Víctor Corchete ^*

• University of Almeria, Almería, Spain

The knowledge of the Martian climate is the key to facilitate its human exploration, and to determine if Mars could be a habitable planet for humans in the future. Analyses of ground-based and satellite observations from the missions developed in more than 50 years, have led to a robust knowledge of this climate, which is regulated by the CO 2 , dust and H 2 O cycles, coupled to radiative and dynamical processes. On a planetary scale, the Martian atmospheric circulation is strongly affected by the seasonal sublimation and deposition of CO 2 at the polar caps, because the CO 2 is the principal component of the Martian atmosphere (95.3 % CO 2 , 2.7 % N 2 , 1.6 % argon (Ar) and 0.4 % other gases; Belton et al., 2024), and it condenses during the autumn and winter seasons (and precipitates as frosted CO 2 ), and sublimes during the spring and summer seasons due to the solar radiation. The northern seasonal frosted CO 2 dissipates completely during its spring and summer seasons, but the South Pole keeps a thin permanent cover of frosted CO 2 . In this paper, the total mass involved in the cycle of CO 2 (exchanged between the Martian atmosphere and surface) is determined from the pressure data, provided by the Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport (InSight) mission. This mass will be compared with that determined by Kelly et al. (2006) from gamma ray and neutron data (measured by components of the gamma ray spectrometer instrument suite on Mars Odyssey the 2001), and that predicted by the General Circulation Models (GCMs): the NASA Ames Research Center (NARC) and the Mars Climate Database V6.1 (MCD). The pressure data used in this study has been provided by pressure sensor of InSight. This sensor is designed to produce a valid output between pressures of about 560 Pa and 1000 Pa, which are expected to be the extreme pressures that will be experienced at the InSight landing site. The sensor is specifically designed to minimize noise, with a typical RMS of about 10 mPa on any particular reading (Banfield et al., 2019). However, according to the recent study performed by Lange et al. (2022) about the recalibration of the InSight pressure data, the RMS is about 1.5 Pa. They provided the correction on this dataset, which has been considered in the pressure data used in this study. Supplement 1 shows the pressure data corresponding to the sols 500-510 (sol is a Martian day, i.e., 88775.244 s or 24.66 hours; Hansen et al., 2024). From all pressure data available from InSight, the daily maximum and minimum are picked (Supplement 1) to build the curves show in Supplement 2a, i.e., the curves of daily maximum and minimum pressure. These curves given versus time in sol are converted to curves with time given in solar longitude (Supplement 2b; the solar longitude of Mars in its orbit, Ls, Martínez et al., 2017;Hansen et al., 2024), and then, the curve of mean pressure (Supplement 2b, green dots) is determined from the mean of the above-mentioned curves of maximum and minimum pressure. This curve is also interpolated to fill small gaps present in data.This curve can be considered as the daily mean atmospheric pressure at InSight landing site, and it follows an annual repeatable cycle, as observed in other previous datasets (e.g., the Viking and Curiosity datasets; Martínez et al., 2017). This cycle shows two pressure dips (Supplement 2b): the first dip is the minimum of this cycle reached during southern winter (northern summer), and the second dip is a more minor dip reached in the northern winter. The minimum of the cycle is reached during southern winter, because it is longer and colder than the northern winter, i.e., the deepest minimum of this pressure cycle is associated to the more extensive coverage by seasonal frosted CO 2 in the South Pole (Martínez et al., 2017;Hansen et al., 2024). In Supplement 2b, it is observed that variations in the bulk atmospheric mass due to the condensation and sublimation of the CO 2 in the seasonal polar caps, cause the observed large pressure variations. This total atmospheric mass (M atm ) can be calculated from the mean atmospheric pressure (p atm ) curve (Supplement 2b, green dots) scaling this curve by means of the formulaM atm = m 0 + a(p atm -p 0 ) (1)where a is a constant and (m 0 , p 0 ) are the mean values of the atmospheric mass and pressure in a Martian year (0º-360º of Ls), respectively. The total mass (M frost ) of the frosted CO 2 in the polar caps can be determined from the M atm , assuming that M atm  M frost  2710 15 kg (Kelly et al., 2006).The value of p 0 is determined from the pressure data shown in Supplement 2b (green dots) as 717.73Pa. The values of a and m 0 are calculated numerically by (1), as the values that gives the best fit of the total mass of frosted CO 2 in the polar caps calculated by Kelly et al. (2006). These values are a  0.0310 15 kg/Pa and m 0  23.510 15 kg. Figure 1 shows the values of the M atm and M frost determined in the present study as a function of Ls, and the values of M frost calculated by Kelly et al. (2006). The values of M atm and M frost predicted by the GCMs are also shown in Figure 1, for comparison. In Figure 1b, it is very noticeable the well-fit existing between the values determined in this study for M frost and those calculated by Kelly et al. (2006). It shows that the pressure data (e.g., provided by InSight) can be used as a valid method to determine the total mass exchanged between the Martian atmosphere and surface (i.e., the total mass involved in the cycle of CO 2 ). Also, a good fit is found in Figure 1b between the M frost calculated here and that predicted by the GCMs. However, it is observed that the NARC model slightly overestimates the M frost values from 40º to 190º Ls, and the MCD model clearly underestimates M frost from 240º to 340º Ls. This misfit of the GCMs may be due to the dust storms occurred on Mars, because they have important effects on the meteorology and climatology of Mars. It is observed that the errors bars in Figure 1b are in general very small (for some measures the error is smaller than the symbol used to plot this measure), i.e., the uncertainties in Figure 1b shows that the misfit is due to the dust (which is not represented correctly in GCM models), and it is not simply uncertainty of results. Dust, once lifted into the atmosphere remains suspended for a long period, absorbing and scattering solar radiation heating the atmosphere (Martín-Rubio et al., 2024). This feature of the dust storms affect the sublimation and condensation processes of the CO 2 in the Polar Regions (He et al., 2024). Unfortunately, the mechanisms of the large storm growth and development already remain poorly understood (Wang et al., 2023). A better understanding of the dust storms is needed to understand the Martian meteorology and climatology (Guha et al., 2024). Modeling efforts have been performed in the recent decades incorporating these effects to the GCMs, such as the NARC and the MCD models. However, more work is already needed to model correctly the dust storms with a GCM. Probably, this task could be feasible when more data will be available provided by future missions.