46th Lunar and Planetary Science Conference (2015) 2423.pdf

TRITON’S PLUMES – SOLAR-DRIVEN LIKE MARS OR ENDOGENIC LIKE ENCELADUS? C. J. Han- sen1 and R. Kirk2, 1Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, Tucson, AZ 85719, cjhan- [email protected], 2United States Geological Survey, 2255 N. Gemini Dr., Flagstaff, AZ 86001, [email protected].

Introduction: Triton’s young surface with rela- Source of the plumes: Are Triton’s plumes solar- tively few craters stands out among moons in the solar driven or do they come from a subsurface ocean? Are system and puts it in a class with Io, Europa, Titan and they more like Enceladus or the seasonal gas jets of Enceladus – other moons with active surface processes Mars? today. Particulate plumes rising 8 km above the sur- Solar-driven activity – the Mars analogy. Triton’s face were imaged by Voyager in 1989, in Triton’s nitrogen atmosphere is in vapor pressure equilibrium southern spring [1]. Dark fans deposited on the sur- with surface ices, and will form polar caps in the win- face were attributed to deposits from similar, no- ter. The solar-driven model for Triton’s plumes relies longer-active plumes, as shown in Figure 1. The on a solid state greenhouse forming in/below a sea- plumes were subsequently modeled as solar-driven sonal layer of nitrogen ice. A 4 K rise in temperature expulsions of nitrogen carrying particles entrained causes a 10x increase in vapor pressure, and this tem- from the surface [2, 3]. perature difference is easily achieved [3]. The detec- Triton’s warm interior: Triton’s surface age of tion of plumes by Voyager in late southern spring is <10 MY is derived from the lack of craters on its sur- consistent with the timing expected for solar-driven face [4], likely erased by surface yielding, deformation jets. and viscous relaxation. New models of Triton’s inte- The Voyager imaging team immediately noted the rior suggest that heating is ongoing and could not be a similarity to fans seen seasonally in Mars’ southern remnant from Triton’s capture into orbit around Nep- polar region. The discovery of the fans and modeling tune [5]. A liquid mantle was first suggested as a re- of the plumes on Triton later inspired the solar-driven sult of Triton’s capture into orbit around Neptune model for the origin of the fan-shaped deposits imaged [summarized in 6]. Later work showed that with a on Mars’ seasonal CO2 polar caps [8]. This model sufficient ammonia component a liquid layer could postulates that gas from basal sublimation of a sea- persist to present time [7]. The new model of the inte- sonal ice layer is trapped beneath impermeable translu- rior of Triton shows that the combination of radiogenic cent ice. Eventually when the pressure is high enough heating with tidal heating due to Triton’s obliquity the ice will rupture and the gas will escape, entraining could sustain a long-lived subsurface ocean, and slug- surface particles. The particulates fall out onto the top gish convection, even without invoking substantial of the ice layer in fan-shaped deposits oriented by the ammonia [5]. 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 [9]. The combination of HiRISE images and updated models of the jets have allowed us to quantify parame- ters 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 [10].

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 Figure 2. Fans deposited on the seasonal CO2 po- vertically from the surface, then being bent by ambient lar caps every martian spring are captured in this winds. HiRISE image (ESP_011960_0925).

46th Lunar and Planetary Science Conference (2015) 2423.pdf

Eruption from the interior – the Enceladus anal- mass flux in particular, estimated at up to 400 kg/sec, ogy. We now have another possible comparison, with is more similar to Enceladus than to the jets at Mars. the Cassini discovery that Saturn’s moon Enceladus spews water vapor and ice particles from fissures Triton’s plumes reach 8 km altitude, erupting across its south pole [11, 12, 13], shown in Figure 3. through an ambient atmosphere, before being carried Enceladus showed us that it is possible to have region- away horizontally by the ambient wind. Although this ally confined geophysical activity, likely driven by can be achieved with solar-driven plumes, perhaps tidal energy [14, summarized in 15]. Triton’s eruptions come from a deeper source. An The most recent Cassini radio science data show interesting test will be provided by New Horizons that there is a subsurface gravity anomaly consistent Pluto observations. Pluto does not experience obliq- with a body of liquid water 30 to 40 km below the uity tides and is thus unlikely to have a young surface south pole, extending up to ~50S latitude [16]. Other similar to Triton [5]. It does however have a nitrogen recent observations give a source size of 9 m [17], with atmosphere in vapor pressure equilibrium with surface vapor exiting at speeds up to 1-2 km/sec in collimated ice, that will form polar caps in the winter. If we see jets [18], consistent with the postulate that warm vapor fans and/or plumes at Pluto in the north polar region from a subsurface ocean exits through a nozzle-like now experiencing spring it will bolster the solar-driven openings to the surface [19]. Vapor mass flux is on the hypothesis. order of 200 kg/sec [18]. Solid particle flux is ~50 kg/sec [20]. References: [1] Smith et al. (1989) Science, 246, 1422-1450. [2] Soderblom, L. et al. (1990) Science, 250, 410-415. [3] Kirk, R. L. et al. (1990) Science, 250, 424-428. [4] Schenk, P. and K. Zahnle (2007) Icarus, 192, 135-149. [5] Nimmo, F. and J. Spencer (2014) Icarus, in press. [6] McKinnon, W. et al. (1995) in Neptune and Triton, ch. 17. [7] Hussmann, H. et al. (2006) Icarus, 185, 258-273. [8] Kieffer, H. H. et al. (2006) Nature 442, 793-796. [9] Hansen, C. J. et al (2010) Icarus, 205, 283-295. [10] Thomas, N. et al. (2011) Icarus, 212, 66-85. [11] Dougherty, M. et al, Figure 3. Cassini images show ice particles being (2006) Science, 311, 1406-1409. [12] Hansen, C. J. et erupted from fissures across Enceladus’ south pole al. (2006) Science, 311, 1422-1425. [13] Porco, C. et [13]. al. (2006) Science, 311, 1393-1400. [14] Hedman, M. M. et al. (2013) Nature, 500, 182-184. [15] Spencer, J. Summary: The solar-driven model has been the and F. Nimmo (2013) Annual Reviews of Earth and accepted explanation for many years for Triton’s Planetary Science, 41, 695-717. [16] Iess, L. et al. plumes. The distribution of fans is consistent with that (2014) Science, 344, 78-80. [17] Goguen, J. et al. model, the timing of the eruptions coincided with (2013) Icarus, 226, 1128-1137. [18] Hansen, C. J. et southern spring, and it is eminently plausible in terms al. (2011) GRL, 38, L11202. [19] Schmidt, J. et al of energetics. Challenges with gas storage and the (2008) Nature, 451, 685-688. [20] Ingersoll, A. and S. required layered surface structure were considered P. Ewald (2011) Icarus, 216, 492-506. surmountable [3]. 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 mod- els 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 measure- ments such as vapor mass flux and exit speeds have expanded possible scenarios for Triton. The vapor