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The relatively unconstrained internal structure of Venus is a missing piece in our understanding of the formation and evolution of the Solar System. Detection of seismic waves generated by venusquakes is crucial to determine the seismic structure of Venus' interior, as recently shown by the new seismic and geodetic constraints on Mars' interior obtained by the InSight mission. In the next decade multiple missions will fly to Venus to explore its tectonic and volcanic activity, but they will not be able to conclusively detect seismic waves, despite their potential to detect fault movements. Looking toward the next fleet of Venus missions after the ones already decided, various concepts to measure seismic waves have been proposed. These detection methods include typical geophysical ground sensors already deployed on Earth, the Moon, and Mars; pressure sensors on balloons; and imagers of high altitude emissions (airglow) on orbiters. The latter two methods target the detection of the infrasound signals generated by seismic waves and amplified during their upward propagation. Here, we provide a first comparison between the detection capabilities of these different measurement techniques and recent estimates of Venus' seismic activity. In addition, we discuss the performance requirements and measurement durations required to detect seismic waves with the various detection methods. Our study clearly presents the advantages and limitations of the different seismic wave detection techniques and can be used to drive the design of future mission concepts aiming to study the seismicity of Venus. Plain Language Summary: We do not really know what the interior of Venus looks like. Even the first‐order structure of the size of Venus' core is plagued with large uncertainties. For other planets, such as the Earth and Mars, the interior structure is much better constrained. This is largely thanks to the seismological investigations performed on these planets that revealed their interior structure by studying the seismic waves caused by quakes. In the next decade, new missions will fly to Venus to explore its tectonic and volcanic activity, which is interesting to estimate seismicity. But these missions will not be able to detect any seismic waves. In order to help design future mission concepts, we discuss instruments that could record seismic waves, as already used on the Earth, the Moon, and Mars; instruments on balloons that could float in the Venusian atmosphere; and instruments on spacecrafts that monitor the variations of atmospheric emissions caused by seismic waves originating at the surface. We compare all these different techniques with each other and with recent estimates of Venus' seismic activity to see which of them works best in different scenarios. Key Points: The capabilities of various measurement concepts to detect quakes on Venus are estimated and compared to recent Venus seismicity estimatesGround sensors are limited by their short measurement duration, but also by an atmosphere induced noise that may be above their self noiseAtmospheric seismology concepts are limited to large quake magnitudes, and airglow imagers are favored relative to balloon measurements [ABSTRACT FROM AUTHOR]

As a fundamental physical process in the ionosphere, photoionization and the associated photoelectrons play vital roles in determining the ionospheric electron density and temperature for Earth and other planets with atmospheres such as Mars and Venus. The production and transport of ionospheric photoelectrons have been widely examined on Earth, but relatively less studied for other terrestrial planets, such as Mars and Venus. In this study, a two‐stream photoelectron transport model for Mars and Venus is constructed, in which the photoelectron fluxes, photoelectron heating rates, primary and secondary ionization rates are calculated. The simulated photoelectron fluxes agree with Mars Atmosphere and Volatile Evolution (MAVEN) observations at various altitudes, with the input of solar spectrum irradiance, electron density and temperature, neutral density and temperature observed by MAVEN. Moreover, by parametrically fitting the simulation results for various solar zenith angles and solar activities, we obtain empirical parameterized formulas for ionization and heating efficiencies which can potentially be adapted to planetary ionospheric models for the community. Key Points: A two‐stream model for Venus and Mars provides photoelectron flux, secondary ionization and photoelectron heating ratesThe photoelectron fluxes simulated by the two‐stream model agree with the Mars Atmosphere and Volatile Evolution observationsParameterized models of ionization and heating efficiencies are available for the community [ABSTRACT FROM AUTHOR]

Observations of young rapidly rotating G-type stars show that a large number of super-powerful flares occur on them. According to the early Sun activity model proposed by (Airapetian et al., 2016), it is assumed that during the first 700 million years there were 250 flares per day with an energy release comparable to the Carrington event of 1859. Within the framework of this model, the high intensity of irradiation of the Earth's atmosphere by solar cosmic rays (SCR) led to the formation of a large amount of greenhouse gases in ion-molecular reactions. Their high concentration in the atmosphere of the early Earth made it possible to solve the so-called "early Sun" paradox, where high climate temperatures existed on Earth at a reduced luminosity (by 20–30%). We have estimated the possible parameters of early Sun activity from data on the isotope composition of gases in the atmospheres of terrestrial planets. As a result of nuclear reactions of SCR protons with atmospheric gases, the 13C/12C and 15N/14N isotope ratios increase in the atmospheres of planets, since the yields of heavy and light C and N isotopes in nuclear spallation reactions are close, and the initial 13C/12C and 15N/14N ratios in planetary atmospheres are less than 0.01. The production of isotopes depends on the shape of the proton spectrum, the intensity of SCR fluxes, and the composition of early atmospheres. Therefore, modern 13C/12C and 15N/14N isotope ratios in the atmospheres of Venus and Mars give a limit to the activity of the early Sun. Our calculations show that in the Martian atmosphere the 13C/12C ratio should increase by tens of percent, and 15N/14N by several times, which contradicts the measurements. In the atmosphere of Venus, the change in isotope ratios under the influence of SCR is within the limits of the measurement error. Possible explanations of the obtained results are considered, including restrictions on the level of activity of the early Sun and SCR spectra, and the possible composition and mass of the early atmospheres of Mars and Venus. [ABSTRACT FROM AUTHOR]

