SETI Institute
  • Mountain View, CA, United States
Recent publications
The deuterium-to-hydrogen (D/H or ²H/¹H) ratio of Martian atmospheric water (∼6× standard mean ocean water, SMOW) is higher than that of known sources, requiring planetary enrichment. A recent measurement by NASA's Mars Science Laboratory rover Curiosity of Hesperian-era (>3 Ga) clays yields a D/H ratio ∼3×SMOW, demonstrating that most of the enrichment occurs early in Mars's history, reinforcing the conclusions of Martian meteorite studies. As on Venus, Mars's D/H enrichment is widely thought to reflect preferential loss to space of ¹H (protium) relative to ²H (deuterium), but both the cause and the global environmental context of large and early hydrogen losses remain to be determined. Here, we apply a recent model of primordial atmosphere evolution to Mars, link the magma ocean of the accretion epoch with a subsequent water-ocean epoch, and calculate the behavior of deuterium for comparison with the observed record. In contrast to earlier works that consider Martian D/H fractionation in atmospheres in which hydrogen reservoirs are present exclusively as H2O or H2, here we consider 2-component (H2O-H2) outgassed atmospheres in which both condensing (H2O) and escaping (H2) components – and their interaction – are explicitly calculated. We find that a ≈2-3× hydrospheric deuterium-enrichment is produced rapidly if the Martian magma ocean is chemically reducing at last equilibration with the primordial atmosphere, making H2 and CO the initially dominant species, with minor abundances of H2O and CO2. Reducing gases – in particular H2 – can cause substantial greenhouse warming and prevent a water ocean from freezing immediately after the magma ocean epoch. We find that greenhouse warming due to plausible H2 inventories (pH=21−102 bars) yields surface temperatures high enough (T=s290−560 K) to stabilize a water ocean and produce an early hydrological cycle through which surface water can be circulated. Moreover, the pressure-temperature conditions are high enough to produce ocean-atmosphere H2O-H2 isotopic equilibrium through gas-phase deuterium exchange such that surface H2O strongly concentrates deuterium relative to H2, which preferentially takes up protium and escapes from the primordial atmosphere. The efficient physical separation of deuterium-rich (H2O) and deuterium-poor (H2) species via condensation permits equilibrium isotopic partitioning and early atmospheric escape to be recorded in modern crustal reservoirs. The proposed scenario of primordial H2-CO-rich outgassing and escape suggests significant durations (>Myr) of chemical conditions on the Martian surface conducive to prebiotic chemistry immediately following magma ocean crystallization.
We examine the 2.21-μm band from 19 disk-integrated Charon spectra measured by New Horizons/LEISA in the week leading up to its closest encounter with the Pluto system. These observations cover one Charon rotation period. Additionally, we analyze Charon’s 2.21-μm band from 22 Earth-based spectra obtained over the last two decades. We measure the equivalent width of the 2.21-μm band from all observations and study it as a function of sub-observer longitude. We find no significant variation in the 2.21-μm band as Charon rotates. Compared to the same band seen on Nix and Hydra, Charon’s 2.21-μm band is several times weaker. We attribute the 2.21-μm band to NH4Cl based on the appearance of a weaker band at 2.24-μm. Furthermore, we see two never-before-reported absorption features in Charon’s spectrum at 1.60 and 1.63-μm, which may also be due to NH4Cl. If NH3-H2O-ice mixtures are present on Charon, they must be a small fraction of the disk-average composition to be consistent with the spectrum at 1.99-μm.
