Research Projects

Supported Research Projects

Every year the Caltech Center for Comparative Planetary Evolution funds a selection of new proposals designed to strengthen collaboration across the Center, build interdisciplinary ties, and advance the science of comparative planetary evolution. Projects supported in the past are shown below.

Elucidating the Limits of Microbial Activity Under Ultrahigh Pressure

Jennifer Jackson, Victoria Orphan, John Magyar, Rebecca Wipfler

Diamond anvil cells (DACs) can generate static pressures in excess of 300 gigapascals, the pressure within Earth's core, by compressing a medium (such as a liquid) between two cut diamonds, thus providing a transparent ‘window' to examine planetary materials in the created high-pressure chamber using a suite of analytical techniques. Generally used in geophysics and other high-pressure fields of research, we will use a DAC system to analyze the activity of cultured polyextremophile microorganisms (archaea and bacteria) under a range of physicochemical conditions, including variable temperature and pressure, salinity, acidity, and nutrient limitation. The design of the DAC gasket available in Jackson's lab allows for four replicate chambers (200 μm diameter) to be spectroscopically examined during incubation without the need for depressurization of microbes or removal from the DAC. In-situ real time tracking of single-cell activity and metabolic products at high-pressures will be tracked using fluorescence and novel x-ray scattering techniques. While limits to single extreme environmental conditions are known for some microbes, the effects of combined multiple extreme conditions, as are often found in natural habitats, are less constrained. This project will examine and quantify the limits of microbial life by establishing how simultaneous extreme environmental conditions affect cellular viability and activity at the single-cell level over time without the need for decompression or removal from the incubation chamber.

Hot Rocks on Highly Irradiated Super-Earths: A New High Temperature Spectral Library

Heather A. Knutson, George Rossman, Bethany Ehlmann

Surveys of nearby stars have revealed that a majority host rocky planets larger than the Earth ('super-Earths') on close-in orbits. Some of these planets appear to have relatively tenuous atmospheres, potentially allowing astronomers to measure thermal emission from their surfaces and constrain their surface compositions for the first time. In order to interpret these observations, we require a spectral library spanning a wide range of bulk mineral compositions. However, current spectral libraries were measured at room temperature, and we expect that the spectral features of some minerals will change as their temperatures increase. We will heat a representative set of rock samples up to 1000 K (1300 degrees F) and measure their temperature-dependent emission spectra in order to create a new high-temperature spectral library. We will then use these data to interpret upcoming observations of highly irradiated super-Earths using the James Webb Space Telescope, which is the first telescope capable of detecting surface spectral features from rocky planets outside the solar system.

_{^{Image credit: NASA, ESA, CSA, Dani Player (STScI).}}

Insights into the Evolution of the Early Solar System from Isotopes and Disk Modeling

Francois Tissot, Konstantin Batygin, and Tony Yap

Prevailing models for the origin of the salient isotopic dichotomy between Solar System (SS) materials rely on heterogeneous infall of molecular cloud material onto the innermost regions of the protoplanetary disk: i.e., close to the nascent Sun where the SS's first condensates, known as Ca-Al-rich Inclusions (CAIs), were synthesized. As such, CAIs offer the potential to probe the processes that took place at a critical time and space in early SS evolution. Despite their ubiquity in primitive carbonaceous chondrites, there is considerable uncertainty regarding how CAIs inherited their diverse physical and isotopic properties, as well as the mechanisms that facilitated their transport to the outer SS, where they were incorporated into carbonaceous chondrites. We are measuring Ti nucleosynthetic isotope anomalies in fine-grained CAI leachates from the CV3 Allende meteorite to gain insights into the physico-chemical characteristics of the mineral phases hosting these anomalies. In other words, we are determining which minerals are favored by presolar carriers enriched in 46Ti and 50Ti. As different minerals condense at different times within the CAI formation region, according to time-dependent pressure and temperature conditions therein, the potential discovery that 46Ti- and 50Ti-carriers are found in separate phases would point to the temporal isotopic evolution of cloud infall. Our results will also evaluate the role of thermal processing in establishing the correlations observed in Ti anomalies of CAIs and genetically-related refractory objects, which remains a topic of debate in the community today. To complement and aid in the interpretation of our measurements, we are developing a disk model with emphasis on the role of winds in the magnetically active CAI formation region at the SS's inner edge.

