Evolution of the Martian atmosphere and hydrosphere: Solar wind erosion studied by ASPERA-3 on Mars Express

The evolution of the Martian atmosphere and the potential existence of a past hydrosphere is a scientific issue of great interest in planetary research. Although the first missions to Mars had a focus on surface features and atmospheric properties, some of the missions (e.g., The Soviet Mars 2, 3 and 5) also carried instruments addressing the solar wind interaction with the Martian atmosphere and ionosphere and the potential existence of an intrinsic magnetic field on Mars. However, it took until 1989 before a spacecraft, Phobos-2, was able to carry out a more detailed investigation of the solar wind interaction with Mars. Phobos-2 gave valuable data on the Solar wind interaction with Mars during about 2 months of operations, leading to a better understanding of the solar wind impact on a weakly magnetized planet. However, Phobos-2 also raised a number of critical issues that has left science without adequate data since 1989.Investigations planned for Mars Express will cast new light on important aspects of the solar wind interaction with Mars. ASPERA-3 (Analyzer of Space Plasma and Energetic Atoms) on Mars Express will focus on the overall plasma outflow and monitor remotely the outflow and inflow of energetic neutral atoms produced by charge exchange processes. This report will discuss some of the unsolved issues about the solar wind interaction with Mars and how we plan to address these issues with Mars Express.     

Planetary ENA Imaging: Venus and a comparison with Mars

We present simulated images of energetic neutral atoms (ENAs) produced in charge exchange collisions between solar wind protons and neutral atoms in the exosphere of Venus, and make a comparison with earlier results for Mars. The images are found to be dominated by two local maxima. One produced by charge exchange collisions in the solar wind, upstream of the bow shock, and the other close to the dayside ionopause. The simulated ENA fluxes at Venus are lower than those obtained in similar simulations of ENA images at Mars at solar minimum conditions, and close to the fluxes at Mars at solar maximum. Our numerical study shows that the ENA flux decreases with an increasing ionopause altitude. The influence of the Venus nighttime hydrogen bulge on the ENA emission is small.     

Initial analysis of ion fluxes in the magnetotail of Mars based on simultaneous measurements on Mars Express and Maven

Simultaneous operation of two Mars satellites, equipped with instruments for the study of the plasma environment close to Mars, the European satellite Mars Express and American satellite MAVEN, allows one to investigate the influence of the interplanetary environment on the Martian magnetosphere and atmospheric losses, induced by the solar wind, for the first time, with a sufficient degree of confidence. In this paper, the data from measurements on the Mars Express satellite (MEX) of heavy ion losses are analyzed in comparison with the solar wind and magnetic field measurements on the MAVEN satellite. The main issue is the spatial structure of the escaping ion flux and the influence of the nonstationarity of the solar wind flux on the escape rate.     

Magnetic fields of mars and venus: Solar wind interactions

Recent U.S.S.R. studies of the magnetic field and solar wind flow in the vicinity of Mars and Venus confirm earlier U.S.A. reports of a bow shock wave developed as the solar wind interacts with these planets. Mars 2 and 3 magnetometer experiments report the existence of an intrinsic planetary magnetic field, sufficiently strong to form a magnetopause, deflecting the solar wind around the planet and its ionosphere. This is in contrast to the case for Venus, where it is assumed to be the ionosphere and processes therein which are responsible for the solar wind deflection. An empirical relationship appears to exist between planetary dipole magnetic moments and their angular momentum for Moon, Mars, Venus, Earth and Jupiter. Implications for the magnetic fields of Mercury and Saturn are discussed.Paper presented at the Lunar Science Institute Conference on Geophysical and Geochemical Exploration of the Moon and Planets, January 10–12, 1973     

