Surface roughness and structure

The first radar echoes from Mars were obtained in 1963 and the technique has been used during most subsequent oppositions (Simpson et al., 1992). The round-trip echo flight time provides information on the distance between Earth and Mars and topographic variations on the martian surface. The dispersion of the returned echo provides an estimate of surface roughness and the strength of the echo constrains the reflectivity of the surface. The polarization of the returned signal allows estimation of small-scale surface structure. Thus, elevations, slopes, textures, and material properties can be obtained through radar observations.

Ground-based radar observations revealed the diversity of terrains on Mars, including the icy nature of the polar caps, large regions covered by dust mantles, rough lava flows, and indications that lava flows have infilled at least some channels (Simpson et al., 1992; Harmon et al., 1999). A region largely corresponding to the

Medusae Fossae Formation southwest of Tharsis displays almost no radar return (Muhleman et al., 1995; Harmon et al., 1999). Radar data of this "Stealth" region provide evidence that this region is covered by thick layers of fine-grained material (Edgett et al., 1997). Radar data have been used to constrain surface material properties at proposed landing sites and complement the information obtained from orbiting spacecraft on surface roughness, rock abundances, and dust coverings (Section 4.4.2) (e.g., Golombek et al., 2003). These predictions are generally consistent with the actual conditions encountered at the landing sites (e.g., Golombek etal., 1999b, 2005).

Decimeter-scale roughness variations are obtained from ground-based radar (Harmon et al., 1999), and are complemented by kilometer-scale surface roughness estimates from MOLA (Figure 4.3). MOLA reveals that the southern highlands are rougher than the northern plains, largely resulting from the southern hemisphere' s heavily cratered nature. The smoothness of the northern plains reflects the sedimentary mantle which overlies this region (Kreslavsky and Head, 2000).

Details about the subsurface properties and structure to a depth of 3-5km are being revealed by the MARSIS ground-penetrating radar (Picardi et al., 2005; Watters et al., 2006). Attenuation of the radar return provides constraints on the dielectric constant of the subsurface materials, which allows mapping of compositional variations. Preliminary results from the north polar cap are consistent with H2O-ice comprising the summer (remnant) cap and much of the polar layered deposits. MARSIS has revealed the thickness of these polar deposits with a sharp contrast in radar reflectivity corresponding to the base of the layer near 1.8 km (Figure 4.4a). Smaller variations in subsurface properties and compositions are being revealed by MRO's higher-resolution SHARAD ground-penetrating radar (Figure 4.4b) (Seu et al., 2004).

4.3 Crustal composition

The compositional diversity of the martian crust has been revealed through remote sensing observations, analysis of martian meteorites, and in situ investigations. Most of the compositional information has been obtained through various types of spectroscopic analyses.

4.3.1 Methods of compositional analysis

Remote sensing data include observations from Earth-based telescopes, Hubble Space Telescope (HST), and Mars orbiting spacecraft. All of these techniques utilize reflection spectroscopy to determine compositional information. Sunlight consists of a blackbody spectrum modified with absorption lines created by gases in the Sun's

Point-to-Point Median Slope Roughness (deg), 35 km window

Figure 4.3 MOLA analysis provides estimates of the roughness of the martian surface. Lhe southern highlands are rougher than the northern plains. (Image PIA02808, NASA/GSFC.) See also color plate.

Figure 4.4 The ground-penetrating radars on MEx and MRO provide information on compositional variations within the near-surface. (a) This MEx MARSIS profile extends from the northern plains (left) to the north polar cap (right). The split in the radar return on the right demonstrates the thickness of the polar cap. (ASI/ESA/ University of Rome.) (b) MRO's Shallow Radar (SHARAD) reveals fine layering within the south polar layered deposits. This profile is 650km long and the arrow length corresponds to an 800 m thick segment. The bottom of the arrow indicates the base of the polar layered deposits. (NASA/JPL-Caltech/ASI/University of Rome/Washington University in St. Louis.)

