EXPERIMENTAL CONSTRAINTS ON METHANE EVAPORATION AT THE LOW LATITUDES OFTITAN A. Luspay-Kuti1, V.F. Chevrier1, F.C. Wasiak1, L. A. Roe1, W.D.D.P. Welivitiya1, T. Cornet2, S.Singh1, E.G. Rivera-Valentin3 1Arkansas Center for Space and Planetary Sciences (FELD 202 Univer-sity of Arkansas, Fayetteville, AR 72701 USA; aluspayk@uark.edu), 2Laboratoire de Planetologie etGeodynamique de Nantes, (UMR 6112, CNRS, OSUNA, 2 rue de la Houssiniere, Nantes, FRANCE),3Brown University (324 Brook St. Box 1846, Providence, RI 02912 USA)

Introduction: The surface of Titan exhibits acharacteristic morphological dichotomy betweenthe polar regions and their well developed lakes[1], and the equatorial regions characterized byvast dune fields [2] and fluvial channels [3]. RecentCassini observations of a low latitude storm fol-lowed by extensive surface changes [4] show thatliquid methane could currently shape the equa-torial regions - a process also suggested by re-cent circulation models [5]. However, this pro-cess is mostly dependent on theoretical estimatesof methane precipitation and evaporation rates[6, 7]. Here we report the first experimental sim-ulations on the evaporation rate of liquid methaneunder simulated Titan conditions similar to thoseobserved at the Huygens landing site [8]. We dis-cuss the implications of our results on the evapo-ration process and stability of low latitude liquidson the surface of Titan.

Figure 1: Schematic diagram of the Titan simulation fa-cility and its various components. Numbers indicate thelocations of K-type thermocouples. ’T1’ is a T-type ther-mocouple used to monitor the temperature of the scale.

Methods: The experimental facility is specifi-cally designed to simulate the surface environmentof Titan [9]. Surface temperatures of 90-95 K arecreated by flowing liquid N2 through coils withinthe Titan module, while a 1.5 bar atmosphere ismaintained with pressurized N2. Once the re-quired temperature and pressure are achieved, weintroduce methane into a condenser, then drain the

liquid methane into a pan with a diameter of 15 cm(Fig. 1).

The mass loss of liquid methane over time, alongwith temperatures at various locations within theTitan module are continuously recorded. We usea gas chromatograph flame ionization detector tomeasure CH4 concentration inside the chamber,from which we calculate the methane partial pres-sure, thus the relative humidity. We maintain aCH4 mole fraction of ∼2×10−2.

Results: Evaporation rates are calculated byfirst finding the linear portion in the mass vs. timecurves (Fig. 2b) where the atmospheric tempera-ture in the chamber remains below 95 K. A least-squares fit to the data is then performed in orderto determine the evaporation rate for a given tem-perature range as the slope of the regression linewith corresponding uncertainties to a 95% confi-dence interval. Based on various runs with thesame experimental setup we determined an evap-oration rate of (3.1 ± 0.6)×10−4 kg s−1m−2.

Discussion: To apply the results for Titan, weuse a theoretical approach based on the diffusionof lighter CH4 molecules into a heavier N2 atmo-sphere. Buoyancy is expected to affect evaporationand therefore modify simple diffusion. We use amodified mass flux equation originally developedfor Martian ice [10]:

J = 0.17DCH4/N2∆η

( ∆ρρsurf

g

ν2

) 13

, (1)

where DCH4/N2 is the diffusion coefficient of CH4

gas in N2 , ∆η is the difference between the densityof CH4 above the liquid layer and the density ofCH4 in the ambient atmosphere, ∆ρ is the differ-ence between the density of the ambient gas andthe gas at the surface, g is the gravitational acceler-ation on Titan, and ν is the kinematic viscosity ofCH4.

Since our experiments are performed underEarth gravity, we correct for the difference in thegravity of Earth and Titan. From Eq. 1 we usea factor of ETitan/EEarth = (gTitan/gEarth)1/3.The resulting buoyancy driven evaporation rateapplied for Titan is (1.6±0.3)×10−4 kg s−1m−2.

