3.4 Local time variations.
Diurnal temperature variations in the Martian atmosphere has been observed by many spacecraft such as TES/MGS, MCS/MRO, PFS/MEX and ACS TIRVIM/TGO (Wilson et al., 2000; Heavens et al., 2011; Kleinbohl et al., 2013; Giuranna et al., 2021; Guerlet et al., 2022). These data have allowed the study of thermal tides driven by diurnal solar forcing (Forbes et al., 2020;). Thermal tides dominate the temperature daily cycle over most of the atmospheric column. Along with diurnal tides, semi-diurnal tides dominate the temperature structure of the middle atmosphere throughout the year from high to low latitudes (Kleinböhl et al., 2013). The morning to evening difference in temperature should therefore also impact water vapor and ice, as they are captured by ACS occultation at both terminators.
The saturation state should also be sensitive to the presence of dust as condensation nuclei in the atmosphere. Observations by PFS have revealed local time variations of dust opacity in the atmosphere. Wolkenberg & Giuranna (2021) have found that aphelion dust opacities are minimum at night and early morning, with perceptible variations during the rest of the day. During the dusty season, no local time variation was found (Giuranna et al., 2021). Diurnal variation of dust was also found during the GDS of MY34 by the Mars Climate Sounder (Kleinböhl et al., 2020) when dust was found at the highest altitudes in the late afternoon and at the lowest altitudes late at night. In addition, dust diurnal cycle was observed to vary with latitude.
The ACS dataset allows us to study the local time variations of water vapor by comparing occultation measurements in the morning and evening terminators. Typically, measurements on the sunset and sunrise belong to different hemispheres, precluding a meaningful comparison of their respective atmospheric state. However, from time to time, a range of close latitudes was probed in the morning and evening terminators within short Ls intervals (Fig. 8). To examine the morning to evening contrasts, we chose seven intervals: two in MY34 (230-250°, 290-310°); four in MY35 (115-135°, 200-220°, 270-290°, 330-350°), and one in MY36 (50-70°). Within each bin, the local time varies along the latitude (see Fig. 8, bottom panels).