Figure 4.4 The ground-penetrating radars on MEx and MRO provide information on compositional variations within the near-surface. (a) This MEx MARSIS profile extends from the northern plains (left) to the north polar cap (right). The split in the radar return on the right demonstrates the thickness of the polar cap. (ASI/ESA/ University of Rome.) (b) MRO's Shallow Radar (SHARAD) reveals fine layering within the south polar layered deposits. This profile is 650km long and the arrow length corresponds to an 800 m thick segment. The bottom of the arrow indicates the base of the polar layered deposits. (NASA/JPL-Caltech/ASI/University of Rome/Washington University in St. Louis.)

photosphere. As sunlight is reflected off of a planetary surface, minerals comprising its surface will absorb additional wavelengths of energy due to vibrational motions within the crystal lattices (Christensen et al., 2001a). The wavelengths of these absorption lines are diagnostic of the minerals doing the absorbing. Removing the solar spectrum and atmospheric absorptions from the reflected light provides us with the absorptions due to the planet, which, when compared with laboratory spectra, allow us to constrain the mineral composition of the surface (Figure 4.5). Many of the important mineral and atmospheric absorptions occur in the infrared (IR) (Hanel et al., 2003), where the energy has frequencies between 3 X1011 Hz to 3 X1014 Hz, or wavelengths between 1 pm and 1 mm. Infrared is subdivided into near-IR (~0.7 to 5 pm), mid-IR (5 to ~30 pm), and far-IR (~30 to 350 pm). Infrared energy between 5 and 100 pm results from the body's heat and is therefore called thermal IR. Infrared observations are often reported in wavenumbers rather than wavelengths or frequencies. Wavenumber is the inverse of wavelength and its units in planetary studies are usually cm-1. Thus IR energy with a wavelength of 10 pm corresponds to a wavenumber of 103 cm-1.

Earth-based IR observations are limited to the near-IR because our atmosphere absorbs the longer wavelengths. The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument on Hubble Space Telescope (HST) also is limited to the near-IR, observing in the 0.8 to 2.5 pm region. Mars-orbiting spacecraft obtain observations over a wider range of wavelengths and produce

Figure 4.5 Laboratory spectra of several geologically important minerals are shown in this image. Differences in band shape and the wavelengths of specific absorptions distinguish one mineral from another and allow mineralogical analysis from remote sensing platforms. (Reprinted by permission of the American Geophysical Union, Christensen et al. [2000b], Copyright 2000.)

Figure 4.5 Laboratory spectra of several geologically important minerals are shown in this image. Differences in band shape and the wavelengths of specific absorptions distinguish one mineral from another and allow mineralogical analysis from remote sensing platforms. (Reprinted by permission of the American Geophysical Union, Christensen et al. [2000b], Copyright 2000.)

higher-resolution data due to their proximity to Mars. Table 4.1 lists the wavelength ranges covered by spectrometers on several of the Mars missions. Many of these spectrometers observe only specific bands within the wavelength range, selected to enhance identification of minerals which are of particular interest. For example, THEMIS investigates the martian surface using IR bands centered at 6.78, 7.93, 8.56, 9.35, 10.21, 11.04, 11.79, 12.57, and 14.88 |im (Christensen et al., 2004a). These bands were selected because absorptions from various silicates (including feldspars and pyroxenes), salts, and carbonates are located near these wavelengths.

Reflection spectroscopy is the major technique utilized in remote sensing observations (Clark and Roush, 1984), but it is not the only way to obtain compositional information. MO's GRS system consists of a gamma-ray spectrometer (GRS), a neutron spectrometer (NS), and a high-energy neutron detector (HEND) (Boynton et al., 2004). GRS detects gamma rays emitted through decay of radioactive elements such as potassium (K), uranium (U), and thorium (Th) as well as gamma rays induced by cosmic rays interacting with non-radioactive elements such as chlorine

Table 4.1 Wavelengths of observations by selected spacecraft missions

Mission/ins trument

Wavelengths

Mariner 6/7

UV Spectrometer IR Spectrometer Mariner 9

UV Spectrometer IRIS (Infrared Interferometer Spectrometer) Viking 1/2 Orbiter

IRTM Phobos 2 ISM KRFM Thermoscan Visible Infrared Mars Global Surveyor TES

Spectrometer Bolometer Albedo Mars Odyssey THEMIS Infrared

Visible Mars Express OMEGA Near-IR Visible PFS

SPICAM Ultraviolet Infrared Mars Reconnaissance Orbiter

CRISM (Compact Reconnaissance Imaging Spectrometer for Mars)

6.62, 7.78, 8.56, 9.30, 10.11, 11.03, 11.78, 12.58, 14.96 |m 423, 553, 652, 751, 870 nm

(Cl), iron (Fe), and carbon (C). NS and HEND detect thermal (<0.4 eV), epithermal (0.4 eV to 0.7 MeV), and fast (0.7-1.6 MeV) neutrons emitted through cosmic ray interactions with surface minerals. These neutrons can pass through the surface largely unaffected or can be absorbed, depending on surface composition. Comparison of thermal, epithermal, and fast neutron fluxes emitted from the surface provides constraints on the composition within ~ 1m of the surface. For example, the

Energy (keV)

Figure 4.6 Data from Spirit's APXS instrument reveal emissions associated with specific elements. The elements identified in this spectrum have been determined primarily from the X-ray analysis. (Image PIA05114, NASA/JPL/Max-Planck Institute for Chemistry.)

