Ice Formation, Exsolution, and Multiphase Equilibria in the Methane–Ethane–Nitrogen System at Titan Surface Conditions
Year: 2024
Type: article
Abstract: Titan is unique among the icy satellites in that it has a thick atmosphere, stable surficial bodies of liquid, and a precipitation system that promotes interactions between the two. Although Titan’s surface conditions are typically assumed to be above the freezing point temperatures of the major constituent species of the climate system (methane, ethane, and nitrogen), conditions may be sufficiently cool across parts of Titan to allow for ice formation alongside known liquid-vapor phases. In this study, we used Raman spectroscopy, visual inspection, and the CRYOCHEM 2.0 equation of state to map the appearance of first ice and to quantify the amount of nitrogen dissolution into liquid in the methane–ethane–nitrogen system along a 1.5 bar isobaric cooling path in the temperature range 80–95 K. This was with the intent of (1) determining the effects nitrogen has on the phase behaviors of the methane–ethane binary system, and (2) establishing the temperatures and ternary mixing ratios needed for ice formation on Titan’s surface. We found that ethane-rich mixtures enter a three-phase solid–liquid–vapor equilibrium and are characterized by nitrogen-rich exsolution upon freezing and ice that form at the bottom of the sample. With sufficient methane content, the mixtures cross a univariant four-phase solid–liquid–liquid–vapor boundary, which contributes to a distinct isothermal freezing point profile and ice that forms starting at the liquid–liquid interface. Our results generally agree with findings from previous studies of the methane–ethane–nitrogen system and are intended to add to our current understanding of Titan’s geochemical processes.
Source: The Planetary Science Journal
DOI: 10.3847/psj/ad6764
Language: English
Topic: Astro and Planetary Science
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,800
Awards: G1380325028, G2268656542, PREVIOUS STUDIES HAVE SHOWN THAT THE LAKES OF TITAN ARE COMPOSED OF METHANE AND/OR ETHANE BUT THE RELATIVE PROPORTIONS ARE MOSTLY UNCLEAR. UNDERSTANDING THE PAST AND CURRENT STABILITY OF THESE LAKES REQUIRES CHARACTERIZING THE INTERACTIONS OF LIQUID METHANE AND ETHANE ALONG WITH NITROGEN. PRIOR INVESTIGATIONS FOUND THAT THE FREEZING POINT OF METHANE IS DEPRESSED WHEN MIXED WITH NITROGEN. RECENT WORK SHOWS THAT ALTHOUGH METHANE AND ETHANE HAVE SIMILAR FREEZING POINTS WHEN MIXED THEY CAN REMAIN LIQUID DOWN TO 72 K. CONCURRENTLY WITH THE FREEZING POINT MEASUREMENTS WE ACQUIRE TRANSMISSION SPECTRA OF THESE MIXTURES TO UNDERSTAND HOW THE SPECTRAL FEATURES CHANGE WITH CONCENTRATION AND TEMPERATURE. ANY MIXING OF THESE TWO SPECIES TOGETHER WILL DEPRESS THE FREEZING POINT OF THE LAKE BELOW TITAN S SURFACE TEMPERATURE PREVENTING THEM FROM FREEZING. THE NEXT STEP IS TO DETERMINE HOW THE METHANE-ETHANE SYSTEM BEHAVES IN THE PRESENCE OF A NITROGEN ATMOSPHERE. PRELIMINARY RESULTS INDICATE THAT AT HIGH PRESSURE THE TERNARY SYSTEM CREATES TWO SEPARATE LIQUID PHASES. THROUGH SPECTROSCOPY WE DETERMINED THE BOTTOM LAYER TO BE NITROGEN RICH AND THE TOP LAYER TO BE ETHANE RICH. UNDERSTANDING THE FREEZING POINTS OF COMBINATIONS OF THESE SPECIES HAS IMPLICATIONS FOR NOT ONLY THE LAKES ON THE SURFACE OF TITAN BUT ALSO FOR THE EVAPORATION/CONDENSATION/CLOUD CYCLE IN THE ATMOSPHERE AS WELL AS THE STABILITY OF THESE SPECIES ON OTHER OUTER SOLAR SYSTEM BODIES. THESE RESULTS WILL GUIDE CURRENT THEORETICAL MODELS AND INTERPRETATION OF FUTURE OBSERVATIONAL DATA. WE WILL USE THE CRYOGENIC ASTROPHYSICAL ICE LAB AT NORTHERN ARIZONA UNIVERSITY TO MEASURE THE COMPOSITION AND STABILITY