TRITON, MOON OF NEPTUNE

Draft version April 18, 2017 Preprint typeset using L A TEX style emulateapj v. 12/16/11 TRITON, MOON OF NEPTUNE Kyle A. Pearson Department of Planetary Sciences, Lunar and Planetary Laboratory, University of Arizona, 1629 East University Boulevard, University of Arizona, Tucson, AZ, 85721, USA 1. INTRODUCTION Before Voyager, Triton s atmosphere could only be inferred from the presence of CH 4 and N 2 frosts (Cruikshank et al. 1977). It is now clear Triton has an extended atmosphere that resembles, in some ways, features from Pluto and Titan. Airglow and occulatation measurements show the major gas constituent in the atmosphere is N 2. Methane is also present but at a much lower abundance than either Pluto or Titan. Atmospheric temperatures vary between 38K near the surface to 95K in the exosphere (Broadfoot et al. 1989). Differences in the atmospheric structure from other outer satellites can be attributed to the planet s low temperature which stems from the high albedo (A=0.719) and lack of IR-active molecules (e.g. CH 4, CO 2 ) which make radiative processes negligible. The thermal structure of the upper atmosphere is dominated by thermal conduction. Where as in the lower atmosphere, convection and the release of latent heat dominate the heat budget near the surface. Triton s atmospheric pressure-temperature grid is plotted in figure 1. Triton lacks an intervening radiative layer or stratosphere that separates the upper and lower atmosphere. This is due to the low abundance of active molecules that absorb sunlight, and heat stratospheres (e.g. CH 4 on Titan and O 3 on Earth). The pressure-temperature grid of Triton was derived from both emission spectra of airglow in the near-uv and occulation measurements infront of the sun and β Canis Majoris. The transmission spectrum is shows in figure 2. 2. DRY LAPSE RATE In the lower part of the atmosphere where convection is the dominant mechanism for heat transfer we can explain the temperature gradient as dt dz = g (1) C p where g is the gravitational acceleration, and C p is the specific heat at constant pressure (kj / Kg K). The dry lapse rate for Trition is -0.188 K/km using a calculated gravitational acceleration of 0.195 m/s 2 and specific heat of N 2 of 1.039 (kj / Kg K). N 2 is the primary consituent in the atmosphere. However methane is also present. Since methane has a larger specific heat than N 2 it will slow the lapse rate. To first order I am going to ignore the presence of CH 4. The dry lapse rate is only able to explain the temperature profile very close to the surface, perhaps to an altitude of one scale height or less. The temperature profile is mostly increasing in temperature from an altitude of 10km to 400km and then is isothermal above that. In the troposphere we assume parcels of air rise adiabatically such that no energy leaves or enters the parcel. Under such conditions convection will dominate heat transfer and we can safely estimate the dry lapse rate with Equation 1. Figure 1 shows the temperature as the T curve and we can see that temperature decreases as the alitude decreases. The authors use a conduction model assuming some heat source at the top of the atmosphere which propogates heat into the lower part of the atmosphere. Lower than 400 km I can esimate the temperature curve using the opposite slope (-dt/dz) from my lapse rate however it is unlikely for convection to explain this temperature slope because of the low number density and weak vertical mixing. 3. PLANETARY EMISSION The equilibrium temperature for Triton is calculated using Teq 4 L(1 A) = 4πD 2 (2) σ where A is the albedo of the moon, D is the distance from the sun, L is the solar luminosity, σ is the Stephanboltzmann constant. I find that the moon s temperature from balancing power in and power out is 52 K using an albedo value of 0.719 (Hicks & Buratti 2004). This value is not unreasonable and is within the limits of the TP profile shown in 1. It does not accurately capture the temperature in the exosphere of the moon but that is fine because non-lte will dominate in that region and this estimate assumes LTE. The peak emission wavelength from the moon assuming a temperature somewhere between 50 100 K would be between 57.9 µm and 28.9 µm (using Wien s law). At these wavelengths we would not have to take into consideration the flux from the Sun to model the outgoing flux of the planet because the Sun does not emit much light there. Wavelengths where the brightness temperature of the planet is much lower than that of the equilibrium temperature we would need to take into account the flux from the Sun because that radiation is being absorbed or scattered in the atmosphere. However, emission can occur if you measure a brightness temperature greater than the equilibrium temperature. Emission occurs because the optical depth of the atmosphere will absorb and scatter light to a greater extent, trapping the sun s radiation in the atmosphere, similar to a greenhouse effect. Even though Triton is closer to the Sun than Pluto, Pluto has a warmer atmosphere because it has a larger abundance of CH 4 than Triton. 4. ATMOSPHERIC COMPOSITION Spectropic measurements of Triton have revealed N 2, CH 4 and CO to be the most abundant gasses (Broadfoot et al. (1989); Lellouch et al. (2010)). Other measurements from surface reflection spectra have detected the