In recent years atomic oxygen (AO) has become a major challenge for the space community. AO is the most dominant species of the residual atmosphere in low Earth orbit (LEO). In addition, AO was found at the top of the Venusian and Martian atmospheres. As a result of exposure to AO, organic space-facing materials erode, leading to changes in their roughness, thermo-optical properties, and mass loss. In the past years, there has been a growing interest in very low Earth orbit missions, below an altitude of 350 km, where AO density and thus, AO flux, are extremely high. At these low altitudes, material degradation by AO erosion poses a great risk to the integrity of any space-facing material, unless well protected. As such, measuring the AO flux and the consequential material erosion by AO, in real-time while under actual space conditions, are of great interest to the space community. Thus, in this work we present real-time AO material erosion measurements that were conducted, for the first-time in LEO, using the on-orbit material degradation detector (ORMADD). The ORMADD is a low-cost and simple to operate detector, comprised of photovoltaic cells coated with semi-transparent AO sensitive coatings. Two ORMADDs were integrated on TAUSAT-1, a 3U CubeSat, deployed into LEO from the international space station. The first ORMADD was coated with a thin amorphous-carbon coating and the second was covered with a thick pyrolytic Kapton film. Using ORMADD, we were able to follow the erosion of the coatings in real-time conditions and calculate the AO flux in TAUSAT-1 orbit. The average in-orbit flux, derived from ORMADD data, was found to be comparable to the theoretical average AO flux calculated by the space environment information system (SPENVIS) software. This work demonstrates the abilities of ORMADD as a space environment detector, as well as a space material degradation research instrument. • Atomic oxygen measurements were done by the on-orbit material degradation detector. • The on-orbit material degradation detector is based on coated photovoltaic cells. • The erosion of the coating was measured and the atomic oxygen flux was derived. • Atomic oxygen flux was also simulated by the SPENVIS simulation software. • Space measured atomic oxygen flux is comparable to the flux simulated by SPENVIS. [ABSTRACT FROM AUTHOR]

Several important processes of the Mars upper atmosphere are regulated by exospheric temperature (Texo) variations, including atmospheric escape rates. From the MAVEN mission, significant and largely periodic variability of Mars dayside Texo is now revealed by Neutral Gas and Ion Mass Spectrometer (NGIMS) data sets collected throughout solar cycle 24. Two complementary methods are utilized to extract temperatures from NGIMS data sets. These NGIMS dayside mean exospheric temperatures are shown to vary by ∼80 K (∼180–260 K) over solar cycle 24. This corresponds to a ΔTexo/ΔEUV sensitivity of ∼38 K m2 mW−1, where Lyman‐α is the solar index. Previous Mars Global Surveyor derived Texo values for solar cycle 23 yielded a sensitivity of ∼45 in the same units. This close correspondence suggests that the underlying dayside energy balances are similar yet slightly different over these two solar cycles. Corresponding Mars Global Ionosphere‐Thermosphere Model simulations show that molecular thermal conduction largely balances EUV heating for the Mars dayside thermosphere, while CO2 15‐µm cooling is secondary in importance, along with global winds. It is valuable to compare this Mars sensitivity to that computed for the dayside thermosphere of Venus. Pioneer Venus dayside data sets imply a sensitivity of ∼17.5 K m2 mW−1 units for solar cycle 21, a factor of ∼2.2 smaller than computed for Mars. This is because Venus CO2 cooling strongly balances EUV heating near its peak, thereby serving as a more efficient thermostat regulating dayside temperatures. CO2 cooling is much weaker for Mars. Plain Language Summary: Modern Earth global warming of its lower atmosphere is complemented by enhanced CO2 15‐μm cooling in its upper atmosphere. This CO2 cooling process also has a role in the CO2 dominated upper atmospheres of Venus and Mars. From the MAVEN mission, significant and largely periodic variability of Mars dayside exospheric temperatures is revealed by Neutral Gas and Ion Mass Spectrometer (NGIMS) data sets collected during the declining phase of the solar cycle (2014–2019). These NGIMS dayside mean exospheric temperatures vary by ∼80 K (180–260 K) over this weak (low sunspot number) solar cycle 24. Corresponding Global Climate Model (GCM) simulations of the Mars upper atmosphere reproduce this 80 K variability, revealing Extreme Ultraviolet heating is largely conducted downward to lower thermosphere altitudes providing net cooling, while CO2 15‐μm cooling is secondary in importance. By contrast, Pioneer Venus Orbiter dayside data sets (1978–1992) also imply a similar ∼80 K variability at Venus, now for the very strong (∼3‐times larger sunspot number) solar cycle 21. GCM simulations for Venus determine that CO2 cooling strongly balances EUV heating near its peak, thereby serving as an efficient thermostat regulating dayside temperatures. CO2 cooling is much weaker for Mars, yielding larger solar cycle variations in exospheric temperatures. Key Points: MAVEN dayside temperatures are shown to vary by 80 K (∼180–260 K) over solar cycle 24, reduced from the Mars Global Surveyor (MGS) value of 120 KMars temperature versus EUV sensitivity is similar for MAVEN and MGS exospheric measurements at 38 and 45 K m2 mW−1Mars thermal conduction balances EUV heating (160–180 km) for solar maximum conditions, while CO2 cooling is secondary [ABSTRACT FROM AUTHOR]

