Mars Radii Research Articles

GJ 238 b: A 0.57 Earth Radius Planet Orbiting an M2.5 Dwarf Star at 15.2 pc

We report the discovery of the transiting planet GJ 238 b, with a radius of 0.566 ± 0.014 R ⊕ (1.064 ± 0.026 times the radius of Mars) and an orbital period of 1.74 days. The transit signal was detected by the TESS mission and designated TOI-486.01. The star’s position close to the southern ecliptic pole allows for almost continuous observations by TESS when it is observing the southern sky. The host star is an M2.5 dwarf with V = 11.57 ± 0.02 mag, K = 7.030 ± 0.023 mag, a distance of 15.2156 ± 0.0030 pc, a mass of 0.4193−0.0098+0.0095 M ☉, a radius of 0.4314−0.0071+0.0075 R ☉, and an effective temperature of 3485 ± 140 K. We validate the planet candidate by ruling out or rendering highly unlikely each of the false positive scenarios, based on archival data and ground-based follow-up observations. Validation was facilitated by the host star’s small size and high proper motion of 892.633 ± 0.025 mas yr–1.

EMM EMUS Observations of Hot Oxygen Corona at Mars: Radial Distribution and Temporal Variability

AbstractWe present the first observations of the dayside coronal oxygen emission in far ultraviolet (FUV) measured by the Emirates Mars Ultraviolet Spectrometer (EMUS) onboard the Emirates Mars Mission (EMM). The high sensitivity of EMUS is providing an opportunity to observe the tenuous oxygen corona in FUV, which is otherwise difficult to observe. Oxygen resonance fluorescence emission at 130.4 nm provides a measurement of the upper atmospheric and exospheric oxygen. More than 500 oxygen corona profiles are constructed using the long–exposure time cross–exospheric mode (OS4) of EMUS observations. These profiles range from ∼200 km altitude up to several Mars radii (>6 RM) across all seasons and for two Mars years. Our analysis shows that OI 130.4 nm is highly correlated with solar irradiance (solar photoionizing and 130.4 nm illuminating irradiances) as well as changes in the Sun–Mars distance. The prominent short term periodicity in oxygen corona brightness is consistent with the solar rotation period (quasi–27–day). A comparison between the perihelion seasons of Mars Year (MY) 36 and MY 37 shows interannual variability with enhanced emission intensities during MY 37, due to the rise of Solar Cycle 25. These observations show a highly variable oxygen corona, which has significant implications on constraining the photochemical escape of atomic oxygen from Mars.

M5 — Mars Magnetospheric Multipoint Measurement Mission: A multi-spacecraft plasma physics mission to Mars

Mars, lacking an intrinsic dynamo, is an ideal laboratory to comparatively study induced magnetospheres, which can be found in other terrestrial bodies as well as comets. Additionally, Mars is of particular interest to further exploration due to its loss of habitability by atmospheric escape and possible future human exploration. In this context, we propose the Mars Magnetospheric Multipoint Measurement Mission (M5), a multi-spacecraft mission to study the dynamics and energy transport of the Martian induced magnetosphere comprehensively. Particular focus is dedicated to the largely unexplored magnetotail region, where signatures of magnetic reconnection have been found. Furthermore, a reliable knowledge of the upstream solar wind conditions is needed to study the dynamics of the Martian magnetosphere, especially the different dayside boundary regions but also for energy transport phenomena like the current system and plasma waves. This will aid the study of atmospheric escape processes of planets with induced magnetospheres. In order to resolve the three-dimensional structures varying both in time and space, multi-point measurements are required. Thus, M5 is a five spacecraft mission, with one solar wind monitor orbiting Mars in a circular orbit at 5 Martian radii, and four smaller spacecraft in a tetrahedral configuration orbiting Mars in an elliptical orbit, spanning the far magnetotail up to 6 Mars radii with a periapsis just outside the Martian magnetosphere of 1.8 Mars radii. We not only present a detailed assessment of the scientific need for such a mission but also show the resulting mission and spacecraft design taking into account all aspects of the mission requirements and constraints such as mass, power, and link budgets. Additionally, different aspects of the mission programmatics like a possible mission timeline, cost estimates, or public outreach are shown. The common requirements for acceptance for an ESA mission are considered. The mission outlined in this paper was developed during the Alpbach Summer School 2022 on the topic of “Comparative Plasma Physics in the Universe”.

