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The first samples collected by the Mars 2020 mission represent units exposed on the Jezero Crater floor, from the potentially oldest Séítah formation outcrops to the potentially youngest rocks of the heavily cratered Máaz formation. Surface investigations reveal landscape-to-microscopic textural, mineralogical, and geochemical evidence for igneous lithologies, some possibly emplaced as lava flows. The samples contain major rock-forming minerals such as pyroxene, olivine, and feldspar, accessory minerals including oxides and phosphates, and evidence for various degrees of aqueous activity in the form of water-soluble salt, carbonate, sulfate, iron oxide, and iron silicate minerals. Following sample return, the compositions and ages of these variably altered igneous rocks are expected to reveal the geophysical and geochemical nature of the planet’s interior at the time of emplacement, characterize martian magmatism, and place timing constraints on geologic processes, both in Jezero Crater and more widely on Mars. Petrographic observations and geochemical analyses, coupled with geochronology of secondary minerals, can also reveal the timing of aqueous activity as well as constrain the chemical and physical conditions of the environments in which these minerals precipitated, and the nature and composition of organic compounds preserved in association with these phases. Returned samples from these units will help constrain the crater chronology of Mars and the global evolution of the planet’s interior, for understanding the processes that formed Jezero Crater floor units, and for constraining the style and duration of aqueous activity in Jezero Crater, past habitability, and cycling of organic elements in Jezero Crater.

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International audience; The Curiosity rover discovered fine-grained sedimentary rocks, which are inferred to represent an ancient lake and preserve evidence of an environment that would have been suited to support a martian biosphere founded on chemolithoautotrophy. This aqueous environment was characterized by neutral pH, low salinity, and variable redox states of both iron and sulfur species. Carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus were measured directly as key biogenic elements; by inference, phosphorus is assumed to have been available. The environment probably had a minimum duration of hundreds to tens of thousands of years. These results highlight the biological viability of fluvial-lacustrine environments in the post-Noachian history of Mars.

This paper describes methods employed to make realistic models of neutral- acid- and base-catalyzed silica aerogels and carbonized resorcinol-formaldehyde aerogels for use in studies of transport and material properties. The objective was to produce models consistent with experimental data including scattering data, microphotographs, and density measurements. To attain this goal, aggregation techniques were used that have not been previously associated with the chemistry of neutral- and acid-catalyzed silica. The appropriateness of these techniques is argued but not addressed rigorously. The methods are unified in a single code with control parameters that allow the user to create a complete range of geometries. Characterization of the computer models is done both by calculation of the scattering function and by visualization, methods that compare directly with experimental data. The computer program embodies a combination of diffusion-limited cluster-cluster aggregation and multi-site, multi-particle diffusion-limited aggregation.

We present, for the first time, a study comparing phonon quench echoes in supercooled liquid argon with a normal mode analysis of the echoes. The phonon echoes are produced via molecular dynamics simulation. We model the echoes by considering a collection of harmonic oscillators whose frequency distribution is defined by the density of normal modes in the liquid under consideration. We find for times less than 300 fs, the fluid behaves in a relatively harmonic manner. We also find that low temperatures and short times are best suited to harmonic analysis. We comment on the agreement and deviation between simulation and model particularly as dephasing processes are concerned. Future approaches to this phenomenon are addressed as well.

A new method to solve the reference HNC equations is developed to treat systems with both asymmetric short-range and long-range interactions. This method is motivated by the work of Patey and co-workers and uses Lado's free-energy minimizing optimization criteria for the reference HNC approximation. The properties of several fluids composed of linear triatomic molecules with various dipole moments or hard-sphere molecules with different-length dipoles are investigated.

Most chemically stable organic materials age through changes on the mesoscopic scale (from 10 nm to 10 mm). Examples include the slow crystallization of polymers, changes in particle size distributions with age, and the swelling of pressed powders. We began the Mesochem project to develop a new mesoscopic modeling capability for organic materials, including polymers, molecular crystals, and filled polymer composites. Our goal was to develop and validate novel mesoscopic modeling techniques that are well suited for materials of interest to LLNL's nation security mission, such as high explosives, binding agents, and foams.

