Natasha M. Johnson

A Self-Perpetuating Catalyst for the Production of Complex Organic Molecules in Protostellar Nebulae

When hydrogen, nitrogen, and CO are exposed to amorphous iron silicate surfaces at temperatures between 500 and 900 K a carbonaceous coating forms via Fischer-Tropsch-type reactions. Under normal circumstances such a coating would impede or stop further reaction. However, we find that this coating is a better catalyst than the amorphous iron silicates that initiate these reactions. Formation of a self-perpetuating catalytic coating on grain surfaces could explain the rich deposits of macromolecular carbon found in primitive meteorites and would imply that protostellar nebulae should be rich in organic material.

The Formation of Graphite Whiskers in the Primitive Solar Nebula

It has been suggested that carbonaceous grains are efficiently destroyed in the interstellar medium and must either reform in situ at very low pressures and temperatures or in an alternative environment more conducive to grain growth. Graphite whiskers have been discovered associated with high-temperature phases in meteorites such as calcium aluminum inclusions and chondrules, and it has been suggested that the expulsion of such material from protostellar nebulae could significantly affect the optical properties of the average interstellar grain population. We have experimentally studied the potential for Fischer-Tropsch and Haber-Bosch type reactions to produce organic materials in protostellar systems from the abundant H{sub 2}, CO, and N{sub 2} reacting on the surfaces of available silicate grains. When graphite grains are repeatedly exposed to H{sub 2}, CO, and N{sub 2} at 875 K abundant graphite whiskers are observed to form on or from the surfaces of the graphite grains. In a dense, turbulent nebula, such extended whiskers are very likely to be broken off, and fragments could be ejected either in polar jets or by photon pressure after transport to the outer reaches of the nebula.

A Mechanism for the Equilibrium Growth of Mineral Crystals in AGB Stars and Red Giants on 105 yr Timescales

We show that for particle sizes ranging from a few hundred angstroms up to several tens of microns in diameter the force exerted by radiation pressure in some red giant and AGB stars exceeds the force of gravity and thus offers the potential for graphite, SiC, corundum, and spinel grains to grow to the size range observed in primitive meteorites (e.g., up to ~25 μm). In the highest mass AGB stars radiation pressure on growing grains greatly exceeds the force of gravity and thus ejects a grain from the star before it can grow larger than a few tens of nanometers. Only in very low mass AGB stars (less than 3 M☉) does the radiative force balance the gravitational force to such a fine degree that the net acceleration on individual particles ranging from a few nanometers up to about 25 μm produces particle velocities that are comparable to atmospheric turbulence. Our analysis shows that the large graphite, SiC, corundum, and spinel crystals found in primitive meteorites can only have formed in the atmospheres of the lowest mass red giant and AGB stars, where particle growth is able to occur on timescales of a hundred thousand years under near-equilibrium conditions. We note that this suggestion is contrary to the standard assumption that grains can only form in stellar winds and implies that there may be a class of grains that can form in chemical equilibrium deep within the stellar atmosphere, just above the photosphere.

Complex Protostellar Chemistry

Numerical models show that protoplanetary nebulae are sites of chemical activity even in the cold outer disk. Two decades ago, our understanding of the chemistry in protostars was simple—matter either fell into the central star or was trapped in planetary-scale objects. Some minor chemical changes might occur as the dust and gas fell inward, but such effects were overwhelmed by the much largerscale processes that occurred even in bodies as small as asteroids. The chemistry that did occur in the nebula was relatively easy to model because the fall from the cold molecular cloud into the growing star was a one-way trip down a well-known temperature-pressure gradient; the only free variable was time. However, just over 10 years ago it was suggested that some material could be processed in the inner nebula, flow outward, and become incorporated into comets (1, 2). This outward flow was confirmed when the Stardust mission returned crystalline mineral fragments (3) from Comet Wild 2 that must have been processed close to the Sun before they were incorporated into the comet. On page 452 of this issue, Ciesla and Sandford (4) demonstrate that even the outermost regions of the solar nebula can be a chemically active environment. Their finding could have consequences for the rest of the nebula.