Ion escape from Mars as a function of solar wind conditions: A statistical study

The influence of solar EUV and solar wind conditions on ion escape at Mars is investigated using ion data from the Aspera-3 instrument on Mars Express, combined with solar wind proxy data obtained from the Mars Global Surveyor (MGS) spacecraft. A solar EUV flux proxy based on data from the Earth position, scaled and shifted in time for Mars, is used to study relatively long time scale changes related to solar EUV variability. Data from May 2004 until November 2005 has been used. A clear dependence on the strength of the subsolar magnetic field as inferred from MGS measurements is seen in the ion data. The region of significant heavy ion flows is compressed and the heavy ion flux density is higher for high subsolar magnetic field strength. Because of the difference in outflow area, the difference in estimated total outflow is somewhat less than the difference in average flux density. We confirm previous findings that escaping planetary ions are mainly seen in the hemisphere into which the solar wind electric field is pointed. The effect is more pronounced for the high subsolar magnetic field case.The average ion motion has a consistent bias towards the direction of the solar wind electric field, but the main motion is in the antisunward direction. The antisunward flow velocity increases with tailward distance, reaching above at 2 to 3 martian radii downtail from Mars for O⁺ ions. Different ion species reach approximately the same bulk flow energy. We did not find any clear correlation between the solar EUV flux and the ion escape distribution or rate, probably because the variation of the solar EUV flux over our study interval was too small. The results indicate that the solar wind and its magnetic field directly interacts with the ionosphere of Mars, removing more ions for high subsolar magnetic field strength. The interaction region and the tail heavy ion flow region are not perfectly shielded from the solar wind electric field, which accelerates particles over relatively large tail distances.     

Magnetic storms on Mars

Based on data from the Mars Global Surveyor magnetometer we examine periods of significantly enhanced magnetic disturbances in the martian space environment. Using almost seven years of observations during the maximum and early declining phase of the previous solar cycle the occurrence pattern and typical time profile of such periods is investigated and compared to solar wind measurements at Earth. Typical durations of the events are 20-40 h, and there is a tendency for large events to last longer, but a large spread in duration and intensity are found. The large and medium intensity events at Mars are found to occur predominantly in association with interplanetary sector boundaries, with solar wind dynamic pressure enhancements being the most likely interplanetary driver. In addition it is found that, on time scales of months to several years, the dominant cause of global variability of the magnetic field disturbance at Mars is solar wind dynamic pressure variations associated with the eccentricity of the martian orbit around the Sun.     

Observations of magnetic anomaly signatures in Mars Express ASPERA-3 ELS data

Mars Express (MEX) Analyser of Space Plasmas and Energetic Atoms (ASPERA-3) data is providing insights into atmospheric loss on Mars via the solar wind interaction. This process is influenced by both the interplanetary magnetic field (IMF) in the solar wind and by the magnetic ‘anomaly’ regions of the martian crust. We analyse observations from the ASPERA-3 Electron Spectrometer near to such crustal anomalies. We find that the electrons near remanent magnetic fields either increase in flux to form intensified signatures or significantly reduce in flux to form plasma voids. We suggest that cusps intervening neighbouring magnetic anomalies may provide a location for enhanced escape of planetary plasma. Initial statistical analysis shows that intensified signatures are mainly a dayside phenomenon whereas voids are a feature of the night hemisphere.     

A plasma flow velocity boundary at Mars from the disappearance of electron plasma oscillations

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express (MEX) spacecraft is capable of measuring ionospheric electron density by the use of two main methods: remote radar sounding and from the excitation of local plasma oscillations. The frequency of the locally excited electron plasma oscillations is used to measure the local electron density. However, plasma oscillations are not observed when the plasma flow velocity is higher than about 160 km/s, which occurs mainly in the solar wind and magnetosheath. As a consequence, in many passes, there is a sudden disappearance of the plasma oscillations as the spacecraft enters into the magnetosheath. This fact allows us to identify a flow velocity boundary on the dayside, between the ionosphere of Mars and the shocked solar wind. This paper summarizes the results of the measurement of 552 orbits mostly over a period from August 4, 2005 to August 17, 2007. The boundary points found using MARSIS have been verified by measurements from the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) Electron Spectrometer (ELS) instrument on Mars Express. The average position of the flow velocity boundary is compared to flow velocity simulations computed using hybrid model and other boundaries. The boundary altitude is slightly lower than the magnetic pile-up boundary determined using Phobos 2 and Mars Global Surveyor (MGS) crossings, but it is in good agreement with the induced magnetospheric boundary determined by ASPERA-3. Investigation of the effect of the crustal magnetic field revealed that the flow velocity boundary is raised at the locations with strong crustal magnetic fields.     