Volcanic synthesis of polyphosphate as a source of energy for early life

Amanda Bednarick, Ed Stolper, Woody Fischer

What energy sources powered the early biosphere? Polyphosphate is present in every living cell today. Because the phospho-anhydride bonds in a polyphosphate molecule are extremely high-energy (30 kJ/mol), it is hypothesized that polyphosphate compounds could have been key components of a very primitive means of energy transduction for the first lifeforms on an early Earth. Prior work established the measurement of polyphosphate in a natural fumarole sample from Mt. Usu, Japan demonstrates the Earth's abiotic production of soluble phosphate compounds today (Yamagata et al, 1991). Our research seeks to expand upon and scale these data to obtain a rough approximation for global (terrestrial) volcanic polyphosphate flux, with the intention of placing this value in the context of prebiotic chemistry and opportunities to fuel a nascent biosphere on a rocky planet. Preliminary calculations suggest volcanoes on Earth emit on the order of 108 moles of polyphosphate per year, which equates to enough energy to power the population of Cody, WY.

_{Field photograph of Amanda collecting a gas/condensate sample from Obsidian Pool, Yellowstone National Park, US.}

Uranium in ophiolites as a record of deep ocean oxygenation

Claire E. Bucholz, François L.H. Tissot

In the modern oxygenated ocean, uranium (U) is present as a dissolved constituent in its oxidized 6+ form. During hydrothermal alteration of oceanic crust at mid-ocean ridges, U is both enriched (taken up from seawater) and isotopically fractionated. Although the present-day atmosphere and oceans are oxygenated, this has not been true throughout Earth history. Consequently, the marine U cycle and alteration of oceanic crust has also varied due to the redox sensitivity of U, although the details are poorly understood. Here we propose to explore the U cycle during hydrothermal alteration of oceanic crust using the ophiolite record as a tool to understand deep ocean marine oxygen concentrations through Earth history. Ophiolites are fragments of oceanic crust that have been preserved in the rock record and have long been used to interrogate marine geochemistry as circulation of seawater through newly formed igneous oceanic crust represents one of the primary means by which the fluid and solid Earth interact and exchange both heat and chemical components. We will analyze both U concentrations and isotopes in three previously well characterized ophiolites from distinct times in Earth history reflecting different phases of ocean oxygenation and U cycling. Defining when the deep oceans became oxygenated is important to a variety of problems including the early evolution and diversification of animals, as well as, understanding how the composition of subducted materials may have varied through time and influence the deep Earth.

What does stromatolite morphology teach us about the biology, chemistry, and physics of the ancient seafloor?

Emily Geyman, John Grotzinger

Throughout the Archean and Proterozoic Eons (a 3.5-billion-year span), some of our primary macroscopic evidence of life comes in the form of stromatolites, which are laminated, accretionary growth structures that are thought to represent the interactions between microbial mats, carbonate precipitation/sedimentation, and hydrodynamics at the ancient seafloor. It has long been hypothesized that the size, shape, and texture of stromatolites records information about the ancient sedimentary environment. However, we still have no models to translate stromatolite morphology into quantitative constraints on the biology, chemistry, and physics of the ancient seafloor. This lack of a mechanistic understanding of stromatolite morphogenesis is important for two reasons. First, it means we cannot verify whether the earliest stromatolites are in fact biogenic. Second, the diversity of stromatolite form presents the exciting possibility that, if decoded, stromatolite morphology could tell us about ancient water depth, wave or current energy, sedimentation rates, seawater chemistry, and biological metabolisms. In other words, stromatolites could be sensitive recorders of the properties that geologists most seek to constrain in order to piece together the co-evolution of Earth's life and environment. Here, we develop simple, first-principle-based numerical models to describe how growth limitation via physical processes such as transport of ions and metabolites across a diffusive boundary layer, light availability, and erosion can give rise to a diverse array of growth forms. We use these numerical models to develop a set of testable hypotheses whereby analysis of the slope, curvature, and surface-normal thickness of stromatolite laminations can be used to constrain the chemical kinetics and biological function (metabolisms) associated with stromatolite formation at the ancient seafloor. We test these models using stromatolites from the 1.9-billion-year-old Rocknest Formation in Arctic Canada.

