Martha W. Schaefer

Aqueous geochemistry on early Mars

A geochemical cycle model is presented for the interaction between the atmosphere, hydrosphere, and regolith of Mars. It was developed to study how this interaction might have produced the present Martian environment from a primitive Martian environment much like that of the primitive Earth. The model is a simple system, consisting of an unweathered starting material (calcium-bearing and magnesium-bearing silicates), a CO2 atmosphere, an ocean of water in contact with both the atmosphere and the unweathering starting material, and both calcite and dolomite precipitates. Several interesting points arise from this model. A 1-bar CO2 atmosphere can be removed by carbonate precipitation alone in about half a billion years. This is roughly fifty times longer than earlier estimates, which were not based on time-varying models (Fanale et al., 1982; Carr, 1986; Pollack et al., 1987). One of the chief problems in Martian geology has been how to explain the large number and wide variety of surface features that were apparently formed by aqueous erosion. This longer atmospheric lifetime may be enough to explain the large number of channels seen on older Martian terrain. If the atmosphere started out with more than 1 bar of CO2, it would take correspondingly longer to remove it. If there should be no other means to remove CO2 from the atmosphere, this long time constant would indicate that the atmosphere could never have contained more than a few bars of CO2, or else there would still be remnants present today. The increase in alkalinity of the ocean as the atmosphere disappears, even without the effects of reduction in the amount of water available, indicates that evaporite deposits may have formed on Mars. If these deposits are still present, they may even yet contain some liquid water.

THE YOUTHFUL APPEARANCE OF THE 2003 EL61 COLLISIONAL FAMILY

We present new solar phase curve observations of the 2003 EL61 collisional family showing that all the members have light-scattering properties similar to the bright icy satellites of the giant planets and dwarf planets. Compared with other Kuiper Belt objects (KBOs), the five family members we observe (2003 EL61, 2002 TX300, 2003 OP32, 2005 RR43, and 1995 SM55) have conspicuously neutral color (V – I = 0.6-0.8 mag) and flat phase curves at small phase angles (phase coefficients of 0.0-0.1 mag deg–1). Comparing the phase curves we observe for other icy KBOs with the phase curves of icy satellites, we find that the flat phase curves of the 2003 EL61 family are an indication that they have high albedo surfaces coated with fresh ice in the last ~100 Myr. We examine possible resurfacing processes and find none that are plausible. To avoid the influence of cosmic radiation that darkens and reddens most icy surfaces on times scales 100 Myr, the family members must be unusually depleted in carbon, or the collision that created the family occurred so recently that the parent body and fragments have not had time to darken. We also find a rotation period of 4.854 (±0.003) h with amplitude 0.26 (±0.04) mag for 2003 OP32.

Mössbauer spectroscopy on the surface of Mars: constraints and expectations

Abstract Mossbauer spectrometers will be included on the upcoming MER/Athena and Mars Express/Beagle rovers. These instruments have the exciting potential to characterize the surface mineralogy of Mars, but there are several complicating factors. It is difficult to differentiate minerals with similar structures, particularly if the rock or soil is multimineralic and the peaks overlap. Mineral modes cannot be determined from Mossbauer spectra alone. The temperature dependence of Mossbauer parameters for common rock-forming minerals and glasses is known for only a few minerals, but it is strong. No database of Mars surface temperature Mossbauer spectra of minerals or glasses currently exists, with which Martian data can be compared. Peak areas in Mossbauer spectroscopy do not directly correspond to the abundances of the minerals that give rise to them, and the calibration/correction factors to compensate for this problem are currently known for only a few minerals. We assess here the advantages and disadvantages of Mossbauer spectroscopy for remote data acquisition, and describe work currently under way in our laboratory to address these potential complications.

Nereid Has Complex Large-Amplitude Photometric Variability

Abstract We report on 224 photometric measurements of Nereid, a small outer satellite of Neptune with a 360-day orbit of high eccentricity (0.751). Our photometry covers 64 nights from 1987 to 1997 and is primarily in the V-band, although we also have 20 measurements in the U, B, R, and I bands. (1) Nereid displays large-amplitude brightness variations with a total amplitude of 1.83 mag on time scales ranging from a few hours to roughly a year. (2) During the 12 days of the Voyager encounter with Neptune, Nereid did not display any short-term variations; however, large-amplitude long-term variations could easily be hidden by the large phase effects and the short duration of observation. (3) Nereids variability is caused by high contrast albedo features, i.e., a dark hemisphere, along with rotational modulation. (4) The character of the brightness variations changed around 1991 from fast and large-amplitude to comparatively slow and low-amplitude. This demonstrates that the direction and magnitude of Nereids rotational angular momentum vector is changing on time scales comparable to its orbital period. (5) Large changes in the magnitude and direction of Nereids angular momentum vector are predicted to arise from chaotic rotation during every periapse passage provided that Nereid is more than ∼1% nonspherical and is spinning slowly. The match between prediction and observation could be taken as strong evidence for chaotic rotation of Nereid. However, the intranight variability cannot be readily explained by chaotic rotation. (6) The colors of Nereid are U-V=0.84±0.05, B-V=0.71±0.04, V-R=0.44±0.03, V-I=0.72±0.05, and V-K=1.6, indicating a nearly flat reflectance spectrum from 0.36 to 2.2 μm. We identify asteroids, inner satellites, and centaurs with similar spectra. (7) Nereid is likely either an inner moon of Neptune kicked to its current orbit or a captured Kuiper Belt object or centaur, with the latter possibility being strongly preferred.