Atmospheric ion wakes of Venus and Mars in the solar wind

A model has been developed for the currents induced in the ionospheres of Venus and Mars by the flowing magnetized solar wind in a previous paper (Cloutier and Daniell, 1973). The altitudes of the ionopauses on both planets, determined from the electrodynamical models of the previous paper, are used here to calculate the total rates of atmospheric mass loss to the solar wind for Venus and Mars. These loss rates are compared to the rates calculated by Michel (1971) based upon the limit of mass loading of the solar wind flow determined from hydrodynamic constraints. The distributions of planetary ions in the downstream wakes of Venus and Mars are calculated, and the interpretation of ion spectrometer measurements from close planetary encounters is discussed.     

First ENA observations at Mars: Solar-wind ENAs on the nightside

We present measurements with an Energetic Neutral Atom (ENA) imager on board Mars Express when the spacecraft moves into Mars eclipse. Solar wind ions charge exchange with the extended Mars exosphere to produce ENAs that can spread into the eclipse of Mars due to the ions' thermal spread. Our measurements show a lingering signal from the Sun direction for several minutes as the spacecraft moves into the eclipse. However, our ENA imager is also sensitive to UV photons and we compare the measurements to ENA simulations and a simplified model of UV scattering in the exosphere. Simulations and further comparisons with an electron spectrometer sensitive to photoelectrons generated when UV photons interact with the spacecraft suggest that what we are seeing in Mars' eclipse are ENAs from upstream of the bow shock produced in charge exchange with solar wind ions with a non-zero temperature. The measurements are a precursor to a new technique called ENA sounding to measure solar wind and planetary exosphere properties in the future.     

ENA detection in the dayside of Mars: ASPERA-3 NPD statistical study

The Analyzer of Space Plasma and EneRgetic Atoms (ASPERA-3) on board Mars Express is designed to study the interaction between the solar wind and the atmosphere of Mars and to characterize the plasma and neutral gas environment in near-Mars space. Neutral Particle Detectors (NPD-1 and 2), which form part of the ASPERA-3 instrument suite, are Energetic Neutral Atom (ENA) detectors which use the time-of-flight (ToF) technique to resolve the energy of detected particles. In the present study, we perform a statistical analysis of NPD ToF data collected between 14 March 2004 and 17 June 2004 when Mars Express was located at the dayside of Mars looking toward the planet. After pre-processing and removal of UV contamination, the ToF spectra were fitted with simple analytical functions so as to derive a set of parameters. The behavior of these parameters, as a function of spacecraft position and attitude, is compared with a model, which describes ENA generation by charge exchange between shocked solar wind protons and extended Martian exosphere. The observations and the model agree well, indicating that the recorded signals are charge-exchanged shocked solar wind.     

The magnetic field draping direction at Mars from April 1999 through August 2004

Using more than five years of data from the magnetometer and electron reflectometer (MAG/ER) on Mars Global Surveyor (MGS), we derive the draping direction of the magnetic field above a given latitude band in the northern hemisphere. The draping direction varies on timescales associated with the orbital period of Mars and with the solar rotation period. We find that there is a strongly preferred draping direction when Mars is in one solar wind sector, but the opposite direction is not preferred as strongly for the other solar wind sector. This asymmetry occurs at or below the magnetic pileup boundary (MPB), is observed preferentially on field lines that connect to the collisional ionosphere, and is independent of planetary longitude. The observations could be explained by a hemispherical asymmetry in the access of field lines to the low-altitude ionosphere, or possibly from global modification of the low-altitude solar wind interaction by crustal magnetic fields. We show that the draping direction affects both the penetration of sheath plasma to 400 km altitudes on the martian dayside and the radial component of the magnetic field on the planetary night side.     