Inner Disk Temperature Profiles in Outbursting Young Stellar Objects

Lynne A. Hillenbrand

Improving our knowledge of the details of inner disk accretion physics, both in standard low-state accretion, and during accretion outbursts, is broadly important -- not only for star formation theory but also for planet formation and migration theory (Becker et al., 2021). Specifically, outbursts have dramatic consequences on protoplanetary disk heating and on chemical and physical disk evolution. As demonstrated in the Figure, exoplanets pile up in the regions corresponding to the innermost disk. There is also radial segregation by mass which has been hypothesized as caused by the influence of tidal interactions on the larger planets. However, this innermost region is also where the temperature history in protoplanetary disks is strongly affected by accretion outbursts. Our proposal is to evaluate in detail the temperature profiles in FU Ori outbursts, and to study the influence of temperature increases during extreme high-state accretion on young exoplanets. High spectral resolution data allows investigation of spectral line morphology, including assessment of disk-dominated vs wind-affected lines. We will measure wavelength-dependent line broadening, and thus constrain v(r) and vmax, and hence M/R and disk inclination. We will measure wavelength-dependent temperature, and thus constrain T(r) and Tmax, and hence the disk accretion rate. Our aim is to empirically characterize the innermost regions of planet-forming disks -- where most fully-formed planets eventually wind up.

Volatile Ingassing by Shock Processes in Martian Rocks

Paul Asimow, Jinping Hu

Glassy pockets in Martian meteorites are quenched from impact-derived melts that often incorporate significant concentrations of volatiles such as water, of which only a small portion is magmatic in origin¹. The bulk of it is derived from the near-surface environment on Mars, potentially preserving the history of aqueous activity in the shallow subsurface up to the moment of their ejection from Mars. However, it is unclear whether the impact glasses incorporate (sub)surficial volatiles by melting aqueously-altered phases or by partitioning from the Martian atmosphere directly¹. We are performing shock-recovery experiments on Martian analogue basalt mixed with of ¹³C-labeled CO₂ and ²H-labeled brucite to measure the incorporation of these solid-derived and gas-derived components into experimental shock melts and thereby solve this puzzle. In the first set of experiments, we used CO₂ gas with δ¹³C PDB of +40529 ‰ and 2 bar pressure to simulate an extreme environment. The recovered basalt shows pervasive bubbles, indicating absolute saturation of gas atmosphere, as expected. Mineral phases and impact melt show a rough positive correlation between CO₂ content and δ¹³C, with up to 200 ppm of CO₂ and δ¹³C = 4000 ‰. The results indicate that, even with unrealistically high (for modern Mars, at least) initial CO₂ pressure condition, the ingassing of CO₂ from the atmosphere is limited to hundreds of ppm. Hence Martian impact melts with similar CO₂ contents more likely remelted aqueously-altered phases in the subsurface, as opposed to direct capture from the atmosphere. The next stage is to repeat the experiment with deuterium-enriched brucite with the intent to show much more efficient capture of volatiles from a condensed-phase source.

¹Liu, Yang, Yang Chen, Yunbin Guan, Chi Ma, George R. Rossman, John M. Eiler, and Youxue Zhang. (2018) "Impact-Melt Hygrometer for Mars: The Case of Shergottite Elephant Moraine (EETA) 79001." Earth and Planetary Science Letters 490: 206–15.

An Isotopic Test for the Roles of Nebular CO and H2O in Prebiotic Synthesis

Geoff Blake, John Eiler, Surjyendu Bhattacharjee

The primitive, carbonaceous meteorites contain abundant and diverse organic molecules that are also synthesized by terrestrial life, meaning the asteroids from which these rocks are derived were potential sources of prebiotic compounds to the early Earth and other planets, and also present us with examples of natural prebiotic chemistry that may illuminate processes that could have been endogenous to early planetary environments. We are examining the role of nebular volatiles — principally CO and H2O — in the origin of these compounds by taking advantage of the marked differences in oxygen isotope composition between these species, and between them and rocky materials, in the early circum-solar disk. So far, we have established a method for precise and accurate oxygen isotope analysis of a variety of individual oxygen-bearing organic compounds, such as ketones and aldehydes, explored through first-principles chemical physics models the oxygen isotope fractionations associated with relevant to nebular and asteroidal chemical processes, and mapped out the methods we will use to extract, purify and analyze select molecules of interest in carbonaceous meteorites — measurements we intend to make during the latter half of 2022.

