The vast majority of planets are too distant to visit, so remote observation and subsequent analysis are essential to the search for extrasolar life. Telescopes such as the James Webb Space Telescope are providing an important new opportunity to directly observe exoplanet atmospheres for signs of life, but we currently lack a quantitative framework for understanding what observations of a planet’s atmosphere provide compelling evidence for life on the underlying planet. Developing a framework that allows one to infer whether or not a planet is inhabited is a two-step process: understand atmospheres in the absence of life (abiotic baselines), and understand how life modifies an atmosphere (atmospheric biosignatures).  This grant renews support for a team of modelers and experimentalists -the AEThER collaboration (Atmospheric Empirical, Theoretical, and Experimental Research)- to tackle the former question.AEThER seeks to develop a framework to quantify the abiotic atmospheric baseline for rocky planets commonly found in our galaxy. Developing such a framework will provide a flexible tool for quantifying how different conditions driving the formation and evolution of a planet lead to different abiotic atmospheric baselines.  Funded activities under this grant include a series of experiments to broaden our understanding of how readily so-called “volatile” elements and compounds—which include nitrogen gas, oxygen gas, hydrogen gas, water, ammonia, and carbon and sulfur dioxide—dissolve into magmas and liquid metals at the high temperatures and pressures common during planetary formation and evolution. The solubility of these molecules plays a key role in determining the viscosity and possible solidification of a planet’s mantle, with significant implications for heat transfer throughout the planet and atmosphere, as well as gas release back to the atmosphere, and thus habitability. In addition and informed by this experimental work, AEThER will continue to develop their theoretical models, including modeling the impacts of atmospheric hazes (suspended small particles) on planetary evolution, which, under different conditions, can either raise or lower planetary temperatures appreciably. When completed, the funded grant work will represent a notable advance in our understanding of planetary processes, and serve as an important complement to research aimed at identifying atmospheric biosignatures. 

Identifying signs of life on another planet would rank among the top scientific discoveries in history. Two of the most daunting challenges to achieving such a discovery are developing instrumentation to study exoplanet atmospheres and developing a quantitative framework to assess whether life creates distinctive atmospheric biosignatures. Here, David Catling will address the latter challenge, aiming to develop a quantitative framework to identify atmospheric signatures of life on distant planets. While billions have been invested in telescopes capable of studying exoplanet atmospheres, we lack robust methods to interpret this data for signs of life.   The project focuses on assessing how living organisms impact a planet’s atmosphere. As such, the research team makes assumptions about life. Firstly, they assume Earth-familiar microbial life because Earth contains the only known examples of life, and because it’s thought that if life exists elsewhere in the universe, it’s more likely to be microbial rather than plant- or animal-based. Secondly, they assume redox-based metabolism would be universal to any life form because reduction / oxidation (redox) reactions are the only class of chemical reactions that release enough energy to satisfy the high energy demands of organisms. Rather than looking for individual gases that might indicate living systems, Catling proposes examining chemical disequilibrium—multiple gases coexisting that should normally react and eliminate each other—as a more reliable biosignature.   The research team will build an integrated model simulating planetary evolution from lifeless to hosting various biospheres. They'll quantify two potential biosignatures: free energy dissipation (which should increase dramatically with biological activity) and the information content of atmospheric disequilibrium. The final step involves determining which measurable gas abundances and fluxes most reliably indicate biological activity.

This grant will support Professor Zhiyue Lu to develop a theoretical framework to understand biochemical networks that operate far from equilibrium and outside steady states. Living systems exist in far-from-equilibrium states; this characteristic is one of the most striking distinctions between living and non-living systems.   Unlike non-living matter, living organisms demonstrate remarkable sensitivity and complex responses to environmental changes. While most scientific understanding focuses on near-equilibrium or steady-state systems, Lu aims to explore how biochemical networks respond to time-varying environmental conditions.   The research focuses on three key properties of living systems: sensitivity and robustness to environmental changes, ability to manipulate energy to power various processes, and capacity to extract energy from fluctuating environments. Through mathematical modeling and simulation, Lu's team will investigate how network structure influences these properties under different temporal patterns of environmental change. Specifically, they'll study how networks process complex information patterns, combine weak energy sources to power energy-intensive processes (or distribute energy from one source to multiple processes), and extract energy from environments that fluctuate at different timescales.

