Constraining the Habitability of Mars When Water Flowed on Its Surface
William Cassata | 20-LW-008
Our research aimed to constrain the timing and magnitude of atmospheric inputs (associated with the delivery of volatiles by impactors and planetary outgassing) and outputs (associated with atmospheric escape processes) on Mars in the past. The interplay between inputs and outputs determines past atmospheric pressures on Mars, which in turn are essential to constraining ancient climate conditions. Our approach to addressing the problem was to measure the elemental and isotopic compositions of atmospheric noble gases trapped in Martian meteorites. Noble gases are excellent tracers of atmospheric inputs and outputs, as they are chemically inert and their elemental and isotopic abundances are differentially modified by planetary outgassing and escape processes. These measurements can thus provide diagnostic fingerprints of atmospheric evolution in the past.
Based on measurements of argon isotopes in an ancient Martian meteorite, we found that (1) widespread loss (>95%) of argon and lighter atmospheric constituents occurred within the first 100 to 150 million years of planetary formation and (2) the present inventories of these gases are derived from planetary outgassing and/or the delivery of volatiles by impactors. Based on measurements of xenon isotopes in an ancient Martian meteorite, we found that xenon escaped as an ion coupled to a partially ionized hydrogen or oxygen wind for hundreds of millions of years after planetary formation. This finding has implications for the loss of other atmospheric constituents early in Martian history and will constrain future models of atmospheric escape on early Mars.
The project has impacted the mission through the (1) publication of novel science that can form the basis for program development, and (2) development of an analytical capability to meet future national security challenges. Research on the Martian atmosphere is directly aligned with current and planned Mars rover and orbiter missions, the goals of which are to search for signs of ancient life and study atmospheric dynamics. Our results may be used to develop future proposals for submission to NASA's Solar System Workings programs. We developed combined in situ laser ablation and electron microprobe analysis methods to measure gases in samples that have been imaged for chemical composition. This capability has applications to programmatic work requiring spatially resolved measurements. This research advances Lawrence Livermore National Laboratory's core competency in nuclear, chemical, and isotopic science and technology.
Publications, Presentations, and Patents
The following manuscripts are the direct result of this LDRD:
Cassata, W.S., Zahnle, K.J., Samperton, K.S., Stephenson, P.C., and Wimpenny, J., 2021, "Xenon isotope constraints on ancient Martian atmospheric escape." Accepted for publication. (LLNL-JRNL-824791)
The following manuscripts were partially supported by this LDRD, through funding for the PI:
Tremblay, M.M., and Cassata, W.S., "Noble gas thermochronology of extraterrestrial materials," Elements 16, 331-336. (LLNL-JRNL-809319)