Mars Global Surveyor Observations Research Articles

Recovery and validation of Mars ionospheric electron density profiles from Mariner 9

Electron density profiles from the ionosphere of Mars that were obtained by the Mariner 9 radio occultation experiment in 1971–1972 have unique scientific value because they extend to higher altitudes than comparable datasets and were acquired during a tremendous dust storm that had substantial and poorly understood effects on the ionosphere. Yet these profiles are not publicly available in an accessible format. Here, we describe the recovery of these profiles, which are made available as part of this article. The validity of the profiles was tested by using them to explore the effects of a dust storm on the topside ionosphere, the morphology of the topside ionosphere, the behavior of the M1 layer, and possible meteoric layers. The dust storm that waned over the course of the primary mission (November–December 1971) had major effects on the ionosphere of Mars. It elevated the M1 and M2 layers of the ionosphere by 20–30 km, but the separation of the two layers stayed fixed throughout the primary mission, which suggests that the neutral atmosphere at these altitudes was not heated during the dust storm. However, the altitude of the 1500 cm −3 density level, a proxy for the top of the ionosphere, decreased steadily by 74±12 km over the course of the primary mission. Mariner 9 observations of the topside ionosphere differ from comparable Mars Express observations. Compared to Mars Express, the Mariner 9 data, which were acquired during a period of relatively high solar wind dynamic pressure, have lower densities at high altitudes. They are also more likely to have a “one scale height” morphology than a “two scale height” morphology. The peak density of the M1 layer depends on solar zenith angle and solar irradiance similarly to previous studies with Mars Global Surveyor observations, which indicates that dust storms do not affect the behavior of the peak density. No clear meteoric layers were identified.

Temporal variability of waves at the proton cyclotron frequency upstream from Mars: Implications for Mars distant hydrogen exosphere

AbstractWe report on the temporal variability of the occurrence of waves at the local proton cyclotron frequency upstream from the Martian bow shock from Mars Global Surveyor observations during the first aerobraking and science phasing orbit periods. Observations at high southern latitudes during minimum‐to‐mean solar activity show that the wave occurrence rate is significantly higher around perihelion/southern summer solstice than around the spring and autumn equinoxes. A similar trend is observed in the hydrogen (H) exospheric density profiles over the Martian dayside and South Pole obtained from a model including UV thermospheric heating effects. In spite of the complexity in the ion pickup and plasma wave generation and evolution processes, these results support the idea that variations in the occurrence of waves could be used to study the temporal evolution of the distant Martian H corona and its coupling with the thermosphere at altitudes currently inaccessible to direct measurements.

Energetic particles detected by the Electron Reflectometer instrument on the Mars Global Surveyor, 1999–2006

We report the observation of galactic cosmic rays and solar energetic particles by the Electron Reflectometer instrument aboard the Mars Global Surveyor (MGS) spacecraft from May of 1999 to the mission conclusion in November 2006. Originally designed to detect low‐energy electrons, the Electron Reflectometer also measured particles with energies >30 MeV that penetrated the aluminum housing of the instrument and were detected directly by microchannel plates in the instrument interior. Using a combination of theoretical and experimental results, we show how the Electron Reflectometer microchannel plates recorded high energy galactic cosmic rays with ∼45% efficiency. Comparisons of this data to galactic cosmic ray proton fluxes obtained from the Advanced Composition Explorer yield agreement to within 10% and reveal the expected solar cycle modulation as well as shorter timescale variations. Solar energetic particles were detected by the same mechanism as galactic cosmic rays; however, their flux levels are far more uncertain due to shielding effects and the energy‐dependent response of the microchannel plates. Using the solar energetic particle data, we have developed a catalog of energetic particle events at Mars associated with solar flares and coronal mass ejections, which includes the identification of interplanetary shocks. MGS observations of energetic particles at varying geometries between the Earth and Mars that include shocks produced by halo, limb, and backsided events provide a unique data set for use by the heliophysics modeling community.