OF LIQUIDS AND ICES UNDER VARIOUS TEMPERATURES AND PRESSURES. OUR CRYOGENIC LABORATORY SETUP ALLOWS US TO EXPLORE ICES DOWN TO 30 K THROUGH IMAGING AND TRANSMISSION SPECTROSCOPY. SAMPLES WILL INCLUDE MIXTURES OF METHANE ETHANE AND NITROGEN. WE WILL ALSO PERFORM MODELING TO UNDERSTAND THE CHEMISTRY INTERACTIONS CAUSING THESE PHENOMENA. THE PROPOSED WORK WILL BE DIRECTLY APPLICABLE TO NASA S SOLAR SYSTEM WORKINGS OBJECTIVES OF CHARACTERIZING AND UNDERSTANDING THE CHEMICAL MINERALOGICAL AND PHYSICAL FEATURES OF PLANETARY SURFACES AND FLUID INVENTORIES THAT INTERACT WITH THE SURFACE. WE WILL ALSO DEVELOP THEORETICAL AND EXPERIMENTAL BASES FOR UNDERSTANDING THESE FEATURES IN THE CONTEXT OF THE VARYING CONDITIONS THROUGH TIME AFTER FORMATION., G549985663, BASED ON RECENT MODELING DISSOLVED NITROGEN APPEARS TO SIGNIFICANTLY REDUCE THE EVAPORATION RATE OF LIQUID METHANE ON THE SURFACE OF TITAN. WE HAVE DEVELOPED A NEW METHOD FOR CALCULATING THE EVAPORATION RATE OF LIQUID METHANE SATURATED WITH NITROGEN UNDER TITAN S 1.5 BAR ATMOSPHERE. THE NEWLY DERIVED EVAPORATION RATES WERE COMPARED TO EVAPORATION RATES CALCULATED FOR PURE METHANE AS A FUNCTION OF BOTH TEMPERATURE AND RELATIVE HUMIDITY. WE FIND THAT THE PURE METHANE ASSUMPTION SIGNIFICANTLY OVERESTIMATES THE METHANE EVAPORATION RATE FROM SURFACE POOLS ON TITAN. FOR EXAMPLE AT 94 K AND 60% HUMIDITY THE PURE METHANE ASSUMPTION OVERESTIMATES THE METHANE EVAPORATION RATE BY MORE THAN 40%. THIS PREVIOUS OVERESTIMATION OF EVAPORATION RATES LIKELY HAS A PROFOUND IMPACT ON PREVIOUS INTERPRETATIONS OF CASSINI VIMS OBSERVATIONS OF TITAN S SURFACE AND OF CLOUDS IN TITAN S ATMOSPHERE. FOR EXAMPLE RECENT OBSERVATIONS OF NORTHERN SUMMER CLOUDS IN TITAN S TROPOSPHERE HAVE BEEN SPARSER AND LESS FREQUENT THAN PREDICTED BY GENERAL CIRCULATION MODELS (GCMS). AN OVERESTIMATION OF METHANE EVAPORATION FROM TITAN S LAKES AND SEAS COULD EXPLAIN THIS DISCREPANCY BETWEEN OBSERVATIONS AND EXISTING PREDICTIONS. THE SEASONAL EVOLUTION OF THE METHANE AND ETHANE SEAS DEPEND ON NOT JUST THE EVAPORATION OF METHANE SATURATED WITH NITROGEN BUT ALSO ON THE EVAPORATIVE PROCESSES ASSOCIATED WITH NITROGEN-METHANE-ETHANE MIXTURES. WE PROPOSE TO MODEL THE EVAPORATIVE PROCESSES OF BINARY MIXTURES OF NITROGEN-METHANE AND TERNARY MIXTURES OF NITROGEN-METHANEETHANE AND TO THEN INCORPORATE THESE RESULTS INTO A GENERAL CIRCULATION MODEL (GCM) OF TITAN. WE WILL COMPARE THE GCM PREDICTIONS OF CLOUD FORMATION DERIVED USING THE NEW EVAPORATIVE RELATIONSHIPS TO CASSINI VIMS OBSERVATIONS OF CLOUDS (E.G. BROWN ET AL. 2009 AP.J 706; RODRIGUEZ ET AL. 2011 ICARUS 216; TURTLE ET AL. 2011 GRL 38). OUR PROPOSAL IS RELEVANT TO THE CDAP PROGRAM. IN PARTICULAR CDAP SOLICITS RESEARCH PROPOSALS TO CONDUCT SCIENTIFIC INVESTIGATIONS UTILIZING OR ENHANCING THE UTILIZATION OF DATA OBTAINED BY THE CASSINI MISSION. TASKS RESPONSIVE TO THIS CALL INCLUDE . . . 3) NONDATAANALYSIS TASKS THAT SIGNIFICANTLY ENHANCE THE USE OR FACILITATE THE INTERPRETATION OF MISSION DATA. OUR INVESTIGATION WILL ENHANCE THE USE OF CASSINI DATA AND FACILITATE THE INTERPRETATION OF CASSINI DATA. SPECIFICALLY OUR INVESTIGATION WILL PROVIDE NEW INSIGHT INTO CASSINI VIMS OBSERVATIONS OF TROPOSPHERIC CLOUDS. WE WILL IMPROVE THE INTERPRETATION OF THIS DATA THROUGH THE USE OF NEW STATE-OF-THE-ART MODELING OF THE EVAPORATIVE PHYSICS OF BINARY AND TERNARY MIXTURES IN THE LAKES/SEAS AND ON THE SURFACE OF TITAN. BECAUSE OUR PROPOSED WORK INCLUDES THE REINTERPRETATION OF CASSINI VIMS OBSERVATIONS AND IS SO CLOSELY TIED TO AND CONSTRAINED BY CASSINI OBSERVATIONS IT IS NOT WITHIN THE SCOPE OF OTHER NASA PROGRAMS INCLUDING SOLAR SYSTEM WORKINGS OR EMERGING WORLDS., G9000200692
OpenAlex ID: W4403275532