triton in its originally eccentric orbit, and the satellite might even have been liquid for as long as one billion years after its capture by Neptune.

However, presently Triton is quite cold, with a surface temperature of -235ºC (-391ºF, 38ºK), and an extremely thin atmosphere (the atmospheric pressure at Triton's surface is about 14 microbars, 1/70,000th the surface pressure on Earth). Nitrogen ice particles might form thin clouds a few kilometers above the surface (click on photo at the right for an enlarged vies of tenuous clouds in Triton's atmosphere).

Triton has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. All the close-up images we have are from that encounter. Those images showed active geyser-like eruptions spewing nitrogen gas and dark dust particles several kilometers into the atmosphere. Voyager 2 found fascinating terrain, a surface scarred by enormous cracks, a thin atmosphere, and even evidence for ice volcanoes.

Science Overview

The atmosphere of Triton, like that of Mars and Pluto, is in vapor-pressure equilibrium with the frost on its surface. For Triton, changes in subsolar latitude, and the resulting changes in the insolation patterns, are predicted to lead to changes in the surface pressure of factors of 10 or more (see Yelle et al. 1995 for a review). Furthermore, because frost migrates from sunlit to unilluminated areas, the changing seasons affects the global appearance of Triton's surface. Because of Neptune's inclined orbit around the sun, and Triton's inclined orbit around Neptune, Triton's subsolar point varies with time in a complex manner. The sub-solar point reached -50º latitude in late 2000, the first time in over 350 years that the subsolar point has more than 30º from the equator. The seasonal changes should result in observable changes in Triton's atmospheric structure, photometric properties, and surface composition and microphysical structure.

The photometric changes should be easily detectable by modest ground-based telescopes, if observations are frequent enough. The Triton Watch program is designed to use the expertise of amateur and professional astronomers distributed around the globe who volunteer to observe Triton on a regular basis in an attempt to ``catch it in the act'' of large-scale change.

Predictions and Recent Observations

Triton's surface (e.g., Smith et al. 1989; McEwen 1990; Stern & McKinnon 1999) is widely thought to be younger, and more active, than almost any other planetary satellite. Its icy surface contains the volatile frosts N2, CO, and CH4, as well as less volatile CO2, coloring agents, and absorbers (see Brown & Cruikshank 1997). Triton's orbit and Neptune's tilt cause Triton to experience extreme seasonal variations in subsolar latitude; because of the exponentially sensitive temperature dependence of the volatility of the frosts, this leads to dramatic seasonal effects, including widescale frost migration and dramatic changes in atmospheric pressure (e.g., Trafton 1984; Spencer & Moore 1992). As volatile frosts migrate from sunlit to darker areas on Triton, they should expose underlying layers and change surface frost optical properties (e.g., Trafton 1984; Spencer 1990), affecting Triton's albedo, color, and spectral signature. Because of the object's high bond albedo (A=pq=0.89), the surface energy balance is extremely sensitive to these photometric changes. Ever since the first seasonal models of Triton (Trafton 1984), planetary astronomers have waited to detect observable effects of the predicted change as Triton approaches a ``major summer'' (which occurs every 640 years due to the precession of Triton's orbit; lesser solstices occur at ~200 yr intervals).

As a result of the global change, seasonally-deposited ices should sublime into the atmosphere, altering the photometric properties of Triton's surface, particularly in the blue. In addition to this seasonal change, repeated episodes of dramatic reddening of Triton's visible spectrum have been reported by ground-based observers, with timescales less than a year.

Both groundbased and Hubble Space Telescope (HST) data indicate that Triton appears to have begun its long-awaited epoch of global seasonal change.

Occultations from 1995 to 1998 show Triton's pressure at 50 km altitude increased by 40% between mid-1995 and late-1997 (Elliot et al. 2000a, 2000b). Visible photometry and spectroscopy show changes in Triton's visible colors (Figure 1) that last roughly one year (Buratti et al. 1999). Ultraviolet spectra obtained with the HST Space Telescope Imaging Spectrograh show that Triton's albedo and spectral slope in the UV appears extremely variable longward of 2400 Å (Young & Stern, 2001). Finally, near infrared spectra (1.4 to 2.5 µm) show pronounced changes in the strength of CH4 absorption features from 1980 to 1992, but little change between 1995 and 1998 (Brown et al. 1995; Hilbert et al. 1998).

A Selection of Triton-related Links

References

Triton Watch Project (TritonWatch@boulder.swri.edu)