This paper provides a short overview of our current understanding of the upper atmosphere/ionosphere of Mars including the escaping neutral atmosphere to space that plays a key role in the current state of the Mars upper atmosphere. The proper definition of the word “aeronomy” relates to the upper atmosphere where ionization is important. Currently there is a paucity of measurements of the internal physical structure of the Martian upper atmosphere/ionosphere. Much that we know has been deduced from theoretical models that predict many more things than thus far measured. The newest Mars orbital missions, the US MAVEN and Indian MOM missions, just beginning their science analyses, will provide the measurements needed to fully characterize the aeronomy of Mars.
The term aeronomy has been used since the 1950’s, but its origin has been largely forgotten. Sydney Chapman first suggested, in a letter to Nature , that the word aeronomy should replace meteorology, given that the association of meteors is misleading. This suggestion was not followed so a number of years later  he modified his original idea and suggested that aeronomy instead of “being used to signify the study of the atmosphere in general, should be adopted with the restricted sense of the science of the upper atmosphere”. This use of the term was accepted and has been in use ever since. In 1960 Chapman restated this definition  in a slightly different form by suggesting that aeronomy is the science of the upper region of the atmosphere where dissociation and ionization are important.
Our current understanding of the aeronomy of Mars is very limited. The two Viking landers each carried a mass spectrometer  and a retarding potential analyzer . Thus we have insitu neutral gas composition and plasma density, composition and temperature information from only the equivalent to a single sounding rocket flight. The other in-situ plasma composition data, comes from the ASPERA instrument  on Mars Express (MEX), which has been providing information on the density, composition and energy of ions and electrons of energies between about 10 eV to ~36 keV at altitudes beyond about 300 km. Much electron density data exists from the MARSIS instrument on MEX  from in situ and remote wave sounding - spottily from radio occultation observations obtained on early missions (e. g. Mariners, Viking, Russian “Mars” missions) and more comprehensively from the radio science limb occultation experiments on the more recent Mars Express  and Mars Global Surveyor  missions. The same Mars missions also provided some remote and inferred atmosphere information from UV occultation, from low altitude aerobreaking-phase satellite drag data (MRO, MGS, Odyssey) and more extensively from Mars Global Surveyor (MGS), reflectometer measurements of atmosphere-scattered electron pitch angle distributions (e.g.,). Finally the analogy between Mars and Venus, with similar atmosphere composition and regions with no intrinsic surface magnetic fields, has also provided important insights about the aeronomy of this poorly studied region of Mars. This is a lot of information, particularly of the electron density distributions. However, we have only glimpses of the overall complexity of the atmosphere and plasma states - there is, as of now, almost a complete absence of the thermal, composition and dynamical parameters that define the underlying physics of the Martian aeronomy.
Neutral upper atmosphere
The Viking 1 and 2 mass spectrometers on entry obtained mass spectra from 1 to 49 amu . They sampled the atmosphere under not very different solar illumination locations and solar activity. The composition data from Viking 1 is shown in Figure 1. Atomic oxygen, a very important neutral gas species for understanding the aeronomy, was not measured by these instruments. The density of O, which has been widely used, was deduced from the presumed chemistry of the measured ion composition. Figure 2 shows results from a theoretical model, indicating that many other minor species are expected to be present. The temperatures derived from the Viking data  are shown in Figure 3, which implies the presence of significant wave activity.
Given this very limited direct observation of the atmospheric altitude variation, plus globally patchy statistical measurements of the density from accelerometer measurements, comprehensive 3D thermospheric circulation models (e.g.,[13,14]), constrained by the measurements, have been developed during the last decade. Such models solve the coupled continuity, momentum and energy equations and provide the densities, temperature and velocities of the major species. These models have been very helpful to achieve a global understanding of the behavior of the upper atmosphere. Figure 4 shows results from such a model which are consistent with measured parameters, but the model predicts spatial changes in the distributions and provide clues for parameters, such as neutral velocities for which no direct data is available.
As indicated in the introduction, each of the two Viking landers carried a retarding potential analyzer, which provided information on the major ion densities and plasma temperatures in the ionosphere up to an altitude of about 300 km as shown in Figures 5 and 6. Note that two spacecraft vertical profiles are very similar and equivalent to data from a single sounding rocket flight and that is all the data available to date. In order to attempt to understand the implication of this extremely minimal data set, we need to use extensive model calculations and invoke the similarities between the ionospheres of Mars and Venus. Carbon dioxide is the major neutral constituent of both planets, with atomic oxygen becoming dominant above about 200 and 150 km, respectively. The Viking RPA’s measurements detected ion flows of ~100 m/s at high altitudes and the fact that Mars has remnant crustal magnetic fields introduces complexity in the transport processes above the chemical equilibrium region (≤200 km), but the basic parent sources of the two ionosphere regions are very similar. The controlling chemical processes are:
This ion chemistry leads to the, initially unexpected result, that in an atmosphere that basically has no molecular oxygen, the major ion is O2+. Figure 7 shows the results from a 1D chemical-diffusion model () that reproduces the Viking observations (assuming a certain topside ion outflow) and also shows that many important minor ion species are also expected to be present.