Nuclear resonant vibrational spectroscopy (NRVS) is an excellent modern vibrational spectroscopy, in particular, for revealing site-specific information inside complicated molecules, such as enzymes. There are two different concepts about the energy calibration for a beamline or a monochromator (including a high resolution monochromator): the absolute energy calibration and the practical energy calibration. While the former pursues an as-fine-as-possible and as-repeatable-as-possible result, the latter includes the environment influenced variation from scan to scan, which often needs an in situ calibration measurement to track. However, an in situ measurement often shares a weak beam intensity and therefore has a noisy NRVS spectrum at the calibration sample location, not leading to a better energy calibration/correction in most cases. NRVS users for a long time have noticed that there are energy drifts in the vibrational spectra’s zero-energy positions from scan to scan (ΔEi), but their trend has not been explored and utilized in the past. In this publication, after providing a brief introduction to the critical issue(s) in practical NRVS energy calibrations, we have evaluated the trend and the mechanism for these zero-energy drifts (ΔEi) and explored their link to the energy scales (αi) from scan to scan. Via detailed analyses, we have established a new stepwise procedure for carrying out practical energy calibrations, which includes the correction for the scan-dependent energy variations using ΔEi values rather than running additional in situ calibration measurements. We also proved that one additional instrument-fixed scaling constant (α0) exists to convert such “calibrated” energy axis (E′) to the real energy axis (Ereal). The “calibrated” real energy axis (Ereal) has a preliminary error bar of ±0.1% (the 2σE divided by the vibrational energy position), which is 4–8 times better than that from the current practical energy calibration procedure.
Astrotourism and related citizen science activities are becoming a major trend of a sustainable, high-quality tourism segment, core elements to the protection of Dark skies in many countries. In the Summer of 2020, in the middle of COVID pandemics, we started an initiative to train young students - Cyber-Cosmos - using an Unistellar eVscope, a smart, compact and user-friendly digital telescope that offers unprecedented opportunities for deep-sky observation and citizen science campaigns. Sponsored by the Ciência Viva Summer program, this was probably the first continuous application of this equipment in a pedagogical and citizen-science context, and in a pandemic context. Pampilhosa da Serra, home to a certified Dark Sky destination (Aldeias do Xisto) in central Portugal, was the chosen location for this project, where we expect astrotourism and citizen science to flourish and contribute to space sciences education.
A highly accurate CO2 ab initio dipole moment surface (DMS), Ames-2021, is reported along with 12C16O2 infrared (IR) intensity comparisons approaching a 1-4‰ level of agreement and uncertainty. The Ames-2021 DMS was accurately fitted from CCSD(T) finite-field dipoles computed with the aug-cc-pVXZ (X = T, Q, 5) basis for C atom and the d-aug-cc-pVXZ (X = T, Q, 5) basis for O atoms, and extrapolated to the one particle basis set limit. Fitting σrms is 3.8 × 10-7 au for 4443 geometries below 15 000 cm-1. The corresponding IR intensity, SAmes-2021, are computed using the Ames-2 potential energy surface (PES), which is the best PES available for CO2. Compared to high accuracy IR studies for 2001i-00001 and 3001i-00001 bands, SAmes-2021 matches NIST experiment-based intensities [SNIST-HIT16 or SHIT20] to -1.0 ± 1.3‰, or matches DLR experiment-based intensities [SDLR-HIT16/UCL/Ames] to 1.9 ± 3.7‰. This indicates the systematic deviations and uncertainties have been significantly reduced in SAmes-2021. The SUCL2015 (or SHITRAN2016) have larger deviations (vs SDLR) and uncertainties (vs SDLR, SNIST) which are attributed to the less accurate Ames-1 PES adopted in UCL-296 line list calculation. The SAmes-2021 intensity of 12C16O2 and 13C16O2 is utilized to derive new absolute 13C/12C ratios for Vienna PeeDee Belemnite (VPDB) with uncertainty reduced by 1/3 or 2/3. Further evaluation of SAmes-2021 intensities are carried out on those CO2 bands discussed in the HITRAN2020 update paper. Consistent improvements and better accuracies are found in band-by-band analysis, except for those bands strongly affected by Coriolis couplings, or very weak bands measured with relatively larger experimental uncertainties. The Ames-2021 296 K IR line lists are generated for 13 CO2 isotopologues, with 18 000 cm-1 and S296 K > 1 × 10-31 cm/molecule cutoff and then combined with CDSD line positions (except 14C16O2). The Ames-2021 DMS and 296 K IR line lists represent a major improvement over previous CO2 theoretical IR intensity studies, including Ames-2016, UCL-296, and recent UCL DMS 2021 update. A real 1 permille level of agreement and uncertainty will definitely require both more accurate PES and more accurate DMS.
