Soon Sam Kim

Magnesium and titanium partitioning between anorthite and Type B CAI liquid: Dependence on oxygen fugacity and liquid composition

Experiments were conducted in air and at low oxygen fugacity (f_O_2)) to evaluate Mg and Ti partitioning between anorthite and liquid (D_(Mg) and D_(Ti) in a synthetic composition similar to that of a Type B Ca, AI-rich inclusion (CAI). The starting material showed a range of compositions, which allowed assessment of the composition dependence of D_(Mg) and D_(Ti) in this system. Additional experiments using a homogeneous split of the same material investigated the effect of oxygen fugacity on the partitioning of Ti^(3+) and Ti^(4+) between anorthite and liquid. The low foe charges were purple, consistent with the presence of significant amounts of Ti^(3+).This was verified by electron spin resonance (ESR) spectra, and quantitative estimates of Ti^(3+) contents were obtained using ESR. The Ti and Mg partition coefficients in the air run using the homogeneous starting material are similar (0.034 and 0.036, respectively) and consistent with those determined in other studies. However, D_(Ti) at low f_(O_2) is slightly greater than D_(Ti) in the air experiments. Using Ti^(3+)/total Ti from the ESR measurements, D_(Ti^(3+)) is calculated to be about 0.040. The range of compositions reveal a clustering of D_(Mg) and D_(Ti) within charges, but a wide range of D_s between charges of different composition. A well-defined inverse correlation exists between D_(Mg) and D_(Ti). This variation is not due to temperature-dependence, but is instead due to the dependence of D_(Mg) and D_(Ti) on liquid composition (Si and Al in particular). D_(Mg) correlates positively with Si content and negatively with Al content, while D_(Ti) shows the opposite correlations. The results of these experiments have interesting implications for the petrogenesis of Type B CAIs and for substitution mechanisms of Mg, Ti^(4+), and Ti^(3+) into anorthite. Crystallization models for Type B CAIs permit certain predictions concerning trace element systematics in plagioclase. The Mg and Ti systematics are best explained by a fractional crystallization model where plagioclase crystallizes very late (>95% crystallization), and D_(Ti^(3+)). is equal to D_(Ti^(4+)). The results from our experiments support this model for the relative partitioning of Ti^(4+) and Ti^(3+) between plagioclase and liquid. In addition, the dependence of D_(Mg), and D_(Ti) on the Si content of a Type B CAI liquid helps explain systematics expected during late-stage crystallization of plagioclase. The composition dependence of D_(Mg) and D_(Ti) also allows assessment of substitution mechanisms in anorthite using a crystallization reaction approach. Using these methods, a plausible mechanism for Mg involves substitution for tetrahedral A1 by the reaction Mg^(2+) + Si^(4+) = 2AI^(3+), consistent with that proposed by previous workers. The systematics are also consistent with Ti^(4+) and Ti^(3+) substitution for tetrahedral Si^(4+) by the reactions 2Al^(3+) + Ti^(4+) = Ca^(2+) + 2Si^(4+) and Al^(3+) + Ti^(3+) = Ca^(2+) + Si^(4+), respectively.

Searching for biosignatures using electron paramagnetic resonance (EPR) analysis of manganese oxides.

Manganese oxide (Mn oxide) minerals from bacterial sources produce electron paramagnetic resonance (EPR) spectral signatures that are mostly distinct from those of synthetic simulants and abiogenic mineral Mn oxides. Biogenic Mn oxides exhibit only narrow EPR spectral linewidths (∼500 G), whereas abiogenic Mn oxides produce spectral linewidths that are 2-6 times broader and range from 1200 to 3000 G. This distinction is consistent with X-ray structural observations that biogenic Mn oxides have abundant layer site vacancies and edge terminations and are mostly of single ionic species [i.e., Mn(IV)], all of which favor narrow EPR linewidths. In contrast, abiogenic Mn oxides have fewer lattice vacancies, larger particle sizes, and mixed ionic species [Mn(III) and Mn(IV)], which lead to the broader linewidths. These properties could be utilized in the search for extraterrestrial physicochemical biosignatures, for example, on Mars missions that include a miniature version of an EPR spectrometer.

Mars miniature science instruments

For robotic Mars in-situ missions, all the science information is gathered through on-board miniature instruments that have been developed through many years of R&D. Compared to laboratory counterparts, the rover instruments require miniaturization, such as low mass (1-2 kg), low power (less than 10 W) and compact (1-2 liters), yet with comparable sensitivity. Since early 1990s, NASA recognized the need for the miniature instruments and launched several instrument R&D programs, e.g., PIDDP (Planetary Instrument Definition and Development). However, until 1998, most of the instrument R&D programs supported only up to a breadboard level (TRL 3, 4) and there was a need to carry such instruments to flight qualifiable status (TRL 5, 6) to respond to flight AOs (Announcement of Opportunity). Most flight AOs have only limited time and financial resources, and cannot afford such instrument development processes. To bridge the gap between instrument R&D programs and the flight instrument needs, NASAs Mars Technology Program (MTP) created an advanced instrumentation program called Mars Instrument Development Project (MIDP). MIDP candidate instruments are selected through NASA Research Announcement (NRA) process. For example, MIDP I (1998-2000) selected and developed 10 instruments, MIDP II (2003-2005) 16 instruments, and MIDP III (2004-2006) 11 instruments. Working with Pis, JPL has been managing the MIDP tasks since September 1998. All the instruments being developed under MIDP have been selected through a highly competitive NRA process, and employ state-of-the-art technology. So far, four MIDP funded instruments have been selected by two Mars missions (these instruments have been discussed in this paper)