Effects of solar X-ray flares in the E region ionosphere of Mars: First model results

[1] We have used radio occultation data obtained from Mars Global Surveyor (MGS) at high latitudes (65.3°–65.6°N, 69.3°–69.6°N and 76.4°–77.5°N) to study the effects of X-ray flares and CMEs on Total Electron Content (TEC) in the E region of the Martian ionosphere in response to solar events that occurred on 29 and 31 May 2003 and on 17 January and 13 May 2005. Modeling of flare induced solar X-ray fluxes, ion production rates, electron densities and TEC are carried out, in each case based on GOES data. The estimated TEC values are compared with the in situ MGS observations. It is found that solar flare caused enhancements in the TEC of Mars by a factor of ∼5. Also, a 3D kinetic solar wind model (Hakamada-Akasofu-Fry Version 2/HAFv.2) is used to predict the arrivals of CME shocks at Mars associated with the flare event of May 2005. These predicted shock arrivals were associated with in situ enhancements in TEC by a factor of ∼2.

Role of solar X-ray flares and CME in the E region ionosphere of Mars: MGS observations

The relationship of the solar flare and the Coronal Mass Ejection (CME) with the ionosphere is a key problem in all planetary atmospheres. Earth is the only planet where ground based observations can be used to study this relationship on a continuous basis. In this paper we have used radio occultation data obtained from Mars Global Surveyor (MGS) for the period of 12–18 May, 2005 to address the responses of X-ray flare and CME in Total Electron Content (TEC) of E region ionosphere of Mars during a violent solar event that occurred on 13th May. It was found that the TEC increased by factors of 6 in the E region ionosphere of Mars during this flare. The response of mass ejections from the Sun's corona is registered in the E region ionosphere of Mars after ∼38h. Our results suggest that TEC can increase by factor of ∼3.5–4.5 during the arrival of CME on Mars. This increase in TEC can be attributed to ionization by energetic particles associated with magnetic storms. We report that shocked waves driven by CME compressed the magnetosheath of Mars by 15km during the magnetic storms.

On the responses to solar X‐ray flare and coronal mass ejection in the ionospheres of Mars and Earth

We have used Mars Global Surveyor (MGS) observations for the period 12 to 18 May 2005 to address the effect of X‐ray flare and Coronal Mass Ejection (CME) on the Total Electron Content (TEC) of the E region ionosphere of Mars during a violent solar event that occurred on 13 May. Modeling of E region TEC was also carried out for the time of the flare using the observed X‐ray flux. We report that the solar flare caused enhancements in the TEC of Mars by factors of 6 to 10. A response was also observed in the E region plasma frequency of the Earth's ionosphere at nearly same time as the MGS observations. Our results suggest that sporadic E layer plasma frequency and TEC can increase by factors of 3 to 6 during the arrival of a CME at both Earth and Mars.

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.

High‐resolution multifluid simulations of flux ropes in the Martian magnetosphere

Three‐dimensional multifluid simulations of the Martian magnetosphere show the development and dynamics of flux ropes. One flux rope, which is analyzed in detail, initiates at a reconnection region near the dusk terminator and travels tailward with a speed on the order of 40 km s−1. The reconnection region forms close to the planet at an altitude of 700 km. Both the location of the reconnection and energy spectra of the plasma in the reconnection region agree with Mars Global Surveyor observations of reconnection. The largest flux ropes have a spatial extent on the order of 2000 km. Energy spectra taken through the flux ropes show an inverted‐V type structure similar to those measured by Mars Express, suggesting that some inverted‐V observations may be transits through flux ropes. The simulations indicate that the formation of flux ropes can lead to enhanced loss of heavy ions from the atmosphere.

Model calculations of electron precipitation induced ionization patches on the nightside of Mars

Using an electron transport model, we investigate the effect of electron precipitation on the electron density and total electron content in the nightside ionosphere of Mars. As input we use Mars Global Surveyor observations of a typical tail electron spectrum and an auroral‐like electron spectrum. The accelerated electron spectrum increases the maximum number density and total electron content by a factor of 3 over that produced by the typical tail spectrum. Our calculations show a secondary electron density peak due to precipitation of several keV electrons not seen in previous modeling efforts. Regions of enhanced ionization are expected to be localized in space, corresponding to magnetic cusps formed by the interaction of crustal sources with the interplanetary magnetic field. Radio and radar measurements from both Mars Global Surveyor and Mars Express agree with this expectation. The horizontally inhomogeneous regions of ionization can affect signals used for subsurface sounding from orbit.

Interannual variability of Martian dust storms in assimilation of several years of Mars global surveyor observations

We study the interannual variability of dust storms on Mars in an assimilation of thermal profiles and dust opacity observations into a general circulation model for the Martian atmosphere. The observations have been provided by the thermal emission spectrometer aboard the Mars Global Surveyor spacecraft during the scientific mapping phase over more than two complete Martian years, which include three dusty seasons in southern spring. A comparison between dust seasons which are characterized only by regional storms and the global, planet-encircling dust storm of 2001 is performed, focusing on the meteorological conditions which can trigger the onset and development of the global storm, and its effects on the global circulation.

