# Probing Dust and Water in Martian Atmosphere with Far-Infrared Frequency Spacecraft Occultation
## 4 Discussion and Caveats  
Signal attenuation by global and regional dust storms can be well resolved from atmospheric sounding at far-infrared frequencies. The dust mass mixing ratio and number density can be retrieved from the absorptivity using the inverse Abel transform of atmospheric losses corrected for background absorption (Equations (10)-(16)). The comparison of atmospheric absorption losses with trace species reveals that the background absorbers can contribute to the losses in the range of instrument sensitivity; in particular, CO\({}_{2}\)-CO\({}_{2}\) CIA, water vapor, and ozone reach magnitudes greater than 0.001 dB/km at frequencies near 1 THz at near-surface conditions. However, in the case of water vapor and ozone, there are windows near 1 THz frequency at which the absorbers do not exhibit significant absorption loss. These bands can be considered as an effective choice of frequency to reduce the contribution of water vapor and ozone below the S- and X-band sensitivity. The contribution of CIA can be removed from the temperature structure and mixing ratio of CO\({}_{2}\). In addition, multiple frequency channels can be used to deduce the contributions of water vapor and ozone, given that the contributions of absorbers in those bands fall within the instrument sensitivity. Too high loss would lead to complete attenuation of the signal, while too less opacity cannot be resolved from total signal loss. It would allow the receiver to distinguish the effect of water ice and provide vertical distribution of water vapor abundance, given the fact that water vapor is one of the major contributors to gas absorption in the 1-10 THz frequency band (Figure 3). Knowledge of the vertical profile of temperature, dust opacity, and water vapor abundance will provide a better understanding of the evolution of dust storms and cloud formation processes. The spacecraft occultation method at far-infrared provides higher-resolution data than the infrared limb observations as discussed in the previous section. Cross-link spacecraft occultations are the only option at such high frequencies due to high free space loss and atmospheric absorption loss in Earth's atmosphere. Further, phase measurement and clock noise can be optimized through cross-link occultation. A constellation  
Figure 6: Expected signal loss (dB) for Martian dust storms at 1 and 10 THz during signal transmission from occultation estimated from Abel transform of signal attenuation from dust storms. of small satellites could provide a large set of observations over Martian topography to characterize the development of dust storms. A constellation of four to eight satellites can provide \(\sim\)200-2300 occultation events per sol depending on the configuration of satellite orbits [47; 61]. The vertical resolution is important with regard to understanding the convective and radiative feedback in the near-surface environment and lowest scale heights. Spatial coverage is important to generate large amounts of data required for data assimilation, and cross-link spacecraft occultations can be used to provide the spatial coverage required for data assimilation. In addition, the method will provide dust opacity and water vapor concentrations in the PBL in a wide range of dust opacity conditions. Knowledge of the physical conditions in PBL under varying dust opacity conditions is essential to study the radiative-convective processes and influence global scale models engineering teams use to safely land assets on the surface during EDL. The merit of dust retrieval using far-infrared occultation is that it can provide high-resolution co-located measurements of atmospheric thermal structure and dust opacity, even during high-opacity conditions like regional and planet-encircling dust storms.