Energy (keV)

Figure 4.6 Data from Spirit's APXS instrument reveal emissions associated with specific elements. The elements identified in this spectrum have been determined primarily from the X-ray analysis. (Image PIA05114, NASA/JPL/Max-Planck Institute for Chemistry.)

presence of hydrogen (H) in the surface, typically found in the form of H2O in terrestrial planets, will absorb epithermal neutrons but not thermal neutrons. Carbon dioxide (CO2) ice will allow both thermal and epithermal neutrons to pass. Thus the low flux of epithermal neutrons and high flux of thermal neutrons in the martian polar regions during the summer suggest the presence of H2O rather than CO2 ice.

The rovers on MPF and MER each carried an Alpha Particle X-Ray Spectrometer (APXS) to determine elemental compositions of surface rocks and soil (Rieder et al., 1997a, 2003) (Figure 4.6). The APXS bombards a surface sample with alpha particles and X-rays emitted from radioactive curium-244. The alpha particles are back-scattered by the sample atoms with energies diagnostic of the atom. X-rays, whose energies are again diagnostic of the sample atoms, are emitted by the sample as ionizations caused by the initial bombardment recombine, a process called X-ray fluorescence (the Viking landers also carried X-ray fluorescence spectrometers). The two techniques are combined because the alpha backscattering technique is superior at detecting lighter elements such as carbon and oxygen while the X-ray fluorescence technique is sensitive to heavier elements (elements heavier than sodium). Iron minerals are further investigated using the MER Mossbauer spectrometer, which bombards samples with gamma rays emitted from a cobalt-57 source and measures the amount of gamma-ray re-emission from the sample (Klingelhofer et al., 2003).

4.3.2 Crustal composition from remote sensing observations

Earth-based telescopic observations cover a limited wavelength range and are of relatively low resolution, but they provided the first insights into martian crustal

Wavelength (mm)

Figure 4.7 Reflectance spectra of the bright versus dark regions of Mars show important differences related to composition. These include variations in iron oxidation state, presence of pyroxenes, and existence of H2O and OH. (Data from US Geological Survey Spectral Laboratory, speclab.cr.usgs.gov.)

Wavelength (mm)

Figure 4.7 Reflectance spectra of the bright versus dark regions of Mars show important differences related to composition. These include variations in iron oxidation state, presence of pyroxenes, and existence of H2O and OH. (Data from US Geological Survey Spectral Laboratory, speclab.cr.usgs.gov.)

composition and allow long-term continuous monitoring of the planet (Singer, 1985; Bell et al., 1994; Erard, 2000). Figure 4.7 shows reflectance spectra of the bright and dark regions of Mars. The reflectance increase between 0.3 and 0.75 ^m is attributed to ferric iron (Fe3+) as is the shallow absorption near 0.86 ^m in the

3 i bright regions. The spectra are inconsistent with Fe occurring in crystalline form, leading to early suggestions of amorphous forms of iron oxides such as palagonite (Singer, 1985). The peak at 0.75 ^m in the dark region spectrum is indicative of ferrous iron (Fe2+), indicating differences in oxidation among the two regions. Mafic (iron- and magnesium-rich) minerals such as pyroxenes are suggested by the absorptions near 1 ^m. Absorptions at 1.4,1.9, and 3.0 ^m are diagnostic of OH and H2O, suggesting the presence of clays. The reddish color of the dark regions is consistent with mafic materials covered with thin alteration coatings.

Hubble Space Telescope observations provide improved spatial resolution over ground-based investigations but are limited in temporal coverage (Bell et al., 1997; Noe Dobrea et al., 2003). Color-ratioing of HST data reveals regional variations in composition, such as higher and/or fresher pyroxene concentrations in Syrtis Major compared to Acidalia and Utopia Planitiae (Bell et al., 1997) and hemispheric differences in the distribution of hydrated minerals (Noe Dobrea et al., 2003).