Scientists believe that before complex cellular life emerged on Earth, simpler “containers” may have served as early "protocells." Droplets containing various chemical systems are one candidate for such cell-like containers, but we currently have a poor understanding of whether such a protocell could have grown and divided—a fundamental requirement for life. Using a technique called electrochemistry, which applies electric current to drive chemical reactions, Professor Julio Alvarez will study how electrical charges affect the behavior of tiny droplets. When droplets become sufficiently charged, they can become unstable and split apart, potentially mimicking cellular division. The research team will use ultra-small electrodes to precisely control chemical reactions that modify droplet charge states. They'll examine various factors including different surfactants (molecules that stabilize droplets), solution environments, and conditions that mimic the crowded, viscous environment inside modern cells. By understanding what controls droplet growth and division, this research could illuminate a critical step in the transition from non-living chemistry to life on early Earth. The findings may also contribute to modern synthetic biology efforts to build artificial cells from scratch.

This grant funds a collaboration between Sara Walker, a Professor in the School of Earth and Space Exploration and Deputy Director of the Beyond Center for Fundamental Concepts in Science at Arizona State University, and Leroy Cronin, the Regius Professor of Chemistry at the University of Glasgow, to advance the development of Assembly Theory (AT) by formalizing the mathematical structure of the theory and extending its applicability. Assembly Theory is a framework developed to quantify the complexity of molecules and objects by assessing the minimal number of steps required to assemble them from fundamental building blocks. The theory assigns an assembly index to objects, which serves as a measure of their structural complexity. It is one promising framework for quantifying, understanding, and predicting the emergence and evolution of varied types of complex objects. Walker and Cronin will work towards a general AT framework by determining a suite of mathematical relationships that hold across any assembly space. This will involves applying AT to other substrates, in this case, minerals and genomes / proteomes. The team will develop an assembly theory for minerals and they propose to trace out the evolutionary history of minerals on Earth by combining mineral AT with existing phylogenetic methods that reveal evolutionary connections between objects. The team will also apply AT to large molecules made from nucleic acid building blocks (e.g. DNA / RNA) and from amino acid building blocks (proteins). The plan is to combine AT with phylogenetic techniques to gain insights into the evolutionary history of the modern-day transcription and translation systems used by all known life. Walker and Cronin will also attempt to establish connections between assembly theory and thermodynamics. By developing a bridged framework—Assembly Thermodynamics—they expect to quantify how limits on free energy constrain when a selection phase transition takes place, and to make predictions about such transitions that can be tested in laboratory experiments. If successful, this project will uncover mathematical relationships that apply to all versions of AT (irrespective of the type of object undergoing complexification), allow AT to describe complex minerals and polymers (DNA, RNA, proteins), and make connections between AT and thermodynamics.

The information stored in genes plays a huge role in directing the cellular processes underlying life, but stored information alone is inadequate to explain how cells function. Cellular forces and associated motions also play a decisive role in determining whether and when genetic information is expressed because DNA can only be copied when a chromosome is in its decompacted state and because forces and other cellular dynamics drive the transition between compacted and decompacted chromosome states. Forces are also central to the creation and migration of chromosomal density fluctuations, pockets of compaction in a nominally decompacted chromosome region (or of decompaction in a compacted region). These migrating density fluctuations can, in different scenarios, contribute both to biological function and to disfunction. Funds from this grant support Andrew Spakowitz, a Professor of Chemical Engineering and of Materials Science & Engineering at Stanford University, to develop theoretical models that will advance our understanding of how forces/dynamics impact the organization and function of chromosomes in eukaryotic cells. Spakowitz plans to achieve a multi-scale model via a staged progression of model development that describes the coupled chemical/mechanical dynamics starting at the molecular scale, he will then expand to intermediate-scale chromosome dynamics (about a tenth of a chromosome), and end with an exploration of dynamics at the scale of an entire chromosome or group of chromosomes. Forces to be modeled include constraining forces that arise from chemical bond formation between two typically distant segments of a chromosome that happen to come into proximity owing to the wiggling motion of a chromosome in the aqueous environment of a cell and which, in turn, promote chromosome compaction; thermal agitation forces (owing to collisions with water molecules) that drive chromosome decompaction; and the force exerted on DNA (by RNA polymerase) during transcription (reading of genetic information).  if successful, the project will improve our understanding of how coupled mechanical and chemical interactions at the molecular-scale drive the organizational dynamics observed at the much larger length scale of a chromosome or group of chromosomes, thereby providing insight into how forces mediate access to and use of genetically encoded information.