Here are my refereed papers and some of my non-refereed papers. To find a complete list of all of my publications, refereed, non-refereed, and abstracts, please see my Google Scholar profile. If you are having a hard time accessing any of my papers, send me a direct message on Twitter and I will try to help out.
My growing publication list, which reflects my ecletic taste in science.
- Soto, A., Fronk, R. G., Neal, K., Ehresmann, B., Bellinger, S. L., Shoffner, M., & McGregor, D. S. (2020). A semiconductor-based neutron detection system for planetary exploration. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 966, 163852. https://doi.org/10.1016/j.nima.2020.163852
We explore the use of microstructured semiconductor neutron detectors (MSNDs) to map the ratio between thermal neutrons and higher energy neutrons. The system consists of alternating layers of modular neutron detectors (MNDs), each comprising arrays of twenty-four MSNDs, and high-density polyethylene moderators (HDPE) with gadolinium shielding to filter between thermal neutrons and higher energy neutrons. We experimentally measured the performance of three different configurations and demonstrated that the sensor system prototypes detect and differentiate thermal and epithermal neutrons. We discuss future planetary exploration applications of this compact, semiconductor-based low-energy neutron detection system.
- Farnsworth, K. K., Chevrier, V. F., Steckloff, J. K., Laxton, D., Singh, S., Soto, A., & Soderblom, J. M. (2019). Nitrogen Exsolution and Bubble Formation in Titan’s Lakes. Geophysical Research Letters. https://doi.org/10.1029/2019gl084792
Titan’s surface liquids are composed primarily of methane, ethane, and dissolved atmospheric nitrogen. The nitrogen content depends on the alkane composition and temperature, and exsolves as bubbles when these parameters are sufficiently perturbed. Herein, we present an experimental study of nitrogen bubbles in methane–ethane liquids, and propose that both methane and ethane are required for bubbles to form under Titan conditions. Bubbles occur when methane composes 40–95 mol% of the alkanes within the liquid. We identify two mechanisms that produce bubbles: ethane mediated titration and temperature‐induced stratification. Both of these mechanisms produce a metastable nitrogen supersaturation within the liquid; equilibration triggers rapid nitrogen exsolution in the form of bubbles. Such equilibration could cause bubble events in Titan’s lakes, possibly explaining the transient “Magic Island” features seen by Cassini RADAR (bubbles within the liquid column), and the presence of deltas in Ontario Lacus.
- McDonald, G. D., Hayes, A. G., Ewing, R. C., Lora, J. M., Newman, C. E., Tokano, T., et al. (2016). Variations in Titan’s dune orientations as a result of orbital forcing. Icarus, 270, 197–210. https://doi.org/10.1016/j.icarus.2015.11.036
Wind-blown dunes are a record of the climatic history in Titan’s equatorial region. Through modeling of the climatic conditions associated with Titan’s historical orbital configurations (arising from apsidal precessions of Saturn’s orbit), we present evidence that the orientations of the dunes are influenced by orbital forcing. Analysis of 3 Titan general circulation models (GCMs) in conjunction with a sediment transport model provides the first direct intercomparison of results from different Titan GCMs. We report variability in the dune orientations predicted for different orbital epochs of up to 70°. Although the response of the GCMs to orbital forcing varies, the orbital influence on the dune orientations is found to be significant across all models. Furthermore, there is near agreement among the two models run with surface topography, with 3 out of the 5 dune fields matching observation for the most recent orbital cycle. Through comparison with observations by Cassini, we find situations in which the observed dune orientations are in best agreement with those modeled for previous orbital configurations or combinations thereof, representing a larger portion of the cycle. We conclude that orbital forcing could be an important factor in governing the present-day dune orientations observed on Titan and should be considered when modeling dune evolution.