Solar wind driven influences on the Martian oxygen corona: Constraints on atmospheric sputtering from a synthesis of MAVEN measurements during solar minimum

The prolonged low solar extreme ultraviolet (EUV) flux conditions provided by the deep solar minimum between solar cycles 24 and 25 can be expected to have amplified the relative influence of any solar wind driven effects on the Martian corona. As such, this solar minimum has provided an opportunity to study solar wind effects on processes that produce the energetic neutral particles that populate the oxygen corona, particularly atmospheric sputtering by precipitating energetic ions, and dissociative recombination of O2+. To probe the corona, we use a novel, fully empirical divergence-based technique for estimating coronal oxygen densities. This method solves the continuity equation for ions to obtain local ionization rates that are, in turn, converted to neutral densities by accounting for ionization via electron impact, charge exchange, and photoionization. We apply this technique to combined measurements of ion flux, electron flux, solar EUV flux, and magnetic fields from the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter at Mars. We separately combine MAVEN measurements during relatively low and high upstream solar wind flux, Fsw, conditions and derive the relevant fluxes for each range of conditions. We find that fluxes of planetward energetic O+ upstream of Mars increase by roughly ×3 during high Fsw conditions. We derive radial density profiles in the range 2.0–2.7 Mars radii and find that coronal hot oxygen densities scale with radial distance, R, as R−2 from 300cm−3 to 100cm−3 in this range. We perform χ2 parameter-space searches for exobase densities and effective temperatures to evaluate any potential differences in the hot O populations that populate the corona. Neither the radial profiles, nor the χ2 search, reveal any significant effects of increased Fsw on the Martian oxygen corona. The best estimates for exobase properties center around (6–7)×103atoms/cm3 and 0.3 eV/kB for both cases, with large correlated uncertainties. Based on the uncertainties inherent to the method and the measurements, we conclude that solar wind driven effects must be limited to roughly a factor of 2 and that further investigations to empirically constrain the corona over a wider range of altitudes are needed to investigate solar wind driven effects.

Two Types of Martian Magnetotail Current Sheets: MAVEN Observations of Ion Composition

AbstractUsing measurements from the Mars Atmosphere and Volatile EvolutioN mission, we investigate the densities of H+ (), O+ (), and O2+ (), respectively, in the Martian magnetotail current sheet. We find that the current sheet when it is closer to the terminator than 0.75 Mars radii is mostly dominated by heavy ions (()>2 ), regardless of the variation of the upstream solar wind, but that it is sometimes dominated by H+ ( >2()) at downstream distances exceeding 0.75 Mars radii. The occurrence rate of the dominant H+ weakly increases (and that of the heavy ions decreases) with solar wind density and dynamic pressure. Our results suggest that solar wind protons could enter the Martian tail and may become the dominant ion species in the current sheet, particularly when the solar wind density or dynamic pressure is high.

Galactic Cosmic Rays at Mars and Venus: Temporal Variations from Hours to Decades Measured as the Background Signal of Onboard Microchannel Plates

A microchannel plate (MCP) is a component widely used for counting particles in space. Using the background counts from MCPs on the Mars Express and Venus Express orbiters—operating over 17 yr and 8 yr, respectively—we investigated the galactic cosmic ray (GCR) characteristics of the inner solar system. The MCP background counts at Mars and Venus, on a solar cycle timescale, exhibited clear anticorrelation with the sunspot number. We concluded that the measured MCP background counts contained GCR information. The GCR characteristics measured using the MCP background counts at Mars showed features consistent with measurements on Earth in Solar Cycle 24. The time lag between the sunspot number and the MCP background counts was found to be ∼9 months at Mars. The shorter-term background data recorded along the orbits (with a timescale of several hours) also showed evident depletion of the background counts, due to absorption of the GCR particles by the planets. Thanks to the visible planetary size change along an orbit, we developed a model to separate the GCR contribution to the MCP background counts from the internal contribution caused by the β-decay of radioactive elements in the MCP glass. Our statistical analysis of the GCR absorption signatures at Mars implies that the effective absorption radius of Mars for the GCR particles is >100 km larger than the radius of the planet. However, the cause remains an open question.