Algal-bacterial interactions provide clues to algal physiology, but mutualistic interactions are complicated by dynamic exchange. We characterized the response of Chlamydomonas reinhardtii to the presence of a putative alga-benefitting commensal bacterium (Arthrobacter strain ‘P2b’). Co-cultivation promoted chlorophyll content, biomass, average cell size, and number of dividing cells, relative to axenic cultures. Addition of bacterial spent medium (whole, size-fractionated and heat-treated) had similar effects, indicating P2b does not require algal interaction to promote growth. Nutrients and pH were excluded as putative effectors, collectively indicating a commensal interaction mediated by Arthrobacter-released small exometabolite(s). Proteogenomic comparison revealed similar response to co-cultivation and spent media, including differential cell cycle regulation, extensive downregulation of flagellar genes and histones, carbonic anhydrase and RubisCO downregulation, upregulation of some chlorophyll, amino acid and carbohydrate biosynthesis genes, and changes to redox and Fe homeostasis. Further, Arthrobacter protein expression indicated some highly expressed putative secondary metabolites. Together, these results revealed that low molecular weight bacterial metabolites can elicit major physiological changes in algal cell cycle regulation, perhaps through a more productive G1 phase, that lead to substantial increases in photosynthetically-produced biomass. This work illustrates that model commensal interactions can be used to shed light on algal response to stimulating bacteria.
Bedforms on Earth and Mars are often preserved in the rock record in the form of sedimentary rock with distinct cross‐bedding. On rare occasions, the full‐surface geometry of a bedform can be preserved through burial and lithification. These features, known as paleobedforms, are found in a variety of geographic locations on Mars. Evidence in the morphology of paleobedforms, such as the retention of impact craters and steep erosional scarps, suggests that these features are well‐lithified and capable of withstanding prolonged weathering and erosion. Here, we present results from thermophysical and compositional analyses on a subset of the best preserved paleobedform candidate fields on Mars. Thermophysical modeling elucidates the changes these bedforms underwent from their unconsolidated, particulate nature to their currently observed properties. Certain paleobedforms have elevated thermal inertias (e.g., ∼300–500 J·m⁻²·s−1/2·K⁻¹) when compared with modern bedforms (∼250 J·m⁻²·s−1/2·K⁻¹), and modeling indicates that they have cement volumes of 0.8%–1.5% even as high as 30%. However, most paleobedform candidates have unexpectedly low thermal inertia when compared with modern dunes. Additionally, compositional analyses reveal a range of spectral characteristics within paleobedforms (e.g., primary and secondary alteration products). These features add to the already existing class of Martian surfaces in which thermal inertia does not seem to correspond to erodibility, cohesion, or mechanical strength. Studying paleobedforms with both raised and nonraised thermal inertia has provided new insights into lithification on Mars and constrained the environmental conditions leading to the formation of these enigmatic features.
Asteroid 2008 TC3 impacted the Earth's atmosphere with a known shape and orientation. Over 600 meteorites were recovered at recorded locations, including meteorites of nonureilite type. From where in the asteroid did these stones originate? Here, we reconstruct the meteor lightcurve and study the breakup dynamics of asteroid 2008 TC3 in 3‐D hydrodynamic modeling. Two fragmentation regimes are found that explain the lightcurve and strewn field. As long as the asteroid created a wake vacuum, the fragments tended to move into that shadow, where they mixed with small relative velocities and surviving meteorites fell along a narrow strip on the ground. But when the surviving part of the backside and bottom of the asteroid finally collapsed at 33 km altitude, it created an end flare and dust cloud, while fragments were dispersed radially with much higher relative speed due to shock–shock interactions with a distorted shock front. Stones that originated in this final collapse tended to survive in a larger size and fell over a wider area at locations on the ground. Those locations to some extent still trace back to the fragment's original position in the asteroid. We classified the stones from this “large mass” area and used this information to glean some insight into the relative location of recovered ureilites and ordinary and enstatite chondrites in 2008 TC3.
It has been a great joint achievement of astronomy, laboratory spectroscopy and quantum chemistry to identify interstellar molecules in various astronomical environments and piece together their origins story from the fragmented evidence. Here the authors provide a sketch of what we know and motivate the asking of open questions on carbon-based molecules in space.
