Anthony Leonid Turkevich - Obituary

Anthony Leonid Turkevich, a nuclear radiochemist and physicist and a professor of chemistry at the University of Chicago, died in his sleep on 7 September 2002 at his home in Lexington, Virginia. He was widely esteemed for his great intellectual powers, deep physical insight, and personal integrity. Tony was born in New York City on 23 July 1916, the son of a Russian Orthodox clergyman who became a head of the Russian Orthodox Church in both North America and Japan. In 1937, Tony earned a BA from Dartmouth College and, in 1940, a PhD in physical chemistry from Princeton University. His doctoral work was on diffusion determination of molecular structures and dielectric investigations of the motion of organic molecules in the solid state. Shortly after his graduation, he was invited to the University of Chicago as a research physicist in the department of physics; he worked on UV spectroscopy and the radiochemical studies of the fission products

Dust Flux Monitor Instrument measurements during Stardust-NExT Flyby of Comet 9P/Tempel 1

The Dust Flux Monitor Instrument (DFMI) on the Stardust and Stardust-NExT missions measured impacts from coma dust particles with masses from ∼10−15 to >10−6 kg using two kinds of sensors – one based on polyvinylidene fluoride (PVDF) thin films and the other on acoustic detectors (ACs) mounted on the front and second layers of the Whipple Bumper Shield. At the higher encounter speed of 10.9 km s−1 at Comet 9P/Tempel 1 compared with 6.12 km s−1 at 81P/Wild 2 encounter, the mass sensitivity of DFMI sensors increased by between a factor of 2 and 12 (depending on the sensor subsystem), but the spatial resolution (⩾1.09 km) decreased to approximately a half. The coma of Comet Tempel 1 exhibits highly non-uniform spatial distribution of dust, as found at comet Wild 2, with bursts of impacts of up to 1000 particles over km scales near closest approach surrounded by void regions of many kilometers with no impacts. These data are consistent with passage through clouds of particles resulting from fragmentation of larger aggregates emitted from the nucleus. These fragmentation products dominate the total dust production of small particles, with only a small contribution likely from direct emission from the nucleus. The derived overall mass distribution is similar to that found at comet Wild 2 with the total mass dominated by large particles. The average cumulative mass index α = 0.65 ± 0.08 (where the particle flux is defined by ϕ (>m) = km−α) but a better fit is obtained with α = 0.85 ± 0.08 for particle masses below 10−10 kg and a significantly lower value for higher masses

Overview of the Opportunity Mars Exploration Rover mission to Meridiani Planum: Eagle crater to Purgatory ripple

The Mars Exploration Rover Opportunity touched down at Meridiani Planum in January 2004 and since then has been conducting observations with the Athena science payload. The rover has traversed more than 5 km, carrying out the first outcrop-scale investigation of sedimentary rocks on Mars. The rocks of Meridiani Planum are sandstones formed by eolian and aqueous reworking of sand grains that are composed of mixed fine-grained siliciclastics and sulfates. The siliciclastic fraction was produced by chemical alteration of a precursor basalt. The sulfates are dominantly Mg-sulfates and also include Ca-sulfates and jarosite. The stratigraphic section observed to date is dominated by eolian bedforms, with subaqueous current ripples exposed near the top of the section. After deposition, interaction with groundwater produced a range of diagenetic features, notably the hematite-rich concretions known as ‘‘blueberries.’’ The bedrock at Meridiani is highly friable and has undergone substantial erosion by wind-transported basaltic sand. This sand, along with concretions and concretion fragments eroded from the rock, makes up a soil cover that thinly and discontinuously buries the bedrock. The soil surface exhibits both ancient and active wind ripples that record past and present wind directions. Loose rocks on the soil surface are rare and include both impact ejecta and meteorites. While Opportunity’s results show that liquid water was once present at Meridiani Planum below and occasionally at the surface, the environmental conditions recorded were dominantly arid, acidic, and oxidizing and would have posed some significant challenges to the origin of life.Additional co-authors: J Farmer, WH Farrand, W Folkner, R Gellert, TD Glotch, M Golombek, S Gorevan, JA Grant, R Greeley, J Grotzinger, KE Herkenhoff, S Hviid, JR Johnson, G Klingelhöfer, AH Knoll, G Landis, M Lemmon, R Li, MB Madsen, MC Malin, SM McLennan, HY McSween, DW Ming, J Moersch, RV Morris, T Parker, JW Rice Jr, L Richter, R Rieder, M Sims, M Smith, P Smith, LA Soderblom, R Sullivan, NJ Tosca, H Wnke, T Wdowiak, M Wolff, A Ye

