The Mössbauer (MB) spectrometer on Opportunity measured the Fe oxidation state, identified Fe‐bearing phases, and measured relative abundances of Fe among those phases at Meridiani Planum, Mars. Eight Fe‐bearing phases were identified: jarosite (K,Na,H3O)(Fe,Al)(OH)6(SO4)2, hematite, olivine, pyroxene, magnetite, nanophase ferric oxides (npOx), an unassigned ferric phase, and metallic Fe (kamacite). Burns Formation outcrop rocks consist of hematite‐rich spherules dispersed throughout S‐rich rock that has nearly constant proportions of Fe3+ from jarosite, hematite, and npOx (29%, 36%, and 20% of total Fe). The high oxidation state of the S‐rich rock (Fe3+/FeT ∼ 0.9) implies that S is present as the sulfate anion. Jarosite is mineralogical evidence for aqueous processes under acid‐sulfate conditions because it has structural hydroxide and sulfate and it forms at low pH. Hematite‐rich spherules, eroded from the outcrop, and their fragments are concentrated as hematite‐rich soils (lag deposits) on ripple crests (up to 68% of total Fe from hematite). Olivine, pyroxene, and magnetite are primarily associated with basaltic soils and are present as thin and locally discontinuous cover over outcrop rocks, commonly forming aeolian bedforms. Basaltic soils are more reduced (Fe3+/FeT ∼ 0.2–0.4), with the fine‐grained and bright aeolian deposits being the most oxidized. Average proportions of total Fe from olivine, pyroxene, npOx, magnetite, and hematite are ∼33%, 38%, 18%, 6%, and 4%, respectively. The MB parameters of outcrop npOx and basaltic‐soil npOx are different, but it is not possible to infer mineralogical information beyond octahedrally coordinated Fe3+. Basaltic soils at Meridiani Planum and Gusev crater have similar Fe‐mineralogical compositions.

The Mössbauer spectrometer on Spirit measured the oxidation state of Fe, identified Fe‐bearing phases, and measured relative abundances of Fe among those phases for surface materials on the plains and in the Columbia Hills of Gusev crater. Eight Fe‐bearing phases were identified: olivine, pyroxene, ilmenite, magnetite, nanophase ferric oxide (npOx), hematite, goethite, and a Fe3+‐sulfate. Adirondack basaltic rocks on the plains are nearly unaltered (Fe3+/FeT < 0.2) with Fe from olivine, pyroxene (Ol > Px), and minor npOx and magnetite. Columbia Hills basaltic rocks are nearly unaltered (Peace and Backstay), moderately altered (WoolyPatch, Wishstone, and Keystone), and pervasively altered (e.g., Clovis, Uchben, Watchtower, Keel, and Paros with Fe3+/FeT ∼ 0.6–0.9). Fe from pyroxene is greater than Fe from olivine (Ol sometimes absent), and Fe2+ from Ol + Px is 40–49% and 9–24% for moderately and pervasively altered materials, respectively. Ilmenite (Fe from Ilm ∼3–6%) is present in Backstay, Wishstone, Keystone, and related rocks along with magnetite (Fe from Mt ∼10–15%). Remaining Fe is present as npOx, hematite, and goethite in variable proportions. Clovis has the highest goethite content (Fe from Gt = 40%). Goethite (α‐FeOOH) is mineralogical evidence for aqueous processes because it has structural hydroxide and is formed under aqueous conditions. Relatively unaltered basaltic soils (Fe3+/FeT ∼ 0.3) occur throughout Gusev crater (∼60–80% Fe from Ol + Px, ∼10–30% from npOx, and ∼10% from Mt). PasoRobles soil in the Columbia Hills has a unique occurrence of high concentrations of Fe3+‐sulfate (∼65% of Fe). Magnetite is identified as a strongly magnetic phase in Martian soil and dust.

Mössbauer spectra measured on Mars by the Spirit rover during the primary mission are characterized by two ferrous iron doublets (olivine and probably pyroxene) and a ferric iron doublet (tentatively associated to nanophase ferric iron oxide). Two sextets resulting from nonstoichiometric magnetite are also present, except for a coating on the rock Mazatzal, where a hematite-like sextet is present. Greater proportions of ferric-bearing phases are associated with undisturbed soils and rock surfaces as compared to fresh rock surfaces exposed by grinding. The ubiquitous presence of olivine in soil suggests that physical rather than chemical weathering processes currently dominate at Gusev crater.