CO-bound forms of nitrogenase are N2-reduction inhibited and likely intermediates in Fischer-Tropsch chemistry. Visible-light photolysis at 7 K was used to interrogate all three known CO-related EPR-active forms as exhibited by the α-H195Q variant of Azotobacter vinelandii nitrogenase MoFe protein. The hi(5)-CO EPR signal converted to the hi-CO EPR signal, which reverted at 10 K. FT-IR monitoring revealed an exquisitely light-sensitive "Hi-2" species with bands at 1932 and 1866 cm-1 that yielded "Hi-1" with bands at 1969 and 1692 cm-1, which reverted to "Hi-2". The similarities of photochemical behavior and recombination kinetics showed, for the first time, that hi-CO EPR and "Hi-1" IR signals arise from one chemical species. hi(5)-CO EPR and "Hi-2" IR signals are from a second species, and lo-CO EPR and "Lo-2" IR signals, formed after prolonged illumination, are from a third species. Comparing FT-IR data with CO-inhibited MoFe-protein crystal structures allowed assignment of CO-bonding geometries in these species.
Plain Language Summary It has become apparent over the last few years that small asteroids and comets are very underdense compared with the materials they are made of. This means that their total porosities are likely quite high, in excess of 70%, both as tiny voids within particles (so‐called microscopic porosity) and spaces between particles (macroscopic porosity). But none are likely as porous as the distant denizens of the Kuiper belt such as Arrokoth (visited by the New Horizons spacecraft in 2019). This paper concerns impact craters on Arrokoth and similar small bodies, and the rather unusual effects expected. Imagine a fluffy (fine powder) snowball striking a much larger fluffy snowball, only that the snow is not pure ice but a mixture of porous icy, rocky, and carbon‐rich particles. Even at high velocities (>100 s of m/s) craters should mostly form by compacting pore space and pushing material away from the impact point, not the traditional blasting of ejecta back into space. Similar to crush‐up of an automobile bumper, compaction helps to protect from the potentially catastrophic effects of large impacts, such as complete disruption of the target or breakup of bilobate bodies like Arrokoth, and should be incorporated in future collisional evolution studies.
Our understanding on the role of amorphous carbon in the interstellar medium to form complex structures, such as carbon nanotube/graphene/fullerene, is limited to date. We have investigated how shocks may induce Physico-chemical transformation in amorphous carbon by subjecting samples of carbon nanopowder to high-temperature shocks, ∼ 7300 K, in a hydrogen-free environment for about 2 ms using a shock tube. The shock conditions achieved in the laboratory mimics low velocity interstellar shocks. Post shock samples were analyzed using Raman Spectroscopy and High Resolution - Transmission Electron Microscopy. We have found compelling evidence for the formation of carbon nanotube, graphene, and various other carbon nanostructures induced by the shock. Our results show that shocks in the interstellar medium can provide important chemical pathways to the formation of fullerenes and suggests that other carbonaceous structures may be present as well.
Early Earth and Mars had analogous environments. While life developed on our planet, the question of whether it did on Mars remains to be answered. Hot spring deposits are compelling targets for exploration because of their high habitability and potential to retain morphological and chemical biosignatures. As a result in this study, we aim to better understand the potential for biosignature preservation in Fe-bearing hydrothermal systems. Understanding oxidation-reduction reactions involving Fe in hot springs is a key step in elucidating the preservation process. Fe reacts readily with reactive oxygen species (ROS), which are produced in hot spring surface waters through photochemical processes. Furthermore, Fe3+ can bind to cell membranes and preserve complex organic molecules (i.e., biomarkers). ROS formation is typically controlled by photoreactions with dissolved organic matter (DOM). However, Fe redox reactions more likely control ROS formation in these Fe-bearing systems. We deconvolved the relationship of ROS with Fe in hot springs and evaluated the role that DOM and dissolved organic sulfur (DOS) may have in ROS production. To better understand these coupled systems, field and laboratory experiments were conducted in hot springs of Yellowstone National Park. In situ H2O2 concentrations observed in these hot springs were comparable to, or higher than, those of other high-temperature systems. Reaction rates determined by measuring concentrations after specified time intervals varied based on water compositions and the presence of particulate or dissolved matter. Fe speciation (photochemical reactivity), concentration, and solubility further determined ROS cycling rates. Specifically, photochemically active Fe enhanced both ROS formation and decay rates depending on incident UV irradiance, and rates increased along with Fe concentration and solubility (i.e., in acidic conditions). Better understanding how ROS and Fe cycle in predominantly abiotic conditions will eventually aid in distinguishing between biosignatures and abiotic substances in the rock record.
