My deep interest in life beyond Earth has led me to research four aspects of astrobiology: the habitability of planetary interiors, the elemental requirements of life, prebiotic synthesis, and atmospheric biosignature detection. While trained in astrophysics and space instrumentation, I have chosen to get a grasp, uniquely obtained through research, on biology, geology, and chemistry. In the Desch group, my research focuses on modeling the geophysics and geochemistry of dwarf planet interiors to assess their habitability.
I have sought to understand the distribution of life on planets by tackling four diverse questions that cross the boundaries between traditional disciplines.
To detect and map trace gases on Mars, possible byproducts of geological or biological activity, I have helped develop an instrument that passively measures the absorption of gases at near-infrared wavelengths. This radiometer has two channels. The first is a reference in which sunlight, being reflected on the surface and traversing the atmosphere, passes through a cell filled with enough gas of interest to saturate its absorption. The second channel is identical except that its gas cell is empty, such that the light intensity is also sensitive to atmospheric trace gas absorption. Both channels are sensitive to spurious variations in Sun-Mars distance and angles, atmospheric thickness, and surface reflectance; but their ratio is sensitive only to trace gas absorption. I carried out proof-of-concept tests to measure CH2O, a photochemical byproduct of methane (CH4), using a lab setup. The compactness of the instrument allows multiple stacked duplicates to look for a variety of gases. I selected six gases by performing a trade study on their measurability and scientific interest (biogenic versus abiotic sources and sinks)9. The instrument is being developed by the PI, E. Wilson, for future missions to solar system bodies.Wilson, Neveu, et al., Measurement Science and Technology, 2011.
Life on Earth uses three large molecules: DNA stores genetic information, RNA reads and executes this blueprint; and proteins carry out the functions of life. This system seems too complex to have arisen from chemistry; rather, it may have evolved from a simpler system that used only one type of molecule. An ideal candidate is RNA, which has both information and functional capacities. Making RNA using the small organics and minerals likely present on early Earth has been a challenge for the past fifty years. I sought to make the “R” of RNA, ribose sugar (C5H10O5), using small amounts of organics delivered by meteorites and a large supply of CH2O, a photolytic product of Earth’s likely early atmosphere of CO2 and H2O. I tried to make ribose in large amounts compared to the meteoritic supply, and to ensure it was the preferred product out of a wide inventory of sugars. I succeeded in obtaining over 10 sugar molecules from one meteoritic precursor. Ribose is the preferred product if boron minerals (borates) are present. However, low cosmic abundances of B made borates scarce on early Earth, likely concentrated only locally in ponds eroding volcanic rocks. Could other minerals work as well? My experiments with P, As, Ge, and V minerals showed no such capacity, making borates the best candidates to guide RNA synthesis.Neveu, Kim, and Benner, Astrobiology, 2013.
All life needs a supply of key elements, such as H, C, N, S, S, P, and trace metals such as Fe. Elemental abundances in common ocean or lake microbes are bound within limited ranges. Are these constraints due to moderate habitats which all offer similar elemental supplies, or do they reveal a deeper attribute of all life? Does the elemental composition of life in environmental extremes differ? I have devised a method to measure major (C, N, P) and trace (e.g. Ni) element abundances in microbes living in temperature, pH, and salinity extremes. This method involves purifying cells from their host sediment (most of the material sampled in the field) whose elements (Fe, Mg) can overwhelm trace cell signals. Using mass spectrometry, I have thus determined elemental contents of extremophile microbes sampled in hot springs and salt-rich ponds.Neveu et al., Limnology and Oceanography: Methods, 2014.
Despite environmental extremes, extremophile microbes seem made of the same elements as common microbes. Thus, life may require elements within universal bounds. This could narrow the range of habitable settings. It may also suggest that life’s elemental needs were set by the early Earth conditions in which it emerged.Neveu et al., Geobiology, 2015.
In the solar system, there seems to be liquid water only on Earth, a few icy moons, and perhaps Mars. These objects form a tiny fraction of the ~90 roughly spherical worlds in the solar system, of which 75% are icy dwarf planets with large amounts of frozen water. Could icy dwarf interiors harbor liquid? If so, our perception that liquid water rarely occurs on planets would be revolutionized. Modeling the persistence of liquid water is complex, as aqueous processes feed back on the very mechanisms that keep water liquid, such as the presence of antifreeze salts or NH3. Many airless icy worlds seem differentiated into a rocky core and a hydrosphere; I have confirmed this using a 1D code that tracks heat conduction through time, heating by the decay of radioisotopes, differentiation, and parameterized convection in the liquid and ice shell. If present, liquid water is sandwiched between the ice shell and the core.
On cold icy worlds, water volcanism replaces silicate magma volcanism. Cryovolcanism is observed on Saturn's moon Enceladus, and perhaps on other moons such as Europa, Titan, Ariel, Triton, and on the dwarf planet Ceres. I have shown that fluid chemistry seems to enable cryovolcanism by extending predictive geochemical models to a previously unexplored range of low temperatures. As fluid ascends through ice fractures, decreasing pressures cause the explosive exsolution of dissolved gases. Efficient gases are CH4 or N2, which have low affinity with water; conversely, polar antifreezes such as NH3 and CH3OH remain dissolved and prevent freezing in the cracks.Neveu et al., Icarus, 2015.
The persistence of liquid water in dwarf planet interiors enables water-rock interactions, which critically alter fluid and rock composition and thermal properties. I have shown that interactions may be widespread by building a detailed model of core micro- and macro-fracturing and rock hydration coupled to a 1D evolution code. Thermal gradients drive fluid circulation through these fractures, which provide a larger interface and higher temperatures than at the seafloor; both effects speed up chemical reactions. Such reactions provide chemical energy that may support life. Fluid circulation cools the core and heats the hydrosphere in brief heat pulse episodes, thus sustaining oceans for tens of millions of years at temperatures and pressures adequate for life, even in the absence of tidal heating.Neveu et al., Journal of Geophysical Research, 2015.