Mössbauer spectra measured by the Opportunity rover revealed four mineralogical components in Meridiani Planum at Eagle crater: jarosite- and hematite-rich outcrop, hematite-rich soil, olivine-bearing basaltic soil, and a pyroxene-bearing basaltic rock (Bounce rock). Spherules, interpreted to be concretions, are hematite-rich and dispersed throughout the outcrop. Hematitic soils both within and outside Eagle crater are dominated by spherules and their fragments. Olivine-bearing basaltic soil is present throughout the region. Bounce rock is probably an impact erratic. Because jarosite is a hydroxide sulfate mineral, its presence at Meridiani Planum is mineralogical evidence for aqueous processes on Mars, probably under acid-sulfate conditions.

Mössbauer spectroscopy is a powerful tool for quantitative mineralogical analysis of Fe‐bearing materials. The miniature Mössbauer spectrometer MIMOS II is a component of the Athena science payload launched to Mars in 2003 on both Mars Exploration Rover missions. The instrument has two major components: (1) a rover‐based electronics board that contains power supplies, a dedicated central processing unit, memory, and associated support electronics and (2) a sensor head that is mounted at the end of the instrument deployment device (IDD) for placement of the instrument in physical contact with soil and rock. The velocity transducer operates at a nominal frequency of ∼25 Hz and is equipped with two 57Co/Rh Mössbauer sources. The reference source (∼5 mCi landed intensity), reference target (α‐Fe2O3 plus α‐Fe0), and PIN‐diode detector are configured in transmission geometry and are internal to the instrument and used for its calibration. The analysis Mössbauer source (∼150 mCi landed intensity) irradiates Martian surface materials with a beam diameter of ∼1.4 cm. The backscatter radiation is measured by four PIN‐diode detectors. Physical contact with surface materials is sensed with a switch‐activated contact plate. The contact plate and reference target are instrumented with temperature sensors. Assuming ∼18% Fe for Martian surface materials, experiment time is 6–12 hours during the night for quality spectra (i.e., good counting statistics); 1–2 hours is sufficient to identify and quantify the most abundant Fe‐bearing phases. Data stored internal to the instrument for selectable return to Earth include Mössbauer and pulse‐height analysis spectra (512 and 256 channels, respectively) for each of the five detectors in up to 13 temperature intervals (65 Mössbauer spectra), engineering data for the velocity transducer, and temperature measurements. The total data volume is ∼150 kB. The mass and power consumption are ∼500 g (∼400 g for the sensor head) and ∼2 W, respectively. The scientific measurement objectives of the Mössbauer investigation are to obtain for rock, soil, and dust (1) the mineralogical identification of iron‐bearing phases (e.g., oxides, silicates, sulfides, sulfates, and carbonates), (2) the quantitative measurement of the distribution of iron among these iron‐bearing phases (e.g., the relative proportions of iron in olivine, pyroxenes, ilmenite, and magnetite in a basalt), (3) the quantitative measurement of the distribution of iron among its oxidation states (e.g., Fe2+, Fe3+, and Fe6+), and (4) the characterization of the size distribution of magnetic particles. Special geologic targets of the Mössbauer investigation are dust collected by the Athena magnets and interior rock and soil surfaces exposed by the Athena Rock Abrasion Tool and by trenching with rover wheels.

After Eu implantation, the phase formation in a surface region of 150 nm of stainless steel was studied non-destructively by a depth resolved Mössbauer technique (DCEMS). The observed iron phases are characterized by the hyperfine interaction of the 57Fe nuclei. Their characteristics and depth distributions strongly depend on fluence and ion energy. A newly formed martensitic phase accompanies the implantation profile of the Eu atoms into a depth of up to about 120 nm. In a near-surface region of 15 nm, the observed phases differ from those found in larger depths. This reveals the substantial role of the surface, where implantation induced diffusion and segregation processes are effective. Concerning the DCEMS analysis, special emphasis is laid on the determination of systematic and statistical errors of the phase depth profiles.