The Influence of Magnetic Field Topology and Orientation on the Distribution of Thermal Electrons in the Martian Magnetotail

AbstractThermal (<1 eV) electron density measurements, derived from the Mars Atmosphere and Volatile Evolution's (MAVEN) Langmuir Probe and Waves (LPW) instrument, are analyzed to produce the first statistical study of the thermal electron population in the Martian magnetotail. Coincident measurements of the local magnetic field are used to demonstrate that close to Mars, the thermal electron population is most likely to be observed at a cylindrical distance of ∼1.1 Mars radii (RM) from the central tail region during times when the magnetic field flares inward toward the central tail, compared to ∼1.3 RM during times when the magnetic field flares outward away from the central tail. Similar patterns are observed further down the magnetotail with greater variability. Thermal electron densities are highly variable throughout the magnetotail; average densities are typically ∼20–50 cm−3 within the optical shadow of Mars and can peak at ∼100 cm−3 just outside of the optical shadow. Standard deviations of 100% are observed for average densities measured throughout the tail. Analysis of the local magnetic field topology suggests that thermal electrons observed within the optical shadow of Mars are likely sourced from the nightside ionosphere, whereas electrons observed just outside of the optical shadow are likely sourced from the dayside ionosphere. Finally, thermal electrons within the optical shadow of Mars are up to 20% more likely to be observed when the strongest crustal magnetic fields point sunward than when they point tailward.

Simulating re-impacts from craters at the deepest location of Phobos to generate its blue spectral units

The spatial distribution pattern of ejecta from an impact crater on the surface of a body in space holds clues to understanding the ejecta launch conditions, the crater excavation process, and the dynamical environment. In particular, focusing on re-impact sites from larger or deeper craters on the surface is important, as they tend to preserve much of the original information. The surface of the Martian satellite Phobos is spectrally divided into red and blue units. The former are globally distributed, while the blue units have been observed inside and outside the Stickney crater—the largest and deepest crater on Phobos. Outside the crater, the blue units are concentrated in the region from the south to the west (south-west blue units) and in the region east of the crater (east blue units). Several models can explain the concentration of east blue units. However, the mode of formation of south-west blue units is still unknown, despite its vast area. We proposed that the combination of ejecta from Stickney and Limtoc (a crater located on the floor of Stickney) contributed to the formation of the blue units. We tested the model using ballistic simulations incorporating, an updated shape model with randomly generated ejecta velocities, different values of orbital radii to Mars, and various launching velocities, to track the emplacement of the ejecta particles. The results showed that the distribution of the two blue units is recreated when the combined re-impact sites of ejecta from Stickney and Limtoc have orbital radius (XPh) values of XPh > 3.34 Mars radii (RM) and XPh < 3.04 RM, respectively. The observations and the results suggest that Phobos comprises an inner blue layer covered by red materials globally and locally excavated blue materials. Using the squared term of the orbital evolution, we calculate the approximate age of Stickney to > 23 Myr, which may be sufficiently old to be estimated from the crater densities. We restrict the age of Limtoc to < 109 Myr, which indicates that this crater is one of the youngest features on Phobos.

Evidence for a Past Martian Ring from the Orbital Inclination of Deimos

Abstract We numerically explore the possibility that the large orbital inclination of the Martian satellite Deimos originated in an orbital resonance with an ancient inner satellite of Mars more massive than Phobos. We find that Deimos’s inclination can be reliably generated by outward evolution of a Martian satellite that is about 20 times more massive than Phobos through the 3:1 mean-motion resonance with Deimos at 3.3 Mars radii. This outward migration, in the opposite direction from tidal evolution within the synchronous radius, requires interaction with a past massive ring of Mars. Our results therefore strongly support the cyclic Martian ring-satellite hypothesis of Hesselbrock &amp; Minton. Our findings, combined with the model of Hesselbrock &amp; Minton, suggest that the age of the surface of Deimos is about 3.5–4 Gyr, and require Phobos to be significantly younger.

Propagation properties of Hot Flow Anomalies at Mars: MAVEN observations

We present a statistical study of propagation characteristics of Hot Flow Anomalies (HFAs) at Mars based on 19 manually selected events registered by the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Based on the number of registered events during the observed time interval we conclude that HFAs are common foreshock transients at Mars with an occurrence rate of 0.6 ± 0.3 events per day.The estimated size of the investigated HFAs starts from 0.2 Mars radii; we are cautious about giving estimates of the upper limit of this value as commonly used techniques produce values up to more than ten of Martian radii which by far exceeds any reasonable dimension of a foreshock transient at Mars. The propagation speed of the observed formations was calculated as the speed of intersection line between the interplanetary current sheet and the surface of the bow shock. Being detected at solar-zenith angles of 53°–76°, almost all of the HFAs presented in this study propagate in the anti-sunward direction.With the use of HFA’s propagation speed we made an upper estimate of the lifetime of each separate anomaly at the moment of its registration assuming that it can’t form before CS’s crossing of the subsolar point at the bow shock. No visible correlation between HFA’s lifetime and size was found, which is consistent with the previous study proving that these structures can be either expanding, contracting or stable (Xiao et al., 2015).

