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date: 01 October 2022

# The Lower Ionosphere of Mars: Modeling and Effect of Dust

• Varun SheelVarun SheelPlanetary Science Division, Physical Research Laboratory, India

### Summary

The study of planetary ionospheres helps us to understand the composition, losses, and electrical properties of the atmosphere. The structure of the ionosphere depends on the neutral gas composition as well. Models based on fundamental equations have been able to simulate the neutral and ion structure of the Martian atmosphere. These models couple chemical, physical, radiative, and dynamical processes at various levels of complexities.

The lower ionosphere (below 80 km) and its composition have not been observed and studied as comprehensively as the upper ionosphere. Most of our current understanding of the plasma environment in the lower atmosphere is based on theoretical models. Models indicate that Mars contains a D region, similar to that in the Earth’s ionosphere, produced primarily due to high-energy galactic cosmic rays that can penetrate to the lower altitudes. The D layer has been simulated to lie in the altitude range of ~25 to 35 km on the dayside ionosphere of Mars. A one-dimensional model, used to calculate the densities of 35 positive and negative ions, predicts hydrated ions to be dominant in the troposphere of Mars. Due to the variability of water vapor, these cluster ions show seasonal variability and can be measured by future experiments on Mars landers.

Dust is an important component of the climate of Mars, wherein dust storms are known to affect the temperatures and winds of the lower atmosphere. The inclusion of ion–dust interactions in the model for the Martian ionosphere has yielded important effects of dust storms on the ionosphere. It has been found that during dust storms, the ion densities can significantly diminish, reducing the total ion conductivity in the troposphere by an order of magnitude. Also, large electric fields could be generated due to the charging of dust in the ionosphere, leading to electric discharges and, possibly, lightning.

### Subjects

• Planetary Atmospheres and Oceans
• Planetary Ionospheres and Magnetospheres

### Introduction

The Martian atmosphere is mostly composed of CO2 (95.7%), while N2 (2.07%) and Ar (2.03%) are the other major species (Franz et al., 2015). The remaining 0.2% of the Martian atmosphere is comprised of gases that are in trace amounts, such as H2O, O3, and CO, but that are important in the photochemistry and stability of the CO2-dominated atmosphere.

Solar photons (extreme ultraviolet [EUV] and soft X-ray) ionize neutral species in the atmosphere into ions and electrons, forming a region of ionized gases called the ionosphere, which on Mars extends from surface to about 400 km. Because the upper ionosphere is formed mainly by solar photo-ionization, it is sensitive to processes like solar flux variations and flares, solar energetic particle (SEP) events, or more local processes, such as interactions with the solar wind and local magnetic fields. The ionosphere below 80 km (lower ionosphere), which is formed mainly by galactic cosmic rays, is affected by processes like gravity waves and dust storms, making it sensitive to drivers from below. Understanding the composition of the ionosphere is critical for understanding several processes occurring on Mars that affect the current state of the atmosphere as well as its evolution.

Dust is an important aerosol component of the Martian atmosphere. Dust heating through absorption of radiation has a strong impact on the thermal and dynamical state of the atmosphere. Every year during perihelion season $(Ls=240∘−360∘)$, a regional dust storm occurs that increases the opacity of the atmosphere up to a range of 0.3 to 0.5 ($9μm$optical depth; Sheel & Haider, 2016; Smith, 2009). The atmospheric dust plays a role in the ion and neutral chemistry of the atmosphere. Dust has been found to be a sink for $HOx$ radicals in the Martian atmosphere (Lefevre et al., 2008), and it acts as a sink for ions in the Martian ionosphere. Both positive and negative ions are destroyed during dust storms due to attachment with dust particles (Sheel & Haider, 2016).