- Soto, A., Mischna, M., Schneider, T., Lee, C., & Richardson, M. (2015). Martian atmospheric collapse: Idealized GCM studies. Icarus, 250, 553–569. https://doi.org/10.1016/j.icarus.2014.11.028
Global energy balance models of the martian atmosphere predict that, for a range of total CO2 inventories, the CO2 atmosphere may condense until a state with a permanent polar cap is reached. This process, which is commonly referred to as atmospheric collapse, may limit the time available for physical and chemical weathering. The global energy balance models that predict atmospheric collapse represent the climate using simplified parameterizations for atmospheric processes such as radiative transfer and atmospheric heat transport. However, a more detailed representation of these atmospheric processes is critical when the atmosphere is near a transition, such as the threshold for collapse. Therefore, we use the Mars Weather Research and Forecasting (MarsWRF) general circulation model (GCM) to investigate how the explicit representation of meridional heat transport and more detailed radiative transfer affects the onset of atmospheric collapse. Using MarsWRF, we find that previous energy balance modeling underestimates the range of CO2 inventories for which the atmosphere collapses and that the obliquity of Mars determines the range of CO2 inventories that can collapse. For a much larger range of CO2 inven- tories than expected, atmospheric heat transport is insufficient to prevent the atmospheric collapse. We show that the condensation of CO2 onto Olympus Mons and adjacent mountains generates a condensa- tion flow. This condensation flow syphons energy that would otherwise be transported poleward, which helps explain the large range of CO2 inventories for which the atmosphere collapses.
- Guo, X., Richardson, M. I., Soto, A., & Toigo, A. (2010). On the mystery of the perennial carbon dioxide cap at the south pole of Mars. Journal of Geophysical Research: Planets, 115(E4), E04005. https://doi.org/10.1029/2009JE003382
A perennial ice cap has long been observed near the south pole of Mars. The surface of this cap is predominantly composed of carbon dioxide ice. The retention of a CO2 ice cap results from the surface energy balance of the latent heat, solar radiation, surface emission, subsurface conduction, and atmospheric sensible heat. While models conventionally treat surface CO2 ice using constant ice albedos and emissivities, such an approach fails to predict the existence of a perennial cap. Here we explore the role of the insolation‐dependent ice albedo, which agrees well with Viking, Mars Global Surveyor, and Mars Express albedo observations. Using a simple parameterization within a general circulation model, in which the albedo of CO2 ice responds linearly to the incident solar insolation, we are able to predict the existence of a perennial CO2 cap at the observed latitude and only in the southern hemisphere. Further experiments with different total CO2 inventories, planetary obliquities, and surface boundary conditions suggest that the location of the residual cap may exchange hemispheres favoring the pole with the highest peak insolation.
- Fesen, R., Rudie, G., Hurford, A., & Soto, A. (2008). Optical Imaging and Spectroscopy of the Galactic Supernova Remnant 3C 58 (G130.7+3.1). The Astrophysical Journal Supplement Series, 174(2), 379. https://doi.org/10.1086/522781
We present a comprehensive imaging and spectroscopic survey of optical emission knots associated with the young Galactic supernova remnant 3C 58. Hα images show hundreds of clumpy filaments and knots arranged in a complex structure covering a nearly circular area roughly 400 ” in diameter. A quite different emission structure is seen in [O III], where the brightest features are less clumpy and largely confined to the remnant’s northwest quadrant. Measured radial velocities of over 450 knots reveal two distinct kinematic populations; one with average and peak expansion velocities of 770 and 1100 km s −1 , respectively, forming a thick shell, and the other showing | v | ≤ 250 km s −1 . High-velocity knots ( | v | ≥ 500 km s −1 ) exhibit a strong bipolar expansion pattern with redshifted and blueshifted knots located in northeastern and southwestern regions, respectively. These knots also show strong [N II]/Hα line emission ratios, suggesting enhanced N/H. In contrast, the slower expanding knot population shows much lower [N II]/Hα line ratios and likely represents circumstellar mass loss material from the 3C 58 progenitor. Proper-motion estimates using images spanning a 28 yr time interval suggest positional shifts of between 0.5 ” and 2.0 ”, implying proper motions of 0.02 ”-0.07 ” yr −1 . These values agree with previous estimates but are much less than the ##IMG## [http://ej.iop.org/icons/Entities/ap.gif] ≈ 0.2 ” yr −1 expected if 3C 58 were associated with historic guest star of 1181 CE.
