Water vapor variability in the north polar region of Mars from Viking MAWD and MGS TES datasets

Atmospheric water vapor abundances in Mars’ north polar region (NPR, from 60° to 90°N) are mapped as function of latitude and longitude for spring and summer seasons, and their spatial, seasonal, and interannual variability is discussed. Water vapor data are from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) and the Viking Orbiter (VO) Mars Atmospheric Water Detector (MAWD). The data cover three complete northern spring-summer seasons in 1977-1978, 2000-2001 and 2002-2003, and shorter periods of spring-summer seasons during 1975, 1999 and 2004. Long term interannual variability in the averaged NPR abundances may exist, with Viking MAWD observations showing twice as much water vapor during summer as the MGS TES observations more than 10 martian years (MY) later. While the averaged abundances are very similar in TES observations for the same season in different years, the spatial distributions in the early summer season do vary significantly year over year. Spatial and temporal variabilities increase between L_s ∼ 80-140°, which may be related to vapor sublimation from the North Polar Residual Cap (NPRC), or to changes in circulation. Spatial variability is observed on scales of ∼100 km and temporal variability is observed on scales of <10 sols during summer. During late spring the TES water vapor spatial distribution is seen to correlate with the low topography/low albedo region of northern Acidalia Planitia (270-360°E), and with the dust spatial distribution across the NPR during late spring-early summer. Non-uniform vertical distribution of water vapor, a regolith source or atmospheric circulation ‘pooling’ of water vapor from the NPRC into the topographic depression may be behind the correlation with low topography/low albedo. Sublimation winds carrying water vapor off the NPRC and lifting surface dust in the areas surrounding the NPRC may explain the correlation between the water vapor and dust spatial distributions. Correlation between water vapor and dust in MAWD data are only observed over low topography/low albedo area. Maximum water vapor abundances are observed at L_s = 105-115° and outside of the NPRC at 75-80°N; the TES data, however, do not extend over the NPRC and thus, this conclusion may be biased. Some water vapor appears to be released in plumes or ‘outbursts’ in the MAWD and TES datasets during late spring and early summer. We propose that the sublimation rate of ice varies across the NPRC with varying surface winds, giving rise to the observed ‘outbursts’ at some seasons.     

Seasonal cycle of water vapor in the atmosphere of mars as revealed from the MAWD/<Emphasis Type="Italic">Viking 1</Emphasis> and <Emphasis Type="Italic">2</Emphasis> experiment

The most detailed information on the seasonal evolution of water vapor distribution in the Martian atmosphere was obtained by the Mars Atmospheric Water Detector (MAWD), which was a five-channel spectrometer operating aboard the Viking 1 and 2 spacecraft in 1977–1980, and from the very recent observations by the Thermal Emission Spectrometer (TES) on the Mars Global Surveyor spacecraft. The TES results show a considerably larger amount of water vapor near the perihelion of Mars (summer in the southern hemisphere) than the estimates based on the MAWD data for this season, which is characterized by the development of global dust storms. The TES and MAWD instruments operated in different spectral regions (20–50 m and 1.38 m, respectively), and this could result in the aforementioned difference because of the effect of aerosol scattering on the intensity of the H₂O bands, which becomes significant at short wavelengths. We considered the effect of aerosol scattering on the water vapor content measured in the 1.38-m band, taking into account the different geometries of observations, and restored the H₂O content from the MAWD data with allowance for multiple scattering. We obtained a seasonal and spatial distribution of water vapor that showed better agreement with the TES data and, thus, indicated the stability of the hydrological cycle on Mars. Periodic structures, which could possibly be associated with the influence of the stationary planetary waves, are reliably revealed in the zonal distribution of the atmospheric water vapor. The seasonal variability of the wave structures correlates with variations of the latitude gradient of water vapor. This could indicate that wave processes contribute considerably to the meridional water transport in the Martian atmosphere.Translated from Astronomicheskii Vestnik, Vol. 38, No. 6, 2004, pp. 483–496.Original Russian Text Copyright © 2004 by Fedorova, Rodin, Baklanova.     