Modeling the neutral and ionic composition of a planetary atmosphere involves representation of chemical, physical (and radiative), and dynamical processes (Sheel, 2010) at various levels of complexities, with the coupled processes being accounted for in advanced models. As an example of coupled processes, chemical constituents of the atmosphere absorb solar radiation, which affects the atmospheric temperature. Chemical constituents are transported by winds that are related to the temperature distribution and are driven by the absorption in the atmosphere of vertically propagating waves. Chemical reaction rates are also temperature dependent. Development of these theoretical models has led to the present understanding of the Martian atmosphere, corroborated by observations (Read & Lewis, 2004). The Martian surface pressure is 7.0 to 9.0 mbar, which is 0.7% of the Earth’s surface pressure (1,013.25 mbar). Unlike the Earth’s atmosphere, the Martian atmosphere has no stratosphere, due to the very low concentration of ozone. This shows there is lower heating through the absorption of UV energy in the atmosphere of Mars than in the Earth’s atmosphere. Modeling has provided better knowledge of the neutral and ion composition and physical structure of the Martian atmosphere compared to what was known earlier. Models based on fundamental equations have been able to simulate the atmospheric constituents of Mars, such as the neutral species CO2, N2, Ar, O2, CO, H2O, H2, O, O3, NO, NO2, and HNO3 and several ionic species.

The progress in understanding the Martian atmosphere made in the last few decades allowed more complex analysis of the neutral and ion composition of the atmosphere. This article focuses mainly on the understanding achieved through models of the lower ionosphere, but it starts with a brief discussion of the models of the neutral atmosphere, which is a precursor to the ionosphere.

### Photochemical Models of the Martian Neutral Atmosphere

The ionosphere of Mars is affected by the chemistry, dynamics, and energetics of the neutral atmosphere. As a result of the ionization of neutral species in the Martian atmosphere caused by solar radiation and particle impact, the atmosphere contains positive and negative ions and electrons at all altitudes, which form the ionosphere. The spatial and temporal variability of these neutral species is predicted by theoretical photochemical models.

Photodissociation and chemical processes in the Martian atmosphere have been modeled through the development of simple one-dimensional (1D) models used to compute the steady-state chemical composition of the atmosphere (Krasnopolsky, 2006; Krasnopolsky et al., 2013; Nair et al., 1994). Such models simulate the altitude variations of species in the atmosphere.

The primary driver of photochemistry on Mars is the photodissociation of the dominant CO2. The end products, CO and O2 (which are the long-lived species in Martian atmosphere), were overestimated by initial models of pure CO2 chemistry. To solve this problem, later models included odd hydrogen species (H, OH, HO2), which are mainly produced by water vapor (Kong & McElroy, 1977).

Models included heterogeneous chemistry and the results compared better with observations. Some models included the heterogeneous loss of OH and HO2 on ice aerosol (Lefèvre et al., 2008). The problem with global mean averaged models is that they cannot account for the variability in temperature, water, and dust that affect many of the neutral species.

One-dimensional time-dependent models can address this issue. These models usually solve the following 1D time-dependent transport equations for the concentration $n$ of a chemical species. Vertical transport is parameterized by the eddy diffusion coefficient $K(z)$.

$Display mathematics$

where $P$ and $L$ are chemical production and loss rates. The vertical number density flux $ϕ$ is computed for each species using a typical diffusion equation based on the eddy diffusion coefficient $K(z)$ and temperature profiles (Chaffin et al., 2017; Rodrigo et al., 1990).

The 1D perspective of the Martian atmosphere is well understood from photochemical models. However, the above models cannot account for the horizontal dynamics; for example, the high abundance of ozone in the polar night is underestimated. Also, the seasonal and geographic variations of atmospheric constituents are linked to many phenomena, such as water cycle, dust cycle, and transport of the atmospheric constituents. Although the photochemistry of Mars is comparatively simple, the orbital properties of Mars and its exposure to solar influx lead to pronounced seasonal and latitudinal variations that the models have to account for. An attempt to account for the horizontal (latitudinal) variations of Mars photochemistry was made in a two-dimensional (2D) zonal-mean model by Moreau et al. (1991). Lefèvre et al. (2004) developed a three-dimensional (3D) model to study spatial and temporal variations of ozone and related neutral species in the Martian atmosphere, as had been done for Earth (Sheel et al., 2014). Such studies need to employ realistic general circulation models (GCMs).