The Seismic and Magnetic Signatures of Saturn's core

Chris Mankovich, Janosz Dewberry, Jim Fuller, Dave Stevenson, and Kimberly Moore

External Collaborators: Sabrine Stanley, Chi Yan

There is enormous uncertainty in the internal structures of giant planets. Large concentrations of rocks and ices are suspected to exist in giant planet cores, but the core-envelope transition is now thought to be smooth such that the core is "dilute". In the case of Saturn, ring seismology data requires this transition to extend throughout most of the planet's interior. However, both seismic data and magnetic dynamo simulations suggest that there is a convective shell in between two regions with stabilizing composition gradients, which may be caused by helium rain-out and a dilute core. We are building suites of models with two convective shells and two stably stratified layers, computing their seismic and gravity signatures, and comparing with data. The best-fit models will reveal how thick each region is, and they will provide what is currently the clearest picture of the interior of a giant planet. This will shed light on the formation history of gas giant planets like Saturn, and it will help us understand how gaseous planets evolve over time

Bridging Mineralogy and Enzymology in the Origin of Life

Yuk Yung, Woody Fischer, Stuart Bartlett, Joshua Goldford, Daniel Zhao, Steffen Buessecker

Understanding the conditions necessary for life to originate provides a useful lens through which to approach comparisons between planets and their secular evolution. But when it comes to the origin of life, there are major gaps in our knowledge that we currently fill by waving a magic wand. This is exactly how we treat the emergence of metabolism—the energy-transducing network of molecular fluxes that define the core mission of any cell. How does/did metabolism emerge? What environmental characteristics enable this? Does it require exotic materials and rare conditions? Or are these likely to be common on rocky planetary bodies? We are approaching this problem with a unique combination of comparative computational analyses of minerals and protein structures, evaluation of metabolic networks found in extant cells to infer constituents of ancient metabolic networks, and chemical analyses of rock samples from the early geological record. Together this will help uncover the materials and environmental conditions that foster early metabolism, and potential for this to occur on other planetary bodies for which we have some mineralogical and chemical data, specifically Mars, Europa, Enceladus and those recorded by certain meteorites.

Io's Sulfur Isotope Cycle: Decoding the History of Tidal Heating

Katherine de Kleer, Ery Hughes, Amy Hoffman, John Eiler

Tides powered by orbital resonances are one of the main drivers of geological activity and heat generation on the Solar System's moons. On Jupiter's moon Io, tidally-driven volcanism dominates the properties of the surface and atmosphere, and this moon has thus long been used as a laboratory for understanding the mechanics of tidal heat generation. However, its high resurfacing rate and young surface, combined with an atmosphere that is dynamically created and lost, leave very few records of its long-term history. The question of how long the galilean satellite orbital resonance has been active and whether the current level of heating is representative or anomalous, including whether Io is currently in thermal equilibrium, remains a fundamental gap in our understanding of Io's thermal history. Isotopic abundances, which can preserve records of processes that occurred over Gyr timescales, present a promising unexplored avenue into filling this gap. Telescope and spacecraft instruments are capable of measuring sulfur isotopologues, but interpretation will rely on an understanding of the isotopic fractionation processes at work on Io. We are developing a box model to characterize and quantify such fractionation in Io's interior and surface environment, including performing some specific petrological and geochemical laboratory experiments to measure fractionation factors that have not been quantified to date under the relevant conditions. The resultant model can be applied to measurements of isotope abundances at Io by telescopes in the near-term and (hopefully) by spacecraft within the next 10-20 years. This work will additionally help us unravel processes occurring in the subsurface of our own planet Earth, such as how and where arc volcanoes acquire their heavy sulphur isotope signal and how to interpret gas measurements collected from volcano monitoring data.