Due to the lower ionospheric thermal pressure and existence of the crustal magnetism at Mars, the Martian ionopause is expected to behave differently from the ionopause at Venus. We study the solar wind interaction and pressure balance at the ionopause of Mars using both in situ and remote sounding measurements from the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument on the Mars Express orbiter. We show that the magnetic pressure usually dominates the thermal pressure to hold off the solar wind at the ionopause at Mars, with only 13% of the cases where the ionospheric thermal pressure plays a more important role in pressure balance. This percentage at Venus, however, is up to 65%. We also find that the ionopause altitude at Mars decreases as the normal component of the solar wind dynamic pressure increases, similar to the altitude variation of the ionopauses at Venus. Moreover, our results show that the ionopause thickness at Mars and Venus is mainly determined by the ion gyromotion and is equivalent to about five ion gyroradii. Plain Language Summary: An ionopause is a sharp decrease in the plasma density at the top of the ionosphere, separating the ionospheric plasma from the shocked solar wind plasma. It was found to be a common feature at Venus, where its variability is well constrained by observations from the Pioneer Venus Orbiter. Past studies have shown that there are many similarities between the ionopauses at Mars and Venus. However, because the thermal pressure of the ionosphere at Mars is lower than that of Venus, the Martian ionopause is also thought to behave differently from the ionopause at Venus. We study the pressure configuration inside the ionopause at Mars and find that most of the time the magnetic pressure is greater than the thermal pressure. We also find that higher solar wind pressure pushes the ionopause downward at Mars. Moreover, we show that the thickness of the ionopauses at Mars equals a few radii of ion circular motion in the magnetic field. Our results provide insight to the process that controls the formation of the ionopause at Mars. Key Points: The ionopauses at Mars are found 13% of the time unmagnetized; this percentage at Venus, however, is up to 65%The ionopause altitude decreases with the solar wind dynamic pressure at Mars, similar to the altitude variation of the ionopauses at VenusThe ionopause thickness at Mars and Venus is mainly determined by the ion gyromotion and equivalent to about five ion gyroradii [ABSTRACT FROM AUTHOR]

Reading Terrestrial Planet Evolution in Isotopes and Element Measurements Edited by Helmut Lammer, Bernard Marty, Aubrey L. Zerkle, Michel Blanc, Hugh O'Neill and Thorsten Kleine Terrestrial planets evolve from primitive material of which remnants are now found in primitive meteorites called chondrites that originated from parent bodies that did not go through the cycle of melting and differentiation. A major finding for the Earth's paleo-atmosphere is that xenon, the heaviest noble gas, is fractionated in Earth's atmosphere. The authors also present a heuristic way to investigate the sensitivity of Earth's current tectonic mode as an example for rocky planets in general to the planet's HPE inventory. [Extracted from the article]

We report microtektites recovered from a large area of the deep seafloor (Central Indian Ocean) that appear to have undergone aerodynamic distortion during re-entry into the atmosphere. Considering the geographic locations, stratigraphic position and chemical compositions these glassy forms belong to the Australasian tektite strewn field. The microtektites are elongated to lengths of cms, sometimes flattened, bent, folded and fused at both ends suggesting that this side could have been the Earth-facing side during the re-entry. The presence of flow lines and distortional features are indicative of high atmospheric pressures experienced by the microtektites. The location where these microtektites were recovered constitute distal ejecta, and the shape distortion, that occurred during re-entry of the ejecta, seems to have affected only a few amongst the extensive collection of microtektites. Most of the specimens contain lechatelierite inclusions and higher volatile oxides, which are indicative of incomplete homogenization after melting and lower temperatures of formation vis-à-vis other specimens at the same location. The element distribution patterns in aerodynamically distorted microtektites suggest that ablation was similar to normal spherical tektites in which volatile elements are preserved. In contrast, aerodynamically ablated forms of Australasian ejecta show skin melting where thin layers of the anterior portions of samples flow back giving rise to the familiar button shapes. Our observation of delicate, elongated, flattened, and viscously deformed specimens is perhaps the first to imply that at the distal end of ejecta, each spot in the specimens has undergone different levels of trajectories, heating and ablation. These investigations could have implications to understand ejecta emplacement characteristics on planetary surfaces that contain appreciable atmospheres such as Mars and Venus. [ABSTRACT FROM AUTHOR]

Later Marov actively collaborated in the VeGa and Phobos projects and worked at NASA on Mars exploration projects. In 2016, Marov was awarded the Keldysh Gold Medal of the RAS "For outstanding scientific work in the field of applied mathematics of mechanics, as well as theoretical research on space exploration." Academician Marov belongs to the school of the great scientist of our country, Academician Mstislav Vsevolodovich Keldysh, whose worthy successor he became. [Extracted from the article]

Water ice clouds form in the mesospheres of terrestrial planets in the solar system (and most likely elsewhere) by vapor deposition at low pressures and temperatures. Under these conditions a range of crystalline and amorphous phases of ice might form. The phase is important because it influences nucleation kinetics, density, vapor pressure over the solid, growth rates and particle shape. In the past, the temperature range over which these different phases exist has been defined on the basis of depositing ice at low temperature and warming it while observing phase changes. However, the direct deposition of ice at a range of temperatures relevant for the terrestrial planets has not been systematically investigated. Here we present X‐ray Diffraction (XRD) measurements of water ice deposited at temperature intervals between 88 and 145 K in a vacuum chamber. XRD patterns showed that low density amorphous ice was formed at ≤120 K, stacking disordered ice I formed from 121 to 135 K and hexagonal ice I formed at 140 and 145 K. Direct deposition results in the stable hexagonal phase at much lower temperatures than when warming stacking disordered ice. All three phases of water ice observed here are possible in clouds in the mesospheres of Earth and Mars, while on Venus only amorphous ice is likely to form. Plain Language Summary: Ice clouds made up of water ice crystals can form in the thin upper atmospheres (mesospheres) of terrestrial planets where it can be extremely cold. Under these conditions a range of crystalline (hexagonal, cubic, or stacking disordered) and amorphous (lacking long range order) phases of ice might form. The phase is important because it influences a range of cloud properties. Experimentally, rather than warming ice formed at low temperature (as has been done in the past) we deposit the ice directly. We did this because an understanding of how the structure of ice evolves on warming as well as what structure forms when ice is deposited at specific temperatures is necessary to understand the properties of these clouds. Measurements showed that the amorphous, stacking disordered and hexagonal phases of water ice (ice I) can form depending on the specific temperature. Direct deposition yields the hexagonal phase at much lower temperatures than when warming ice deposited at lower temperatures. All three phases of water ice observed here are possible in clouds in the mesospheres of Earth and Mars. In contrast, on Venus clouds form at lower temperatures and so only amorphous ice is likely to form. Key Points: Low density amorphous ice was formed at 88–120 K, stacking disordered ice I formed from 121 to 135 K and hexagonal ice I formed at 140–145 KDirect deposition results in the stable hexagonal phase at much lower temperatures than when warming stacking disordered iceAll three phases of ice are possible in clouds in the mesospheres of Earth and Mars, while on Venus only amorphous ice is likely to form [ABSTRACT FROM AUTHOR]