Investigation of water vapor on Mars with PFS/SW of Mars Express

New insight into the seasonal, diurnal and spatial distribution of water vapor on Mars has been obtained from analyzing the spectra of the short-wavelength channel (SW) of the Planetary Fourier Spectrometer (PFS) onboard Mars Express. The processed dataset, recorded between January 2004 and April 2005, covers the seasons from LS=331° of Mars Year 26 to LS=196° of the following year. In this period the mean column density around vernal equinox was 8.2 pr. μm. The maximum values during northern summer were about 65 pr. μm, located around 75° N latitude with a longitudinally inhomogeneous distribution. Regarding the atmospheric transport, the majority of polar water vapor remains in the north polar region while only about a quarter is transported southward. Geographically there are two water vapor maxima visible, over Arabia Terra and the Tharsis plateau, that are most likely caused both by atmosphere-ground interaction and by atmospheric circulation. A comparison with other instruments generally shows a good agreement, only the SPICAM results are systematically lower. Compared to the results from the PFS long-wavelength channel the results of this work are slightly higher. A strong discrepancy is visible northward of about 50° N during the northern summer that is possibly explained by a non-uniform vertical H₂O mixing. In particular, a confinement of the water to the lower few kilometers yields a much better agreement between the retrieved column densities of the two PFS channels.     

Martian water vapor: Mars Express PFS/LW observations

We present the seasonal and geographical variations of the martian water vapor monitored from the Planetary Fourier Spectrometer Long Wavelength Channel aboard the Mars Express spacecraft. Our dataset covers one martian year (end of Mars Year 26, Mars Year 27), but the seasonal coverage is far from complete. The seasonal and latitudinal behavior of the water vapor is globally consistent with previous datasets, Viking Orbiter Mars Atmospheric Water Detectors (MAWD) and Mars Global Surveyor Thermal Emission Spectrometer (MGS/TES), and with simultaneous results obtained from other Mars Express instruments, OMEGA and SPICAM. However, our absolute water columns are lower and higher by a factor of 1.5 than the values obtained by TES and SPICAM, respectively. In particular, we retrieve a Northern midsummer maximum of 60 pr-μm, lower than the 100-pr-μm observed by TES. The geographical distribution of water exhibits two local maxima at low latitudes, located over Tharsis and Arabia. Global Climate Model (GCM) simulations suggest that these local enhancements are controlled by atmospheric dynamics. During Northern spring, we observe a bulge of water vapor over the seasonal polar cap edge, consistent with the northward transport of water from the retreating seasonal cap to the permanent polar cap. In terms of vertical distribution, we find that the water volume mixing ratio over the large volcanos remains constant with the surface altitude within a factor of two. However, on the whole dataset we find that the water column, normalized to a fixed pressure, is anti-correlated with the surface pressure, indicating a vertical distribution intermediate between control by atmospheric saturation and confinement to a surface layer. This anti-correlation is not reproduced by GCM simulations of the water cycle, which do not include exchange between atmospheric and subsurface water. This situation suggests a possible role for regolith-atmosphere exchange in the martian water cycle.     