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Due to the high solubility of N2 in CH4, we findthat the sample liquid will not be pure CH4, butrather a binary mixture of CH4-N2 with mole frac-tions of 0.84 and 0.16, respectively. This is in excel-lent agreement with results from Eq. 1 divided bythe density of the binary mixture (J = 1.52 ×10−4

kg s−1m−2), suggesting that mass transfer in ourexperiments is largely controlled by the concentra-tion difference in the simulated atmosphere andbuoyancy-driven diffusion. At the same time, ac-cording to the model of [11], the daily averagenon-radiative fluxes on Titan are 20 times higherthan previously thought [12], and are theoreti-cally available for convective energy. However,we do not see any evidence for active convectionin the chamber, only for passive convection in theform of buoyancy. Indeed, as shown in Figure2 d and c, the temperature close to the pan re-mains cooler than in the ambient atmosphere ofthe chamber. Moreover, the temperature profilesremain very parallel and show an overall slight de-creasing trend (Fig. 2c). These observations areindicative of heat conduction from the warmer up-per gas downward into the liquid.

Using our experimentally determined evapora-tion rate, the areal coverage of the observed surfacechange and the elapsed time during a low latitudestorm reported by [4] we estimated the maximumdepth of the evaporated liquid resulting from thestorm. We find the depth to be 2.4± 0.5 m and themaximum total mass (5.4±1.2)×1010 kg.

Conclusions: Based on the fact that Eq. 1very closely matches the evaporation rate in thechamber we conclude that evaporation is predom-inantly diffusion-driven in the chamber, althoughwe cannot exclude possible minor additional en-ergy sources that could contribute to the evapora-tion. This is expected to play an important role onTitan as well, resulting in the significantly fasterevaporation of the binary mixture than ternary liq-uids for purely energy controlled models.

Considering the significance of diffusion andbuoyancy in our experiments and the effect of heatbalance in the models, we propose that evapora-tion on Titan is most probably a combination ofthese effects, and so, should be taken into accountin modeling in the future.

At the same time, the results of our experimentsprovide an upper bound to the evaporation of liq-uid CH4 on Titan. Our results are within the modelestimates of 0.5 - 15 mm hr−1, thought to be re-quired for the formation of fluvial features [13].This implies that ponding, and even liquid flowmay be possible depending on the local topogra-

phy, in agreement with the presence of fluvial fea-tures and dunes in spite of the occasional heavystorm events.

Figure 2: Example run of an evaporation experiment.Top left: Captured image of liquid CH4 pouring out ofthe condenser through the solenoid valve (indicated byarrow). a) Mass vs. time throughout the run. The ver-tical lines indicate distinguished phases of the experi-ment. Between sections i and ii there is a plateau causedby ongoing heat transfer between the colder liquid andthe warmer ambient atmosphere. ii-iii: non-steady stateevaporation as the liquid and atmosphere equilibrate. b)Steady-state evaporation (Section iii in panel a)). The lin-ear fit is marked by the red line. Bottom: Correspondingtemperatures (d, c).

Acknowledgements:: This work was fundedby NASA OPR #NNX10AE10G.

References: [1] E. R. Stofan, et al. (2007) Nature445:61. [2] R. D. Lorenz, et al. (2009) GRL36:L03202. [3] R. D. Lorenz, et al. (2006) Science312:724. [4] E. P. Turtle, et al. (2011) Science331:1414. [5] T. Schneider, et al. (2012) Nature481:58. [6] G. Mitri, et al. (2007) Icarus 186:385.[7] J. L. Mitchell (2008) JGR 113:E08015. [8] A.Luspay-Kuti, et al. (2012) GRL 39:L23203. [9] F. C.Wasiak, et al. (2012) ASR In Press. [10] A. P.Ingersoll (1970) Science 168:972. [11] K. E.Williams, et al. (2012) PS&S 60:376. [12] C. P.McKay, et al. (1991) Science 253:1118. [13] J. T.Perron, et al. (2006) JGR 111:E11001.

2256.pdf44th Lunar and Planetary Science Conference (2013)