Orbiting spacecraft provide the highest resolution compositional information for Mars. Early spacecraft, particularly the Phobos 2 mission, gave some insights into surface compositional variations, but the first detailed mineralogic survey of the entire martian surface was performed by MGS's TES, which had a nadir surface footprint

Aram Chaos

Meridiani Planum

Figure 4.8 TES observations revealed a spectral signature indicative of coarsegrained crystalline hematite in the Meridiani Planum and Aram Chaos regions. The large hematite exposure in Meridiani led to the selection of this region for Opportunity's landing site. (Hematite mineralogy map courtesy of ASU/TES team.)

size of 3.15km at the nominal orbital altitude of 378km (Christensen et al, 2001a). TES spectra are dominated by volcanic materials, particularly basalt (Christensen etal, 2000a), consistent with Earth- and HST-based observations. Bright regions tend to be dust-covered, providing sparse information about the composition of the underlying bedrock (Bandfield, 2002). The dark regions comprise two distinct mineralogies with the boundary between these two surface types approximating the hemispheric dichotomy (Bandfield et al., 2000). The type 1 surface unit dominates in the dark regions of the southern hemisphere and is apparently a plagioclase-and clinopyroxene-rich basalt (Bandfield et al., 2000; Mustard and Cooper, 2005). Surface type 2 covers the dark regions of the northern hemisphere and has been interpreted as unaltered basaltic andesite or andesite (Banfield et al., 2000) or a weathered basalt (Wyatt and McSween, 2002; McSween et al., 2003). TES revealed outcrops of crystalline gray hematite (a-Fe2O3) in Terra Meridiani, Aram Chaos, and scattered through Valles Marineris (Figure 4.8), which were interpreted as a chemical precipitate from Fe-enriched aqueous fluids (Christensen et al., 2001b). TES also detected regional concentrations of olivine (Hoefen et al., 2003) and orthopyroxenite (Hamilton et al., 2003). Carbonates, which might be expected from interactions of the CO2-rich atmosphere with past surface water, were not detected nor were sulfates identified (Christensen et al., 2001a; Bandfield, 2002).

MO THEMIS and MEx OMEGA have expanded upon the TES results with higher spectral and spatial resolutions (100m for THEMIS, 0.3-5km for OMEGA) and find greater mineralogic diversity across the surface (Bibring et al., 2005, 2006; Christensen et al., 2005). Olivine-rich basalts occur in widely separated regions, including Nili Patera, Ganges Chasma, and Ares Vallis (Christensen et al., 2005; Mustard et al., 2005) and outcrops of olivine are associated with some impact

Figure 4.9 OMEGA observations have revealed localized outcrops of a variety of minerals. This view of Marwth Vallis shows outcrops (dark regions) of hydrated minerals. (Image SEMDVZULWFE, ESA/OMEGA/HRSC.)

craters and basins (Mustard etal., 2005). Quartz- and plagioclase-rich granitoid rocks are exposed in the floors and central peaks of a few impact craters (Bandfield et al., 2004). While much of the martian surface is composed of basalt, highly evolved high-silica (dacite) lavas have been identified in small regions such as Nili Patera in Syrtis Major (Christensen et al., 2005). High-calcium clinopyroxene dominates in low-albedo volcanic regions, dark sand, and crater ejecta while low-calcium orthopyr-oxene is found in moderate to bright outcrops within the ancient terrain (Mustard et al., 2005). Hydrated minerals (clays), particularly phyllosilicates, are exposed in many of the older terrain units (Bibring et al., 2005; Poulet et al., 2005) (Figure 4.9). Carbonates continue to avoid detection (Bibring et al., 2005), but hydrated sulfates have been detected in the north polar region (Langevin et al., 2005a) and within layered deposits of probable sedimentary origin (Gendrin et al., 2005).

Bibring et al. (2006) noted an age correlation for specific mineralogies and suggested a mineralogy-based evolutionary sequence for Mars. Their earliest period (corresponding to early to middle Noachian (Section 5.2)) experienced large amounts of aqueous alteration, producing the phyllosilicates observed in the ancient terrains. Bibring et al. (2006) call this the "phyllosian period," in reference to the phyllosilicates. The phyllosian period was followed by the "theiikian period," characterized by acidic aqueous alteration processes which produced the sulfur deposits found in localized regions of Mars. Bibring et al. (2006) propose that this evolutionary change was produced by increased volcanic activity near the end of the Noachian period. The theiikian period extended from the late Noachian through the early Hesperian, when it transitioned into the "siderikian period." This latter period extends to the present day and is characterized by lack of liquid water on the global scale and for long time periods, as indicated by the presence of ferric oxides.