Structure of the current sheets in the near-Mars magnetotail. Maven observations

During the last 15 years, the Current Sheets (CSs) have been intensively studied in the tail of the terrestrial magnetosphere, where protons are the dominated ion component. On the contrary, in the Martian magnetotail heavy ions (O+ and+ 0) usually dominate while the abundance of protons can be negligible. Hence it is interesting to study the spatial structure and plasma characteristics of such “oxygen” CSs. MAVEN spacecraft (s/c) currently operating on the Martian orbit with a unique set of scientific instruments allows observation of the magnetic field and three-dimensional distribution functions of various ion components and electrons with a high time resolution. In this paper, we analyse nine intervals of the CSs observed by MAVEN in the near-Mars tail at the distances from the planet ~1.5–1R M , where R M is the radius of Mars. We analyse the spatial structure of the CSs and estimate their thickness for different magnetic configurations and relative abundance of the heavy and light ions in the sheets. It is shown that, similarly to the CSs in the Earth’s magnetotail, the thickness and complexity of the spatial structure of the Maritan CSs (i.e. the presence of embedded and / or peripheral current structures) depend on the magnetic configuration of the sheets, which, in turn, affects the fraction of the quasi-adiabatic particles in the CSs.

X-Rays From Mars

X-rays from Mars were first detected in July 2001 with the satellite Chandra. The main source of this radiation was fluorescent scattering of solar X-rays in its upper atmosphere. In addition, the presence of an extended X-ray halo was indicated, probably resulting from charge exchange interactions between highly charged heavy ions in the solar wind and neutrals in the Martian exosphere. The statistical significance of the X-ray halo, however, was very low. In November 2003, Mars was observed again in X-rays, this time with the satellite XMM-Newton. This observation, characterized by a considerably higher sensitivity, confirmed the presence of the X-ray halo and proved that charge exchange is indeed the origin of the emission. This was the first definite detection of charge exchange induced X-ray emission from the exosphere of another planet. Previously, this kind of emission had been detected from comets (which are largely exospheres) and from the terrestrial exosphere. Because charge exchange interactions between atmospheric constituents and solar wind ions are considered as an important nonthermal escape mechanism, probably responsible for a significant loss of the Martian atmosphere, X-ray observations may lead to a better understanding of the present state of the Martian atmosphere and its evolution. X-ray images of the Martian exosphere in specific emission lines exhibited a highly anisotropic morphology, varying with individual ions and ionization states. With its capability to trace the X-ray emission out to at least 8 Mars radii, XMM-Newton can explore exospheric regions far beyond those that have been observationally explored to date. Thus, X-ray observations provide a novel method for studying processes in the Martian exosphere on a global scale.

First observation of Mars with XMM-Newton

In the first observation of Mars with XMM-Newton, on 20-21 November 2003, this planet is clearly detected as an X-ray source. High resolution X-ray spectroscopy with the Reflection Grating Spectrometer (RGS) confirms that the X-ray radiation from Mars is composed of two different components: one due to fluorescent scattering of solar X-rays in its upper atmosphere and the other one due to solar wind charge exchange in its exosphere. Close to Mars, the RGS spectrum is dominated by two pronounced CO 2 fluorescence lines at 23.5 A and 23.7 A. Fluorescence from N 2 at 31.5 A is also observed. With increasing distance from Mars, these lines fade, while numerous (∼12) emission lines become prominent at the positions expected for de-excitation of highly ionized C, N, O, and Ne atoms, strongly resembling a cometary X-ray spectrum. The He-like O 6+ multiplet is resolved and is dominated by the spin-forbidden magnetic dipole transition 2 3 S 1 → 1 1 S 0 , confirming charge exchange as the origin of the emission, while the resonance line 2 1 P 1 → 1 1 S 0 increases in intensity closer to Mars, where the density of the exosphere is higher. The high spectral dispersion and throughput of XMM-Newton/RGS make it possible to produce X-ray images of the Martian exosphere in individual emission lines, free from fluorescent radiation. They show extended emission out to ∼8 Mars radii, with morphological differences between individual ions and ionization states. This is the first definite detection of charge exchange induced X-ray emission from the exosphere of another planet, providing a direct link to cometary X-ray emission.