A Novel Isotopic Tracer for Impact Events

Francois Tissot, Gerrit Budde, Ren Marquez

The Cretaceous-Paleocene (K/Pg, also known as K/T) boundary saw one of the largest ecological upheavals in Earth's history. The rocks from this time show marked enrichments in iridium (Ir), an element that is typically depleted on the Earth's surface but relatively abundant in some meteorites. This sparked a then largely controversial hypothesis that a large asteroid hit the Earth during this time. While its role in the subsequent mass extinction remains debatable, multiple lines of evidence have since confirmed that a giant impact event did occur around 66 million years ago. In this study, we are going to leverage novel isotopic tracers in K/Pg Boundary samples to identify the type of impactor that hit the Earth during this transition. Impactor contribution preserved in the rock record will be identified as isotopic deviations from a terrestrial/standard composition and have the potential to enable identification of where in the Early Solar System the K/Pg impactor was formed. More importantly, this study will serve as a first step in developing a powerful new tracer for detecting impact events in the sedimentary record.

In-Situ Studies of Polyextremophiles Under Ultrahigh Pressure

Jennifer Jackson, Victoria Orphan, Johannes Buchen, Rebecca Wipfler

Diamond anvil cells (DACs) can generate static pressures in excess of 300 gigapascals, the pressure within Earth's core, by compressing a medium (such as a liquid) between two cut diamonds, thus providing a transparent ‘window' to examine planetary materials in the created high-pressure chamber using a suite of analytical techniques. Generally used in geophysics and other high-pressure fields of research, we will use a DAC system to analyze the activity of cultured polyextremophile microorganisms (archaea and bacteria) under a range of physicochemical conditions, including high and low temperatures, high pressures, acidity, and nutrient limitation. The design of the DAC gasket available in Jackson's lab allows for four replicate chambers (200 μm diameter) to be spectroscopically examined during incubation without the need for depressurization of microbes or removal from the DAC. Single-cell microbial activity in the DAC will be assessed using redox-sensitive fluorescence tracers, and metabolite production will be tracked via Raman spectrophotometry. While limits to single extreme environmental conditions are known for some microbes, the effects of combined multiple extreme conditions, as are often found in natural habitats, are less constrained. This project will examine and quantify the limits of microbial life by establishing how simultaneous extreme environmental conditions affect cellular viability and activity at the single-cell level over time without the need for decompression or removal from the incubation chamber.

Environmental Controls on the Evolution of Complex Life on Earth

Alex Session, John Eiler, Ilya Bobrovsky

The appearance of eukaryotes roughly 600-800 million years is a singular event in the evolution of Earth's biosphere, and indeed the planet itself. It marks the rapid and virtually complete transition from a planet dominated by single-celled bacterial life, to one dominated by complex, multicellular plants and animals. However, the reasons for this sudden appearance of complex life some 3.7 billion years after the Earth formed are still hotly debated. Potential explanations range from evolutionary innovations that suddenly made complex body forms possible, to environmental determinants in the form of O2 or trace metals, or even population bottlenecks due to mass extinctions during the Snowball glaciations. Uncertainty stems in large part from our limited ability – in the absence of most fossils – to discern which specific organisms were present, where they were living, and what they were doing. Our project will seek to develop new methods for the analysis of carbon isotopes in biomarker compounds specific to eukaryotes. Such data should provide new insights into the metabolism and physiology of ancient organisms, and the environment to which they were responding. This in turn should allow us to test specific ideas about the origins of complex life on Earth.

Formation of Giant Planet Satellites

Konstantin Batygin, Phil Hopkins

Four hundred years ago, the astronomer Galileo Galilei announced his discovery of four moons orbiting around the planet Jupiter, each seen as a distinct white dot through his telescope. However, it is only in the span of the last four decades that astronomers have been able to study the Jovian moons in detail to reveal that the four—Io, Europa, Ganymede, and Callisto—are fascinating worlds of their own. The exact story of how these satellites formed, however, remains imperfectly understood. In particular, identifying the specific physical mechanism responsible for the conversion of icy dust into the building blocks of satellites presents a considerable challenge to the theory of satellite formation. To address this problem, we are employing large-scale numerical simulations of resonant drag instabilities — a process through which dust clouds can grow, and eventually coalesce into solid satellitesimals within a differentially rotating gaseous disk. As continued exploration of icy moons within the solar system ventures forward, these calculations will shed light on the origins of these remarkable worlds.