Trapped Ar isotopes in meteorite ALH 84001 indicate Mars did not have a thick ancient atmosphere

Abstract

Water is not currently stable in liquid form on the martian surface due to the present mean atmospheric pressure of ∼7 mbar and mean global temperature of ∼220 K. However, geomorphic features and hydrated mineral assemblages suggest that Mars’ climate was once warmer and liquid water flowed on the surface. These observations may indicate a substantially more massive atmosphere in the past, but there have been few observational constraints on paleoatmospheric pressures. Here we show how the [superscript 40]Ar/[superscript 36]Ar ratios of trapped gases within martian meteorite ALH 84001 constrain paleoatmospheric pressure on Mars during the Noachian era [∼4.56–3.8 billion years (Ga)]. Our model indicates that atmospheric pressures did not exceed ∼1.5 bar during the first 400 million years (Ma) of the Noachian era, and were <400 mbar by 4.16 Ga. Such pressures of CO[subscript 2] are only sufficient to stabilize liquid water on Mars’ surface at low latitudes during seasonally warm periods. Other greenhouse gases like SO[superscript 2] and water vapor may have played an important role in intermittently stabilizing liquid water at higher latitudes following major volcanic eruptions or impact events.