# Probing Dust and Water in Martian Atmosphere with Far-Infrared Frequency Spacecraft Occultation
## 1 Introduction  
Dust activity in the Martian atmosphere is an important component of the radiative-dynamic coupling processes [1; 2; 3]. Measurements from landers and orbiters indicate that dust activity occurs at many length scales [1; 4; 5; 6; 7]. Dust storms influence regional and global climate and weather processes, including seasonal and annual variations of radiative and dynamical properties [1]. Dust activity occurs on length and time scales ranging from those of small transient dust devils to seasonal global dust storms. Contact electrification processes in dust devils and dust storms might also trigger electrochemical reactions that impact the composition of the atmosphere [8]. Furthermore, dust activity could modulate the cloud formation of water [9] and CO\({}_{2}\), as well as the destruction of atmospheric ozone [10]. Therefore, dust activity likely couples radiative-dynamical feedback, cloud microphysics [11], and chemical processes on Mars [12]. Moreover, knowledge of dust activity and the thermal structure of the atmosphere is crucial to understanding the evolution of the atmosphere and to allow accurate predictions to plan entry, descent, and landing (EDL) events [13]. Electrostatic discharge (ESD) events present an unknown risk to humans and assets on the surface [14]. Uncertainties in the atmospheric conditions can cause errors in EDL events such as the one that caused the crash of the Schiaparelli Entry Demonstrator Module [13]. Therefore, a consistent framework of incorporating atmospheric dust opacity observations into global circulation models (GCMs) is required to provide accurate predictions for future EDL events. Mars atmospheric models like MarsWRF [15] and LMD GCM [16] are used to simulate the impact of dust activity on meteorological quantities like temperature, convective activity, and wind speeds. However, current models cannot resolve the formation ofdust storms. Previous studies show that dust distribution is sensitive to Mars climate model parameters and affects the prediction of both thermal and dynamic properties of the atmosphere, including the nature of Hadley circulation and gravity waves [1]. Current Mars climate models can produce dust storm events with the aid of prescribed dust profiles from observations of actual dust storm events. The Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) [5; 6] and the Mars Reconnaissance Orbiter (MRO) Mars Climate Sounder (MCS) [17; 18] instruments provided limb viewing observations of dust opacity. The dust opacities retrieved from TES and MCS have been used to study weather conditions during dust storms [1]. Both MCS and TES have played a key role in providing observations of dust activity over multiple Martian years, including the coverage of global and regional dust storms. TES, Thermal Emission Imaging System (THEMIS) [19], CRISM [20], OMEGA [21], and MCS have provided observations of dust opacity profiles and optical depth. However, the dust opacity profile retrieval methods of these instruments are distinct from each other, and the use of different dust opacity scenarios produces significant variations in simulation results [1; 22]. Radio occultation experiments have been conducted at Mars to study the properties of the ionosphere and neutral atmosphere in the near-surface region [23; 24]. In radio science experiments, the spacecraft transmits radio frequency signals to the receiver (Earth or another satellite) through the Martian atmosphere and undergoes bending to atmospheric refraction before reaching the receiver. The phase and Doppler shift measurements at the receiver can be used to retrieve temperature, pressure, and neutral and electron densities of the atmosphere [25; 26]. Radio science experiments at Mars have provided accurate observations of the neutral atmosphere and ionosphere of Mars.