46th Lunar and Planetary Science Conference (2015) 2423.pdf TRITON’S PLUMES – SOLAR-DRIVEN LIKE MARS OR ENDOGENIC LIKE ENCELADUS? C. J. Hansen1 and R. Kirk2, 1Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, Tucson, AZ 85719, [email protected], 2United States Geological Survey, 2255 N. Gemini Dr., Flagstaff, AZ 86001, [email protected] Introduction: Triton’s young surface with relatively few craters stands out among moons in the solar system and puts it in a class with Io, Europa, Titan and Enceladus – other moons with active surface processes today. Particulate plumes rising 8 km above the surface were imaged by Voyager in 1989, in Triton’s southern spring . Dark fans deposited on the surface were attributed to deposits from similar, nolonger-active plumes, as shown in Figure 1. The plumes were subsequently modeled as solar-driven expulsions of nitrogen carrying particles entrained from the surface [2, 3]. Triton’s warm interior: Triton’s surface age of <10 MY is derived from the lack of craters on its surface , likely erased by surface yielding, deformation and viscous relaxation. New models of Triton’s interior suggest that heating is ongoing and could not be a remnant from Triton’s capture into orbit around Neptune . A liquid mantle was first suggested as a result of Triton’s capture into orbit around Neptune [summarized in 6]. Later work showed that with a sufficient ammonia component a liquid layer could persist to present time . The new model of the interior of Triton shows that the combination of radiogenic heating with tidal heating due to Triton’s obliquity could sustain a long-lived subsurface ocean, and sluggish convection, even without invoking substantial ammonia . Figure 1. Dark fans of material are deposited across the south polar region of Triton in this Voyager image. Both of the active plumes can be seen rising vertically from the surface, then being bent by ambient winds. Source of the plumes: Are Triton’s plumes solardriven or do they come from a subsurface ocean? Are they more like Enceladus or the seasonal gas jets of Mars? Solar-driven activity – the Mars analogy. Triton’s nitrogen atmosphere is in vapor pressure equilibrium with surface ices, and will form polar caps in the winter. The solar-driven model for Triton’s plumes relies on a solid state greenhouse forming in/below a seasonal layer of nitrogen ice. A 4 K rise in temperature causes a 10x increase in vapor pressure, and this temperature difference is easily achieved . The detection of plumes by Voyager in late southern spring is consistent with the timing expected for solar-driven jets. The Voyager imaging team immediately noted the similarity to fans seen seasonally in Mars’ southern polar region. The discovery of the fans and modeling of the plumes on Triton later inspired the solar-driven model for the origin of the fan-shaped deposits imaged on Mars’ seasonal CO2 polar caps . This model postulates that gas from basal sublimation of a seasonal ice layer is trapped beneath impermeable translucent ice. Eventually when the pressure is high enough the ice will rupture and the gas will escape, entraining surface particles. The particulates fall out onto the top of the ice layer in fan-shaped deposits oriented by the ambient wind. The Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (HiRISE) images, shown in Figure 2, taken every spring have largely substantiated this model . The combination of HiRISE images and updated models of the jets have allowed us to quantify parameters such as gas exit speeds (~20-300 m/sec), mass flux (30-150 gm/sec), height achieved (50-100m), volatile storage requirements, and lifetimes < 2 hr . Figure 2. Fans deposited on the seasonal CO2 polar caps every martian spring are captured in this HiRISE image (ESP_011960_0925). 46th Lunar and Planetary Science Conference (2015) Eruption from the interior – the Enceladus analogy. We now have another possible comparison, with the Cassini discovery that Saturn’s moon Enceladus spews water vapor and ice particles from fissures across its south pole [11, 12, 13], shown in Figure 3. Enceladus showed us that it is possible to have regionally confined geophysical activity, likely driven by tidal energy [14, summarized in 15]. The most recent Cassini radio science data show that there is a subsurface gravity anomaly consistent with a body of liquid water 30 to 40 km below the south pole, extending up to ~50S latitude . Other recent observations give a source size of 9 m , with vapor exiting at speeds up to 1-2 km/sec in collimated jets , consistent with the postulate that warm vapor from a subsurface ocean exits through a nozzle-like openings to the surface . Vapor mass flux is on the order of 200 kg/sec . Solid particle flux is ~50 kg/sec . Figure 3. Cassini images show ice particles being erupted from fissures across Enceladus’ south pole . Summary: The solar-driven model has been the accepted explanation for many years for Triton’s plumes. The distribution of fans is consistent with that model, the timing of the eruptions coincided with southern spring, and it is eminently plausible in terms of energetics. Challenges with gas storage and the required layered surface structure were considered surmountable . More recent data and models however motivate a re-examination of the source of Triton’s plumes. The age estimate for Triton’s surface and recent tidal models incorporating obliquity were not available in the Voyager era. Study of Mars’ jets has allowed us to characterize and quantify solar-driven processes on that planet. The discovery of tidally-driven eruptions confined geographically on Enceladus and measurements such as vapor mass flux and exit speeds have expanded possible scenarios for Triton. The vapor 2423.pdf mass flux in particular, estimated at up to 400 kg/sec, is more similar to Enceladus than to the jets at Mars. Triton’s plumes reach 8 km altitude, erupting through an ambient atmosphere, before being carried away horizontally by the ambient wind. Although this can be achieved with solar-driven plumes, perhaps Triton’s eruptions come from a deeper source. An interesting test will be provided by New Horizons Pluto observations. Pluto does not experience obliquity tides and is thus unlikely to have a young surface similar to Triton . It does however have a nitrogen atmosphere in vapor pressure equilibrium with surface ice, that will form polar caps in the winter. If we see fans and/or plumes at Pluto in the north polar region now experiencing spring it will bolster the solar-driven hypothesis. References:  Smith et al. (1989) Science, 246, 1422-1450.  Soderblom, L. et al. (1990) Science, 250, 410-415.  Kirk, R. L. et al. (1990) Science, 250, 424-428.  Schenk, P. and K. Zahnle (2007) Icarus, 192, 135-149.  Nimmo, F. and J. Spencer (2014) Icarus, in press.  McKinnon, W. et al. (1995) in Neptune and Triton, ch. 17.  Hussmann, H. et al. (2006) Icarus, 185, 258-273.  Kieffer, H. H. et al. (2006) Nature 442, 793-796.  Hansen, C. J. et al (2010) Icarus, 205, 283-295.  Thomas, N. et al. (2011) Icarus, 212, 66-85.  Dougherty, M. et al, (2006) Science, 311, 1406-1409.  Hansen, C. J. et al. (2006) Science, 311, 1422-1425.  Porco, C. et al. (2006) Science, 311, 1393-1400.  Hedman, M. M. et al. (2013) Nature, 500, 182-184.  Spencer, J. and F. Nimmo (2013) Annual Reviews of Earth and Planetary Science, 41, 695-717.  Iess, L. et al. (2014) Science, 344, 78-80.  Goguen, J. et al. (2013) Icarus, 226, 1128-1137.  Hansen, C. J. et al. (2011) GRL, 38, L11202.  Schmidt, J. et al (2008) Nature, 451, 685-688.  Ingersoll, A. and S. P. Ewald (2011) Icarus, 216, 492-506.
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