The present‐day Venusian atmosphere is dry, yet, in its earlier history a significant amount of water evidently existed. One important water loss process comes from the energy and momentum transfer from the solar wind to the atmospheric particles. Here, we used measurements from the Ion Mass Analyzer onboard Venus Express to derive a relation between the power in the upstream solar wind and the power leaving the atmosphere through oxygen ion escape in the Venusian magnetotail. We find that on average 0.01% of the available power is transferred, and that the percentage decreases as the available energy increases. For Mars the trend is similar, but the efficiency is higher. At Earth, the ion escape does not behave similarly, as the ion escape only increases after a threshold in the available energy is reached. These results indicate that the Venusian induced magnetosphere efficiently screens the atmosphere from the solar wind. Plain Language Summary: Today, there is barely any water on Venus, but presumably a large amount existed in its earlier history. Therefore, the water must have been lost over the course of the Venusian history. An important process for removal of water is escape to space, induced by the interaction between the Venusian atmosphere and the solar wind (a fast stream of particles ejected from the Sun). In this study, we investigated the connection between the energy available in the upstream solar wind and the energy leaving Venus in the form of oxygen ion escape. By characterizing this relation, we investigate how the Venusian atmosphere reacts to changes in the upstream solar wind and how well it protects itself from atmospheric loss caused by the solar wind. We find that the energy transfer decreases as the available upstream energy increases, a trend that is very similar to that found at Mars. However, the Venusian atmosphere seems to absorb less energy from the solar wind than Mars. This indicates that the Venusian induced magnetosphere efficiently screens the atmosphere from the solar wind. This is important for the understanding of the effect of the solar wind on the Venusian atmospheric evolution. Key Points: The total escaping power from Venus does not increase linearly with increasing available power in the solar windThe coupling coefficient, that is, ratio between power out and in, decreases with increasing solar wind energy fluxThe trend of the coupling coefficient with the upstream parameters for Venus is similar to that of Mars but different from that of Earth [ABSTRACT FROM AUTHOR]

The concept of residence time of energy in a planetary atmosphere τ푅, recently introduced and computed for the Earth's atmosphere (Osácar et al., 2020), is here extended to the atmospheres of Venus, Mars and Titan. After a global thermal perturbation, τ푅is the time scale the atmosphere needs to return to equilibrium. The residence times of energy in the atmospheres of Venus, Earth, Mars and Titan have been computed. In the cases of Venus, Mars and Titan, these are mere lower bounds due to a lack of some energy data. [ABSTRACT FROM AUTHOR]

Low lunar gravity is not able to hold much atmosphere presumably created by degassing its solid body. Not like massive rocky planets-Venus, Earth, and Mars. They have atmospheres with their masses roughly proportional to their orbital frequencies (more frequency-more expelled volatiles from the planets depths by systematic wave shaking). Nevertheless, very thin exosphere is observed around the Moon. Its origin is related to the solar wind destructive action in the very thin surface film, meteorite impacts and, likely, due to constant trembling in the microwave diapason (modulated diapason due to the Moon orbiting around Earth and in Galaxy). By various methods in the lunar exosphere are found Argon 20000-100000 atoms in cm3, helium 500-30000, neon up to 20000; sodium 70, potassium 17, hydrogen fever than 17 atoms in cm3; methane and carbon also were found. There is a kind of equilibrium between supply of atoms from the solid lunar body and expelling them to space. Another result of constant trembling is in regolith layering. [ABSTRACT FROM AUTHOR]

The CO2(010)–O(3P) vibrational energy transfer (VET) efficiency is a key input to aeronomical models of the energy budget of the upper atmospheres of Earth, Venus, and Mars. This work addresses the physical mechanisms responsible for the high efficiency of the VET process at the thermal energies existing in the terrestrial upper atmosphere (150 K≤=T≤=550 K). We present a quantum-mechanical study of the process within a reduced-dimensionality approach. In this model, all the particles remain along a plane and the O(3P) atom collides along the C2v symmetry axis of CO2, which can present bending oscillations around the linear arrangement, while the stretching C–O coordinates are kept fixed at their equilibrium values. Two kinds of scattering calculations are performed on high-quality ab initio potential energy surfaces (PESs). In the first approach, the calculations are carried out separately for each one of the three PESs correlating to O(3P). In the second approach, nonadiabatic effects induced by spin-orbit couplings (SOC) are also accounted for. The results presented here provide an explanation to some of the questions raised by the experiments and aeronomical observations. At thermal energies, nonadiabatic transitions induced by SOC play a key role in causing large VET efficiencies, the process being highly sensitive to the initial fine-structure level of oxygen. At higher energies, the two above-mentioned approaches tend to coincide towards an impulsive Landau-Teller mechanism of the vibrational to translational (V-T) energy transfer. [ABSTRACT FROM AUTHOR]

We compare dynamics of planetary ions in the induced magnetospheres of Venus and Mars in a global hybrid simulation to study factors controlling the ion escape at unmagnetized planets. In the simulation we find that the finite Larmor radius (FLR) effects of escaping heavy ions are stronger at Mars than Venus under nominal solar wind conditions. But, varying upstream conditions, especially the IMF, affects the strength of the FLR effects. We classify three basic types of planetary ion dynamics in an induced magnetosphere. First, light ions such as hydrogen follow the E×B drift, and escape in the wake in the hemisphere where the solar wind convection electric field is pointing towards the planet. Second, heavy ions like oxygen undergo FLR effects, and escape mainly outside of the wake in the hemisphere where the solar wind convection electric field is pointing away from the planet. Third, ion species between light and heavy ions can have both the E×B and FLR type dynamics in the same time. [ABSTRACT FROM AUTHOR]