Tracking the edge of the south seasonal polar cap of Mars

The advance and retreat of the polar caps were one of the first observations that indicated Mars had seasons. Because a large portion of the atmosphere is cycled in and out of the seasonal caps during the year, the frost deposits play a significant role in regional and global atmospheric circulation. Understanding the nature of the seasonal polar caps is imperative if we are to understand the current Martian climate. In this study, we track the seasonal cap edges as a function of season and longitude for the fall and winter seasons (MY27), using data from the Planetary Fourier Spectrometer (PFS) onboard the Mars Express (MEX) ESA mission. Making use of the rapid rise (decrease) in surface temperature that occurs when CO₂ ice is removed (deposited), in a first approach, we defined the advancing cap edge to be where the surface temperature drops below 150 K, and the retreating cap edge where the surface temperature rises above 160 K. In this case, starting from Ls∼50°, the edge progression speed start to be longitude dependent. In the hemisphere that extends form the eastern limit of the Hellas basin to the western limit of the Argyrae basin (and containing the two) the edges progression speed is about a half than that of the other hemisphere; the cap is thus asymmetric and, unexpectedly, no CO₂ ice seems to be present inside the basins. This is because the above mentioned surface temperatures used in this approach to detect the cap edges are not adequate (too low) for the high-pressure regions inside the basins where, following the Clausius–Clapeyron's law, the CO₂ condensation temperature can be several degrees higher than that of the adjacent lower-pressure regions. In the second, final approach, special attention has been given to this aspect and the advancing and retreating cap edges are defined where, respectively, the surface temperatures drop below and rise above the CO₂ condensation temperature for the actual surface pressure values. Now, the results show an opposite situation than the previous one, with the progression speed being higher and the cap more extended (up to −30° latitude) in the hemisphere containing the two major Martian basins. During the fall season, up to Ls∼50° the South Martian polar cap consists of CO₂ frost deposits that advance towards lower latitudes at a constant speed of 10° of latitude per 15 degrees of Ls. The maximum extension (−40° latitude) of the South polar cap occurs somewhere in the 80°–90° Ls range. At the winter solstice, when the edges of the polar night start moving poleward, the cap recession has already started, in response to seasonal changes in insolation. The CO₂ ice South polar cap will recede with a constant speed of ∼5° of latitude every 25° degrees of Ls during the whole winter. The longitudinal asymmetries reduce during the cap retreat and completely disappear around Ls=145°.     

Observations of atmospheric water vapor above the Tharsis volcanoes on Mars with the OMEGA/MEx imaging spectrometer

The OMEGA imaging spectrometer onboard the Mars Express spacecraft is particularly well suited to study in detail specific regions of Mars, thanks to its high spatial resolution and its high signal-to-noise ratio. We investigate the behavior of atmospheric water vapor over the four big volcanoes located on the Tharsis plateau (Olympus, Ascraeus, Pavonis and Arsia Mons) using the 2.6 μm band, which is the strongest and most sensitive H₂O band in the OMEGA spectral range. Our data sample covers the end of MY26 and the whole MY27, with gaps only in the late northern spring and in northern autumn. The most striking result of our retrievals is the increase of water vapor mixing ratio from the valley to the summit of volcanoes. Corresponding column density is often almost constant, despite a factor of ∼5 decrease in air mass from the bottom to the top. This peculiar water enrichment on the volcanoes is present in 75% of the orbits in our sample. The seasonal distribution of such enrichment hints at a seasonal dependence, with a minimum during the northern summer and a maximum around the northern spring equinox. The enrichment possibly also has a diurnal trend, being the orbits with a high degree of enrichment concentrated in the early morning. However, the season and the solar time of the observations, due to the motion of the spacecraft, are correlated, then the two dependences cannot be clearly disentangled. Several orbits exhibit also spatially localized enrichment structures, usually ring- or crescent-shaped. We retrieve also the height of the saturation level over the volcanoes. The results show a strong minimum around the aphelion season, due to the low temperatures, while it raises quickly before and after this period. The enrichment is possibly generated by the local circulation characteristic of the volcano region, which can transport upslope significant quantities of water vapor. The low altitude of the saturation level during the early summer can then hinder the transport of water during this season. The influence of the coupling between atmosphere and surface, due mainly to the action of the regoliths, can also contribute partially to the observed phenomenon.     

Mars atmospheric water vapor abundance: 1996-1997

Measurements of martian atmospheric water vapor made throughout L_s = 18.0°-146.4° (October 3, 1996-July 12, 1997) show changes in Mars humidity on hourly, daily, and seasonal time scales. Because our observing program during the 1996-1997 Mars apparition did not include concomitant measurement of nearby CO₂ bands, high northern latitude data were corrected for dust and aerosol extinction assuming an optical depth of 0.8, consistent with ground-based and HST imaging of northern dust storms. All other measurements with airmass greater than 3.5 were corrected using a total optical depth of 0.5. Three dominant results from this data set are as follows: (1) pre- and post-opposition measurements made with the slit crossing many hours of local time on Mars’ Earth-facing disk show a distinct diurnal pattern with highest abundances around and slightly after noon with low abundances in the late afternoon, (2) measurements of water vapor over the Mars Pathfinder landing site (Carl Sagan Memorial Station) on July 12, 1997, found 21 ppt μm in the spatial sector centered near 19° latitude, 36° longitude while abundances around the site varied from as low as 6 to as high as 28 ppt μm, and (3) water vapor abundance is patchy on hourly and daily time scales but follows the usual seasonal trends.     

Water-ice clouds and dust in the north polar region of Mars using MGS TES data

Water-ice and dust optical depths in Mars’ north polar region are mapped as function of season, latitude and longitude, and their characteristics and variability on a geographic, seasonal, and interannual basis are discussed. We use water-ice and dust optical depth data provided by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES), covering nearly three northern spring and summer periods. We find that interannual variability exists in both the water ice and dust behavior, although there are trends that repeat year to year as well. The optical thickness of the north polar hood (NPH) exhibits interannually varying longitudinal structure, both during springtime recession and late-summer onset. We define the characteristics associated with the transition to and from the NPH and find that the disappearance occurs near L_s=75° and the reappearance near L_s=160-165°. We find that the late spring to early summer time frame is characterized by very low water-ice optical depths and enhanced dust activity, with a preference for lower water-ice and higher dust optical depths in the 0-90°W quadrant. We see possible evidence for stationary wavenumber 2 systems in a few of the maps examined.     

Polar warming in the middle atmosphere of Mars

We report ground-based laser heterodyne spectroscopy of non-thermal emission in the cores of the 10.33-μmR(8) and 10.72-μmP(32) lines of ¹²C¹⁶O₂, obtained at 23 locations on the disk of Mars during the 1984 opposition, at L_s = 130°. The data were obtained at a sub-Doppler spectral resolution, and the temperature of the middle Martian atmosphere (50–85 km) is derived from the frequency width and intensity of the R(8) emission, and from the total intensity of the P(32) emission. We find that the temperature of the middle Martian atmosphere varies with latitude. Near the subsolar latitude, the average 50- to 85-km temperature is close to the radiative equilibrium value for a CO₂ atmosphere. However, at high latitudes in both the northern (summer) and southern (winter) hemispheres the 50- to 85-km temperature exceeds the CO₂ radiative equilibrium value; a meridional gradient in the range of 0.4 – 0.9°K per degree of latitude is indicated by our data. The highest temperatures are seen at high latitudes in the winter hemisphere, reminiscent of the seasonal effects seen at the Earth's mesopause. As in the terrestrial case, this winter polar warming in the Martian middle atmosphere necessitates departures from radiative equilibrium; dynamical heating of order 4 × 10² ergs g⁻¹ sec⁻¹ is required at the edge of the winter polar night. A comparison with 2-D circulation models shows that the presence of atmospheric dust may enhance this dynamical heating at high winter latitudes, and may also account for heating at high latitudes in the summer hemisphere.     

Martian atmospheric water vapor observations: 1972–1974 apparition

The patrol of Martian water vapor carried out with the echelle-coudé scanner at McDonald Observatory during the 1972–1974 apparition has produced 469 individual photoelectric scans of Doppler-shifted Martian H₂O lines. Almost an entire Martian year was covered during the 1972–1974 period (L_s = 118?269° and 301?80°). Three types of coverage have been obtained: (1) regular—the slit placed pole to pole on the central meridian; (2) latitudinal—the slit placed parallel to the Martian equator at various latitudes; (3) diurnal—the slit placed parallel to the terminator at several times during a Martian day measured from local noon.Both the seasonal and diurnal effects seem to be controlled by the insolation and not the local topography with respect to the 6.1 mb surface. A slight negative correlation with elevation was noted which improved during the seasons of greater H₂O content. The previous seasonal behavior has been confirmed and amplified. The following are the primary conclusions: (1) The planetwide abundance is low (5?15 μm of ppt H₂O) during both equinoctical periods. (2) The maximum abundance of about 40 μm occurs in each hemisphere after solstice at about 40° latitude in that hemisphere. (3) The latitude of the maximum amount in the N-S distribution precedes the latitude of maximum insolation by 10–20° of latitude. (4) During the “drier” seasons (5–20 μm) near the equinoxes on Mars, the atmospheric water vapor changes by a factor of 2–3x over a diurnal cycle with the maximum near local noon. (5) The effects of the 1973 dust storm during the southern summer reduced the amount of water vapor over the southern hemisphere regions to 3–8 μm.     

Remnant Massifs of Layered Deposits at High Northern Latitudes of Mars

Ice deposits of the permanent northern polar cap of Mars exhibit a clearly expressed layered structure connected with consecutive accumulation of a solid phase of H₂O (ice) with different impurity fractions of dust. The observed structure of the northern polar cap deposits is direct evidence of the uneven accumulation of ice associated with climate variations on Mars over a long period of time. Outside the boundaries of the present northern polar cap, there are isolated remnant massifs of bright deposits with a clearly distinguishable layered structure. In this paper, we analyze the morphology, spatial distribution, and material composition of remnant massifs within the latitude belt of 60°–85°. The similarity in the structure and material composition found in the remnant massifs and the layered deposits of the northern polar cap of Mars apparently suggests that these formations were genetically connected in the past climatic epochs.     

Mariner 9 ultraviolet spectrometer experiment: Observations of ozone on Mars

The Mariner 9 Ultraviolet Spectrometer has observed the 2550 Å ozone spectral absorption feature on Mars. This absorption was previously detected in the south polar region by Mariner 7 in 1969. Mariner 9 did not observe ozone at any time in the equatorial region, nor at the south polar cap during its summer season. However, ozone was found in the north polar region beginning at a latitude of 45°N and extending northward. Ozone later appeared in the southern hemisphere southward of 50°S as the Mars autumnal equinox approached. The presence of ozone on Mars seems to be coupled to the water vapor content of its atmosphere.     

Viking observation of water vapor on Mars: Revision from up-to-date spectroscopy and atmospheric models

A reanalysis of the Mars Atmospheric Water Detector (MAWD, Viking 1 and 2 Orbiters) Planetary Data System (PDS) data set (Jakosky, B.M., Farmer, C.B. [1982]. J. Geophys. Res. 87 (B4), 2999-3019) is presented taking into account a new spectroscopic database and improved atmospheric model assumptions. Starting from HITRAN 2004 edition and later (Rothman, L.S., and 29 colleagues [2005]. J. Quant. Spectrosc. Radiat. Trans. 96, 139-204), the number of lines in the 1.38-μm band has been significantly increased, and their parameters have been modified. The implication of this new spectroscopic data and atmospheric model based on Martian Climate Database (MCD, Forget, F., Hourdin, F., Fournier, R., Hourdin, C., Talagrand, O., Collins, M., Lewis, S.R., Read, P.L., Huot, J.-P. [1999]. J. Geophys. Res. 104 (E10), 24155-24176) gives a significant impact on the H₂O retrieval: the total H₂O abundance after the reanalysis has decreased twofold in all seasons and most of geographic locations. Overall decrease is associated with larger cumulative strength of the band in HITRAN 2004; low saturation height of water profiles imposed by MCD significantly contributes to decrease of summer polar maximum. Revised MAWD data are compared with later H₂O measurements on Mars Global Surveyor (MGS) and Mars-Express (MEX). A good agreement with SPICAM/MEX near-IR (1.38-μm band) measurements is found. However, both sets of near-IR measurements are systematically below TES/MGS results obtained in thermal infrared, with a factor of 1.5-2.0. This difference might be associated with remaining ambiguity of the near IR spectral data, and of line broadening in CO₂ in both spectral ranges. The reanalyzed MAWD data are in much better agreement with later measurements suggesting more homogeneous, and significantly dryer water cycle on Mars, with no signature of change between the Viking epoch (MY12-14) and MGS-Mars-Express measurements (MY24-29).     

Seasonal redistribution of water in the surficial Martian regolith: Results from the <Emphasis Type="Italic">Mars Odyssey</Emphasis> high-energy neutron detector (HEND)

The seasonal variation of neutron emissions from Mars in different spectral intervals measured by the HEND neutron detector for the entire Martian year are analyzed. Based on these data, the spatial variations of the neutron emissions from the planet are globally mapped as a function of season, and the dynamics of seasonal variation of neutron fluxes with different energies is analyzed in detail. No differences were found between seasonal regimes of neutron fluxes in different energy ranges in the southern hemisphere of Mars, while the regime of fast neutrons (with higher energies) during the northern winter strongly differs from that during the southern winter. In winter (L _s = 270°–330°), the fast neutron fluxes are noticeably reduced in the northern hemisphere (along with the consecutive thickening of the seasonal cap of solid carbon dioxide). This provides evidence of a temporary increase in the water content in the effective layer of neutron generation. According to the obtained estimates, the observed reduction of the flux of fast neutrons in the effective layer corresponds to an increase in the water abundance of up to 5% in the seasonal polar cap (70°–90°N), about 3% at mid-latitudes, and from 1.5 to 2% at low latitudes. The freezing out of atmospheric water at the planetary surface (at middle and high latitudes) and the hydration of salt minerals composing the Martian soil are considered as the main processes responsible for the temporary increase in the water content in the soil and upper layer of the seasonal polar cap. The meridional atmospheric transport of water vapor from the summer southern to the winter northern hemisphere within the Hadley circulation cell is a basic process that delivers water to the subsurface soil layer and ensures the observed scale of the seasonal increase in water abundance. In the summer northern hemisphere, the similar Hadley circulation cell transports mainly dry air masses to the winter southern hemisphere. The point is that the water vapor becomes saturated at lower heights during aphelion, and the bulk of the atmospheric water mass is captured in the near-equatorial cloudy belt and, thus, is only weakly transferred to the southern hemisphere. This phenomenon, known as the Clancy effect, was suggested by Clancy et al. (1996) as a basic mechanism for the explanation of the interhemispheric asymmetry of water storage in permanent polar caps. The asymmetry of seasonal meridional circulation of the Martian atmosphere seems to be another factor determining the asymmetry of the seasonal water redistribution in the “atmosphere-regolith-seasonal polar caps” system, found in the peculiarities of the seasonal regime of the neutron emission of Mars.     

Water distribution in Martian permafrost regions from joint analysis of HEND (Mars Odyssey) and MOLA (Mars Global Surveyor) data

We jointly analyze data from the High-Energy Neutron Detector (HEND) onboard the NASA Mars Odyssey spacecraft and data from the Mars Orbiter Laser Altimeter (MOLA) onboard the Mars Global Surveyor spacecraft. The former instrument measures the content of hydrogen (in the form of H₂O or OH) in the subsurface layer of soil and the latter instrument measures the surface albedo with respect to the flux of solar energy. We have checked the presence of a correlation between these two data sets in various Martian latitude bands. A significant correlation has been found between these data at latitudes poleward of 40° in the northern hemisphere and at latitudes 40°–60° in the southern hemisphere. This correlation is interpreted as evidence for the presence of stable water ice in these regions under a dry layer of soil whose thickness is determined by the condition for equilibrium between the condensation of water from the atmosphere and its sublimation when heated by solar radiation. For these regions, we have derived an empirical relation between the flux of absorbed solar radiation and the thickness of the top dry layer. It allows the burial depth of the water ice table to be predicted with a sub-kilometer resolution based on near-infrared albedo measurements. We have found no correlation in the southern hemisphere at latitudes >60°, although neutron data also suggest that water ice is present in this region under a layer of dry soil. We conclude that the thickness of the dry layer in this region does not correspond to the equilibrium condition between the water ice table and the atmosphere.     

Observations of the north polar water ice annulus on Mars using THEMIS and TES

The Martian seasonal CO₂ ice caps advance and retreat each year. In the spring, as the CO₂ cap gradually retreats, it leaves behind an extensive defrosting zone from the solid CO₂ cap to the location where all CO₂ frost has sublimated. We have been studying this phenomenon in the north polar region using data from the THermal EMission Imaging System (THEMIS), a visible and infra-red (IR) camera on the Mars Odyssey spacecraft, and the Thermal Emission Spectrometer (TES) on Mars Global Surveyor. Recently, we discovered that some THEMIS images of the CO₂ defrosting zone contain evidence for a distinct defrosting phenomenon: some areas just south of the CO₂ cap edge are too bright in visible wavelengths to be defrosted terrain, but too warm in the IR to be CO₂ ice. We hypothesize that we are seeing evidence for a seasonal annulus of water ice (frost) that recedes with the seasonal CO₂ cap, as predicted by previous workers. In this paper, we describe our observations with THEMIS and compare them to simultaneous observations by TES and OMEGA. All three instruments find that this phenomenon is distinct from the CO₂ cap and most likely composed of water ice. We also find strong evidence that the annulus widens as it recedes. Finally, we show that this annulus can be detected in the raw THEMIS data as it is collected, enabling future long-term onboard monitoring.     

The effect of orbital element variations on the mean seasonal daily insolation on Mars

In this paper we briefly study changes in the mean seasonal insolations on the planet Mars caused by significant large-scale variations in the following orbital elements: the eccentricity (e), the obliquity (ε) and the longitude of perihelion (λ _p ). Three orbital configurations have been investigated. In the first, the eccentricity equals successively 0, 0.075, and 0.15, whereas for the obliquity and the longitude of perihelion we took the present values which amount, respectively to 25° and 250°. In the second situation, ε=15, 25, and 35° for a circular orbit (e=0) and with λ _p =250°. In the last model we have sete=0.075 and ε=25° for λ _p =?90,0, and 90°. Although long-term periodic oscillations ofe (first case) and λ _p (third case) produce, respectively, very small or no variations in the average yearly insolation, fluctuations of the above mentioned planetary data strongly effect the mean summer and winter daily insolations. Indeed, the calculations reveal that between the two extreme values of the orbital elements used, the seasonal insolations exhibit a change in amplitude of about 15 to 20% difference over the entire latitude interval. Considering more particularly the second case it is found that the summertime insolation experiences a nearly similar variation as the mean annual daily insolation — i.e., a decrease of about 7% at the equator and a more than twofold increase at the poles. The corresponding mean winter daily insolation varies maximally by approximately 60% in the 60–80° latitude range.     

Mars: Dual-polarization radar observations with extended coverage

Thirteen-centimeter-wavelength radar observations of Mars made in 1982 at Arecibo Observatory yield accurate measurements of the full backscatter spectrum in two orthogonal polarizations. The data, which were obtained for several widely separated subradar longitudes at 24°N latitude, provide the first global view of the distribution of small-scale surface roughness on Mars. The diffuse component of the echo exhibits strong spatial variations. Areas of maximum depolarization correlate well with volcanic regions (Tharsis and Elysium), while the heavily cratered upland terrain yields relatively low depolarization. Parts of Tharsis give near-complete depolarization (polaziation ratio $\text{μ}{}_{\mathrm{<mtext>c</mtext>}}\text{? 1}$ when viewed at oblique angles of incidence). Northern Martian plains regions (Tharsis, Elysium, and Amazonis) may comprise the most extensive area of severe decimeter-scale surface roughness in the inner Solar System. On the average, the northern Martian tropics yield higher diffuse radar cross sections (σ^D = 0.05–0.12) and a higher of degree disk-integrated depolarization (μ_c = 0.1–0.4) than is found for the Moon, Mercury, and Venus. Comparisons between the Moon and Mars using radar data, ground truth, and simple scattering models suggest that Mars possesses a relatively high average coverage by decimeter-scale rocks. Also discussed are several of the more interesting quasispecular scattering results, the most unsual of which were obtained over the Olympus Mons aureole region.     

Mars atmospheric water vapor abundance: 1991-1999, emphasis 1998-1999

Our ground-based measurements of martian atmospheric water vapor, made throughout Ls=34° to 249°, 24 September 1998 to 23 November 1999, during Mars year 24 (MY 24), show changes in Mars' humidity on hourly, daily, and seasonal timescales. We made concomitant measurement of nearby CO₂ bands, and when possible, results were corrected for aerosol extinction using aerosol optical depths derived from our own CO₂ analysis. Where there is spatial and temporal overlap, similar results are obtained for water vapor abundances and aerosol opacities as those observed from the Thermal Emission Spectrometer on Mars Global Surveyor. In addition some further discussion of our published earlier water vapor measurements (1991-1995) is included. Six results from this data set are: (1) the measured aerosol opacity in Mars atmosphere was variable but not greater than τ=1, with almost no clear atmosphere being observed, (2) measurements made with the slit crossing many hours of local time on Mars' Earth-facing disk show a diurnal pattern with highest abundances at mid-day and low abundance in very early morning and late afternoon for some but not all measurements, (3) water vapor abundance is patchy on hourly and daily time scales but follows the usual seasonal trends seen by instrumentation on the Mars Atmospheric Water Detector on the Viking Orbiters and by the Thermal Emission Spectrometer on Mars Global Surveyor, (4) there is a slight longitudinal correlation with the ground-ice observed by the Gamma Ray Spectrometer on Mars Odyssey, (5) there is evidence of the Low Southern Latitude Summer Minimum in our water vapor measurements but our data set for southern summer is limited, and (6) MY 24 appears to be wetter than MY 22 and MY 23.     

Hydrogen peroxide on Mars: evidence for spatial and seasonal variations

Hydrogen peroxide (H₂O₂) has been suggested as a possible oxidizer of the martian surface. Photochemical models predict a mean column density in the range of 10¹⁵-10¹⁶ cm⁻². However, a stringent upper limit of the H₂O₂ abundance on Mars (9×10¹⁴ cm⁻²) was derived in February 2001 from ground-based infrared spectroscopy, at a time corresponding to a maximum water vapor abundance in the northern summer (30 pr. μm, Ls=112°). Here we report the detection of H₂O₂ on Mars in June 2003, and its mapping over the martian disk using the same technique, during the southern spring (Ls=206°) when the global water vapor abundance was ∼10 pr. μm. The spatial distribution of H₂O₂ shows a maximum in the morning around the sub-solar latitude. The mean H₂O₂ column density (6×10¹⁵ cm⁻²) is significantly greater than our previous upper limit, pointing to seasonal variations. Our new result is globally consistent with the predictions of photochemical models, and also with submillimeter ground-based measurements obtained in September 2003 (Ls=254°), averaged over the martian disk (Clancy et al., 2004, Icarus 168, 116-121).