Photochemical escape of oxygen from Mars: First results from MAVEN in situ data

Abstract

Photochemical escape of atomic oxygen is thought to be one of the dominant channels for Martian atmospheric loss today and played a potentially major role in climate evolution. Mars Atmosphere and Volatile Evolution Mission (MAVEN) is the first mission capable of measuring, in situ, the relevant quantities necessary to calculate photochemical escape fluxes. We utilize 18 months of data from three MAVEN instruments: Langmuir Probe and Waves, Neutral Gas and Ion Mass Spectrometer, and SupraThermal And Thermal Ion Composition. From these data, we calculate altitude profiles of the production rate of hot oxygen atoms from the dissociative recombination of O2+ and the probability that such atoms will escape the Mars atmosphere. From this, we determine escape fluxes for 815 periapsis passes. Derived average dayside hot O escape rates range from 1.2 to 5.5 × 1025 s−1, depending on season and EUV flux, consistent with several pre‐MAVEN predictions and in broad agreement with estimates made with other MAVEN measurements. Hot O escape fluxes do not vary significantly with dayside solar zenith angle or crustal magnetic field strength but depend on CO2 photoionization frequency with a power law whose exponent is 2.6 ± 0.6, an unexpectedly high value which may be partially due to seasonal and geographic sampling. From this dependence and historical EUV measurements over 70 years, we estimate a modern‐era average escape rate of 4.3 × 1025 s−1. Extrapolating this dependence to early solar system, EUV conditions gives total losses of 13, 49, 189, and 483 mbar of oxygen over 1–3 and 3.5 Gyr, respectively, with uncertainties significantly increasing with time in the past.

Estimates of Ionospheric Transport and Ion Loss at Mars

“…Q ion is about 5–10% of the total O loss from photochemistry (cf. Lillis et al, ; Cravens et al, , and references therein). Of more interest is the variation with solar EUV irradiance and with upstream solar wind pressure (i.e., variation goes as the square root).…”

“…A major loss process is the photochemical escape of O due to the dissociative recombination of O 2 + ions that are present in the exosphere. Many papers have been devoted to this topic (see Cravens et al, ; Lillis et al, , , and Fox & Hać, , and the many references therein). Another loss mechanism is transport of ions from the planet.…”

Effect of Mars Atmospheric Loss on Snow Melt Potential in a 3.5 Gyr Mars Climate Evolution Model

“…The current atmospheric pressure on Mars is approximately 600 Pa, but at the temperatures that would allow liquid water on the surface of Mars, the approximately 600 Pa of additional CO 2 currently trapped in Martian polar caps would be released into the atmosphere, and so we assume a modern‐day atmospheric pressure of 1,200 Pa (Bierson et al, ). Atmospheric loss to space was parameterized as a power law where P is the atmospheric pressure in Pa, t is time in Gyr since the Sun formed, t 0 =4.56 Gyr, and k = 1,270 Pa Gyr −1 is a constant chosen to match the parameterization to the current rate of Mars atmospheric loss due to solar UV flux, which we estimated using MAVEN measurements of hot atomic oxygen escape (Lillis et al, ). This estimate of k assumes that the oxygen loss which was measured by MAVEN corresponds entirely to loss of CO 2 over geologic time and not to loss of other atmospheric constituents such as H 2 O.…”

“… Note. α = 3.22 is the mean value of α inferred from initial Mars Atmosphere and Volatile EvolutioN (MAVEN) results, while α = 2.20, 2.71, 3.73, and 4.24 represent values that are −1, −0.5, 0.5, and 1 standard deviations away from this value, respectively (Lillis et al, ; Tu et al, ).…”

“…Our model is zero‐dimensional because this allows fast computation of a full 3.5 Gyr energy balance, whereas previous 3‐D models have been unable to include multi‐Gyr time evolution because of computational limits (Wordsworth et al, ). This model is timely because the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission is currently in orbit around Mars collecting data on rates of atmospheric loss and is presumably providing better constraints on the atmospheric pressure over time on Mars which we can use in our model to constrain the conditions that explain melting on post‐Noachian Mars (Lillis et al, ). We describe our model in section 2.…”