GCMs use physical laws based on fluid dynamics and radiative transfer to compute the evolution of the atmosphere by integrating the equations representing momentum, energy, and mass conservation, from an initial state defined by temperature, pressure, winds, etc. One of the first Mars GCMs to predict the atmospheric condensation of CO2 was adapted from a terrestrial GCM by Leovy and Mintz (1969). Some of the Mars GCMs include the NASA Ames General Circulation Model (Haberle et al., 1999), the Geophysical Fluid Dynamics Laboratory (GFDL) Mars model (Basu et al., 2004), the Oxford-LMD (Laboratoire du Meteorologie Dynamique) Mars model (Forget et al., 1999; Read et al., 2003), a Mars GCM based on the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (Urata & Toona, 2013), and the Mars Weather Research and Forecasting (Mars-WRF) GCM (Richardson et al., 2007). These GCMs have been used in studies of several aspects of the dynamics of the Martian atmosphere, such as the Hadley circulation, baroclinic waves, thermal tides, and water cycle. A comprehensive review can be found in Lewis (2003).

Over the last decade, the increase in computer power has allowed the development of full Martian GCMs coupled with photochemistry. In addition to an adequate repre­sentation of atmospheric transport, Martian GCMs are able to provide a realistic description of the 3D field of water vapor and its variations at all scales, which is a crucial advantage for constraining properly the fast chemistry of the lower atmosphere. This has enabled coupling of photochemistry to the models (Montmessin & Lefèvre, 2013). The seasonal and geographic variation of neutral species like ozone, has been studied in different latitude regions, and the correlation of ozone with water vapor and CO has been studied using—the LMD photochemistry-coupled GCM (LMD-GCM; Modak, 2019; Modak et al., 2019).

The photochemical models successfully simulate many features of the observed neutral atmosphere, for example, the O3–H2O anticorrelation (Lefèvre et al., 2004; Modak et al., 2019), but the difference of a factor of 8 between the observed and calculated CO abundances is still a challenging issue for the models to solve. The other challenging problem is to simulate the spatial and temporal variability of methane constrained by observations and the known photochemistry (Lefèvre & Forget, 2009). Even the sulfur chemistry needs to be simulated and validated.

### Models of the Ionosphere of Mars

The ionosphere of Mars extends right from the surface to the ionopause (which varies from 200 km to 400 km). Models of the upper ionosphere of Mars show that maximum ionization occurs on the dayside at altitudes of ~125 km and ~105 km due to absorption of solar EUV (< 1,025.7 Å) and X-ray photons (< 90 Å), respectively (Haider et al., 2002). The ionospheric models show that the dayside ionosphere of Mars can be divided into D, E, and F layers at altitude ranges of ~25–35, ~100–112, and ~125–145 km owing to the impact of galactic cosmic rays, X-rays (10–90 Å), and solar EUV (90–1026 Å) radiations, respectively. A comprehensive review on the upper ionosphere can be found in Haider et al. (2011). From here on, this article focuses on the lower part of the ionosphere.