Features of the Martian Magnetic Field Structure

Based on the single-fluid MHD model of Mars space simulation, this paper has studied the magnetic field structure in the near-Mars space and investigated the influence of Martian crustal magnetic anomalies on the magnetic field structure. In the process of the solar wind interaction with Mars, the bow shock and magnetic pile-up region are produced. The interplanetary magnetic lines are curved and deformed while they are towed toward the two poles by the solar wind. The majority of magnetic lines bypass the two poles, then leave behind a ‘V-shaped’ structure in the magnetotail behind Mars. In the crust of Mars, the local magnetic anomalies have a noticeable influence on the magnetic field structure. The magnetic anomalies at different positions and in different intensities interact with the solar wind to form the mini-magnetospheres of different structures and morphologies, such as the towed mini-magnetosphere and the mini-magnetosphere with open magnetic lines. The local magnetic anomalies have changed the near-Mars magnetic field structure, and probably changed the plasma distribution as well.     

Asymmetry of plasma fluxes at Mars. ASPERA-3 observations and hybrid simulations

The asymmetry of fluxes of solar wind and planetary ions is studied by using the ASPERA-3 observations onboard the Mars Express spacecraft in February 2004 to March 2006. Due to the small scale of the Martian magnetosphere and its induced origin, the flow pattern near Mars is sensitive to the directions of the interplanetary magnetic and electric (-V×B) fields. Asymmetry of the magnetic field draping produces an asymmetry in plasma flows in the plane containing the IMF. The crustal magnetic fields on Mars also influence the flow pattern. Scavenging of planetary ions is less efficient in the regions of strong crustal magnetization and therefore the escape fluxes of planetary ions in the southern hemisphere are smaller. The results of the observations are compared to simulations based on a 3D hybrid model with several ion species.     

Comparative study of ion cyclotron waves at Mars, Venus and Earth

Ion cyclotron waves are generated in the solar wind when it picks up freshly ionized planetary exospheric ions. These waves grow from the free energy of the highly anisotropic distribution of fresh pickup ions, and are observed in the spacecraft frame with left-handed polarization and a wave frequency near the ion’s gyrofrequency. At Mars and Venus and in the Earth’s polar cusp, the solar wind directly interacts with the planetary exospheres. Ion cyclotron waves with many similar properties are observed in these diverse plasma environments. The ion cyclotron waves at Mars indicate its hydrogen exosphere to be extensive and asymmetric in the direction of the interplanetary electric field. The production of fast neutrals plays an important role in forming an extended exosphere in the shape and size observed. At Venus, the region of exospheric proton cyclotron wave production may be restricted to the magnetosheath. The waves observed in the solar wind at Venus appear to be largely produced by the solar-wind-Venus interaction, with some waves at higher frequencies formed near the Sun and carried outward by the solar wind to Venus. These waves have some similarity to the expected properties of exospherically produced proton pickup waves but are characterized by magnetic connection to the bow shock or by a lack of correlation with local solar wind properties respectively. Any confusion of solar derived waves with exospherically derived ion pickup waves is not an issue at Mars because the solar-produced waves are generally at much higher frequencies than the local pickup waves and the solar waves should be mostly absorbed when convected to Mars distance as the proton cyclotron frequency in the plasma frame approaches the frequency of the solar-produced waves. In the Earth’s polar cusp, the wave properties of ion cyclotron waves are quite variable. Spatial gradients in the magnetic field may cause this variation as the background field changes between the regions in which the fast neutrals are produced and where they are re-ionized and picked up. While these waves were discovered early in the magnetospheric exploration, their generation was not understood until after we had observed similar waves in the exospheres of Mars and Venus.     

Comparison study of magnetic flux ropes in the ionospheres of Venus, Mars and Titan

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.     