Such a 1D model is very important, because it can include the many important chemical processes; however it requires a number of important assumptions (e.g. upper boundary ion flow and plasma temperature). We also have 3D MHD models, which self-consistently solve the plasma continuity, momentum and energy equations from below the main ionosphere well out into the solar wind. However there is always a price to pay; such a model given limited computational resources can only consider the main ions (O2+, O+, CO2+). The results from such a model  are shown in Figure 8.
It was established decades ago, using the large Pioneer Venus data base, that the observed ionosphere temperatures at Venus, required either i) a reasonable, but ad hoc energy inflow at the top of the ionosphere and/or ii) a reduced electron thermal conductivity. The plasma temperatures measured by the RPA’s on Viking shown in Figure 6 also increase enough with increasing altitude to require the ad hoc energy input assumptions, as needed in the case of Venus. Figure 8 shows the results of a model ion composition calculation that used a reasonable topside heat inflow to match the observed ion temperatures from Viking and that agrees with the Viking composition measurements. Currently it is clear that conventional EUV heating and classical thermal conductivity lead to temperature values below the observed ones both at Mars and Venus and there is insufficient information to establish if either or both of the processes suggested to remedy this problem are in effect active. It is quite likely that both play a role, but whether one or the other dominate, or some other process is active, cannot be established at this time. Note that two review papers have been published recently that deal with details of the ionosphere of Mars [20,21].
Exosphere and Atmospheric Escape
At both Mars and Venus a hot oxygen corona has been observed to be present ([22,23]). These fast energetic oxygen atoms are mostly created by dissociative recombination of molecular oxygen ions (some other processes, such as dissociative recombination of CO+ also make a contribution).
Sophisticated 3D Monte Carlo models have been developed to quantify the distribution of these atoms around Mars . An example of such predicted spatial distribution is shown in Figure 9.
The maximum energy of a hot oxygen created by the dissociative recombination of O2+ is about 3.5 eV; this energy is too low to allow it to escape from Venus, but is well above the necessary escape energy of Mars (~2 eV), thus resulting in a significant escape flux. Estimates of oxygen escape are indicated in Table 1 ; note the significant variation with orbital location and solar insolation.
The other process by which the atmosphere can be depleted is ion escape. Ion escape has been measured extensively by the ASPERA instrument on Mars Express ([26,27]) and also widely modeled (e.g. [24,28-30]) and there is reasonably good agreement between the measured and calculated values. Figure 10 shows the measured solar cycle variation obtained from Mars Express and Table 2 shows the results of the most recent calculations by Dong et al. .
As outlined in the introduction the amount of direct information available at this point on the aeronomy of Mars is extremely limited. However, that is changing very rapidly. There are two spacecraft, recently beginning science operations, in orbit around Mars (both were inserted into orbit late September 2014) carrying instruments that will provide a wealth of new information. The Indian Mars Orbiter Mission (MOM) carries three aeronomy related instruments: a methane sensor, a quadrupole neutral mass spectrometer and a Lyman alpha photometer. The US Mars Atmospheric Volatile Escape Mission (MAVEN) has 9 instruments on board all designed to address a wide range of aeronomy related issues. After the completion of the Pioneer Venus Orbiter mission we could confidently state that we knew more about the aeronomy of Venus than any other planet in our solar system except for Earth. We are confident that in the next year or two we will be able to say the same about Mars.
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The authors declare that they have no competing interests.
This paper was presented as a “Distinguished Lecture” at the 2014 AOGS Meeting held in Sapporo, Japan. AFN was the recipient of the “Distinguished Lecture” award, but because of unforeseen circumstances was unable to attend the meeting, so his lecture was presented by JMG. Both authors read and approved the final manuscript.
Professor Nagy has over forty five years of experience in both theoretical and experimental studies of the upper atmospheres, ionospheres and magnetosphere of the Earth and planets. He was principal and co-investigator of a variety of space-borne instruments. He was Interdisciplinary Scientist for the Dynamic Explorer and Pioneer Venus programs. He is a team member of the Radio Science Investigation on Cassini and a co-investigator of the MAVEN mission. He has been a principal or co-author of over 350 papers; he has also authored/co-authored a number of review papers and encyclopedia chapters and a book on the “ionospheres”.
Dr. Grebowsky started his working career analyzing measurements of ion composition measurements from the Atmosphere Explorer and OGO satellites. Subsequently he was principal scientist on ion measurements from the Plasma Diagnostic Package flown twice on the Space Shuttle, and co-investigator for the ion spectrometers on the Midcourse Space Experiment and Pioneer Venus Orbiter., the Earth’s thermosphere/ionosphere, ionosphere-magnetosphere coupling. He is currently NASA’s Project Scientist for the Mars MAVEN mission. His research efforts and publications dealt with Venus, Mars and terrestrial ionospheres and upper atmospheres, spacecraft plasma environments, meteoric ion and neutral species layers at Earth and other planets.
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