In the lower atmosphere of Mars, the ion neutral chemistry is similar to that of the D region of Earth’s ionosphere. Cosmic ray ionization is an important process in the lower atmosphere of Mars (Haider et al., 2007; Molina-Cuberos et al., 2002). Our understanding of the daytime lower ionosphere of Mars is limited due to lack of observations in this region. The first theoretical study of the Martian lower ionosphere was carried out by Whitten et al. (1971), who considered the ionization by cosmic rays and solar radiation in the dayside ionosphere of Mars. Later, Molina-Cuberos et al. (2002) calculated electron density in the nighttime lower ionosphere of Mars. Haider et al. (2007, 2008, 2009a, 2009b) and Sheel and Haider (2012) developed a model to calculate the production rates and densities of 35 positive and negative ions in the troposphere of Mars. These ions include Ar+, O2+, CO2+, O2+CO2, NO+, H3O+, H3O+H2O, H3O+(H2O)2, H3O+(H2O)3, H3O+(H2O)4, H3O+HO, CO2+CO2, CO+, C+, N2+, NO+CO2, N+, O2+(CO2)2, O2+H2O, O2+(H2O)2, O4+, O+, CO3H2O, CO3(H2O)2, CO3, CO4, NO2, NO2H2O, NO2(H2O)2, NO3, NO3H2O, NO3(H2O)2, O2, O3, and O. The impact ionization source was taken as galactic cosmic rays.

#### Production Rate of Ions

In order to calculate ion densities due to impact by galactic cosmic rays on neutrals, the flux of the cosmic rays is required. The high-energy cosmic rays pass through the atmosphere, producing nucleonic cascades. The collision of primary cosmic rays (protons and alpha particles) with atmospheric molecules produces protons, neutrons, and pions. Energetic secondary nucleons increase the production of particles by neutral collisions. Neutral pions quickly decay to gamma rays, and their contribution to the energy deposition is very important in the lower part of the atmosphere. At high altitude, the maximum ion production rates are due to protons.

The ion production rate $p(h,χ)$ at height $h$ and solar zenith angle $χ$ is given by (Haider et al., 2008):

$Display mathematics$

where $Q=35eV$ is the energy required for the formation of an electron ion pair, $F$ is the total differential flux of the galactic cosmic rays being expressed in $cm−2s−1GeV−1ster−1$ at height $h,−$, and $dE/dh$ is the energy loss per unit length as calculated by a loss method (Haider at al., 2008). For some of the models discussed below, the flux of the galactic cosmic rays (impact ionization source), is taken as 103 to 10-5 particles m−2 s−1 GeV−1 ster−1 for energy range 1 to 1,000 GeV (Haider et al., 2008).

#### Ionospheric Processes

Apart from the photo-ionization of the neutral atmosphere, production and loss due to chemical reactions and recombination of ions with electrons contribute to the formation of an ionosphere. How these processes are quantified is discussed next.

The ion density is calculated from the continuity equations, neglecting transport, as follows:

$Display mathematics$

where $qi$ and $li$ are the total production rate (due to cosmic ray ionization and chemical reactions) and loss rate coefficient of $ith$ gas, respectively, and $ni$ is the ion density.

The time-dependent chemical production and loss rates of a general reaction $A++B→C++D$ are:

$Display mathematics$

In this reaction, ion $C+$ is produced and $A+$ is lost with the same chemical rate coefficient $R$. Under steady-state condition, $dni/dt=0$. Using the above three equations, the concentration of positive ion $ni+$, negative ion $ni−$, and electron $ne$ are calculated by the iteration process:

$Display mathematics$

where $Pi$ is the production rate of positive ion $i$ by galactic cosmic rays;$Rk,ji$ is the rate coefficient of the reaction of neutral $k$ with ion $j$ to produce the ion $i$;$γji$is the rate coefficient of production of ion $i$ by photon collision with ion $j$;$Kji$is the rate coefficient of the reaction to remove ions $i$ by reaction with neutral $k$; $αei$ and $αii$ are the ion–electron and ion–ion recombination rate coefficient; $n+$ and $n−$ are the total density of positive and negative ions; $Γi$ is the coefficient rate of loss of ion $i$ by photon collision;$Aji$is the rate coefficient of the production of negative ion $i$ by electron attachment to neutral $k$;$γje$is the rate coefficient of electron production by photodetachment of ion $j$; and $Bj$ is the attachment of electron to molecule $nj$. Because transport time is several orders of magnitude higher than chemical life time, transport of ions is neglected in this model.