Acoustic coupling between solid Earth and atmosphere has been observed since the 1960s, first from ground-based seismic, pressure, and ionospheric sensors and since 20 years with various satellite measurements, including with global positioning system (GPS) satellites. This coupling leads to the excitation of the Rayleigh surface waves by local atmospheric sources such as large natural explosions from volcanoes, meteor atmospheric air-bursts, or artificial explosions. It contributes also in the continuous excitation of Rayleigh waves and associated normal modes by atmospheric winds and pressure fluctuations. The same coupling allows the observation of Rayleigh waves in the thermosphere most of the time through ionospheric monitoring with Doppler sounders or GPS. The authors review briefly in this paper observations made on Earth and describe the general frame of the theory enabling the computation of Rayleigh waves for models of telluric planets with atmosphere. The authors then focus on Mars and Venus and give in both cases the atmospheric properties of the Rayleigh normal modes and associated surface waves compared to Earth. The authors then conclude on the observation perspectives especially for Rayleigh waves excited by atmospheric sources on Mars and for remote ionospheric observations of Rayleigh waves excited by quakes on Venus. [ABSTRACT FROM AUTHOR]

Generic predictions for acoustic dispersion and absorption in the atmospheres of Mars and Venus are presented. For Mars, Pathfinder and Mars Express ambient data and averaged thermophysical parameters are used as inputs to a preliminary model based on the continuum approximation for Mars' thin atmosphere--the need for Boltzmann-based treatment is discussed in the context of Knudsen numbers. Strong absorption constrains acoustic sensing within the Martian planetary boundary layer. For the dense atmosphere of Venus, the van der Waals equation of state is used. The thermophysical and transport parameters were interpolated at the ambient conditions. Acoustic sensing is discussed at 50 km above Venus' surface, a level where aerostats (e.g., European Space Agency's EVE) and manned airships (e.g., NASA's HAVOC) may be deployed in the future. The salient atmospheric characteristics are described in terms of temperature, pressure, and convective stability profiles, followed by wavenumber predictions, and discussions of low- and high-frequency sensing applications. At low frequencies, emphasis is placed on infrasound. A simple generation mechanism by Martian dust devils is presented, yielding fundamental frequencies between 0.1 and 10 Hz. High-frequency sensing is exemplified by ultrasonic anemometry. Of the two environments, Venus is notably more dispersive in the ultrasonic range. [ABSTRACT FROM AUTHOR]

A new Mid-Infrared Laser Heterodyne Instrument (MILAHI) with >10 6 resolving power at 7–12 μm was developed for continuous monitoring of planetary atmospheres by using dedicated ground-based telescopes for planetary science at Mt. Haleakalā, Hawaii. Room-temperature-type quantum cascade lasers (QCLs) that cover wavelength ranges of 7.69–7.73, 9.54–9.59, and 10.28–10.33 μm have been newly installed as local oscillators to allow observation of CO 2 , CH 4 , H 2 O 2 , H 2 O, and HDO. Modeling and predictions by radiative transfer code gave the following scientific capabilities and measurement sensitivities of the MILAHI. (1) Temperature profiles are achieved at altitudes of 65–90 km on Venus, and the ground surface to 30 km on Mars. (2) New wind profiles are provided at altitudes of 75–90 km on Venus, and 5–25 km on Mars. (3) Direct measurements of the mesospheric wind and temperature are obtained from the Doppler-shifted emission line at altitudes of 110 km on Venus and 75 km on Mars. (4) Detections of trace gases and isotopic ratios are performed without any ambiguity of the reproducing the terrestrial atmospheric absorptions in the observed wavelength range. A HDO measurement of twice the Vienna Standard Mean Ocean Water (VSMOW) can be obtained by 15-min integration, while H 2 O of 75 ppm is provided by 3.62-h integration. The detectability of the 100 ppb-CH 4 on Mars corresponds to an integration time of 32 h. [ABSTRACT FROM AUTHOR]

Highlights: [•] We made a new absorption HDO linelist in a wide region between 0 and 25,600cm−1. [•] About 3,000,000 of 720,000 line positions are estimated with experimental accuracy. [•] HDO–CO2 broadening coefficients and their temperature exponents are calculated. [•] Accurate variational intensities are provided for every line. [•] This HDO linelist is optimal for studying the martian and venusians atmospheres. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution vacuum ultraviolet photoabsorption cross sections of CO2 are required for accurate modeling of airglow emissions from the Martian and Venusian atmospheres and for photochemical models of those and Earth''s atmospheres. We report cross section measurements between 91,000 and 115,000cm−1 (87–110nm) at spectral resolutions of 1.15cm−1 (91,000–102,000cm−1) and 0.58cm−1 (102,000–115,000cm−1). These high-resolution cross sections show significant deviations from the results of previous lower-resolution measurements, particularly in regions with sharp spectral structure. [Copyright &y& Elsevier]

The article discusses the Earth's atmosphere, which includes layers such as the troposphere, stratosphere, and mesosphere. The reflection of radiation from the Sun by the atmosphere, weather patterns, and ozone molecules are discussed, as well as the atmospheres of Venus and Mars. Research conducted by weather balloons is also mentioned.