Effect of the solar radiation in the topside atmosphere/ionosphere of Mars: Mars Global Surveyor observations

The peak electron density in the ionosphere of Mars and the neutral atmosphere scale height are proportional to the solar radiation flux. The data of the radio occultation experiments on board Mariner 9 and Viking 1 and 2 have already been analyzed to derive the relationship between the peak electron density and neutral atmosphere scale height and F10.7 flux which was used as a proxy of the solar UV radiation. The data points from Mariner 9 and Viking 1 and 2 missions are distributed from −40° to +38° in latitude and rather nonuniformly distributed in longitude. The experiment with the Mars Global Surveyor (MGS) accelerometer has revealed significant diurnal variations and latitudinal and longitudinal variations in the neutral atmosphere density and scale height. The effect of the solar radiation can be more confidently established if the effects of diurnal, latitudinal, and longitudinal variations are minimized. In the northern hemisphere the 732 electron density profiles derived from the data of the MGS radio science experiment, which were collected during the mapping phase of the mission, are located in the narrow latitude interval from +63° to +77°. In the southern hemisphere, 219 profiles are located in the latitude interval from −69° to −64.5°. These profiles were also obtained within narrow intervals of local time and are practically uniformly distributed in longitude. The peak electron density and the effective scale height of the neutral atmosphere density in the vicinity of the ionization peak have been derived for each of the profiles studied. The daily averages of E10.7 index are derived from the solar radiation fluxes measured near Earth. Their values are then recalculated, accounting for the relative positions of Mars and the Earth, and are used as a proxy of the EUV radiation flux at Mars. For the time period November 2000 to January 2001 the calculated daily averages of the adjusted peak electron density and effective scale height have been compared with the daily averages of E10.7 index. Analysis has shown that Te was, on average, slightly larger in the southern hemisphere than it was in the northern hemisphere. The hot electrons could be actually trapped within the latitude interval from −69° to −64.5° and the strong crustal magnetic fields have detectable but modest effect in the southern hemisphere Te might be, on average, 50–60% larger than Te in the northern hemisphere. In the case of density profiles measured in the period 1–31 January 2001 in the northern hemisphere Te is likely primarily controlled by the solar wind interaction with the Martian atmosphere/ionosphere.

Ionospheric effects upon a satellite navigation system at Mars

Trans‐ionospheric radio propagation effects resulting in ranging errors are examined for a potential orbital network of communications and navigational satellites at Mars. Using recent results from the radio science experiment on board the Mars Global Surveyor (MGS) spacecraft and a photochemical model of Mars' ionosphere, we study the total electron content (TEC) at Mars to investigate how its latitude, local time, and solar cycle patterns would contribute to errors in positioning on the planet. In addition, we examine the relationship between TEC and peak density (Nmax) and find that their ratio, called the equivalent slab thickness, shows that integral preserving distortions of the Ne(h) profile can be rather substantial, implying that neutral atmosphere dynamics can have strong effects upon Mars' photochemical ionosphere. We use MGS observations to validate modeling results and determine the extreme cases for TEC at Mars (i.e., when the planet is at perihelion during solar maximum years and at aphelion during solar minimum years). If a proposed Mars Communication and Navigation (MC&N) System used UHF/L‐band (1–2 GHz) transmission frequencies similar to those used for the terrestrial Global Positioning System (GPS) satellites, upper limits to the magnitude and variability of the martian ionosphere (TEC < ∼few × 1016 el m−2, with σ ∼ 10%) would not be of concern unless extremely precise positional information were required (<1 m). The impact of the ionosphere would be greater along slanted ray paths, and especially if lower frequency UHF beacon frequencies (e.g., 400 MHz) were selected for use. Indeed, such effects could be used as a diagnostic for the global structure of Mars' ionosphere, much in the same way as GPS measurements are used in terrestrial ionospheric physics.