Europan domes are positive relief features that are typically circular to elliptical in planform shape, and have characteristic diameters <16 km. Although it cannot be ruled out that many of these domes may have been formed from the intrusion of diapirs into Europa's crust, a subset of domes have relatively smooth surfaces that do not mimic the surrounding terrain. These domes appear to obscure the preexisting terrain and have distinct margins which may be lobate or rounded. If all domes on Europa's surface represented structures where the icy crust had simply been “punched up” by diapiric intrusions, uplifts with these distinct morphologies would not be expected to exist. In this study, we revisit the hypothesis that a subset of europan domes formed in a manner similar to lava domes on Earth and Venus. Previously, we modeled dome formation as a consequence of the extrusion of viscous cryolava. However, that approach only allowed for the investigation of late-stage eruptive processes far from the vent and provided little insight into how cryovolcanic fluids may have arrived at the surface. Consideration of cryolava dome emplacement as fluids erupt onto Europa's surface is therefore pertinent. A volume flux approach, in which dome formation is modeled as fluid extrudes onto the surface at a constant rate, has been successfully applied to the formation of lava domes on Venus. That study showed that neglecting to consider changes in fluid rheology while a constant flux of lava is actively extruded onto the surface may result in overestimates, by several orders of magnitude, of initial lava viscosities at the time of eruption. Obtaining accurate viscosity estimates for Europa's cryovolcanic fluids is a critical step in understanding the properties of near-surface fluids that have participated in subsurface-surface exchange in the geologically recent past. To place improved constraints on the rheology and composition of europan cryolavas, and to better gauge the potential for dome formation on Europa via effusive eruptions, we apply this new volume flux approach to the formation of putative europan cryolava domes. We present a perturbation solution to the generalized form of the Boussinesq equation for fluid flow in a cylindrical geometry and explore dome formation while fluid is continuously extruding onto the surface. We find that at the time of eruption, dome-forming cryolavas may have had viscosities of 10¹–10³ Pa s. These viscosity values suggest that cryolavas may be briny slurries composed of a mixture of water, salts, and ice crystals, rather than pure water (viscosity ~10⁻³ Pa s) or simple brines (viscosities between 10⁻³ and 10⁻¹ Pa s). Nevertheless, the derived bulk viscosities indicate that dome-forming cryolavas have a rheology more similar to basalt than typical higher-viscosity andesite to rhyolitic dome-forming lavas on Earth. Several of the domes in our study may be connected to liquid reservoirs in Europa's crust, and subsurface-surface exchange may be ongoing today. As such, these domes represent compelling targets for multispectral imaging, radar sounding, and surface sampling by future missions to Europa.
A unified theory of particle transport by wind can explain the observations of aeolian features, like dunes, across the Solar System rocky bodies with atmospheres.
Desert ecosystems are a key repository for important Mars analog habitats and the extant or extinct life within them. We provide an overview of four main desert habitat types—soils, sediments, salts, and rocks—and the extreme microbiology living within them, with a particular focus on the hyperarid Atacama Desert and Dry Valleys of Antarctica, the driest and coldest limits for life on Earth. We construct habitat maps of Mars from an ecological perspective and the first estimates of study sample sizes of key habitats from historical and recent Mars orbiter and lander imagery and data. We review the lessons that can be drawn for the search for life on Mars from decades of microbial ecology work in end-member terrestrial deserts.