- Anderson, M. S., Andringa, J. M., Carlson, R. W., Conrad, P., Hartford, W., Shafer, M., et al. (2005). Fourier transform infrared spectroscopy for Mars science. Review of Scientific Instruments, 76, 4101-+. https://doi.org/10.1063/1.1867012
Presented here is a Fourier transform infrared spectrometer (FTIR) for field studies that serves as a prototype for future Mars science applications. Infrared spectroscopy provides chemical information that is relevant to a number of Mars science questions. This includes mineralogical analysis, nitrogen compound recognition, truth testing of remote sensing measurements, and the ability to detect organic compounds. The challenges and scientific opportunities are given for the in situ FTIR analysis of Mars soil and rock samples. Various FTIR sampling techniques are assessed and compared to other analytical instrumentation. The prototype instrument presented is capable of providing field analysis in a Mars analog Antarctic environment. FTIR analysis of endolithic microbial communities in Antarctic rocks and a Mars meteor are given as analytical examples.
Conference and Workshop Proceedings
Here are a few non-refereed papers that may be of interest.
- Soto, A. (2015). Meridional Transport in the Atmospheres of Earth and Mars. ArXiv e-Prints.
As we continue to discover terrestrial exoplanets, many with orbital and planetary characteristics drastically different from anything encountered in our solar system, we are likely to encounter ’exotic’ atmospheric transport processes. As an example, we show an analysis of meridional transport from simulations Mars. These simulations provide insight into the differences in meridional transport between Earth and Mars, particularly through the role of a condensation flow. The differences between Earth and Mars are a reminder that there may be a wide variety of meridional transport processes at work across the range of observed terrestrial planets.
- Soto, A., Richardson, M. I., & Newman, C. E. (2010). Global constraints on rainfall on ancient Mars: Oceans, lakes, and valley networks. In Lunar and Planetary Institute Science Conference Abstracts (Vol. 41, p. 2397).
- Heverly, M., Dougherty, S., Toon, G. C., Soto, A., & Blavier, J.-F. (2004). A Low Mass Translation Mechanism for Planetary FTIR Spectrometry Using an Ultrasonic Linear Motor. Proceedings Fo the 37th Aerospace Mechanisms Symposium.
One of the key components of a Fourier Transform Infrared Spectrometer (FTIR) is the linear translation stage used to vary the optical path length between the two arms of the interferometer. This translation mechanism must produce extremely constant velocity motion across its entire range of travel to allow the instrument to attain high signal-to-noise ratio and spectral resolving power. A new spectrometer is being developed at the Jet Propulsion Laboratory under NASA’s Planetary Instrument Definition and Development Program (PIDDP). The goal of this project is to build upon existing spaceborne FTIR spectrometer technology to produce a new instrument prototype that has drastically superior spectral resolution and substantially lower mass, making it feasible for planetary exploration. In order to achieve these goals, Alliance Spacesystems, Inc. (ASI) has developed a linear translation mechanism using a novel ultrasonic piezo linear motor in conjunction with a fully kinematic, fault tolerant linear rail system. The piezo motor provides extremely smooth motion, is inherently redundant, and is capable of producing unlimited travel. The kinematic rail uses spherical Vespel rollers and bushings, which eliminates the need for wet lubrication, while providing a fault tolerant platform for smooth linear motion that will not bind under misalignment or structural deformation. This system can produce velocities from 10 – 100 mm/s with less than 1% velocity error over the entire 100-mm length of travel for a total mechanism mass of less than 850 grams. This system has performed over half a million strokes under vacuum without excessive wear or degradation in performance. This paper covers the design, development, and testing of this linear translation mechanism as part of the Planetary Atmosphere Occultation Spectrometer (PAOS) instrument prototype development program.