Triton's plumes - Insights into Triton's ocean world status

Introduction: Triton's young surface with relatively few craters stands out among moons in the solar system and puts it in a class with geologically active Io, Europa, Titan and Enceladus. Particulate plumes rising 8 km above the surface were imaged by Voyager in 1989, in Triton's southern spring [1]. Dark fans deposited on the surface were attributed to deposits from similar, no-longer-active plumes. The plumes were modeled as insolation-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 [4], likely erased by surface yielding, deformation and viscous relaxation. A new model of Triton's interior suggests that the combination of radiogenic heating with ongoing tidal heating due to Triton's obliquity could sustain a long-lived subsurface ocean even without invoking substantial ammonia; thus remnant heat from Triton's capture into orbit around Neptune is not required to sustain endogenic geological activity [5].Source of the plumes: Are Triton's plumes solar-driven or do they come from a subsurface ocean? Are they more like the endogenic eruptions at 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 [3]. The discovery of the fans and modeling of the plumes on Triton inspired the solar-driven model for the origin of the fan-shaped deposits imaged on Mars' sea-sonal CO2 polar caps from seasonal CO2 gas jets [6]. Mars Reconnaissance Orbiter High Resolution Imaging Sci-ence Experiment (HiRISE) images have largely substantiated this model [7]. 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 [8]. Eruption from the interior - the Enceladus analogy. We now have another possible comparison, with the Cas-sini discovery that Saturn's moon Enceladus spews water vapor and ice particles from fissures across its south pole [9, 10, 11]. Enceladus showed that it is possible to have regionally confined geophysical activity, driven by tidal energy [12]. Observations indicate vapor exiting at speeds up to 1-2 km/sec in collimated jets [13]. Vapor mass flux is on the order of 200-300 kg/sec [7]. Solid particle flux is 50 kg/sec [14].Pluto comparison. Pluto has an N2 atmosphere in vapor pressure equilibrium with surface frost, similar to Tri-ton. Although the New Horizons flyby of Pluto was at a season in which solar-driven fans could have been active none were imaged [15]. 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 [3]. More recent data and models however motivate a re-examination of the source of Triton's plumes. The age esti-mate 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 possibility that Triton's plumes could be endogenic and sourced from sub-surface liquid is deserving of further investigation and would solidify Triton's identity as an ocean world.References: [1] Smith et al. (1989) Science 246:1422. [2] Soderblom, L. et al. (1990) Science 250:410. [3] Kirk, R. L. et al. (1990) Science 250:424. [4] Schenk, P. and K. Zahnle (2007) Icarus 192:135. [5] Nimmo, F. and J. Spencer (2015) Icarus 246:2. [6] Kieffer, H. H. et al. (2006) Nature 442:793. [7] Hansen, C. J. et al (2010) Icarus 205:283. [8] Thomas, N. et al. (2011) Icarus, 212:66. [9] Dougherty, M. et al, (2006) Science 311:1406. [10] Hansen, C. J. et al. (2006) Science 311:1422. [11] Porco, C. et al. (2006) Science 311:1393. [12] Hedman, M. M. et al. (2013) Nature 500:182. [13] Hansen, C. J. et al. (2011) GRL, 38, L11202. [14] Ingersoll, A. and S. P. Ewald (2011) Icarus 216:492. [15] Hofgartner, J. D. et al., (2018) Icarus 302:273.