Mossbauer spectroscopy is a powerful tool for quantitative mineralogical analysis of Fe-bearing materials. The miniature Mossbauer spectrometer MIMOS II is a component of the Athena science payload launched to Mars in 2003 on both Mars Exploration Rover missions. The instrument has two major components: (1) a rover-based electronics board that contains power supplies, a dedicated central processing unit, memory, and associated support electronics and (2) a sensor head that is mounted at the end of the instrument deployment device (IDD) for placement of the instrument in physical contact with soil and rock. The velocity transducer operates at a nominal frequency of ∼25 Hz and is equipped with two 57 Co/Rh Mossbauer sources. The reference source (∼5 mCi landed intensity), reference target (α-Fe 2 O 3 plus α-Fe 0 ), and PIN-diode detector are configured in transmission geometry and are internal to the instrument and used for its calibration. The analysis Mossbauer source (∼150 mCi landed intensity) irradiates Martian surface materials with a beam diameter of ∼1.4 cm. The backscatter radiation is measured by four PIN-diode detectors. Physical contact with surface materials is sensed with a switch-activated contact plate. The contact plate and reference target are instrumented with temperature sensors. Assuming ∼18% Fe for Martian surface materials, experiment time is 6--12 hours during the night for quality spectra (i.e., good counting statistics); 1-2 hours is sufficient to identify and quantify the most abundant Fe-bearing phases. Data stored internal to the instrument for selectable return to Earth include Mossbauer and pulse-height analysis spectra (512 and 256 channels, respectively) for each of the five detectors in up to 13 temperature intervals (65 Mossbauer spectra), engineering data for the velocity transducer, and temperature measurements. The total data volume is ∼150 kB. The mass and power consumption are ∼500 g (∼400 g for the sensor head) and ∼2 W, respectively. The scientific measurement objectives of the Mossbauer investigation are to obtain for rock, soil, and dust (1) the mineralogical identification of iron-bearing phases (e.g., oxides, silicates, sulfides, sulfates, and carbonates), (2) the quantitative measurement of the distribution of iron among these iron-bearing phases (e.g., the relative proportions of iron in olivine, pyroxenes, ilmenite, and magnetite in a basalt), (3) the quantitative measurement of the distribution of iron among its oxidation states (e.g., Fe 2+ , Fe 3+ , and Fe 6+ ), and (4) the characterization of the size distribution of magnetic particles. Special geologic targets of the Mossbauer investigation are dust collected by the Athena magnets and interior rock and soil surfaces exposed by the Athena Rock Abrasion Tool and by trenching with rover wheels.

In May and July 2003 both the European space agency ESA and the American space agency NASA will launch space missions to Mars. The ESA lander Beagle 2 and the two NASAMars-Exploration-Rovers (MER) will explore the Martian surface with a set of sophisticated instruments. Part of the payload will be our miniaturized Mössbauer spectrometer MIMOS II. It operates in backscattering geometry and meets the requirements for space application of low mass (⩽500 g), small volume (coke can size), and low power consumption (⩽3 W). Main goals are the determination of the oxidation state of iron and the iron mineralogy on the surface. This information will contribute to a much deeper understanding of the evolution of the planet Mars, its surface and atmosphere, and the history of water. The MIMOS II flight units for MER were delivered in April 2002 to the NASA Jet Propulsion Laboratories (JPL), California, for integration to the Rovers. After some more testing of the complete Rover system the spacecraft will be shipped to the Kennedy Space Center early February 2003. The first launch will be in May 2003 and the second launch in late June on early July 2003. The flight unit for the ESA Mars-Express Beagle lander was delivered to ESA by the end of May 2002 for integration to the lander in late November/early December 2002. The launch is scheduled for June 2003 from Baikonur, Kazakhstan. The instrument MIMOS II is also under consideration for an ESA space mission to Mercury in 2009, and it is part of the ESA exobiology multi-user facility to be launched as part of one of the next lander Mars missions after 2005.

The magnetic properties of a 50-to 2-nm-thick surface layer in hexagonal ferrite BaFe12O19 single crystals are investigated for the first time. Measurements are made on a conversion-electron Mössbauer spectrometer constructed on the basis of a unique ultrahigh-vacuum magnetostatic electron analyzer of the “orange” type. An analysis of the experimental Mössbauer spectra obtained from the surface layer of BaFe12O19 50-to 2-nm-thick single crystals reveals that (1) the spectral line widths are close to the natural widths of Mössbauer lines and neither changes nor a set of the values of effective magnetic fields, as well as a paramagnetic state of iron ions, could appear due to a defect such as the “surface,” and (2) the experimental spectra are best described only under the assumption that the surface of a hexaferrite BaFe12O19 single crystal contains a 2-nm-thick layer, in which the magnetic moments of iron ions are deflected through ∼20° from the crystallographic C axis along which the magnetic moments of ions located in the bulk of the crystal are oriented.