Spatial and temporal patterns of solar eclipses by Phobos on Mars

The spatial and temporal patterns associated with motion of the shadow of Phobos across the surface of Mars are quite different than those associated with solar eclipses on Earth. We present a simple analysis of variations in the position, velocity, size, and shape of the shadow. Simple expressions give reasonably accurate depictions of the shadow motion, which mainly consists of a subdiurnal longitude cycle and an annual latitude cycle. Over most of each year, there are an average of 3.22 shadow transits per day. The duration of the shadow transit depends on latitude. It is maximum at the equator and is then 11.8% of the orbital synodic period. As the subsolar point moves north, the shadow moves south, and vice versa. There is a narrow band, centered on the equator of Mars, within which every point is eclipsed at least once during each semiannual eclipse season. Outside that band, the density of coverage decreases slowly with increasing distance from the equator, until the limiting latitudes are reached. During epochs, like the present, when the obliquity of Mars is in excess of 21.2°, there are portions of each year during which no eclipses occur. As the obliquity increases beyond that transition value, the durations of the eclipse seasons decrease. The minimum possible eclipse season duration, expressed as a fraction of the Mars year, is the same as the maximum shadow transit duration, expressed as a fraction of the Phobos synodic period, since both ratios depend on the same geometry, which is essentially just the radius of the orbit of Phobos, compared to the radius of Mars.

Discovery of X–rays from Mars with Chandra

On 4 July 2001, X–rays from Mars were detected for the first time. The observation was performed with the ACIS–I detector onboard Chandra and yielded data of high spatial and temporal resolution, together with spectral information. Mars is clearly detected as an almost fully illuminated disk, with an indication of limb brightening at the sunward side, accompanied by some fading on the opposite side. The morphology and the X–ray luminosity of ~ are fully consistent with fluorescent scattering of solar X–rays in the upper Mars atmosphere. The X–ray spectrum is dominated by a single narrow emission line, which is most likely caused by O–Kα fluorescence. No evidence for temporal variability is found. This is in agreement with the solar X–ray flux, which was almost constant during the observation. In addition to the X–ray fluorescence, there is evidence for an additional source of X–ray emission, indicated by a faint X–ray halo which can be traced to about three Mars radii, and by an additional component in the X–ray spectrum of Mars, which has a similar spectral shape as the halo. Within the available limited statistics, the spectrum of this component can be characterized by 0.2 keV thermal bremsstrahlung emission. This is indicative of charge exchange interactions between highly charged heavy ions in the solar wind and exospheric hydrogen and oxygen around Mars. Although the observation was performed at the onset of a global dust storm, no evidence for dust–related X–ray emission was found.

The velocity distribution of periodic comets and the meteor shower on Mars

Abstract. ThecollisionvelocitieswithMarsof theknown periodic cometswithperiheliondistances lessthantheorbital radiusof Mars 1.52 au are calculated and statisticallyanalyzed. The velocity distribution shows that short-period comets usually havesmaller collision velocities than the long-period comets whether they are numbered or unnumbered. For comets with period P ≤270 y, most collision velocities with Mars are less than 30 km s −1 . These velocities are smaller than the correspondingcollisionvelocities withEarth.Basedonthesimilarityofthevelocitydistributionsof thecomets andmeteoroidstreams, meteorshowers on Mars are discussed briefly. Key words. celestial mechanics – comets: general – meteors, meteoroids 1. Introduction It is well known that comets, as they near the perihelion oftheir orbits, release large numbers of dust grains (Kirkwood1861) due to the normal process of gas ejection. These parti-cles, namely meteoroids, leave the cometary nucleus at speedswhich are significantly less than the orbital velocity of thecomet(Whipple1951).Inconsequence,thesemeteoroidsmoveon almost the same orbitsas the parentcomets,graduallydrift-ingaheadandbehindthecometbecauseof smalldifferencesinorbital period. After a time interval of a few tens of cometaryperiods, the meteoroids will have spread all the way aroundthe cometary orbit (Steel 1994; Williams 1996). At this stage,meteors(whichareproducedbytheimpactofthestreammete-oroidswiththeatmosphereoftheEarth)canbeobservedwhen-ever the Earthpasses throughthe cometarynodes.In principle,there could also be meteor showers on Mars or any other plan-ets with an atmosphere whenever the planet encounters a me-teoroid streams.MeteoroidsstriketheEarthwitha velocitythatis similar tothe orbital velocity, that is, several tens of kilometers per sec-ond. At such velocities, dust impacts can affect all phases ofspacecraft operation (Beech et al. 1995, 1997; McBride 1997;McBride & McDonnell 1999). For example, the