The interaction of LiD with water vapor in the partial pressure range of 10{sup -7} Torr to 20 Torr has been investigated. The reaction probability of water with pure LiD cleaved in an ultra high vacuum environment was obtained using the modulated molecular beam technique. This probability was 0.11 and independent of LiD surface temperature suggesting a negligible activation energy for the reaction in agreement with quantum chemical calculations. The value gradually reduced, however, to .007 as the surface concentration of oxygen containing product (LiOH), which was monitored in-situ by Auger electron spectroscopy on the reaction zone, approached full coverage. As the hydroxide film grew beyond a monolayer, the phase lag of hydrogen product increased from zero to 20 degrees and the reaction probability reduced further until it approached our detection limit ({approx} 10{sup -4}). This phase lag was attributed to a diffusion limited process in this regime. In separate experiments, the film growth has been studied in nitrogen atmosphere with 100% relative humidity using thermogravimetric analysis (TGA) and in air with 50% relative humidity utilizing scanning electron microscopy (SEM). For exposures to environment with high water concentrations and for micrometer thick films, the reaction probability reduced to 4 xmore » 10{sup -7} and was independent of exposure time, The lattice diffusion through the film was no longer controlling the transport of water to the LiD/LiOH interface. Microcracks generated in the film to release stress provided easier pathways to the interface. A modified microscope, capable of both atomic force microscopy (AFM) and nanoindentation, was employed to investigate the surface morphology of LiOH.H{sub 2}O grown on LiOH at high water vapor partial pressures and the kinetics of this growth.« less

Coherent medium theory for a master equation description of transport on random lattices is extended to includes scattering in clusters larger than two sites (or one bond) exactly. Bond and site-averaged numerical calculations for the square lattice show some features of transport are improved by this method but that larger clusters are needed in order to obtain exact long-time properties.

The dynamics of polar solvent reorganization near an instantaneously created charge are investigated. A linear, Vlasov‐type equation is derived, where the interactions between the polar molecules are treated using a mean field term containing the direct correlation function. For the diffusive limit, a Smoluchowski–Vlasov equation for reorientation is studied. The results, for model solvents, are compared to the results from continuum dielectric theories, to experimental results, and to results from recent computer simulations.

Charge transfer reactions in polar solvents are accompanied by reorganization of the solvent. In many cases, the dynamics of this reorganization determines the rate for charge transfer. In this paper the dynamics of this process is investigated with special emphasis on the molecular nature of the solvent. The reaction coordinate is identified and an equation of motion for this coordinate is derived. The equation describes the diffusion of the reaction coordinate in a bistable potential, which can be treated using Kramers’ theory. The form of the bistable potential is discussed in detail, with special emphasis on the distinction between this effective potential and a nonequilibrium free energy. As an example the resulting expressions for the rate of electron transfer between the charged and uncharged states of a zwitterion are calculated using a simple, approximate model for the fluid structure. The difference between these results and those from the continuum dielectric treatment of the solvent are discussed.

Calculations of the site-site correlation functions for a model of water were performed as a function of density (or pressure) at fixed temperature. These calculations are discussed and compared to neutron-scattering data. The structure of liquid water at high pressure is consistent with a substantially distorted hydrogen-bonding network. It is found that, unlike MeOH, water cannot easily accommodate structures associated with directional attractive forces in the region of several kilobars of pressure.

The behavior of random walkers on a lattice with randomly distributed traps is investigated. Specifically, we use the coherent potential approximation to solve the master equation for nearest-neighbor hopping. The results of this calculation are compared with recent CPA calculations for the diffusion equation with randomly placed spherical traps in a continuum.

The behavior of an impurity spin in a one-dimensional chain is investigated using the Glauber model. Two different types of impurities are considered and exact expressions for the average spin of the impurity, given that the impurity was initially excited out of equilibrium, are found. The behavior of these models is discussed in detail and their relevance to other physical situations is considered.