The presence and stability of ground ice in the southern hemisphere of Mars

We calculate new estimates of ground-ice stability and the depth distribution of the ice table (the depth boundary between ice-free soil above and ice-cemented soil below) and compare these theoretical estimates of the distribution of ground ice with the observed distribution of leakage neutrons measured by the Neutron Spectrometer instrument of the Mars Odyssey spacecraft's Gamma Ray Spectrometer instrument suite. Our calculated ground-ice distribution contains improvements over previous work in that we include the effects of the high thermal conductivity of ice-cemented soil at and below the ice table, we include the surface elevation dependence of the near-surface atmospheric humidity, and we utilize new high resolution maps of thermal inertia, albedo, and elevation from Mars Global Surveyor observations. Results indicate that ground ice should be about 5 times shallower than in previous predictions. While results are dependent on the atmospheric humidity, depths are generally between a few millimeters and a few meters with typical values of a few centimeters. Results are also geographically similar to previous predictions with differences due to the higher resolution of thermal inertia and the inclusion of elevation effects on humidity. Comparison with the measured epithermal-neutron count rates in the southern hemisphere indicate that the geographic distribution of the count rate is best correlated with ground ice in equilibrium with 10 to 20 pr μm (precipitable micrometers) column abundance of atmospheric water, assuming a uniform distribution with CO 2; however, given the uncertainties, 5 to 30 pr μm also may be viable. This water abundance represents a longer-term average over 100 to 1000 yr. There is a high degree of correlation between the depth of the ice table and the epithermal count rate that agrees remarkably well with predicted count rates as a function of ice-table depth. These results indicate that ground ice in the upper meter of the martian soil is in diffusive equilibrium with the atmosphere. Since ground ice in this depth zone is expected to undergo saturation/desiccation cycles with orbital variations, this ice should be younger than about 500 kyr and was emplaced under similar cold and dry climate conditions of today. Remaining differences between the predicted depths of the ice table and those inferred from the neutron data are likely to be due to subpixel heterogeneity in the martian surface including the presence of rocks, slopes, and patches of soil with varying thermophysical properties.

Martian obstacle and bow shock: origins of boundaries anisotropy

The origin of the anisotropy in the shape of the Martian obstacle and bow shock is analyzed using Mars Global Surveyor observations. The influence of MHD or ion pick-up effects on Martian obstacle position was to be small found, however, localized Martian crustal magnetization increases the thickness of the downstream planetary magnetotail by 500–1000 km in agreement with earlier Phobos 2 observations. A new analytical model is presented for Martian obstacle shape variation for different solar wind ram pressure. Elongation of the Martian BS cross-section in the direction perpendicular to IMF was confirmed while the shift of this cross section in the + Y direction of Martian interplanetary medium reference frame was discovered. The shift of BS cross section in the direction of interplanetary electric field was not revealed thus not conforming the idea that mass-loading play some role in BS control.

Modeling day‐to‐day ionospheric variability on Mars

We use a simple photochemical model of Mars' ionosphere to investigate the changes in calculated electron density profiles, Ne(h), under the assumption that the only variable parameter is the solar irradiance. The period under study is 9–27 March 1999, corresponding to a series of Ne(h) observations made by the Mars Global Surveyor (MGS) Radio Science (RS) experiment. The SOLAR 2000 model is used to obtain the solar irradiances during this period. Secondary ionization is included and is expressed as a simple function of photoionization. The model uses several neutral gas species, five of which (CO2, N2, CO, NO, O) experience photoionization. Subsequent ion‐neutral chemistry leads to an ionosphere with five ions species. Using average solar conditions for March 1999, good agreement was obtained between the model's mean electron density profile and the mean electron densities from the MGS observations. Modeling day‐to‐day changes using daily solar irradiance values resulted in ∼6% and 19% variabilities for the major and secondary layer peak densities, respectively, in good agreement with the observed values of 5 and 10%. The variable photon flux from the Sun is thus the primary driver of day‐to‐day changes in the peak electron densities on Mars.

An assessment of the global, seasonal, and interannual spacecraft record of Martian climate in the thermal infrared

