Friday, March 9, 2012

Nitrogen Geysers

Triton is by far the largest moon in orbit around Neptune and with a diameter of 2700 km; it is the seventh largest moon in the Solar System. Triton is also more massive than the combined mass of all known moons in the Solar System that are smaller than Triton itself. The orbit of Triton around Neptune is retrograde and this means that Triton orbits in a direction that is opposite to Neptune’s rotation. For this reason, it is believed that Triton did not form in orbit around Neptune. Instead, Triton is a captured Kuiper belt object. Triton’s surface is largely covered by frozen nitrogen overlying a water ice crust. With a mean density of 2.061 grams per cubic centimetre, the composition of Titan is approximately one-third water ice and two-thirds rock material.

Credit: Walter Myers

The only spacecraft to have ever visited Triton was NASA’s Voyager 2 which made a flyby of Triton in August 1989. During the flyby, Voyager 2 imaged geyser-like eruptions occurring on the surface of Triton. In these images, the geyser-like eruptions appear as columns of dark material rising to an altitude of about 8 km before trailing off over a hundred kilometres downwind. The dark material carried within these plumes gets deposited onto the surface of Triton, explaining the presence of dark streaks seen in the images acquired by Voyager 2. Measurements have shown that Triton has a globally uniform surface temperature of about 38 degrees Kelvin (minus 235 degrees Centigrade) and a predominantly nitrogen atmosphere with a surface pressure of approximately 16 micro-bars.

During the flyby of Voyager 2, Triton is experiencing southern summer where the Sun appears directly overhead at a latitude of 45 degrees south on the surface of Triton. The inclination of Triton’s orbit around Neptune and the inclination of Neptune’s spin axis means that Triton has an overall tilt of 53 degrees with respect to the Sun. In comparison, the Earth has a tilt of 23.5 degrees. The geyser-like eruptions on Triton occur when sunlight is absorbed by dark particles encased within the frozen nitrogen on Triton’s surface. This acts as a form of greenhouse effect which causes the temperature of the nitrogen ice to increase. A rise in temperature of only 4 degrees Kelvin is sufficient to increase the vapour pressure of nitrogen ice by an order of magnitude.

With the overlying layers of ice acting as a seal, the subliming nitrogen ice produces nitrogen gas which migrates into porous regions within the subsurface. Such a process charges the subsurface with pressurized nitrogen gas. When a fracture or weakness in the overlying ice layers is encountered, the nitrogen gas explosively decompresses and launches itself into the atmosphere of Triton. The sudden decompression of nitrogen gas causes a fraction of the nitrogen gas to condense into nitrogen ice crystals. Entrained within the erupting plume are fine dust particles and nitrogen ice crystals. At about 8 km in altitude, the plume trails off due to the shearing effect from high altitude atmospheric winds.

Given a temperature difference of 4 degrees Kelvin, the estimated initial velocity of an erupting plume can be as high as 180 m/s. In fact, an initial velocity of 100 m/s is enough to launch a plume to an altitude of 8 km. The estimated mass flux of nitrogen gas from a single erupting plume is about 400 kg/s and the power required to sustain such a mass flux is on the order of a hundred million watts. Given that the solar radiation incident on Triton is 1.5 watts per square meter and assuming that two-thirds of the incident solar radiation gets absorbed by the subsurface nitrogen ice, a surface area of about 100 square kilometres is needed to collect the power necessary to drive the mass flux.

Reference: L. A. Soderblom, et al., “Triton's Geyser-Like Plumes: Discovery and Basic Characterization”, Science 19 October 1990: Vol. 250 no. 4979 pp. 410-415