It is apparent that the kinetic rate kD influences the effective rate of reaction and that in certain circumstances (kr � kr) it becomes the rate­ limiting step [keff = kD]. Accordingly, the rate coefficient kD' which describes the mass transport rate at which reactants encounter each other, is of central concern to solution kineticists. There is little doubt that the subject of diffusion-controlled reactions has broad application beyond this prototype example. Diffusion-controlled reactions play a prominent role in heterogeneous catalysis (after all, the perfeot catalyst leads to mass transport limitations), polymer chain growth kinetics, colloid or crystal growth, and biophysical chemistry.

A theory of diffusion controlled reactions with randomly distributed penetrable sinks is presented. The model considered includes a reversible reaction between a diffusing species, and a fixed reactive sink. For low reactivity, both steady state and initial value problems are discussed. The dependence on the sink concentration of the rate coefficients and the diffusion coefficients is found. By considering the limit of fast irreversible reaction, the connection is made with previous work.

The extended reference interaction site method is used to calculate the structure of water behind a shock front. The competing effects of increased density and temperature modify the correlation functions in a manner that is qualitatively consistent with both expectation and recent experimental results.

The use of the mean spherical approximation (MSA) in the study of dynamics of polar fluids is further explored in two ways. First, the recent result of Wolynes [J. Chem. Phys. 86, 5133 (1987)] showing that solvation dynamics can be extracted from the static solution of the MSA is further investigated. The solvation of a dipole in a dipolar fluid is calculated, and compared with an ion in a dipolar fluid. Both cases reduce to simple numerical quadrature. Second, the Smoluchowski–Vlasov equation (SVE) of Calef and Wolynes [J. Chem. Phys. 78, 4145 (1983)] is also reexamined. Using the analytic MSA results, a new set of relaxation times can be defined. In both sections, structural relaxation occurs with multiple relaxation times.

The behaviour of water under high pressure was studied using Monte Carlo NPT simulation methods. The results of the simulations were compared to previous experimental and theoretical works. The TIP4P water model reproduced calculated densities for pressures ranging from 0·1 MPa to 1·0 GPa to within 2 per cent of the experimental densities. The trend of a slight decrease in energy as the pressure increases at constant temperature appears to reproduce experimental findings but is found to be model and method dependent. Hydrogen bond and angular distribution functions analyses were performed in an effort to uncover the structural relaxation mechanism of water at high pressures.

The correct choice of boundary conditions for applying hydrodynamics to molecular transport properties has been widely discussed. In this paper, we explore the possibility that flow birefringence data can be used to distinguish between the stick and slip boundary conditions. We present results for the flow birefringence from ellipsoidal particles. We discuss the related phenomena of the Rytov dip in depolarized light scattering. We report calculations for this dip for both boundary conditions. The use of mixed boundary conditions is also discussed.

In epithelial cells, a barrier or tight junction restricts the diffusion of lipid probes from the apical to the basolateral side of the outer membrane bilayer. This phenomenon is studied theoretically with the diffusion equation on planar and spherical surfaces. Two models for the tight junction are considered: a penetrable barrier embedded in a monolayer and an impenetrable obstacle in the outer membrane of a bilayer than must be bypassed by flip-flopping between inner and outer membranes. The rate of passing from one side of the cell to the other is calculated for each of these models under steady state conditions. The results are compared with recent fluorescent photobleaching recovery experiments. The theoretical interpretation indicates that it would be difficult to distinguish experimentally between the flip-flop case and the barrier crossing case. Assuming a flip-flop model, large differences in the magnitude of the flip-flop rates of probes are necessary to explain the experimental results as suggested by Dragsten et al. (Dragsten, P. R., R. Blumenthal, and J. S. Handler, 1981, Nature [Lond.], 294:718--722).