Hydrothermal systems and their deposits are primary targets in the search for fossil evidence of life beyond Earth. However, to learn how to decode fossil biomarker records in ancient hydrothermal deposits, we must first be able to interpret unambiguously modern biosignatures, their distribution patterns, and their association with physicochemical factors. Here, we investigated the molecular and isotopic profile of microbial biomarkers along a thermal gradient (from 29 to 72°C) in a hot spring (labeled Cacao) from El Tatio, a geyser field in the Chilean Andes with abundant opaline silica deposits resembling the nodular and digitate structures discovered on Mars. As a molecular forensic approach, we focused on the analysis of lipid compounds bearing recognized resistance to degradation and the potential to reconstruct the paleobiology of an environment on a broader temporal scale than other, more labile, biomolecules. By exploiting the lipid biomarkers' potential to diagnose biological sources and carbon fixation pathways, we reconstructed the microbial community structure and its ecology along the Cacao hydrothermal transect. The taxonomic adscription of the lipid biomarkers was qualitatively corroborated with DNA sequencing analysis. The forensic capacity of the lipid biomarkers to identify biosources in fresh biofilms was validated down to the genus level for Roseiflexus, Chloroflexus, and Fischerella. We identified lipid biomarkers and DNA of several new cyanobacterial species in El Tatio and reported the first detection of Fischerella biomarkers at a temperature as high as 72 • C. This, together with ecological peculiarities and the proportion of clades being characterized as unclassified, illustrates the ecological singularity of El Tatio and strengthens its astrobiological relevance. The Cacao hydrothermal ecosystem was defined by a succession of microbial communities and metabolic traits associated with a high-(72°C) to low-(29°C) temperature gradient that resembled the inferred metabolic sequence events from the 16S rRNA gene universal phylogenetic tree from thermophilic to anoxygenic photosynthetic species and oxygenic phototrophs. The locally calibrated DNA-validated lipidic profile in the Cacao biofilms provided a modern (molecular and isotopic) end member to facilitate the recognition of past biosources and metabolisms from altered biomarkers records in ancient silica deposits at El Tatio analogous to Martian opaline silica structures.
Multiple layers of lava flows and channels characterize the region adjacent to the eastern slope of Olympus Mons, the largest volcano on Mars. We have mapped this volcanic region to survey and classify individual channel systems and determine their formative processes. As a final output of this mapping effort, we have produced a 1:1.9 million scale channel map that is first published in this paper in both GIS and static formats.
It is commonly accepted that exoplanets with orbital periods shorter than one day, also known as ultra-short-period (USP) planets, formed further out within their natal protoplanetary disks before migrating to their current-day orbits via dynamical interactions. One of the most accepted theories suggests a violent scenario involving high-eccentricity migration followed by tidal circularization. Here we present the discovery of a four-planet system orbiting the bright (V = 10.5) K6 dwarf star TOI-500. The innermost planet is a transiting, Earth-sized USP planet with an orbital period of ~13 hours, a mass of 1.42 ± 0.18 M⊕, a radius of 1.166−0.058+0.061R⊕ and a mean density of 4.89−0.88+1.03gcm−3. Via Doppler spectroscopy, we discovered that the system hosts 3 outer planets on nearly circular orbits with periods of 6.6, 26.2 and 61.3 days and minimum masses of 5.03 ± 0.41 M⊕, 33.12 ± 0.88 M⊕ and 15.05−1.11+1.12M⊕, respectively. The presence of both a USP planet and a low-mass object on a 6.6-day orbit indicates that the architecture of this system can be explained via a scenario in which the planets started on low-eccentricity orbits then moved inwards through a quasi-static secular migration. Our numerical simulations show that this migration channel can bring TOI-500 b to its current location in 2 Gyr, starting from an initial orbit of 0.02 au. TOI-500 is the first four-planet system known to host a USP Earth analogue whose current architecture can be explained via a non-violent migration scenario. TOI-500 hosts at least four planets, the innermost of which is an Earth-sized ultra-short-period body with a density similar to Earth. The architecture of the TOI-500 system can be explained by a slow, secular, low-eccentricity migration scenario.
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65 members
Dale T. Andersen
  • Carl Sagan Center
Douglas A Caldwell
  • Carl Sagan Center
Alfonso F Davila
  • Carl Sagan Center
Susan E. Thompson
  • Kepler Mission
189 Bernardo Ave, Suite 200, 94043, Mountain View, CA, United States
Head of institution
Bill Diamond
(650) 961-6633
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