Figure 3.5 Gravity anomaly map, as measured from the vertical accelerations of MGS. A strong positive gravity anomaly is seen between 240°E and 300°E longitude which corresponds to the Tharsis volcanic province, while a strong negative anomaly in the equatorial region between 300°E and 0°E correlates with Valles Marineris. However, many topographic features do not correlate with gravity anomalies. (Image PIA02054, NASA/Goddard Space Flight Center (GSFC).)

Figure 3.5 Gravity anomaly map, as measured from the vertical accelerations of MGS. A strong positive gravity anomaly is seen between 240°E and 300°E longitude which corresponds to the Tharsis volcanic province, while a strong negative anomaly in the equatorial region between 300°E and 0°E correlates with Valles Marineris. However, many topographic features do not correlate with gravity anomalies. (Image PIA02054, NASA/Goddard Space Flight Center (GSFC).)

Figure 3.6 MOLA revealed the large variations in topography visible across the martian surface. Topography ranges from a high at the summit of Olympus Mons to a low in the bottom of the Hellas Basin. (Image PIA02820, NASA/JPL/GSFC/MGS MOLA team.)

ARCADIA

HELLAS;

ELY SI

West Longitude

Figure 3.8 MGS's MAG/ER experiment revealed remnant magnetization within ancient rocks across the martian surface. The areas of the strongest magnetizations (red/blue colors) correspond to ancient rocks in the Terra Cimmeria and Terra Sirenum regions. Open circles indicate impact craters and filled circles are volcanoes. The dichotomy boundary is indicated by the solid line. (Image PIA02059, NASA/GSFC/MGS MAG/ER team.)

Figure 4.2 Mars displays slight variations in color, as demonstrated in this MOC image taken at LS = 211° (northern autumn/southern spring) in May 2005. The bright south polar cap is visible at the bottom of the image. The dark feature in the center is the volcanic province Syrtis Major Planum and the brighter circular feature below Syrtis Major is the Hellas impact basin. (MOC Release MOC2-1094; NASA/MSSS.)

Figure 4.2 Mars displays slight variations in color, as demonstrated in this MOC image taken at LS = 211° (northern autumn/southern spring) in May 2005. The bright south polar cap is visible at the bottom of the image. The dark feature in the center is the volcanic province Syrtis Major Planum and the brighter circular feature below Syrtis Major is the Hellas impact basin. (MOC Release MOC2-1094; NASA/MSSS.)

Point-to-Point Median Slope Roughness (deg), 35 km window

Figure 4.3 MOLA analysis provides estimates of the roughness of the martian surface. The southern highlands are rougher than the northern plains. (Image PIA02808, NASA/GSFC.)

Lower limit of water mass fraction on Mars 2% 4% 8% 16% 32% >64%

Figure 4.10 Analysis from the neutron experiments on GRS has produced information on the water mass fraction within the upper meter of the martian surface. This map shows that high water concentrations occur near the poles, which likely indicates the presence of H20 ice within the upper meter. The high H20 concentrations seen in the equatorial regions may be due to ice or hydrated minerals. (NASA/MO-GRS/Los Alamos National Laboratory.)

Figure 4.10 Analysis from the neutron experiments on GRS has produced information on the water mass fraction within the upper meter of the martian surface. This map shows that high water concentrations occur near the poles, which likely indicates the presence of H20 ice within the upper meter. The high H20 concentrations seen in the equatorial regions may be due to ice or hydrated minerals. (NASA/MO-GRS/Los Alamos National Laboratory.)

Figure 4.22 Thermal inertia provides information about particle sizes across Mars. This map, derived from TES data, shows the variation in thermal inertia across the planet. Areas of low thermal inertia, such as Tharsis, are dust-covered while regions of high thermal inertia, such as Valles Marineris and the rim of the Hellas impact basin, have high rock abundances. (Image PIA02818, NASA/JPL/ASU.)

Figure 4.22 Thermal inertia provides information about particle sizes across Mars. This map, derived from TES data, shows the variation in thermal inertia across the planet. Areas of low thermal inertia, such as Tharsis, are dust-covered while regions of high thermal inertia, such as Valles Marineris and the rim of the Hellas impact basin, have high rock abundances. (Image PIA02818, NASA/JPL/ASU.)