A self-perpetuating catalyst for the production of complex organic molecules in protostellar nebulae

When hydrogen, nitrogen and CO are exposed to amorphous iron silicate surfaces at temperatures between 500–900 K a carbonaceous coating forms via Fischer-Tropsch type reactions. Under normal circumstances such a coating would impede or stop further reaction. However, we find that this coating is a better catalyst than the amorphous iron silicates that initiate these reactions. Formation of a self-perpetuating catalytic coating on grain surfaces could explain the rich deposits of macromolecular carbon found in primitive meteorites and would imply that protostellar nebulae should be rich in organic material.

On the Use of Fourier Transform Infrared (FT-IR) Spectroscopy and Synthetic Calibration Spectra to Quantify Gas Concentrations in a Fischer-Tropsch Catalyst System

One possible origin of prebiotic organic material is that these compounds were formed via Fischer-Tropsch-type (FTT) reactions of carbon monoxide and hydrogen on silicate and oxide grains in the warm, inner-solar nebula. To investigate this possibility, an experimental system has been built in which the catalytic efficiency of different grain-analog materials can be tested. During such runs, the gas phase above these grain analogs is sampled using Fourier transform infrared (FT-IR) spectroscopy. To provide quantitative estimates of the concentration of these gases, a technique in which high-resolution spectra of the gases are calculated using the High-Resolution Transmission Molecular Absorption (HITRAN) database is used. Next, these spectra are processed via a method that mimics the processes giving rise to the instrumental line shape of the FT-IR spectrometer, including apodization, self-apodization, and broadening due to the finite resolution. The result is a very close match between the measured and computed spectra. This technique was tested using four major gases found in the FTT reactions: carbon monoxide, methane, carbon dioxide, and water. For the ranges typical of the FTT reactions, the carbon monoxide results were found to be accurate to within 5% and the remaining gases accurate to within 10%. These spectra can then be used to generate synthetic calibration data, allowing the rapid computation of the gas concentrations in the FTT experiments.

Low-cost, compact, and robust gas abundance sensor package

Gas Abundance Sensor Package (GASP) is a stand-alone scientific instrument that has the capability to measure the concentration of target gases based on a non-dispersive infrared sensor system along with atmospheric reference parameters. The main objective of this work is to develop a GASP system which takes advantage of available technologies and off-the-shelf components to provide a cost-effective solution for localized sampling of gas concentrations. GASP will enable scientists to study the atmosphere and will identify the conditions of the target’s planetary local environment. Moreover, due to a recent trend of miniaturization of electronic components and thermopiles detectors, a small size and robust instrument with a reduction in power consumption is developed in this work. This allows GASP to be easily integrated into a variety of small space vehicles such as CubeSats or small satellite system, especially the Micro-Reentry Capsule (MIRCA) prototype vehicle. This prototype is one of the most advanced concepts of small satellites that has the capability to survive the rapid dive into the atmosphere of a planet. In this paper, a fully-operational instrument system will be developed and tested in the laboratory environment as well as flight preparation for a field test of the instrument suite will be described.

Exploring the Possible Continuum Between Comets and Asteroids

Abstract There are many indicators that suggest that the differences between asteroids and comets may result from their evolution over several billion years, based on similar accretion processes in the primitive solar nebula rather than in fundamental differences in their accretional histories. Our goal is to review some of the various processes (e.g., headwind drag-induced radial drift, grain–grain collisional dynamics, thermal processing vs. accretion time, etc.) that would act to accrete or to modify the accreted materials as a function of the size of the object. We do not account for the migration of the giant planets on the populations of asteroids and comets as the consequences of the proposed mixing episodes would come much later. We will, however, discuss the potential implications of the formation of the terrestrial planets on the population of meteorite parent bodies in the inner solar system. These same processes do not seem to have been important in or beyond the giant planet region of the solar nebula.