In the absence of solar radiation, the most common photoexcitation process in the terrestrial planets results from atom recombination. Thus, in the Mars and Venus atmospheres recombination of O(3P) + O(3P) and O(3P) + N(4S) leads to emissions of excited states of O2 and NO, respectively. However, in the Earth's atmosphere there are ionospheric emissions that depend on interactions of charged species, for example, the oxygen red and green lines arising from dissociative recombination (DR) of . In the case of the CO2 planets, the predominant ionospheric ion is also , so that in principle DR is important in that environment, where the markers would also be the red and green lines. Whether these can be detected depends on DR rates and the radiating efficiencies of these species. In this report we discuss the history of green line detection in the Venus nightglow as well as the observations of excited O2 molecules, and the lack of appearance of the O2 (b-X) Atmospheric 0-0 band. [ABSTRACT FROM AUTHOR]

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Abstract: Atmospheric angular momentum variations of a planet are associated with the global atmospheric mass redistribution and the wind variability. The exchange of angular momentum between the fluid layers and the solid planet is the main cause for the variations of the planetary rotation at seasonal time scales. In the present study, we investigate the angular momentum variations of the Earth, Mars and Venus, using geodetic observations, output of state-of-the-art global circulation models as well as assimilated data. We discuss the similarities and differences in angular momentum variations, planetary rotation and angular momentum exchange for the three terrestrial planets. We show that the atmospheric angular momentum variations for Mars and Earth are mainly annual and semi-annual whereas they are expected to be “diurnal” on Venus. The wind terms have the largest contributions to the LOD changes of the Earth and Venus whereas the matter term is dominant on Mars due to the CO2 sublimation/condensation. The corresponding LOD variations (ΔLOD) have similar amplitudes on Mars and Earth but are much larger on Venus, though more difficult to observe. [Copyright &y& Elsevier]

Abstract: The sunlit portion of planetary ionospheres is sustained by photoionization. This was first confirmed using measurements and modelling at Earth, but recently the Mars Express, Venus Express and Cassini-Huygens missions have revealed the importance of this process at Mars, Venus and Titan, respectively. The primary neutral atmospheric constituents involved (O and CO2 in the case of Venus and Mars, O and N2 in the case of Earth and N2 in the case of Titan) are ionized at each object by EUV solar photons. This process produces photoelectrons with particular spectral characteristics. The electron spectrometers on Venus Express and Mars Express (part of ASPERA-3 and 4, respectively) were designed with excellent energy resolution (ΔE/E=8%) specifically in order to examine the photoelectron spectrum. In addition, the Cassini CAPS electron spectrometer at Saturn also has adequate resolution (ΔE/E=16.7%) to study this population at Titan. At Earth, photoelectrons are well established by in situ measurements, and are even seen in the magnetosphere at up to 7R E. At Mars, photoelectrons are seen in situ in the ionosphere, but also in the tail at distances out to the Mars Express apoapsis (∼3R M). At both Venus and Titan, photoelectrons are seen in situ in the ionosphere and in the tail (at up to 1.45R V and 6.8R T, respectively). Here, we compare photoelectron measurements at Earth, Venus, Mars and Titan, and in particular show examples of their observation at remote locations from their production point in the dayside ionosphere. This process is found to be common between magnetized and unmagnetized objects. We discuss the role of photoelectrons as tracers of the magnetic connection to the dayside ionosphere, and their possible role in enhancing ion escape. [Copyright &y& Elsevier]

Abstract: This paper deals with two common problems and then considers major aspects of chemistry in the atmospheres of Mars and Venus. (1) The atmospheres of the terrestrial planets have similar origins but different evolutionary pathways because of the different masses and distances to the Sun. Venus lost its water by hydrodynamic escape, Earth lost CO2 that formed carbonates and is strongly affected by life, Mars lost water in the reaction with iron and then most of the atmosphere by the intense meteorite impacts. (2) In spite of the higher solar radiation on Venus, its thermospheric temperatures are similar to those on Mars because of the greater gravity acceleration and the higher production of O by photolysis of CO2. O stimulates cooling by the emission at 15μm in the collisions with CO2. (3) There is a great progress in the observations of photochemical tracers and minor constituents on Mars in the current decade. This progress is supported by progress in photochemical modeling, especially by photochemical GCMs. Main results in these areas are briefly discussed. The problem of methane presents the controversial aspects of its variations and origin. The reported variations of methane cannot be explained by the existing data on gas-phase and heterogeneous chemistry. The lack of current volcanism, SO2, and warm spots on Mars favor the biological origin of methane. (4) Venus’ chemistry is rich and covers a wide range of temperatures and pressures and many species. Photochemical models for the middle atmosphere (58–112km), for the nighttime atmosphere and night airglow at 80–130km, and the kinetic model for the lower atmosphere are briefly discussed. [Copyright &y& Elsevier]

Abstract: Classified as a terrestrial planet, Venus, Mars, and Earth are similar in several aspects such as bulk composition and density. Their atmospheres on the other hand have significant differences. Venus has the densest atmosphere, composed of CO2 mainly, with atmospheric pressure at the planet''s surface 92 times that of the Earth, while Mars has the thinnest atmosphere, composed also essentially of CO2, with only several millibars of atmospheric surface pressure. In the past, both Mars and Venus could have possessed Earth-like climate permitting the presence of surface liquid water reservoirs. Impacts by asteroids and comets could have played a significant role in the evolution of the early atmospheres of the Earth, Mars, and Venus, not only by causing atmospheric erosion but also by delivering material and volatiles to the planets. Here we investigate the atmospheric loss and the delivery of volatiles for the three terrestrial planets using a parameterized model that takes into account the impact simulation results and the flux of impactors given in the literature. We show that the dimensions of the planets, the initial atmospheric surface pressures and the volatiles contents of the impactors are of high importance for the impact delivery and erosion, and that they might be responsible for the differences in the atmospheric evolution of Mars, Earth and Venus. [Copyright &y& Elsevier]

