MAVEN‐IUVS Observations of the CO₂⁺ UV Doublet and CO Cameron Bands in the Martian Thermosphere: Aeronomy, Seasonal, and Latitudinal Distribution

Abstract

We analyze two Martian years of dayglow measurements of the CO Cameron bands and the CO2+ ultraviolet doublet (UVD) at 298–299 nm with the Imaging UltraViolet Spectrograph on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter. We show that the altitude and the brightness of the two emissions peaks are strongly correlated, although data were collected over a wide range of latitudes and seasons. Averaged limb profiles are presented and compared with numerical simulations based on updated calculations of the production of the CO (a3Π) and the CO2+ (B 2Σ) states. The model simulations use the solar flux directly measured on board MAVEN with the Extreme Ultraviolet Monitor and the neutral densities provided by the Mars Climate Database version 5.3, adapted to the conditions of the observations. We show that the altitude and the shape of the sample limb profiles are well reproduced using the Mars Climate Database neutral atmosphere. The simulated peak intensities of the CO2+ UVD and Cameron bands are in good agreement considering the uncertainties on the excitation cross sections and the calibration of the Imaging Ultraviolet Spectrograph (IUVS) and Extreme Ultraviolet Monitor instruments. No significant adjustment of the electron impact cross section on CO2 to produce the a3Π state is needed. Seasonal‐latitudinal maps of the Cameron and UVD peak altitude observed during two Martian years show variations as large as 23 km. Model simulations of the amplitude of these changes are in fair agreement with the observations except during the southern summer dust period (Ls = 270–320°) when the calculated rise of the dayglow layer is underestimated.

Martian Thermospheric Warming Associated With the Planet Encircling Dust Event of 2018

“…The measurements by Mariner 9, Mars Global Surveyor, and SPICAM have shown the responses due to dust storms by the thermospheric structure, especially the neutral and ionospheric densities (Bougher et al, 1999;Cox et al, 2010;Forget et al, 2009;Keating et al, 1998;Stewart & Hanson, 1978;Withers & Pratt, 2013). The upper atmospheric expansion due to the dust storm was also observed in elevated airglow peak heights measured by the MAVEN Imaging Ultraviolet Spectrograph (IUVS) (Gérard et al, 2019) and increased in neutral densities measured by the MAVEN Neutral Gas and Ion Mass Spectrometer (NGIMS) (Elrod et al, 2019;Liu et al, 2018). Forget et al (2009) reported temperatures from altitudes around 130 km using the SPICAM stellar occultation data but did not report any significant effect of dust storms on the temperatures with exception of a few observations.…”

Solar control of CO₂ ⁺ ultraviolet doublet emission on Mars

“…Airglow emission provides important information on the structural variability of a planetary upper atmosphere under both external and internal drivers (Slanger et al, 2008, and references therein). This study is dedicated to a systematic investigation of the solar cycle and SZA variations in UVD emission at Mars, a distinctive feature in a typical Martian dayglow spectrum that has been extensively studied over the past several decades (e.g., Barth et al, 1969, 1971; Stewart, 1972; Stewart et al, 1972; Leblanc et al, 2006; Cox et al, 2010; Jain et al, 2015, 2018; Gérard et al, 2019). Existing studies have established that this emission feature is mainly produced via photoionization and photoelectron impact ionization of atmospheric (e.g., Fox and Dalgarno, 1979).…”

“…Since 1969, the UVD emission on Mars has been measured remotely by the UV spectrometer or spectrograph on board Mariner 6, 7, and 9 (Barth et al, 1969, 1971; Stewart, 1972; Stewart et al, 1972), the Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars (SPICAM) on board the Mars Express (MEx; Leblanc et al, 2006), and the Imaging Ultraviolet Spectrograph (IUVS) on board the Mars Atmosphere and Volatile Evolution (MAVEN; Jain et al, 2015). This feature is mainly caused by the photoionization and photoelectron impact ionization of which produces excited‐state (Fox and Dalgarno, 1979; Cox et al, 2010; Gérard et al, 2019). Fluorescent scattering of solar photons also contributes but is usually negligible at altitudes below 180 km (Fox and Dalgarno, 1979).…”

Laboratory Study of the Cameron Bands, the First Negative Bands, and Fourth Positive Bands in the Middle Ultraviolet 180–280 nm by Electron Impact Upon CO

“…We have started a laboratory aeronomy program at the University of Colorado to study electron impact fluorescence of the CB in the MUV from two parent gases, CO and CO 2 , to match and model the spectra of past, present, and future spacecraft equipped with MUV capabilities to observe the upper atmosphere of Mars and Venus (100–300 km) where both gases are present and abundant (Fox, 2008; Gérard et al., 2019; Krasnopolsky & Feldman, 2002). A thorough understanding of CO CB emissions by the same laboratory technique as performed for the LBH bands (Ajello et al., 2020) from a large chamber will lend greater precision in the spectral analysis of solar system objects.…”

“…Accurate electron impact emission cross sections are the main missing ingredient from ionospheric and cometary forward modeling codes. Many cross sections are unknown or uncertain by a factor of 2 or more (Avakyan et al, 1998;Gérard et al, 2019;Itikawa, 2002;Leblanc et al, 2006Leblanc et al, , 2007Simon et al, 2009). This lack of necessary atomic and molecular data jeopardizes accurate altitude determination of minor species (e.g., CO + , C, and C + ) composition and structure interacting with solar radiation field energy inputs (Hubert et al, 2010;Jain et al, 2015).…”