The contribution to the ionosphere of the processes considered above varies with altitude and species. A detailed study considering a comprehensive chemistry of 35 ions (Sheel & Haider, 2012) showed that out of these, the most important process in the lower atmosphere of Mars is the ion-neutral collisions. The ion–ion recombination, ion–electron recombination, and photodissociation of positive ions are not very important processes, while electron-neutral collisions, electron-detachment of anions, and photodissociation of negative ions are significant for a only very few reactions.

#### Structure of the Lower Ionosphere

The models show that the hydrated hydronium ions—H3O+(H2O)n for $n=1,2,3$, and $4$—and water cluster of NO2- and CO3 —i.e., NO2(H2O)n and CO3(H2O)n for $n=1,2$—are the dominant ions in the Martian troposphere (Molina-Cuberos et al., 2002; Sheel & Haider, 2012, 2016). As can be seen from Figures 1 and 2, the highest concentratsions of these ions occur near the surface and are estimated to be 103 cm−3, which is in a reasonable agreement with Mars 4 and Mars 5 measurements.

CO2+ and O2+ ions are produced initially due to ionization by galactic cosmic rays in the D region (CO2 being the dominant neutral species in the Martian atmosphere). Later, O2+CO2 is formed by three-body recombination, and it is quickly converted into hydrated cluster ions H+(H2O)n by first forming H3O+ ions and by subsequent association with water vapor molecules. Above 30 km, electrons play an important role in the lower ionosphere of Mars. The relative high abundance of oxygen-bearing molecules permits the presence of negative ions in the lower ionosphere. Initially, negative ions O and O2 are produced through electron capture by ozone and molecular oxygen, respectively. Later CO3 and CO4 are produced from three-body reactions. The loss of CO3 with H2O is the major source of production of CO3H2O and CO3(H2O)2. This chemistry is schematically represented in Figure 3. The maximum electron densities in southern winter and northern summer were obtained at altitudes of ~25 km and ~30 km, respectively, due to high efficiency of electron attachment to Ox molecules. The ion densities were found to differ by factors of 5 to 10 between the two seasons (Haider et al., 2008).

The D layer in Earth’s atmosphere is observed at 65 to 70 km due to precipitation of galactic cosmic rays. The mean atmospheric pressure of Mars is nearly 1,000 times less than that on Earth; therefore, galactic cosmic rays penetrate deeper into the Martian atmosphere than into Earth’s atmosphere. By analogy to Earth’s ionosphere, the model results discussed above suggest that the Martian ionosphere has a D layer in the troposphere. The position and value of this layer are different in summer and winter seasons at high northern and southern latitudes, respectively. In the dayside ionosphere, we have neither in situ measurements of low-altitude ion layers nor remote-sensing measurements from the Martian surface. The direct measurements of cluster ions and electron density in the lower ionosphere of Mars can be performed by Sounding rocket and Langmuir probe experiments, respectively, onboard the Mars lander. Sounding rockets carrying quadrupole ion mass spectrometers have been found to be very useful for measuring positive and negative ion densities in the D region of Earth’s ionosphere. The Langmuir probe has been used in rockets and balloons to measure electron density in the lower ionosphere of Earth. Ground-based remote-sensing technologies, such as ionosondes and incoherent scatter radar, are routinely used to measure the D layer on Earth. These two technologies can also be used for Mars from a ground-based laboratory.

#### Ionosphere in Presence of Dust

Dust aerosols play an important role in the lower ionosphere of Mars. Periodical massive dust storms with high surface winds disturb surface sediments and lift large amounts of dust into the atmosphere. Martian dust can acquire electrical charges because ions can charge aerosols by attachment. This can reduce the electrical conductivity on Mars and is also an important loss process for the ions.