On the classification of comet plasma tails

The investigation of plasma tails of comets is an important part of comet research. Different classifications of plasma tails of comets are proposed. Plasma acceleration in the tails is investigated in sufficient detail. Several cometary forms are explained. Plasma tails of Mars and Venus were observed during the first studies of these planets. They are associated with the capture of ionized atoms and exosphere molecules by the solar wind magnetized plasma flow. Distinct plasma tails of Mars and Venus are caused by the mass loading of the solar wind with heavy ions. It was shown that the transverse dimension of the tails of Mars, Venus, and comets can be quite accurately determined by production rate of the obstacle to the solar wind flow. While plasma tails of Mars and Venus are investigated by in situ measurements from spacecraft, observations of comet tails from the Earth make it possible to see the entire object under study and to monitor changes in its structure. A certain similarity of cometary and planetary tails can be explained by the nonmagnetic nature of both types of bodies. Thus, it is reasonable to suppose that investigations of plasma tails of comets can supplement the information obtained by in situ methods of the study of the planets. In this paper, plasma tails of comets, presumably analogous to the plasma tails of Mars and Venus, have been identified on modern photographs of comets (more than 1500 photographs viewed). Only quasi-steady laminar tails are considered. They are divided into two types: double structures and outflows. The paper attempts to define the 3D structure of double structures and to determine certain characteristics of outflows.     

Simultaneous measurements of Martian plasma boundaries by Rosetta and Mars Express

We present the first two-spacecraft near-simultaneous observations of the Martian bow shock (BS), magnetic pileup boundary (MPB) and photo-electron boundary (PEB) obtained by the plasma instruments onboard Rosetta and Mars Express during the Rosetta Mars flyby on February 25, 2007. Our observations are compared with shape models for the BS and MPB derived from previous statistical studies. The MPB is found at its expected position but the BS for this event is found significantly closer to the planet than expected for the rather slow and moderately dense solar wind. Cross-calibration of the density measurements on the two spacecraft gives a density profile through the magnetosheath, indicating an increasing solar wind flux during the Rosetta passage which is consistent with the multiple BS crossings at the Rosetta exit.     

Ionospheric currents induced by solar wind interaction with planetary atmospheres

In a steady-state model for the interaction of the solar wind with the atmosphere of a non-magnetic planet, the magnetized solar wind acts as a dynamo over the dayside of the planet and induces Ohmic currents in the planet's ionosphere. A model for the dynamo mechanism and for the induced current configuration is developed. Based on this model and assumed model atmospheres of Mars and Venus, the distribution of currents entering the ionosphere through the ionopause is calculated. The requirement that the total current be of such a magnitude as to cancel the shock-compressed interplanetary magnetic field fixes the ionopause altitude. The calculations for Venus are in reasonable agreement with observations. The calculations for Mars indicate the possibility of an observable ionopause in the altitude range from 325 to 425 km.     

Solar EUV and Electron-Proton-Hydrogen Atom-Produced Ionosphere on Mars: Comparative Studies of Particle Fluxes and Ion Production Rates Due to Different Processes

The total photoelectron and secondary electron fluxes are calculated at different times and altitudes along the trajectory of Mars Global Surveyor passing through the nightside and dayside martian ionosphere. These results are compared with the electron reflectometer experiment on board Mars Global Surveyor. The calculated electron spectra are in good agreement with this measurement. However, the combined fluxes of proton and hydrogen atom as calculated by E. Kallio and P. Janhunen (2001, J. Geophys. Res.106, 5617-5634) were found to be 1-2 orders of magnitude smaller than the measured spectra. We have also calculated ionization rates and ion and electron densities due to solar EUV, X-ray, and electron-proton-hydrogen atom impacting with atmospheric gases of Mars at solar zenith angles of 75°, 105°, and 127°. In the vicinity of the dayside ionization peak, it is found that the ion production rate caused by the precipitation of proton-hydrogen atom is larger than the X-ray impact ionization rate while at all altitudes, the photoionization rate is always greater than either of the two. Moreover, X-rays contribute greatly to the photoelectron impact ionization rate as compared to the photoion production rate. The calculated electron densities are compared with radio occultation measurements made by Mars Global Surveyor, Viking 1, and Mars 5 spacecraft at these solar zenith angles. The dayside ionosphere produced by proton-hydrogen atom is smaller by an order of magnitude than that produced by solar EUV radiation. X-rays play a significant role in the dayside ionosphere of Mars at the altitude range 100-120 km. Solar wind electrons and protons provide a substantial source for the nightside ionosphere. These calculations are carried out for a solar minimum period using solar wind electron flux, photon flux, neutral densities, and temperatures under nearly the same areophysical conditions as the measurements.