Alteration occurring during the siderikian period has been limited to oxidation from atmospheric weathering and perhaps frost/rock interaction.

Information on the elemental concentrations of the martian crust comes from GRS analysis. Figure 4.10 shows the distribution of H2O, as indicated by the detection of H, within the upper meter of the surface based on thermal, epithermal, and fast neutron analysis (Feldman et al., 2004a). Regions poleward of ~±50° display high amounts of H2O, ranging in concentration from 20% to 100% water equivalent by mass. Arabia and the highlands region south of Tharsis and Elyisum show higher H2O concentrations than the rest of the highlands, with water-equivalent concentrations of 2% to 10% by mass. While the high-latitude reservoirs likely contain substantial concentrations of H2O ice, the high-H equatorial regions could consist of either buried H2O ice or hydrated minerals.

GRS also has mapped the distribution of Cl, Fe, K, Si, and Th across the martian surface (Taylor etal., 2006). Chlorine is highly mobile and can be affected by aqueous processes. High concentrations of Cl are found covering much of Tharsis, perhaps emplaced by volcanic outgassing. Older volcanic regions in eastern Tharsis and Syrtis Major and younger volcanism in Elysium show moderate Cl, Fe, and H concentrations but low K, Si, and Th. Arabia Terra is unique among the highlands regions, showing higher Th concentrations in addition to its high H signature. Most of the southern highlands have higher Si and Th concentrations than Cl, Fe, H, and K. The northern plains display high Si, Fe, K, and Th. The correlation between elemental concentrations and geologic units suggests that the elemental analysis will provide important insights into the evolutionary processes shaping the martian surface.

4.3.3 Crustal composition from martian meteorite analysis

The martian meteorites are geochemically subdivided into an orthopyroxenite (ALH84001), basaltic shergottites, olivine-phyric shergottites, lherzolitic shergot-tites (plagioclase-bearing peridotites), olivine-rich nakhlites, and dunites (chas-signites) (Section 2.2.3) (McSween et al., 2003). ALH84001 samples the early martian cumulate crust, but the other martian meteorites originated from young volcanic regions. All of the martian meteorites have higher concentrations of pyroxene than plagioclase, opposite of the TES results for the dark regions of Mars, which led McSween (2002) to argue that the SNCs originated from the dust-covered young volcanic regions of Tharsis and/or Elysium. If this is correct, the SNCs represent a biased sampling of a small region of Mars, and extrapolation of SNC crustal results to the entire planet is unjustified.

The compositional differences among the shergottites have been attributed to contamination of the magma through inclusion of early crustal material (Borg et al., 1997). Alteration products resulting from fluid-rock interactions have been detected

Lower limit of water mass fraction on Mars 2% 4% 8% 16% 32% >64%

Figure 4.10 Analysis from the neutron experiments on GRS has produced information on the water mass fraction within the upper meter of the martian surface. This map shows that high water concentrations occur near the poles, which likely indicates the presence of H20 ice within the upper meter. The high H20 concentrations seen in the equatorial regions may be due to ice or hydrated minerals. (NASA/MO-GRS/Los Alamos National Laboratory.) See also color plate.

Figure 4.10 Analysis from the neutron experiments on GRS has produced information on the water mass fraction within the upper meter of the martian surface. This map shows that high water concentrations occur near the poles, which likely indicates the presence of H20 ice within the upper meter. The high H20 concentrations seen in the equatorial regions may be due to ice or hydrated minerals. (NASA/MO-GRS/Los Alamos National Laboratory.) See also color plate.

Figure 4.11 The round objects with thin rims are carbonate globules found in ALH84001. The globules are about 200 |m in diameter and consist of calcium- and iron-rich carbonates with magnesium carbonate rims. (Image courtesy of Monica Grady, British Museum of Natural History.)

in several of the martian meteorites. ALH84001 contains large quantities of carbonates (Figure 4.11), which were deposited in rock fractures either by evaporation of brines (Warren, 1998) or through reactions with hydrothermal fluids (Romanek et al., 1994). Alteration products from rock-water interactions are found in several of the martian meteorites (Treiman et al., 1993; Bridges and Grady, 2000). These results indicate that the crustal regions sampled by the martian meteorites are primarily composed of volcanic materials which have interacted with groundwater and/or surface water throughout their histories.

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