Abstract: A study of the CO2 atmospheric emissions at 10-μm in the upper atmospheres of Mars and Venus is performed in order to explain a number of ground-based measurements of these emissions recently taken at very high spectral resolution in both planets. The measurements are normally used to derive atmospheric temperatures and winds, but uncertainties on the actual emission layers were so far a serious drawback for their correct interpretation. The non-LTE models used for Mars and Venus in the present analysis are entirely similar in order to perform consistent comparisons between the two planets. In particular, the same scheme of CO2 states and ro-vibrational bands are used, with similar assumptions on collisional routes and rate coefficients, and also the same radiative transfer approximations. The emissions at 10-μm are produced in both atmospheres by the same excitation mechanism: radiative pumping of the CO2(0001) vibrational state by direct solar absorption(at 4.3μm) and indirect absorption (at 2.7μm, followed by collisional quenching). The computed radiances are specially strong in the upper mesosphere and lower thermosphere of the two planets during maximum solar illumination, producing a population inversion in such conditions with the lower states of the bands, the CO2 (1000) and CO2(0200). We obtained that other population inversions are also possible, involving higher energy CO2 states. The larger solar flux available on Venus is found to produce larger vibrational populations and stronger emissions than equivalent atmospheric layers on Mars, in agreement with the observations. A number of perturbation studies were used to determine the exact emission altitudes, or weighting function peaks, for usual nadir sounding. The sensitivity of the emission to non-LTE model uncertainties and to atmospheric variations in temperature and CO2 density is also presented. The dependence with the solar zenith angle and with the emission angle, as obtained with this model, could also be useful for guiding future observations. [Copyright &y& Elsevier]

Abstract: We report here on CO limb observations in Mars, Venus and Earth and their model simulations. A comparative study of the CO emission, including the most recent spacecraft observations of the three planets’ atmospheres, has been performed. Strong daytime emissions near have been recently observed in the limb of the upper atmosphere of the three terrestrial planets by the Planetary Fourier Spectrometer on Mars Express (), the Visible and Infrared Thermal Imaging Spectrometer on Venus Express (), and the Michelson Interferometer for Passive Atmosphere Sounding on Envisat (). Those emissions are produced by solar pumping of molecular vibrations and non-local thermodynamic equilibrium (non-LTE) models are used to explain them in a consistent framework for the three atmospheres. The maximum of the emission occurs on Venus between 90 and 110km, on Mars above 60km, and on Earth around 68km. The observations show that the fundamental band dominates the CO non-LTE emission on Earth, while on Venus and Mars the larger contribution comes from the first hot band transition. The main goal of this study is to understand those differences and similarities with the help of theoretical simulations with optical thickness considerations, and compare the capability of space instrumentation for the remote sounding of the atmospheres in this spectral region. [Copyright &y& Elsevier]

Abstract: Measurements of water vapor in the atmospheres of Venus or Mars by spectroscopic techniques in the infrared range are being made routinely by instruments onboard the Venus Express and the Mars Reconnaissance Orbiter. The interpretation of these measurements in the 2250–4450cm−1 region is being complicated by the presence of HDO lines absorbing radiation in this region. These spectra cannot be modeled properly because line shape parameters for CO2 broadening (principal gas in these atmospheres) of HDO are not available. Here semi-classical line shape calculations for the HDO–CO2 collision system are made using the Robert–Bonamy formalism for some 2300 rotational band transitions of HDO. From these calculations, the half-width, its temperature dependence, and the line shift are determined to aid in the reduction of the measured spectra. These data will greatly reduce the uncertainty of the reduced profiles from the Venus and Mars measurements and will also allow better estimates of the D/H ratio on these planets. [Copyright &y& Elsevier]

Abstract: Precise modelling of infrared absorption by carbon dioxide is of primary importance for radiative transfer calculations in CO2-rich atmospheres like those of Venus and Mars. Despite various measurements and theoretical models dedicated to this subject, accurate data at different temperatures and pressures are still lacking in numerous spectral regions. In this work, using two Fourier Transform Spectrometers, we have measured spectra of pure CO2 in a large spectral region range, from 750 to 8500cm−1 at various densities (3–57amagat) and temperatures (230–473K). Comparisons between measured dipolar absorption bands and spectra calculated with the widely used Lorentz line shape show very large discrepancies. This result is expected since the Lorentz approach neglects line-coupling effects due to intermolecular collisions which transfer absorption from the wings to the band center. In order to account for this effect, a theoretical approach based on the impact and Energy Corrected Sudden approximations has been developed. Comparisons of this model with numerous laboratory spectra in a wide range of pressure, temperature and spectral domain show satisfactory agreements for band centers and near wing regions where the impact approximation is valid. However, as expected, due to the breakdown of the impact approximation, the model fails when considering far wing regions. In the absence of precise models accounting for line-mixing and finite collision duration (non impact) effects, empirical approximations are proposed in order to model the far wings. [Copyright &y& Elsevier]

Abstract: Magnetic flux ropes are created in the ionosphere of Venus and Mars during the interaction of the solar wind with their ionospheres and also at Titan during the interaction of the Saturnian magnetospheric plasma flow with Titan’s ionosphere. The flux ropes at Venus and Mars were extensively studied from Pioneer Venus Orbiter and Mars Global Surveyor observations respectively during solar maximum. Based on the statistical properties of the observed flux ropes at Venus and Mars, the formation of a flux rope in the ionosphere is thought first to arise near the boundary between the magnetic barrier and the ionosphere and later to sink into the lower ionosphere. Venus flux ropes are also observed during solar minimum by Venus Express and the observations of developing and mature flux ropes are consistent with the proposed mechanism. With the knowledge of flux rope structure in the Venus ionosphere, the twisted fields in the lower ionosphere of Titan from Cassini observations are studied and are found to resemble the Venus flux ropes. [Copyright &y& Elsevier]