The ion scheme given by Haider et al. (2009b) has been extended to include ion–aerosol reactions in the troposphere of Mars (Sheel & Haider, 2016). In presence of dust aerosols, the positive and negative ion concentrations can be calculated, as given by (Cardnell et al., 2016; Haider et al, 2010; Michael et al., 2008; Sheel & Haider, 2016)

$Display mathematics$

where$ρi+$and$ρi−$ are the positive and negative ion densities, respectively, in presence of aerosols; $Ai+$ and $Ai−$are the positive and negative ion aerosol densities, respectively; $βe$ and $αe$ are the ion–aerosol and ion–electron recombination coefficients, respectively; $ρe$ is the electron density; $qi+$ and $qi−$ are total ion production rates of positive and negative ions, respectively; $qA+$ and $qA−$ are production rates of positive and negative ion aerosols, respectively; $α$ is the aerosol ion-aerosol ion recombination coefficient; $α1$ is the positive ion and negative ion aerosol recombination coefficient; $α2$ is the negative ion and positive ion aerosol recombination csoefficient; $Γi$ is the coefficient rate for the loss of $ith$ ion by photon collision; $αii$ is the ion–ion recombination coefficient; $Ki$ is the rate coefficient of the reaction to remove ions $i$ by reaction with $kth$ neutral density $ρk$.We have solved the above coupled equations for the steady-state condition. The production rates of ions and ion aerosol are given by

$Display mathematics$

where $Pi$is the production rate of positive ion $i$ by galactic cossmic rays, $Rkji$is the rate coefficient of the reaction of neutral $k$ with ion $j$ to produce the ion $i$, $γji$ is the rate coefficient of production of ion $i$ by photon collision with ion $j$, $Aji$ is the rate coefficient of the production of negative ion $i$ by electron attachment to neutral $k$, $β$ is the ion–aerosol attachment coefficient, and $A$ is the neutral concentration of dust aerosol. The concentrations$ρ+$, $ρ−$, $A+$, and $A−$are represented in the form of their total ion concentration as

$Display mathematics$

Under the charge neutrality condition, the sum of all positive ion densities is equal to the sum of all negative ion and electron densities, as given below:

$Display mathematics$

The total density $(J)$ is calculated from the sum of $ρ+,ρ−,ρe,A+$, and $A−$.

The ion-dust models in the literature show that the densities of water cluster ions H+(H2O)n, NO2-(H2O)n, and CO3-(H2O)n reduced by up to 2 orders of magnitude in the presence of dust aerosols (Cardnell et al., 2016; Michael et al., 2008; Sheel & Haider, 2016). This indicates that during a dust storm, when optical depth changes considerably, a large hole in the ion densities may appear until this anomalous condition returns to the normal condition after a period of about few days, as shown in Figure 4.

Model results also show that in the dust storm, the total ion conductivity in the troposphere is reduced by an order of magnitude (Haider et al., 2010). The process of dust charging with ions discussed above can produce large electric fields in the atmosphere during a dust storm period (Haider et al., 2010). The possible occurrence of such large E-fields and the associated electric discharges, if any, have potentially important implications for Martian atmospheric chemistry, human exploration, habitability, and even the possible development of life (Atreya et al., 2006).

### Summary

The photochemical model studies of the ion chemistry described in this article show the importance of water vapor in producing hydrated ions. The densities of neutral species (and dust) for the 1D ionospheric models were taken as inputs, calculated from air density measurements by radio occultation experiments. Due to the variability of water vapor, the lower ionosphere is expected to vary seasonally. This variability of the ionosphere due to the seasonal variability of the neutrals and dust has been indicated with the help of the LMD-GCM (Modak, 2019), which provides the seasonal variations of the neutral inputs to the ion model.

The models of the ionosphere can thus provide an initial estimate for future missions to Mars that propose to measure the ion composition of the lower Martian atmosphere. Also, there are no direct measurements of the vertical profiles of atmospheric electrical conductivity on Mars. In the absence of such measurements, the theoretical calculations discussed above can be used as a diagnostic tool for design of electrical payloads.

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