Intercomparison of thermal infrared data collected by Mariner 9, Viking, and Mars Global Surveyor (MGS) is presented with a specific focus on air temperatures, dust opacities, and water ice opacities. Emphasis is placed on creating a uniform data set to most effectively reduce interinstrument biases and offsets. The annual cycle consistently shows a strong asymmetry about the equinoxes, with northern spring and summer exhibiting relatively low temperatures, very high year‐to‐year repeatability, and essentially no short‐term (tens of days) variability. The globally averaged Martian nighttime air temperatures close annually to within a Kelvin during northern spring and summer. Daytime temperatures show more variability (3–6 K). The difference in repeatability of daytime versus nighttime temperatures is not understood. Viking and MGS air temperatures are essentially indistinguishable for this period, suggesting that the Viking and MGS eras are characterized by essentially the same climatic state. Southern summer is characterized by strong dust storm activity and hence strong year‐to‐year air temperature variability. Dust opacity shows a remarkable degree of interannual variability in southern spring and summer, associated with the intermittent activity of regional and planet‐encircling dust storms, but exhibits high year‐to‐year repeatability in northern spring and summer. Specifically, late northern spring and early northern summer dust opacities appear to be completely insensitive to the occurrence (or not) of major dust storms in the previous southern spring or summer. We show that both Viking and MGS data sets exhibit significant (and similar) polar cap edge dust storm activity. The origins of the various major dust storms can be identified in the thermal infrared data from Viking and MGS, including the transport of dust from the northern autumn baroclinic zone into the southern hemisphere tropics, which has also been identified in visible imaging. We also note that the period around Ls = 225° is characterized by very high dust opacities associated with dust storm development or decay in every year thus far observed by spacecraft. Water ice opacities have been retrieved from Viking infrared data for the first time. We show that the northern spring and summer tropical cloud belt structure and evolution are essentially the same in each of the multiple years observed by Viking and MGS. Relatively subtle spatial features recur in the cloud belt from year to year, suggesting the influence of surface topography and thermophysical properties and a reasonably consistent supply of water vapor. The seasonal evolution of the tropical cloud belt through northern spring and summer is shown, with the only significant deviations between years occurring from Ls = 140° to 160°, where opacities fall in the second MGS year associated with a small dust storm. Polar hood clouds are observed in Viking and MGS observations with similar timing and extent. Interactions between dust and water ice were highlighted in the Hellas basin region during the southern spring 1977a and 2001 dust storms. The observations demonstrate that the Martian atmosphere executes a very “repeatable” annual cycle of atmospheric phenomena. However, a major part of this cycle is the occurrence of highly variable and potentially major dust storm events. After such dust storm events the atmosphere rapidly relaxes to its stable, repeatable state.

Magnetic field draping enhancement at the Martian magnetic pileup boundary from Mars global surveyor observations

The Magnetic Pileup Boundary (MPB) is a sharp and permanent plasma boundary located between the bow shock and the ionospheric boundary, reported so far at Mars and comets. We use Mars Global Surveyor Magnetometer data to do a quantitative analysis of the magnetic field geometry in the surroundings of the Martian MPB. As a result, we report for the first time a dramatic enhancement of the magnetic field draping at this boundary. This new feature, already reported at comets, is independent of the presence of the crustal magnetic sources. Comparisons with similar results across the Martian and cometary magnetotails reveal that the MPB and the magnetotail boundary are connected. Moreover, the study of this feature can help understand the physics of the Venusian magnetic barrier.

On the possibility of liquid water on present‐day Mars

Using a validated general circulation model, we determine where and for how long the surface pressure and surface temperature on Mars meet the minimum requirements for the existence of liquid water in the present climate system: pressures and temperatures above the triple point of water but below the boiling point. We find that for pure liquid water, there are five “favorable” regions where these requirements are satisfied: between 0° and 30°N in the plains of Amazonis, Arabia, and Elysium; and in the Southern Hemisphere impact basins of Hellas and Argyre. The combined area of these regions represents 29% of the planet's surface area. In the Amazonis region these requirements are satisfied for a total integrated time of 37 sols each Martian year. In the Hellas basin the number of degree days above zero is 70, which is well above those experienced in the dry valley lake region of Antarctica. These regions are remarkably well correlated with the location of Amazonian paleolakes mapped by Cabrol and Grin [2000] but are poorly correlated with the seepage gullies found by Malin and Edgett [2000]. In both instances, obliquity variations may play a role. For brine solutions the favorable regions expand and could potentially include most of the planet for highly concentrated solutions. Whether liquid water ever forms in these regions depends on the availability of ice and heat and on the evaporation rate. The latter is poorly understood for low‐pressure CO2 environments but is likely to be so high that melting occurs rarely, if at all. However, even rare events of liquid water formation would be significant since they would dominate the chemistry of the soil and would have biological implications as well. It is therefore worth reassessing the potential for liquid water formation on present day Mars, particularly in light of recent Mars Global Surveyor observations.