In view of the low H2O abundance in the present Venusian and Martian atmospheres several observations by spacecraft and studies suggest that both planets should have lost most of their water over the early active period of the young Sun. During the first Gyr after the Sun arrived at the Zero- Age-Main-Sequence high X-ray and EUV fluxes between 10 and 100 times that of the present Sun were responsible for much higher temperatures in the thermosphere-exosphere environments on both planets. By applying a diffusive-gravitational equilibrium and thermal balance model for investigating radiation impact on the early thermospheres by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by CO2 IR emission in the 15μm band we found expanded thermospheres with exobase levels between about 200 km (present) and 2000 km (4.5 Gyr ago). The higher temperatures in the upper atmospheres of both planets could reach “blow-off” conditions for H atoms even at high CO2 mixing ratios of 96%. Lower CO2/N2 mixing ratio or higher contents of H2O vapor in the early atmospheres could have had a dramatic impact from the loss of atmosphere and water on both planets. The duration of this phase of high thermal loss rates essentially depended on the mixing ratios of CO2, N2, and H2O in the early atmospheres and could have lasted between about 150 and several hundred Myr. [ABSTRACT FROM AUTHOR]

Magnetometer measurements on the Pioneer Venus Orbiter (PVO) have shown that Venus is a non-magnetic planet and similar measurements on the Mars Global Surveyor (MGS) have demonstrated that, except for some very localized magnetic fields of crustal origin, Mars intrinsic magnetic field is rather weak. As a consequence, solar wind interacts directly with the atmospheres and ionospheres of these planets. Much has been learnt about the ionosphere of Venus from the large database generated by the various aeronomy experiments on the PVO, especially during the solar maximum. For example, the major ion in the upper ionosphere (topside) of Venus is O+, and is controlled by diffusion, while O2+ dominates in the bottomside (below 200 km) and is in photochemical equilibrium. Mars ionosphere is relatively less explored and most of the information has come from radio occultation experiments, which have provided a large data base on the electron density profiles for various solar activity conditions. Only two direct ion measurements exist due to Viking Landers, which correspond to solar minimum conditions and have shown that O2+ dominates in most part of the Mars ionosphere. However, its height distribution is in variance with that expected theoretically from the diffusive equilibrium state. During high solar activity when Venus ionosphere is robust, solar wind interaction results in a sharp boundary of the ionosphere, called the ionopause. However, during solar minimum, when the ionosphere is weak, the solar wind compresses the Venus ionosphere to its limiting altitude in the photodynamical regime forming a thick ionopause. The diffusion region (or the topside ionosphere) is then nearly extinct. This thick ionopause is also seen at times of high solar wind dynamic pressure (Psw) during solar maximum with O+ still as the dominant ion there. The diffusion region is nearly extinct again. Since Mars ionosphere is weak, particularly during low and moderate solar activity conditions, one expects the solar wind to compress its ionosphere in the photodynamical regime forming a thick ionopause. Consequently, the diffusion region (or the topside ionosphere) would be nearly extinct most of these times. The observed Viking O2+ profiles and the radio occultation Ne profiles, which are indeed greatly compressed provide evidence in this regard. Thus the observed topside ionospheric profiles of Venus and Mars basically represent the ionopause regions, only exception being the solar maximum when an extended topside ionosphere exists at Venus. [Copyright &y& Elsevier]

At the non-magnetized planets Venus and Mars, the solar wind comes in direct contact with the planetary neutrals. This interaction produces energetic neutral atoms (ENAs) and X-rays. In this paper we review some of the different production mechanisms and present examples of simulated ENA and X-ray images. To extract as much information as possible about the ion and neutral distributions in the observed region one has to further process the images. We discuss different approaches to model the production of ENAs and X-rays, and then how to solve the resulting mathematical inverse problems. [Copyright &y& Elsevier]

Current knowledge of solar tides in the middle and upper atmospheres of Mars and Venus is summarized, including Sun-synchronous tides on both planets; nonmigrating tides on Mars due to radiative/topographic interactions near the surface; and acceleration/deceleration of the zonal mean flows on Mars and Venus due to tidal interactions. [Copyright &y& Elsevier]

We review here advances in the aeronomy of Earth''s nearest neighbors, Venus and Mars, including ion and neutral chemistry, airglow, temperature structures, wave activity and the effects of the newly discovered areas of remnant magnetization. We discuss the results of recent modeling and measurements, both ground based and from satellites, with emphasis on progress of the last 3–5 years. [Copyright &y& Elsevier]

Sagan proposed reducing the greenhouse effect by seeding the cooler heights of the Venusian atmosphere with photosynthetic bacteria that would convert carbon dioxide into formaldehyde (CH SB 2 sb O). Among the most influential papers of that heyday was Martyn Fogg's "Terraforming: A review for environmentalists."[2] I asked Fogg via email why scientific interest in terraforming Mars had waned. In 1965 NASA's I Mariner 4 i mission to Mars confirmed that the planet's atmosphere is mostly CO SB 2 sb and that it's too thin to exert a strong greenhouse effect. [Extracted from the article]

New formulations valid for wide ranges of particle diameter and density and gas density are presented for prediction of saltation threshold speed for small particles. A low-air-density wind tunnel was used to extend the range of previous investigations and to separate the effects of Reynolds number and interparticle forces of cohesion. The new formulations are used to predict saltation threshold for atmospheric conditions on the surface of the Earth, Mars, and Venus. [ABSTRACT FROM AUTHOR]

The present paper studies the generation of terahertz (THz) radiation in CO2 in comparison with atmospheric air at a wide range of pressures. We established experimentally and explained theoretically that for these gases there are optimal pressures at about 1 bar for air and 0.5 bar for CO2 under which the efficiency of conversion from near-infrared to THz frequencies is the highest. We consider the possibility of applying femtosecond laser-induced THz generation for the study of the atmosphere of Mars and found that the overall THz yield near the surface of Mars is just a factor of 6 lower than on Earth. Comparable THz energy on the two planets is associated with underdense plasma on Earth (∼10% of neutrals) and full double ionization of carbon dioxide on Mars (∼200% of neutrals), the latter opening great perspective for THz remote sensing of trace gases in the Martian atmosphere. [ABSTRACT FROM AUTHOR]