Karst on Mars? The thumbprint terrain

Abstract Some instances of the so-called “thumbprint” terrain of the northern plains of Mars may be caused by differential solution of large carbonate deposits occupying low-lying areas, and may represent an analog to the arid karst of the Nullarbor Plain in Australia, based on the comparative morphology of the two regions.

Nereid: Light curve for 1999–2006 and a scenario for its variations

Abstract Nereid is a small irregular moon of Neptune that displays large-, moderate-, and small-amplitude photometric variations on both fast and slow time scales. The central mystery of Nereid is now to explain the physical mechanism of these unique brightness changes and why they change with time. To characterize Nereids variability, we have been using the SMARTS telescopes on Cerro Tololo for synoptic monitoring from 1999 to 2006. We present a well-sampled photometric time series of 493 magnitudes on 246 nights mostly in the V-band. In combination with our earlier data (for 774 magnitudes over 362 nights), our 20-year data set is the most comprehensive for any small icy body in our Solar System. Our yearly light curves show that Nereid displays various types of behaviors: large amplitude brightenings and fadings (1987 to 1990); moderate-amplitude variation about the average phase curve (1993–1997, 2003, 2005), moderate-amplitude variation and systematically brighter by roughly one-quarter magnitude throughout the entire season (2004); and nearly constant light curves superimposed on a surprisingly large-amplitude opposition surge (1998, 1999, 2000, 2006). Other than in 2004, Nereids variations were closely centered around a constant phase curve that is well fit with a Hapke model for the coherent backscattering opposition surge mechanism with angular scale of 0.7 ° ± 0.1 ° . In our entire data set from 1987–2006, we find no significant periodicity. We propose that the year-to-year changes in the variability of Nereid are caused by forced precession (caused by tidal forces from Neptune) on the spin axis of a nonspherical Nereid, such that cross-sectional areas and average albedos change as viewed from Earth.

Volcanic recycling of carbonates on Mars

Thermal erosion of carbonate deposits by turbulently-flowing lava is investigated as a means of recycling carbon dioxide back into the atmosphere of Mars. Erosion rates of several meters/day are found, implying that up to hundreds of meters of carbonate could be removed over the lifetime of a flow. A large fraction of the northern plains and other parts of Mars were covered by lava during the Hesperian, and may have released the carbon dioxide trapped in carbonate deposits. This period of time, several times 108 years, is comparable to that for the re-deposition of such carbonate deposits. Therefore, there could have existed a relatively dense atmosphere, and enhanced weathering and erosion, after the Noachian era. This may help explain the apparent observational evidence for late fluvial and lacustrine activity on Mars.

The Characterization of Atomically Precise Nanoclusters Using X-Ray Absorption Spectroscopy

The XAS toolbox (EXAFS, XANES, theoretical calculations, in situ measurements) is used in a variety of applications to determine the structure and electronic properties of atomically precise nanoclusters (APNCs). The analysis using the EXAFS part of the XAS spectrum involves models that are based on atomic packing (e.g., fcc, icosahedra) or surface effects. Theoretical methods based on DFT can improve the understanding of disorder effects on the EXAFS measurements. In comparative experiments, the effect of solvation and of the ligands (density of ligands, different ligands) is discussed. It was found that the strength of the Au–thiol bonding can lead to relaxation effects that reduce the contraction of Au–Au bonds in the core. Changes in structure could be observed for solvation and catalytic reaction with the application of in situ measurements in specifically designed reactors. Even though EXAFS is a powerful method with a number of advantages, such as that no long-range order is necessary, all kinds of materials can be investigated, nondestructively. The analysis of EXAFS data is quite challenging, however, and effects such as structural disorder, if the sample is a mixture of components (not pure) or if the APNCs have several binding ligands, can distort the results. The use of l-DOS based on theoretical XANES calculations that can give information about electronic properties of APNCs is also challenging. In complement with XPS experiments, however, consistent answers can be found.