2014 Annual Science Report
VPL at University of Washington Reporting | SEP 2013 – DEC 2014
Understanding the Early Mars Environment
In this task VPL team members use Mars mission data and atmospheric models to understand the early environment on Mars. Areas of research include: the atmospheric formation of salts that have been found on the Martian surface, Early Mars volcanism and atmospheric composition, and possible atmospheric means of warming early Mars. Several VPL team members are also active on the MSL mission and have contributed to scientific discussions of modern geochemistry and the ancient habitability of Mars.
The history of the Martian climate is still a mystery. Data from Mars Curiosity provide the strongest evidence to-date for sustained, liquid water reservoirs at the surface (Grotzinger et al., 2015). However, because Mars is further from the Sun than the Earth is, and because the Sun used to be less bright than it is now, modelers have had difficulty re-creating a climate that would have allowed for this liquid water to persist. Our team has followed up on our earlier hypothesis that early Mars was kept warm by a H2-rich greenhouse (Ramirez et al., 2013). We have done this by further exploring the chemistry of H2-dominated atmospheres, the related issue of the redox state of the ancient Martian atmosphere, and the escape history of the planet.
Graduate student Natasha Batalha led other VPL team members (Kasting and Domagal-Goldman) on a study on whether or not H2 could have built up to the high concentrations proposed by Ramirez et al. (2013). In a recently submitted paper (Batalha et al., submitted to Icarus), they simulated the sources and sinks for H2, and conclude that these high concentrations were possible, but only if H2 escaped slowly from Mars, the early Martian mantle was very reduced, and if ancient Mars had active element recycling (likely via plate tectonics). There are implications of both the slow escape rate and the need for element recycling for measurements from NASA’s Curiosity rover. If future tests from the rover suggest these conditions were met, it would mean the hypothesized H2-rich greenhouse would have been plausible, and would provide a potential solution to one of the greatest mysteries on ancient Mars.
A separate group of team members (Catling and Smith) studied the history of volcanism on Mars. With past volcanic outgassing in the expected range, they found that the early martian atmosphere could have been anoxic and weakly reducing (Fig. 1), which is favorable for an origin of life and should leave geochemical traces in the soil (Sholes et al 2015). Simulated early atmospheres tend to be rich in carbon monoxide (CO) (e.g., in Fig. 1 there is 10% CO). Pathways involving CO and water have been proposed for prebiotic organic synthesis (Miyakawa et al 2002). Future work will combine this study with that of Batalha et al. to determine whether this amount of reducing power would allow for the H2-rich greenhouses previously proposed.
The amount of H2 in the atmosphere is also a function of the escape of H2 from the top of the atmosphere. Two VPL team members (Conrad and Domagal-Goldman) were on landmark papers that estimated the amount of H2 that escaped early in the history of Mars, by measuring the D/H ratio (Mahaffy et al., 2014) of a sample dated by analysis of noble gases in the same rocks (Farley et al., 2013). The combination of the D/H constraints on the early H loss history and the dating of the rock give a “time stamp” on the early atmospheric evolution of Mars.
Our team has also explored other aspects of habitability beyond climate and it’s relationship to the stability of liquid surface water. Two VPL team members (Conrad and Domagal-Goldman) have contributed to discussions of the modern geochemistry and the ancient habitability of Mars. Conrad wrote a Science perspectives piece on the habitability of ancient Mars (Conrad, 2014), which expands the consideration of habitability significantly beyond the presence/absence of liquid water. Domagal-Goldman presented habitability implications of an H2-rich greenhouse to the MSL team, and is working on developing rover tests for this hypothesis. The approach they are taking traces habitability from sub-surface sources of volatiles, through their release into the atmosphere, potential entrapment in mineral phases, and availability to any biology on the surface. By considering this elemental cycling of elements critical to Martian climate and to biological productivity, this work will form a more comprehensive approach to habitability questions. Thus, this line of thought will be central to the discussion and synthesis of future MSL papers that discuss the ancient habitably of Mars. Additionally, these studies will have implications for the instrumentation and landing site selection for future Mars missions.
Atmospheric chemistry simulations showed how the martian atmosphere could have had a reducing composition in the past because of ancient volcanic outgassing, and how salts should have been produced in an atmosphere similar to present through oxidation of volcanic gases during the last 3 billion years. This work was partially funded by VPL and NASA’s Mars Fundamental Research program. Work also tested the hypothesis that perchlorate and some other salts, sulfates and nitrates, were produced from atmospheric deposition over the last 3 billion years (Smith et al 2014). Although such deposition works well for sulfates, we found that gas phase reactions for making perchlorates are insufficient to explain levels of ~0.5wt% in soil found by NASA’s Phoenix Mars Lander. Thus, unknown gas-solid reactions are required. Work was also published that places such results in the broader context of the evolution of Mars and the general behavior of planetary atmospheres (Catling 2014, Catling 2015, Haberle et al 2015). Conrad is leading the chlorine working group for Mars Science Laboratory, which will place the work of Phoenix in a broader geological context through MSL analysis of chlorine chemistry in Gale Crater.
VPL co-I Conrad is a co-investigator to the Mars Environmental Dynamics Analyzer, which measures the present climate of Mars, launching on 2020. That instrument was selected for the payload during this period of performance, continuing to provide constraints on present climate from which we work backward to understand martian climate change since its formation.
Fig. 1. VPL has studied the consequences of volcanism on early Mars, and the relationship to the composition of the atmosphere.
Here, volcanic gas composition is based on a magma buffer for eruption into an atmosphere of low pressure similar to modern Mars (Fayalite-Magnetite-Quartz-0.5 log10 units, 0.4 wt% H2O, P=0.01 bar). Symbols are mixing ratios and map to the left-hand axis. Solid lines are fluxes (dep = deposition to surface; loss = escape to space for H2) and map to the right axis. As volcanic fluxes are increased, O2 levels decline and Mars’s atmosphere becomes anoxic and weakly reducing. For comparison, Earth’s magma flux is ~20-30 km3/year.
PROJECT INVESTIGATORS:Mark Claire
PROJECT MEMBERS:David Catling
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.