2 Figure 1. Model of Triton s atmosphere fitted to the measured concentrations and temperature (bold lines). Temperatures, shown at right, were obtained empirically over the 450- to 700- km range and extended downward by a conduction model, on the assumption of a heat source of 1.6 10 3 erg cm 3 s 1 concentrated in a thin layer at 400 km. Sudden changes in slope in the profiles oftemperature and scale height are due to this simplifying assumption. If there is a troposphere below 9 km, the temperature could break away as sketched. The variation of gravity with height has a major effect on the scale heights. Figure and Caption from Broadfoot et al. (1989). Figure 2. Triton solar occulation (A and B) Ingress and egress light curves at short wavelengths (569 634 A), where extinction is due to the N 2 ionization continuum. The solid lines show the transmission calculated from models with a temperature of 95K above 400 km. (C and D) Ingress and egress curves at longer wavelengths (1171 1236 A) where CH 4 is the dominant absorber. In the models used to compute the solid lines, the CH 4 scaleheights are 9.5km (ingress) and 8.5km (egress). The inset shows measured number densities and temperatures. The CH 4 abundance is shown for the entrance occultation. Figure and Caption from Broadfoot et al. (1989)

3 presence of H 2 O and CO 2 alongside N 2, CH 4 and CO, however constraints on the mixing ratios were not obtained (Holler et al. 2016). N 2, CH 4 and CO are the only molecules with estimated mixing ratios on Triton. The mixing ratios for CO and CH 4 are on the order of 10 4 relative to N 2 (Broadfoot et al. (1989); citelellouch2010). The methane and nitrogen abundances were measured from occultations of the moon in front of the sun and another star. The density of various species can be constrained with the transmission because the transmission places an upper limit on the column abundance. Voyager 2 measures transmission as a function of altitude and, since it can reliably resolve the surface of the moon we know the altitude of our measurements very well, unlike exoplanet observations. Assuming hydrostatic equilibrium and the ideal gas law we can then derive the pressure as a function of altitude. Estimating the pressure and density can then yield the temperature via the ideal gas law. Additional measurements of CH 4 and CO were made using high-resolution spectroscopy in 2.32-2.37 µm range using CRIRES at the VLT. These measurements were made of the surface/atmosphere composition and compared to various radiative transfer simulations to derive the expected column abundance of CO and CH 4. These measurements esimate the mixing ratio of ice CH 4 to be 2.4e-3 and CO to be 6e-4 relative to N 2 Lellouch et al. (2010). The ice mixing ratio for CH 4 is larger by a factor of 6 than the atmospheric mixing ratio measured by Voyager. The authors attribute this difference to the supply of CH 4 into the atmosphere most likely from sublimating CH 4 ice on the surface. Various assumptions went into estimating the mixing ratio for CH 4 and CO. The vertical temperature structure of the atmosphere was assumed to be constant over time and the authors of Lellouch et al. (2010) take the TP profile from occulation measurements in Broadfoot et al. (1989). This profile assumes the abundance of N 2 is not changing over time. Such changes to either the N 2 or other carbon-species could be caused by photochemical effects or the sublimation of ice on the surface back into the atmosphere, thus changing the mixing ratio. Uncertainties on these mixing ratios were not quoted directly in the paper but the authors do find an allowable range of values shown in Figure 3. The authors use a lineby-line radiative transfer code and compare their output models to their observations. Pressure broadening in the features is negligible because the surface pressure on the moon is very small 40µbar. 5. ATMOSPHERIC CLOUDS AND HAZE The atmosphere of Triton is very tenuous with models predicting the surface pressure to be on the order of a few 10s of microbars. The far orbital distance of 30 AU from the Sun creates an equilibrium temperature for the moon to be 50 K. This cold temperature does not allow for many molecular species (e.g. H 2 O, CO 2 ) to be present in the gas phase because they will condense out of the atmosphere. Methane has been observed in Triton s atmosphere from occultation measurements however it only exists at low altitudes ( 20 km and below) because the condensation temperature for CH 4 is 41 K (Lodders 2003). The low condensation temperature will only allow CH 4 to be present in the warmer lower part of the atmosphere because it will condense out at larger but cooler altitudes. While it could be possible for photolysis of methane to produce a haze on Triton it is unlikely to be very dense because the atmosphere has a very low pressure. Perhaps an alternate carbon baring species like CO could help supply the haze production since Neptune lies right at the snowline for CO ( 30 AU; Piso et al. (2015)). Hazes and clouds have been found on Triton at an altitude of roughly 100 km above the surface (see Figure 4). This large altitude suggests CH 4 will already be condensated out and that N 2 is the primary absorber. The boiling point of nitrogen is 77 K at standard atmospheric pressure and density and it will have a smaller condensation temperature at much lower pressures suggesting N 2 is a valid candidate to produce clouds. Seasonal variations on the moon will cause either the north or south pole to undergo sublimination of surfaces ices which feed into the atmosphere. An alternate mechanism to feed particulates into the atmosphere of Triton are the geyser-like plumes. Triton has active geyser-like eruptions that were discovered in Voyager 2 images that erupt unknown dark material into the atmosphere roughly 8 km high (Soderblom et al. 1990). This ejected material feeds into dark clouds that can drift as far as 100 km. The main mechanism for feeding the geysers involves heating nitrogen ice in a subsurface solid-state greenhouse environment (Cruikshank et al. 1995). The nitrogen gas becomes pressurized by solar heating and explosively erupts to the surface, carrying clouds of ice and dark particles into the atmosphere. The dark particles are thought to be remnants of primordial dark hydrocarbons that accreted onto Triton s surface or created with photolysis of organic polymers from CH 4. 6. ANNOTATED BIBLIOGRAPHY Broadfoot et al. (1989) - Ultraviolet spectrometer observations of Neptune and Triton, Atmospheric Properties of Triton Thomas (2000) The shape of Triton from Limb Profiles, yields the mean radius which can be used to calculate the surface gravity. Hicks & Buratti (2004) The spectral variability of Triton from 1997-2000, Albedo constraints, Geometric Albedo = 0.719 Prockter et al. (2005) A shear heating origin for ridges on Triton, Mechanisms for surface feature formations Jacobson (2009) The Orbits of the Neptunian Satellites and the Orientation of the Pole of Neptune, Orbital parameters for Neptunian satellites (3) REFERENCES Broadfoot, A. L., Atreya, S. K., Bertaux, J. L., Blamont, J. E., Dessler, A. J., Donahue, T. M., Forrester, W. T., Hall, D. T., Herbert, F., Holberg, J. B., Hunten, D. M., Krasnopolsky, V. A., Linick, S., Lunine, J. I., Mcconnell, J. C., Moos, H. W., Sandel, B. R., Schneider, N. M., Shemansky, D. E., Smith, G. R., Strobel, D. F., & Yelle, R. V. 1989, Science, 246, 1459

4 Figure 3. Atmospheric mixing ratios and composition of the ice boundary layer (film) in the detailed balancing model. (Top): CO-N2 system. A surface pressure of 40 µbar is assumed, as estimated for 2009. The dark blue line is the CO/N2 mixing ratio expected for pure ices. The green curve shows the CO mole fraction in the ice surface film, and the black curve is the CO/N2 atmospheric mixing ratio derived from the composition of the ice film by applying Raoults law. The range of CO/N2 atmospheric mixing ratios inferred in this work for this pressure ((218) 104) is indicated by the blue-colored region. It implies a CO/N2 mixing ratio in the surface film of (1.412)103 (see text). The surface film is therefore largely dominated by N2, and the total pressure is defined by N2 equilibrium at 39.075 K. (Bottom): CH4-N2 system. Calculations are here performed for a 14 bar pressure, appropriate for the Voyager conditions. The Voyager-determined CH4/N2 atmospheric mixing ratio at the surface level ((1.13) 104) is indicated by the yellow region. The colored lines have the same meaning as in the top panel, with CH4 replacing CO. The intersection of the black line with the colored area shows that explaining the observed CH4 mixing ratio and the total pressure requires a 38.339.6 K surface temperature and would imply a very high CH4 mole fraction (5080%) in the surface film, well beyond the solubility limit of CH4 in N2. The same conclusion is reached if the CH4 amounts measured in 2009 are used. Note that these diagrams remain similar at other surface pressures, the only change being the required ice temperature to sustain the total pressure. Figure and Caption from Lellouch et al. (2010)

5 Figure 4. A Voyager 2 image of the limb of Triton. The tenuous cloud at the top portion of the limb is thought to originate from geyser-like eruptions of pressurized N 2 beneath the surface. The temperature nitrogen beneath the surface only needs to increase 4K to drive the plumes to roughly an 8km altitude. Cruikshank, D. P., Matthews, M. S., & Schumann, A. M. 1995, Neptune and Triton, Miscellaneous investigations series / U.S. Department of the Interior, U.S. Geological Survey (University of Arizona Press) Cruikshank, D. P., Morrison, D., & Pilcher, C. B. 1977, ApJ, 217, 1006 Hicks, M. D., & Buratti, B. J. 2004, Icarus, 171, 210 Holler, B. J., Young, L. A., Grundy, W. M., & Olkin, C. B. 2016, Icarus, 267, 255 Jacobson, R. A. 2009, AJ, 137, 4322 Lellouch, E., de Bergh, C., Sicardy, B., Ferron, S., & Käufl, H.-U. 2010, A&A, 512, L8 Lodders, K. 2003, ApJ, 591, 1220 Piso, A.-M. A., Öberg, K. I., Birnstiel, T., & Murray-Clay, R. A. 2015, ApJ, 815, 109 Prockter, L. M., Nimmo, F., & Pappalardo, R. T. 2005, Geophys. Res. Lett., 32, L14202 Soderblom, L. A., Kieffer, S. W., Becker, T. L., Brown, R. H., Cook, A. F., Hansen, C. J., Johnson, T. V., Kirk, R. L., & Shoemaker, E. M. 1990, Science, 250, 410 Thomas, P. C. 2000, Icarus, 148, 587