First measurements of the effective magnetic fields as a function of the depth at which the iron ions are in the surface layer of α-Fe 2 O 3 and FeBO 3 single crystals are reported. The method used is the depth-selective conversion-electron Mössbauer spectroscopy. An analysis of experimental spectra revealed that the magnetic properties of the crystal surface vary smoothly from the bulk to surface characteristics within a layer ∼100 nm thick. The layers lying below ∼100 nm from the surface are similar in properties to the bulk of the crystal, and their spectra consist of narrow lines. The spectral linewidths increase smoothly as one approaches the crystal surface. The spectra obtained from a ∼10-nm thick surface layer consist of broad lines indicating a broad distribution of effective magnetic fields. Calculations show that the field distribution width in this layer is δ=2.1(3) T, for an average value H eff =32.2(4) T. It has been experimentally established that, at room temperature (291 K), the effective magnetic fields smoothly decrease as one approaches the crystal surface. The effective fields in a 2.4(9)-nm surface layer of α-Fe 2 O 3 crystals are lower by 0.7(2)% than the fields at the ion nuclei in the bulk of the sample. In the case of FeBO 3 , the effective fields decrease by 1.2(3)% in a surface layer 4.9(9) nm thick.

An FeBO3 single crystal was studied from 291 K up to the Neel temperature T(N) = 348.35 K by depth selective conversion electron Mossbauer spectroscopy in ultrahigh vacuum (10(-9) mbar). A new magnetic near-surface phase was found. Its thickness D diverges on approaching T(N) and gives a critical exponent for the correlation length of nu = 0.59(4). The phase boundary between the bulk and near-surface phase could be identified.

Depth-selective conversion electron Mössbauer spectroscopy was used to study magnetic properties of the thin surface layers of the α-Fe 2 O 3 and FeBO 3 single crystals. An analysis of the experimental spectra indicates that the magnetic properties of the layers at a depth of more than ∼100 nm from the surface are similar to the properties of crystal bulk, and the corresponding spectra consist of narrow lines. The lines gradually broaden as the crystal surface is approached. The spectra of the ∼10-nm-thick surface layers consist of broad lines, indicating a wide distribution δ=2.1 T of the effective magnetic field about its mean value of 32.2(4) T. The experimental spectra were used to determine the effective magnetic fields ( H eff ) for the iron ions situated in the surface layers of thickness ∼100 nm. The effective fields in these layers were found to gradually decrease at room temperature (291 K) as the crystal surface was approached. The H eff values in the 2.4(9)-nm-thick surface layer of the α-Fe 2 O 3 crystal and 4.9(9)-nm layer of FeBO 3 are 0.7(2) and 1.2(3)%, respectively, smaller than for the nuclei of the ions in the bulk of these crystals.

The behavior of the surface and near-surface layers of macroscopic FeBO 3 single crystals is studied over the temperature range from 291 K to Neél temperature ( T N ) using depth-selective conversion-electron Mössbauer spectroscopy. Three different phases or states, namely, an antiferromagnetically ordered phase (similar to the crystal bulk state), a surface phase, and a transition layer between them coexist near the Neél point in a surface layer ∼500 nm thick. The critical parameters found for the bulk phase agree well with the theoretical critical index ν th ≅0.63 predicted by the 3D Ising model. As the crystal surface is approached, the critical parameter β increases to 0.51(2) but remains smaller than the value of β=0.8 for the surface of a semi-infinite Heisenberg model. Therefore, the effective dimensionality of the system, being equal to 3 in the bulk, decreases at the crystal surface.

The surface magnetic ordering of an α- 57 Fe 2 O 3 and 57 FeBO 3 single crystal was studied by Depth Selective Conversion Electron Mössbauer Spectroscopy (DCEMS). The depth selective Mössbauer data of 57 FeBO 3 up to the Néel temperature ( T N = 348.35 K) reveal the coexistence of the canted antiferromagnetic bulk phase and a magnetic surface phase that shows time relaxation. Both phases are separated by a wall (interim layer) and the thickness of the magnetic surface layer increases as the crystal is heated to T N .