The Mars D/H Fractionation Factor as a Function of Temperature and Water Vapor
Abstract
Water once ran on the surface of Mars, but most has escaped to space over the planet's history. Geological and atmospheric modeling estimates of the lost water do not fully agree, ranging from 10s to 1000s of meters global equivalent layer (GEL). Key to improving the accuracy of atmospheric modeling estimates is the fraction factor f , which measures the efficiency of escape of deuterium (D) to hydrogen (H) via Jeans escape. Because water (H2O or HDO) is the main reservoir of H and D on Mars, understanding H and D escape rates leads to understanding water loss.The fractionation factor, in combination with the D/H ratio of the present atmosphere, can be used in the Rayleigh distillation equation to estimate change in water inventory over time. Past studies by Yung et al. (1988) and Krasnopolsky (2000, 2002) produced "best estimate" values for f , but f as a function of atmospheric properties has not been previously explored. In this work, we use a 1D photochemical model of the martian atmosphere, modified from the original by Chaffin et al. (2017) to include D chemistry, to study how f varies with atmospheric temperature and water vapor and the implications for integrated water loss. Our temperature profiles are constructed to represent global mean conditions with small variations, constrained by output from the Mars Climate Database and data from the MAVEN orbiter and the MSL Curiosity Rover. Water vapor profiles represent conditions from a nominal day to global dust storms. Using these profiles as model input, we find that f is 1-3 orders of magnitude smaller than previous estimates. The results are consistent with water loss between 30 and 170 m GEL (depending on the D/H ratio), in agreement with the more conservative past estimates. In addition, we find that the quantity of water lost is most sensitive to the D/H ratio when f is small (<0.01), and most sensitive to f i tself when f is large (>0.01). Future work will include incorporation of thermal balance to the model to allow for study of the time variation of f , allowing us to more tightly constrain water loss.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.P52C..04C
- Keywords:
-
- 0343 Planetary atmospheres;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 6225 Mars;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 5405 Atmospheres;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS;
- 5445 Meteorology;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS