Dust aerosols above the south polar cap of Mars as seen by OMEGA

The time evolution of atmospheric dust at high southern latitudes on Mars has been determined using observations of the south seasonal cap acquired in the near infrared (1-2.65 μm) by OMEGA/Mars Express in 2005. Observations at different solar zenith angles and one EPF sequence demonstrate that the reflectance in the 2.64 μm saturated absorption band of the surface CO₂ ice is mainly due to the light scattered by aerosols above most places of the seasonal cap. We have mapped the total optical depth of dust aerosols in the near-IR above the south seasonal cap of Mars from mid-spring to early summer with a time resolution ranging from one day to one week and a spatial resolution of a few kilometers. The optical depth above the south perennial cap is determined on a longer time range covering southern spring and summer. A constant set of optical properties of dust aerosols is consistent with OMEGA observations during the analyzed period. Strong variations of the optical depth are observed over small horizontal and temporal scales, corresponding in part to moving dust clouds. The late summer peak in dust opacity observed by Opportunity in 2005 propagated to the south pole contrarily to that observed in mid spring. This may be linked to evidence for dust scavenging by water ice-rich clouds circulating at high southern latitudes at this season.     

Yearly and seasonal variations of low albedo surfaces on Mars in the OMEGA/MEx dataset: Constraints on aerosols properties and dust deposits

The time variations of spectral properties of dark martian surface features are investigated using the OMEGA near-IR dataset. The analyzed period covers two Mars years, spanning from early 2004 to early 2008 (includes the 2007 global dust event). Radiative transfer modeling indicates that the apparent albedo variations of low to mid-latitude dark regions are consistent with those produced by the varying optical depth of atmospheric dust as measured simultaneously from the ground by the Mars Exploration Rovers. We observe only a few significant albedo changes that can be attributed to surface phenomena. They are small-scaled and located at the boundaries between bright and dark regions. We then investigate the variations of the mean particle size of aerosols using the evolution of the observed dark region spectra between 1 and 2.5 μm. Overall, we find that the observed changes in the spectral slope are consistent with a mean particle size of aerosols varying with time between 1 and 2 μm. Observations with different solar zenith angles make it possible to characterize the aerosol layer at different altitudes, revealing a decrease of the particle size of aerosols as altitude increases.     

Mean thermal and albedo behavior of the Mars surface and atmosphere over a Martian year

A Mars average data set (MADS) has been constructed from thermal and albedo measurements of the Viking Infrared Thermal Mapper; by merging information from all longitudes; and, ensuring reasonably complete longitudinal sampling, a representation of mean Mars behavior is obtained. Brightness temperatures at 7, 9, 11, 15, and 20 μm and albedo information in the band 0.3–3.0 μm have been binned using 2° latitude strips, 24 times of day, 3 emission angle intervals, and 23 nonoverlapping L_s periods covering 1.43 Mars years starting at L_s = 84°. The MADS is ideally suited to parametric study of latitudinal, diurnal, angular, and seasonal dependences. Data are presented for surface thermal and albedo behavior in clear and dusty atmospheric conditions; the thermal response of the atmospheric temperature to a major dust storm is found to be consistent with Mariner 9 data from the 1971 storm. Examples of use of the MADS, which is available through the Mars Consortium, indicate how averaged data reveal specific surface and atmospheric phenomena.     

Albedo and photometric study of Mars with the Planetary Fourier Spectrometer on-board the Mars Express mission

The Short Wavelength Channel of the Planetary Fourier Spectrometer (PFS) covers the 8333-1750 cm⁻¹ (1.2-5.7 μm) spectral range, that is well suited to study the reflectance properties of the martian soil. These properties vary with time due to the dust dynamics in the martian environment. Wind can blow off dust exposing soil and fresh rocks and can support grain mobility inducing local dust settling. We have analyzed PFS data from January 2004 to April 2005. A detailed photometric study of the radiance acquired from the planet has been performed in order to compare correctly measurements obtained at different viewing geometries and to produce a mosaic image of the planet. The results show good agreement with data from the Thermal Emission Spectrometer (on-board NASA Mars Global Surveyor orbiter), although some variations are observed. Some albedo changes could be due to small to medium scale dust storms. A very accurate estimation of the limb-darkening parameter has been computed from the analyzed data. The obtained values are compared with a surface roughness and a thermal inertia map in order to assess the relation between the limb-darkening parameter and the physical properties of surface.     

An improvement to the volcano-scan algorithm for atmospheric correction of CRISM and OMEGA spectral data

The observations of Mars by the CRISM and OMEGA hyperspectral imaging spectrometers require correction for photometric, atmospheric and thermal effects prior to the interpretation of possible mineralogical features in the spectra. Here, we report on a simple, yet non-trivial, adaptation to the commonly-used volcano-scan correction technique for atmospheric CO₂, which allows for the improved detection of minerals with intrinsic absorption bands at wavelengths between 1.9 and 2.1 μm. This volcano-scan technique removes the absorption bands of CO₂ by ensuring that the Lambert albedo is the same at two wavelengths: 1.890 and 2.011 μm, with the first wavelength outside the CO₂ gas bands and the second wavelength deep inside the CO₂ gas bands. Our adaptation to the volcano-scan technique moves the first wavelength from 1.890 μm to be instead within the gas bands at 1.980 μm, and for CRISM data, our adaptation shifts the second wavelength slightly, to 2.007 μm. We also report on our efforts to account for a slight ∼0.001 μm shift in wavelengths due to thermal effects in the CRISM instrument.     

High spectral resolution UV to near-IR observations of Mars using HST/STIS

We acquired high spectral and spatial resolution hyperspectral imaging spectrometer observations of Mars from near-UV to near-IR wavelengths (∼300 to 1020 nm) using the STIS instrument on the Hubble Space Telescope during the 1999, 2001, and 2003 oppositions. The data sets have been calibrated to radiance factor (I/F) and map-projected for comparison to each other and to other Mars remote sensing measurements. We searched for and (where detected) mapped a variety of iron-bearing mineral signatures within the data. The strong and smooth increase in I/F from the near-UV to the visible that gives Mars its distinctive reddish color indicates that poorly crystalline ferric oxides dominate the spectral properties of the high albedo regions (as well as many intermediate and low albedo regions), a result consistent with previous remote sensing studies of Mars at these wavelengths. In the near-IR, low albedo regions with a negative spectral slope and/or a distinctive ∼900 nm absorption feature are consistent with, but not unique indicators of, the presence of high-Ca pyroxene or possibly olivine. Mixed ferric-ferrous minerals could also be responsible for the ∼900 nm feature, especially in higher albedo regions with a stronger visible spectral slope. We searched for the presence of several known diagnostic absorption features from the hydrated ferric sulfate mineral jarosite, but did not find any unique evidence for its occurrence at the spatial scale of our observations. We identified a UV contrast reversal in some dark region spectra: at wavelengths shorter than about 340 nm these regions are actually brighter than classical bright regions. This contrast reversal may be indicative of extremely “clean” low albedo surfaces having very little ferric dust contamination. Ratios between the same regions observed during the planet-encircling dust storm of 2001 and during much clearer atmospheric conditions in 2003 provide a good direct estimate of the UV to visible spectral characteristics of airborne dust aerosols. These HST observations can help support the calibration of current and future Mars orbital UV to near-IR spectrometers, and they also provide a dramatic demonstration that even at the highest spatial resolution possible to achieve from the Earth, spectral variations on Mars at these wavelengths are subtle at best.     

Complex refractive index of Martian dust: Wavelength dependence and composition

Mariner 9 ultraviolet spectra of the 1971 dust clouds were analyzed to obtain the phase function times single scattering albedo of the dust particles. The phase functions were matched with Mie scattering calculations for size distributions of spheres of homogeneous and isotropic material. The absorption index of the dust particles was found to increase with decreasing wavelenght from 350 nm down to about 210 nm, and drop off shortward of this wavelength. A structural shoulder occurs in the absorption spectrum between 240 and 250 nm. Titanium dioxide (anatase) has the correct transitions at 210 and 250 nm to match the absorption curve of Martian dust, and is proposed as a candidate constituent in Martian surface material. The spectral neutrality of TiO₂ between 0.5 and 4 μm is consistent with visible and infrared observations of Mars. The high refractive index of TiO₂ can explain the large refringence of Martian dust. The titanium dioxide content of the dust particles is estimated to be a few percent or less. Uncertainties in the results due to limitations in the model and data are discussed.     

Titan's surface: Search for spectral diversity and composition using the Cassini VIMS investigation

The surface composition of Titan is of great importance for understanding both the internal evolution of Titan and its atmosphere. The Visual and Infrared Mapping Spectrometer (VIMS) investigation on Cassini is observing Titan from 0.35 to 5.11 μm with spatial resolution down to a few kilometers during each flyby of the spacecraft as it orbits Saturn. Our search for spectral diversity using seven methane transmission windows in the near infrared suggests that spectrally distinct units exist on the surface of Titan and that most of the surface can be modeled using only a few distinct spectral units: water frost, CO₂ frost, atmospheric scattering, and an unknown material bright at 2 μm. A dark, spectrally neutral material is also implied. Use of an atmospheric scattering component with spectral mixing analysis may provide a method for partially removing atmospheric effects. In some locations, atmospheric scattering accounts for the majority of the signal. There are also small regions with unusual spectra that may be due to low signal and high noise and/or may be exotic materials of interest. Further, we searched within the methane windows for spectral features associated with Titan's surface. Only the 5-μm and, to a lesser extent, the 2-μm window provide a reasonable opportunity for this, as the shorter-wavelength windows are too narrow and the 2.8-μm window is cluttered with an unknown atmospheric constituent. We find evidence for only one spectral feature: near 4.92 μm for the 5-μm bright Tui Regio region. CO₂ frost with grains smaller than about 10 μm is the best candidate we have found so far to explain this absorption as well as the feature's spectral contrast between the 2.7- and the 2.8-μm atmosphere subwindows. This suggested CO₂ identification is supported by the presence of an endmember in the spectral mixture analysis that is consistent with CO₂ frost with large grain sizes. We find no other absorption features that are statistically significant, including those reported earlier by others. These results are consistent with but greatly extend our early analysis that treated only the Ta data set [McCord, T.B., et al., 2006a. Planet. Space Sci. 54, 1524-1539]. In the spectral feature search process, we explored in detail the noise characteristics of the VIMS data within the 5-μm window, which has generally very low signal (4-20 DN), due to the measurement conditions and low illumination levels. We find noise of nearly Gaussian statistics except for some erratic darks and noise spikes, and the data set seems generally well behaved. We present examples of our attempt to improve on the standard VIMS pipeline data calibration.     

Quantitative compositional analysis of martian mafic regions using the MEx/OMEGA reflectance data 1. Methodology, uncertainties and examples of application

The Mars Express Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) collected an unprecedented visible and near-infrared hyperspectral dataset covering the low albedo regions of Mars. We investigate the ability to infer modal abundance of surfaces of these regions from a radiative transfer model developed by Shkuratov et al. [Shkuratov, Y., Starukhina, L., Hoffmann, H., Arnold, G., 1999. Icarus 137, 235-246] and adapted to basaltic surfaces by Poulet and Erard [Poulet F., Erard, S., 2004. J. Geophys. Res. 109 (E2), doi:10.1029/2003JE002179]. From OMEGA measurements of mafic surfaces, we develop several sensitivity tests to assess the extent to which the model can be applied to predict pyroxene composition (high-calcium phase and low-calcium phase), abundance of almost neutral components (plagioclase) in the near-infrared wavelength as well as grain sizes, by using a library of selected end-members. Results of the sensitivity tests indicate that the scattering model can estimate both abundances and grain sizes of major basaltic materials of low albedo regions within uncertainties (±5 to 15 vol%). The model is then applied to data from dissected cratered terrains located in Terra Meridiani. The derived grain size including uncertainties is in the 50-500 μm range. This is consistent with the thermal inertia and albedo of this region, which indicates a fine sand-sized surface with little dust. The abundances of plagioclase (43-57%) and pyroxenes (35-45±10%, including 11±5% of low-calcium phase) are in good agreement with previous basalt-like compositions of low albedo regions from thermal infrared spectral measurements. The method presented in this paper will provide a valuable tool for evaluating the modal mineralogy of other mafic regions of Mars observed in the near-infrared wavelength range.     

Photometric imaging of Mars during the 1973 opposition

Images of Mars were obtained at 20 wavelengths between 0.35 and 1.10 μm during the 1973 opposition using the MIT MKIII silicon vidicon imaging system at the 88-in. telescope of the Mauna Kea Observatory, Hawaii. The observations covered a 3-hr interval on Day 2 of the 1973 Solis Lacus dust storm. At blue wavelengths two huge crescent-shaped anomalous brightenings surrounded the central dust storm in the early morning but faded during the morning; they were probably deposits of H₂O frost or low level haze that was degassed from the regolith in Solis Lacus. The north polar hood, which was nonsymmetrically oriented about the north pole, faded and receded during the observing period. Regions of known dust storm activity could be clearly distinguished, and the images revealed dust cloud activity in Chryse as well. Relative reflectance spectra generated from the images showed compositional variations within the Erythraeum M. dark area complex: Margaritifer S. and Eastern Erythraeum M. have similar ferrosilicate mineralogy, but have stronger ferrosilicate absorption bands than Pyrrhae R. and Eos. Western Erythraeum M. and northern Eos were contaminated by high albedo dust. Reflectance spectra obtained from orbit around Mars could provide higher spatial resolution and more complete coverage of the dark areas, yielding an opportunity to correlate geologic features with crustal petrology. An orbiting spectrometer could also provide spectra in the compositionally diagnostic wavelength range 1–4 μm, where the Earth's atmosphere contains intense absorption bands.     

Modelling of atmospheric dust extinction and surface reflectance of Mars applying a radiative transfer simulation in the 2.0 and 2.7 μm CO2 bands

Infrared radiation spectra of Mars which can be measured by an orbiting Planetary Fourier Spectrometer (PFS) have been simulated in the spectral region from 1 to 50 μm. The radiative transfer simulation technique considers absorption, emission and multiple scattering by molecular (CO₂, H₂O, CO) and particulate (palagonite) species. It is shown that the contribution from atmospheric dust extinction and surface reflectance can be separated in the region of the CO₂ bands at 2.0 and 2.7 μm. Quantitative results of simultaneous surface pressure, reflectance and aerosol optical depth retrievals are discussed.     

On the nature of the blue and ultraviolet absorption in the Jovian atmosphere

Spatially resolved reflectivities from 3000 to 6600 Å of three positions from the center to the limb of the Jovian Equator, North Equatorial Belt, and North Tropical Zone are analyzed to determine the vertical distribution and wavelength dependence of various sources of blue and uv absorption. Six different models of the distribution of absorbing dust particles are examined. In each model, the variation of dust optical depth and cloud single-scattering albedo are determined. Only those models having dust above the upper NH₃ cloud layer will fit the data. The high altitude dust distribution is approximately uniform over the three regions examined. The contrast in reflectivity of the belts and zones may be modeled by a different cloud single-scattering albedo in the different regions.     

Ultraviolet observations of Mars and Saturn by the TDIA and OAO-2 satellites

Ultraviolet photometric and spectrophotometric observations of Mars and Saturn obtained by two Earth-orbiting satellites are combined in this report. High-resolution data from the S59 experiment aboard TD1A reveal no definite absorption features in the spectra of either planet. The absence of a prominent absorption in the Mars data near 2150 Å can be reconciled with the preliminary Viking measurement of NO only if that gas is preferentially concentrated at high Martian altitudes. Broadband photometry from OAO-2 shows that atmospheric dust on Mars during the great dust storm of 1971–1972 reduced the ultraviolet geometric albedo by a factor of ?3 at the height of the storm. This atmospheric energy deposition is probably an important mechanism in the storm dynamics. Diurnal variation in the ultraviolet brightness of Mars appears to be marginally detectable during the dust storm. A real brightness variation during a clear season is observed. The combined Saturn data from the two satellites strongly suggest that NH₃ does not influence the ultraviolet spectrum of Saturn, but that some other absorber does. A candidate for such an absorber, H₂S, is investigated. OAO-2 broadband photometry of Jupiter and of Saturn demonstrate that these planets have very similar albedos from 2100 to 2500 Å. This implies a common ultraviolet absorber on both planets, other than NH₃.     

Multispectral imaging observations of Neptune’s cloud structure with Gemini-North

Observations of Neptune were made in September 2009 with the Gemini-North Telescope in Hawaii, using the NIFS instrument in the H-band covering the wavelength range 1.477–1.803 μm. Observations were acquired in adaptive optics mode and have a spatial resolution of approximately 0.15–0.25″.The observations were analysed with a multiple-scattering retrieval algorithm to determine the opacity of clouds at different levels in Neptune’s atmosphere. We find that the observed spectra at all locations are very well fit with a model that has two thin cloud layers, one at a pressure level of ∼2 bar all over the planet and an upper cloud whose pressure level varies from 0.02 to 0.08 bar in the bright mid-latitude region at 20–40°S to as deep as 0.2 bar near the equator. The opacity of the upper cloud is found to vary greatly with position, but the opacity of the lower cloud deck appears remarkably uniform, except for localised bright spots near 60°S and a possible slight clearing near the equator.A limb-darkening analysis of the observations suggests that the single-scattering albedo of the upper cloud particles varies from ∼0.4 in regions of low overall albedo to close to 1.0 in bright regions, while the lower cloud is consistent with particles that have a single-scattering albedo of ∼0.75 at this wavelength, similar to the value determined for the main cloud deck in Uranus’ atmosphere. The Henyey-Greenstein scattering particle asymmetry of particles in the upper cloud deck are found to be in the range g ∼ 0.6–0.7 (i.e. reasonably strongly forward scattering).Numerous bright clouds are seen near Neptune’s south pole at a range of pressure levels and at latitudes between 60 and 70°S. Discrete clouds were seen at the pressure level of the main cloud deck (∼2 bar) at 60°S on three of the six nights observed. Assuming they are the same feature we estimate the rotation rate at this latitude and pressure to be 13.2 ± 0.1 h. However, the observations are not entirely consistent with a single non-evolving cloud feature, which suggests that the cloud opacity or albedo may vary very rapidly at this level at a rate not seen in any other giant-planet atmosphere.     

Effect of atmospheric dust loading on martian albedo measurements

This work is devoted to the analysis of the variation of albedo measured by orbiting instruments with atmospheric opacity on Mars. The study has been conduced by analysing Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) data from martian regions with different surface albedo.In support of these data, synthetic spectra with different surface albedo and atmospheric opacities have been computed, so that a comparison has been performed. The synthetic spectra have been retrieved by using two different grain sizes for suspended dust (0.5 and 1.2 μm), allowing a comparison between the two models and the observations.Using the DCI, a parameter describing the quantity of dust deposited on the surface, the effectiveness of the single scattering approximation has been tested for low atmospheric opacity by analysing the quality of the linear fit up to different atmospheric opacity.For more opaque conditions two kinds of fits have been applied to the data, linear and second-order degree polynomial. In this case, we found that the polynomial fit better describes the observations.The analysis of these data made it possible to notice a peculiar trend, already reported by Christensen (1988), of the albedo over Syrtis Major after the occurrence of dust storms, but, differently from that work, now the study of DCI together with atmospheric opacity and albedo allowed us to robustly confirm the hypothesis made by Christensen.Finally, the comparison between observations and synthetic spectra computed with models with different particles grain sizes indicates that dust particles of 0.5 μm diameter are the most effective to change the aerosol atmospheric opacity on Mars.     

The effects and characteristics of atmospheric dust during martian global dust storm 2001A

We present retrieved trends in dust optical depth, dust effective radius and surface temperature from our analysis of Mars Global Surveyor Thermal Emission Spectrometer daytime data from global dust storm 2001A, and describe their significance for the martian dust cycle. The dust optical depth becomes correlated with surface pressure during southern spring and summer in years both with and without a global dust storm, indicating that global dust mixing processes are important at those seasons. The correlation is low at other times of the year. We found that the observed decay of optical depths at the later stages of the dust storm match, to first-order, theoretical values of clearing from Stokes–Cunningham fallout of the dust. Zonally averaged effective radius is constant within standard deviation of results (between 1.2 and 2.0 μm, with a global mean for all seasons of 1.7 μm), at all latitudes and seasons except at southern latitudes of 35° and higher around equinoxes in both martian years, where it is larger than average (2–3 μm). The emergence and disappearance of these larger particles correlates with observations of polar cap edge storms at those latitudes. Northern latitude observations under similar conditions did not yield a similar trend of larger average effective radii during the equinoxes. We also report on a linear correlation between daytime surface temperature drop and rise in optical depth during the global dust storm. Global dust storm 2001A produced a significant optical depth and surface temperature change.     

Spectral properties of simulated impact glasses produced from martian soil analogue JSC Mars-1

To simulate the formation of impact glasses on Mars, an analogue of martian bright soil (altered volcanic soil JSC Mars-1) was melted at relevant oxygen fugacities using a pulsed laser and a resistance furnace. Reduction of Fe³⁺ to Fe²⁺ and in some cases formation of nanophase Fe⁰ in the glasses were documented by Mössbauer spectroscopy and TEM studies. Reflectance spectra for several size fractions of the JSC Mars-1 sample and the glasses were acquired between 0.3 and 25 μm. The glasses produced from the JSC Mars-1 soil show significant spectral variability depending on the method of production and the cooling rate. In general, they are dark and less red in the visible compared to the original JSC Mars-1 soil. Their spectra do not have absorption bands due to bound water and structural OH, have positive spectral slopes in the near-infrared range, and show two broad bands centered near 1.05 and 1.9 μm, typical of glasses rich in ferrous iron. The latter bands and low albedo partly mimic the spectral properties of martian dark regions, and may easily be confused with mafic materials containing olivine and low-Ca pyroxene. Due to their disordered structures and vesicular textures, the glasses show relatively weak absorption features from the visible to the thermal infrared. These weak absorption bands may be masked by the stronger bands of mafic minerals. Positive near-infrared spectral slopes typical of fresh iron-bearing impact or volcanic glasses may be masked either by oxide/dust coatings or by aerosols in the Mars' atmosphere. As a result, impact glasses may be present on the surface of Mars in significant quantities that have been either misidentified as other phases or masked by phases with stronger infrared features. Spectrometers with sufficient spatial resolution and wavelength coverage may detect impact glasses at certain locations, e.g., in the vicinity of fresh impact craters. Such dark materials are usually interpreted as accumulations of mafic volcanic sand, but the possibility of an impact melt origin of such materials also should be considered. In addition, our data suggest that high contents of feldspars or zeolites are not necessary to produce the transparency feature at 12.1 μm typical of martian dust spectra.     

A simulation of the OMEGA/Mars Express observations: Analysis of the atmospheric contribution

Spectral images of Mars obtained by the Mars Express/OMEGA experiment in the near infrared are the result of a complex combination of atmospheric, aerosol and ground features. Retrieving the atmospheric information from the data is important, not only to decorrelate mineralogical against atmospheric features, but also to retrieve the atmospheric variability. Once the illumination conditions have been taken into account, the main source of variation on the CO₂ absorption is due to the altitude of the surface, which governs atmospheric pressure variation by more than an order of magnitude between the summit of Olympus Mons down to the bottom of Valles Marineris. In this article we present a simplified atmospheric spectral model without scattering, specially developed for the OMEGA observations, which is used to retrieve the local topography through the analysis of the CO₂ band. OMEGA atmospheric observations increase the horizontal resolution compared to MOLA altimetry measurements, and therefore complement the mineralogical studies from the same instrument. Finally, residual variations of the pressure can be related to atmospheric structure variation.     

Mineralogy of the Phoenix landing site from OMEGA observations and how that relates to in situ Phoenix measurements

We present an analysis comparing observations acquired by the Mars Express Observatoire pour la Minéralogie l’Eau, les Glaces et l’Activité (OMEGA) and Phoenix lander measurements. Analysis of OMEGA data provides evidence for hydrous and ferric phases at the Phoenix landing site and the surrounding regions. The 3 μm hydration band deepens with increasing latitude, along with the appearance and deepening of a 1.9 μm H₂O band as latitude increases ∼60° polewards. A water content of 10-11% is derived from the OMEGA data for the optical surface at the Phoenix landing site compared to 1-2% derived for subsurface soil by Phoenix lander measurements. The hydration of these regions is best explained by surface adsorbed water onto soil grains. No evidence for carbonate or perchlorate-bearing phases is evident from OMEGA data, consistent with the relatively small abundances of these phases detected by Phoenix. The identification of spectral features consistent with hydrated phases (possibly zeolites) from OMEGA data covering regions outside the landing site and the ubiquitous ferric absorption edge suggest that chemical weathering may play a role in the arctic soils.     

Radiance factor calibration of near-infrared spectral images of Mars

We present radiometrically calibrated spectrophotometric images of Mars taken at the NASA Infrared Telescope Facility (IRTF) near and during the 1995 and 1999 oppositions. Absolute intensity and radiance factor (r_F = I/F) values have been calculated for approximately 95% of the surface over all longitudes between −70° to 90° latitude in the 1.5- to 4.1-μm spectral region at a spectral resolution (Δλ/λ) of 1.5%. Values of radiance factor range from r_F = 0.4 to 0.6 at 2.2 μm for the bright regions such as Moab and Arabia to r_F = 0.12 to 0.3 at 2.2 μm around the dark regions Syrtis Major and Acidalia Planitia. Variations are seen due to seasonal dust and/or condensate cloud cover and viewing geometry. Our results are generally consistent with the few reported previous radiance factor determinations for Mars. These data are unique among ground-based data in their relatively high spatial resolution (?200 km/pixel at the sub-Earth point) and coverage combined with their spectral resolution and coverage. These radiometrically calibrated observations can be used as input to studies focusing on spectral unmixing of surface and atmospheric components, radiative transfer modeling of disk and limb radiances, and photometric modeling of the martian phase function.