Geochemical properties of rocks and soils in Gusev Crater, Mars: Results of the Alpha Particle X-Ray Spectrometer from Cumberland Ridge to Home Plate

Geochemical diversity of rocks and soils has been discovered by the Alpha Particle X-Ray Spectrometer (APXS) during Spirit’s journey over Husband Hill and down into the Inner Basin from sol 470 to 1368. The APXS continues to operate nominally with no changes in calibration or spectral degradation over the course of the mission. Germanium has been added to the Spirit APXS data set with the confirmation that it occurs at elevated levels in many rocks and soils around Home Plate. Twelve new rock classes and two new soil classes have been identified at the Spirit landing site since sol 470 on the basis of the diversity in APXS geochemistry. The new rock classes are Irvine (alkaline basalt), Independence (low Fe outcrop), Descartes (outcrop similar to Independence with higher Fe and Mn), Algonquin (mafic-ultramafic igneous sequence), Barnhill (volcaniclastic sediments enriched in Zn, Cl, and Ge), Fuzzy Smith (high Si and Ti rock), Elizabeth Mahon (high Si, Ni, and Zn outcrop and rock), Halley (hematite-rich outcrop and rock), Montalva (high K, hematite-rich rock), Everett (high Mg, magnetite-rich rock), Good Question (high Si, low Mn rock), and Torquas (high K, Zn, and Ni magnetite-rich rock). New soil classes are Gertrude Weise (very high Si soil) and Eileen Dean (high Mg, magnetite-rich soil). Aqueous processes have played a major role in the formation and alteration of rocks and soils on Husband Hill and in the Inner Basin

Nickel on Mars: Constraints on meteoritic material at the surface

Impact craters and the discovery of meteorites on Mars indicate clearly that there is meteoritic material at the Martian surface. The Alpha Particle X-ray Spectrometers (APXS) on board the Mars Exploration Rovers measure the elemental chemistry of Martian samples, enabling an assessment of the magnitude of the meteoritic contribution. Nickel, an element that is greatly enhanced in meteoritic material relative to samples of the Martian crust, is directly detected by the APXS and is observed to be geochemically mobile at the Martian surface. Correlations between nickel and other measured elements are used to constrain the quantity of meteoritic material present in Martian soil and sedimentary rock samples. Results indicate that analyzed soils samples and certain sedimentary rocks contain an average of 1% to 3% contamination from meteoritic debris

Modeling the nucleus and jets of comet 81P/Wild 2 based on the startdust encounter data

We interpret the nucleus properties and jet activity from the Stardust spacecraft imaging and the onboard dust monitoring system data. Triangulation of 20 jets shows that 2 emanate from the nucleus dark side and 16 emanate from sources that are on slopes where the Sun's elevation is greater than predicted from the fitted triaxial ellipsoid. Seven sources, including five in the Mayo depression, coincide with relatively bright surface spots. Fitting the imaged jets, the spikelike temporal distribution of dust impacts indicates that the spacecraft crossed thin, densely populated sheets of particulate ejecta extending from small sources on the rotating nucleus, consistent with an emission cone model

Dust measurements in the Coma of Comet 81P/Wild 2 by the Dust Flux Monitor Instrument

We present measurements of the dust particle flux and mass distribution from the Stardust Dust Flux Monitor Instrument (DFMI) throughout the flyby of comet 81P/Wild 2. In the particle mass regime from 10–14 to 10–7 kilograms, the spacecraft encountered regions of intense swarms of particles, together with bursts of activity corresponding to clouds of particles only a few hundred meters across. This fine-scale structure can be explained by particle fragmentation. We estimate that 2800 ± 500 particles of diameter 15 micrometers or larger impacted the aerogel collectors, the largest being 6 x 10–7 kilograms, which dominates the total collected mass