connected nwlusters ' [u1 —W,i] connected decompositions [u1 —W,s] The problem of transport of a localized particle in a random network has been the subject of many investigations, ' ' which have been applicable either to very long or very short times. In this paper, we present a new calculation which bridges the gap between short and long times, by using both lowand high-density expansions in a novel two-point Pade approximant technique. " As an example, we compute the probability of remaining on the initial site, Po(r), as a function of time for a two-dimensional triangular lattice. Using scaling arguments based on the recent results of Coniglio for the structure of the percolating cluster, we examine both the critical and noncritical parts of Po(r) near the percolation edge, and finally we address the question of anomalous diffusion on this cluster. ' The model we consider is a triangular lattice with randomly populated sites, on which the transport is governed by a master equation with transfer matrix 8', whose elements W&= tv(g&. Here, w is the nonrandom nearest-neighbor jump rate and g, is a random variable for occupation of site i: (& = 0 if i is unoccupied, (;= 1 if it is occupied, and (g, ) = p (the concentration of occupied sites), where the bracket represents an average over all configurations consistent with p. The probability of occupation of a site at time t is governed by the averaged Green's function, G(t) = (exp Wr), or by its Laplace transform, G(u) = ([ul —W] '). The probability that the particle is at the initial site is given by Gas(t), which is, of course, a function of p. We write the low-density expansion of G (u ) as The first term on the right-hand side of Eq. (2) corresponds to solving the master equation on a given cluster of n sites; the second term removes from this all processes involving fewer than n sites, leaving only processes contributing to p". All possible locations of the n cluster are summed over in Eq. (2). To compute Gpp( ),uwe only consider connected clusters containing the origin and sum over all locations by taking the trace for each unique cluster. We enumerate the clusters and perform the matrix inversions on a computer. In the same spirit, we write the high-density expansion as

A simple method is introduced to estimate the barrier to dissociation of nitrogen under shock conditions. A free-energy curve is constructed from quantum-mechanical calculations of the ground-state potential well for ${\mathrm{N}}_{2}$, the model exponential-six potential for the dissociated state, and the additional chemical potential (due to the surrounding dense media) in the hard-sphere approximation. The curves are adjusted to yield behavior in agreement with equilibrium thermodynamic calculations. The barriers are several electron volts, decreasing with pressure. Estimates for the prefactor in an Arrhenius rate expression are based on the recent computer simulations of nitrogen vibrational relaxation [B. Holian, J. Chem. Phys. 84, 3138 (1986)]. Time scales for reaction vary from microseconds to tenths of nanoseconds with increasing shock pressure.

Publisher Summary This chapter discusses the stability or chaos in discrete epidemic models. It describes a study on certain systems of difference equations that are formulated as deterministic epidemic models. The simplest of these is the equation x n +1 = (1− x n ) (1− e − A x n ). The model underlying this equation is a variation of the classical Reed–Frost model and A is a parameter that can be interpreted as the infectivity of the disease. There is a strong stability for all A > 0 and chaos does not appear. The chapter presents the analysis of the equation x n +1 = (1− x n ) (1− e −Ax n ) and the complete proofs of stability properties.

A lattice random-walk model is used to simulate diffusion in a porous polymer. This model may be useful for the practical design of drug-release systems. Both interacting and noninteracting particles (random walkers) were allowed to diffuse through a pore with a single exit hole. It was found that the specific interactions among the diffusing particles have little influence on the overall release rate. Diffusion through more complicated structures was investigated by simulating the diffusion of particles through two pores connected by a constricted channel whose length and width were varied. The overall rate of release was found to be proportional to the width of the constricted channel. When the length of the channel was greater than or equal to the length of the pore, the rate of release was also inversely proportional to the channel length. From a practical standpoint, release rates can be decreased (and times for release increased) by one or two orders of magnitude by decreasing the width and expanding the length of the interconnecting channels in the polymer matrix.

The parameters Km and kcat were determined for 16 methyl hippurates (CH3OCOCH2NHCOC6H4-X) hydrolyzed by papain. A simple linear relationship is found between log 1 K m and the hydrophobic substituent constant π. It is found that log kcat is parabolically related to π. The results with papain are compared with results obtained by Hawkins and Williams with the enzyme bromelain. The two enzymes behave in a similar fashion.