Equilibrium Configurations of Solid Cohesionless Bodies

The bodies of the Solar System exist in a variety of irregular shapes. Studies of those shapes are conducted to infer information about the internal composition, structure, and history of those bodies. However, such inferences require knowing how the composition and structure or history relates to the shape and internal forces. That connection is known only for fluid bodies, where the permissible equilibrium states were discovered centuries ago by Newton, Maclaurin, Jacobi, Poincaré, and Roche. While others have given results for linear elastic solid bodies, the elastic problem is not uniquely posed, since elastic solutions depend on an implicit assumption about the existence and shape of an initial stress-free state. The present states of Solar System bodies are a culmination of complicated past histories, possibly involving collisions, disruption, melting, accumulation, and large-scale yielding and reshaping. Such processes create underlying residual stress fields that cannot be known. Here I present an approach in the same spirit as for the fluid bodies: limits on equilibrium shapes are determined. Results are obtained for a cohesionless elastic–plastic solid with a Mohr–Coloumb yield criteria. That model is commonly used in soil mechanics and is appropriate for “rubble pile” reaccumulated asteroids that have negligible cohesive forces. It is possible to determine limit equilibrium stress fields and shapes that are independent of past histories, using the approaches of limit analyses of elastic–plastic theories. The results show that for these bodies there exists a region of permissible combinations of shape and spin rates, centered about the unique equilibrium fluid states of Maclaurin and Jacobi. The database on asteroids is compared to those equilibrium states. Few asteroids are outside the limit shape envelopes according to this theory. The application of the analysis to Phobos is also presented, assuming that the rubble-pile model is appropriate. The deformation that would occur as it moves closer to Mars is determined; it is shown to be unstable and globally catastrophic at about 2.1 Mars radii.

Planned observation of Phobos/Deimos by Mars imaging camera (MIC) on NOZOMI

The large elongated orbit planned for NOZOMI around Mars, i.e. a periapsis of 150 km and an apoapsis of 15 R M (R M denotes the radius of Mars), will provide many occasions for encounters of NOZOMI with two Martian satellites, Phobos and Deimos, where NOZOMI is the former Planet-B meaning “Hope” in Japanese. We present a plan for imaging the two satellites by the Mars Imaging Camera (MIC) on board NOZOMI at such encounters during the mission lifetime of two years from October 1999. An Autonomous Tracking Mode is available for fly-by imaging of satellites. MIC scans the azimuth direction (orthogonal to the CCD line arrays) using the spacecraft spin at a rotation rate of 7.5 rpm, and has an image resolution of 80 arc second in both elevation and azimuth directions. The main science objectives of MIC, related to the two satellites, are (i) to study the size/spatial distributions of craters on both satellites, (ii) to examine the groove structure on Phobos, (iii) to image areas not yet seen areas of Deimos, and (iv) to derive its whole shape. We will, furthermore, search for the dust rings along the orbits of these two satellites in the forward scattering region of sunlight. The capability of MIC to execute these objectives are briefly summarized.

PLANET-B magnetic fields investigation

Some scientific topics relating to magnetic field measurements around Mars are briefly described and specification of the magnetic field instrument onboard PLANET-B is presented. The highly elliptical orbit of PLANET-B with a periapsis altitude of 150 km and an apoapsis of 15 RM (Mars radii) will enable us to observe magnetic environments in various regions from the ionospheric altitudes to upstream and tail regions. One of the most important objectives is to resolve the controversial arguments about the magnitude of the magnetic moment of Mars. Combination of measurements with other instruments onboard PLANET-B will give us important information on unsolved problems such as the pressure balance in the dayside ionosphere and acceleration processes for ionospheric ions.

Constraints on the structure and composition of Mars from thermal evolution models

A numerical model of the thermal evolution of Mars was developed, solving simultaneously the equations for energy conservation and hydrostatic equilibrium throughout the planet, and using an equation of state that includes the computation of the thermal pressure according to the Debye theory. Based on the known values of the size and mass of Mars, and choosing the value of 0.365 for the inertia factor, we determine the initial mass fractions of different materials constituting the core and mantle of Mars. The abundance ratios of the components are described by three parameters. Two of these parameters are fixed by the radius and inertia factor of Mars, while the third remains free. Several models that match the present radius and inertia factor were found, distinguished by their initial composition. The thermal evolution and present internal state of these models were evaluated, in order to establish constraints on the composition and internal structure of Mars.