An orbital window into the ancient Sun's mass

Models of the Sun's long-term evolution suggest that its luminosity was substantially reduced 2-4 billion years ago, which is inconsistent with substantial evidence for warm and wet conditions in the geological records of both ancient Earth and Mars. Typical solutions to this so-called "faint young Sun paradox" consider changes in the atmospheric composition of Earth and Mars, and while attractive, geological verification of these ideas is generally lacking-particularly for Mars. One possible underexplored solution to the faint young Sun paradox is that the Sun has simply lost a few percent of its mass during its lifetime. If correct, this would slow, or potentially even offset the increase in luminosity expected from a constant-mass model. However, this hypothesis is challenging to test. Here, we propose a novel observational proxy of the Sun's ancient mass that may be readily measured from accumulation patterns in sedimentary rocks on Earth and Mars. We show that the orbital parameters of the Solar system planets undergo quasi-cyclic oscillations at a frequency, given by secular mode g_2-g_5, that scales approximately linearly with the Sun's mass. Thus by examining the cadence of sediment accumulation in ancient basins, it is possible distinguish between the cases of a constant mass Sun and a more massive ancient Sun to a precision of greater than about 1 per cent. This approach provides an avenue toward verification, or of falsification, of the massive early Sun hypothesis.Comment: 7 pages, 4 Figures. Accepted to The Astrophysical Journal Letter

The resilience of Kepler systems to stellar obliquity

The Kepler mission and its successor K2 have brought forth a cascade of transiting planets. Many of these planetary systems exhibit multiple members, but a large fraction possess only a single transiting example. This overabundance of singles has lead to the suggestion that up to half of Kepler systems might possess significant mutual inclinations between orbits, reducing the transiting number (the so-called "Kepler Dichotomy"). In a recent paper, Spalding & Batygin (2016) demonstrated that the quadrupole moment arising from a young, oblate star is capable of misaligning the constituent orbits of a close-in planetary system enough to reduce their transit number, provided that the stellar spin axis is sufficiently misaligned with respect to the planetary orbital plane. Moreover, tightly packed planetary systems were shown to be susceptible to becoming destabilized during this process. Here, we investigate the ubiquity of the stellar obliquity-driven instability within systems with a range of multiplicities. We find that most planetary systems analysed, including those possessing only 2 planets, underwent instability for stellar spin periods below ~3 days and stellar tilts of order 30 degrees. Moreover, we are able to place upper limits on the stellar obliquity in systems such as K2-38 (obliquity <20 degrees), where other methods of measuring spin-orbit misalignment are not currently available. Given the known parameters of T-Tauri stars, we predict that up to 1/2 of super-Earth mass systems may encounter the instability, in general agreement with the fraction typically proposed to explain the observed abundance of single-transiting systems.Comment: 13 pages, 8 figures, accepted to The Astronomical Journa

A shorter Archean day-length biases interpretations of the early Earth's climate

Earth's earliest sedimentary record contains evidence that surface temperatures were similar to, or perhaps even warmer than modern. In contrast, standard Solar models suggest the Sun was 25% less luminous at this ancient epoch, implying a cold, frozen planet—all else kept equal. This discrepancy, known as the Faint Young Sun Paradox, remains unresolved. Most proposed solutions invoke high concentrations of greenhouse gases in the early atmosphere to offset for the fainter Sun, though current geological constraints are insufficient to verify or falsify these scenarios. In this work, we examined several simple mechanisms that involve the role played by Earth's spin rate, which was significantly faster during Archean time. This faster spin rate enhances the equator-to-pole temperature gradient, facilitating a warm equator, while maintaining cold poles. Results show that such an enhanced meridional gradient augments the meridional gradient in carbonate deposition, which biases the surviving geological record away from the global mean, toward warmer waters. Moreover, using simple atmospheric models, we found that the faster-spinning Earth was less sensitive to ice-albedo feedbacks, facilitating larger meridional temperature gradients before succumbing to global glaciation. We show that within the faster-spinning regime, the greenhouse warming required to generate an ice-free Earth can differ from that required to generate an Earth with permanent ice caps by the equivalent of 1–2 orders of magnitude of pCO_2. Accordingly, the resolution of the Faint Young Sun problem depends significantly on whether the early Earth was ever, or even at times, ice-free

A shorter Archean day-length biases interpretations of the early Earth's climate

Earth's earliest sedimentary record contains evidence that surface temperatures were similar to, or perhaps even warmer than modern. In contrast, standard Solar models suggest the Sun was 25% less luminous at this ancient epoch, implying a cold, frozen planet-all else kept equal. This discrepancy, known as the Faint Young Sun Paradox, remains unresolved. Most proposed solutions invoke high concentrations of greenhouse gases in the early atmosphere to offset for the fainter Sun, though current geological constraints are insufficient to verify or falsify these scenarios. In this work, we examined several simple mechanisms that involve the role played by Earth's spin rate, which was significantly faster during Archean time. This faster spin rate enhances the equator-to-pole temperature gradient, facilitating a warm equator, while maintaining cold poles. Results show that such an enhanced meridional gradient augments the meridional gradient in carbonate deposition, which biases the surviving geological record away from the global mean, toward warmer waters. Moreover, using simple atmospheric models, we found that the faster-spinning Earth was less sensitive to ice-albedo feedbacks, facilitating larger meridional temperature gradients before succumbing to global glaciation. We show that within the faster-spinning regime, the greenhouse warming required to generate an ice-free Earth can differ from that required to generate an Earth with permanent ice caps by the equivalent of 1-2 orders of magnitude of pCO2. Accordingly, the resolution of the Faint Young Sun problem depends significantly on whether the early Earth was ever, or even at times, ice-free.Comment: 15 pages. 7 Figures. Accepted for publication in Earth and Planetary Science Letter

Energetic Costs of Calcification Under Ocean Acidification

Anthropogenic ocean acidification threatens to negatively impact marine organisms that precipitate calcium carbonate skeletons. Past geological events, such as the Permian-Triassic Mass Extinction, together with modern experiments generally support these concerns. However, the physiological costs of producing a calcium carbonate skeleton under different acidification scenarios remain poorly understood. Here we present an idealized mathematical model to quantify whole-skeleton costs, concluding that they rise only modestly (up to ∼10%) under acidification expected for 2100. The modest magnitude of this effect reflects in part the low energetic cost of inorganic, calcium carbonate relative to the proteinaceous organic matrix component of skeletons. Our analysis does, however, point to an important kinetic constraint that depends on seawater carbonate chemistry, and we hypothesize that the impact of acidification is more likely to cause extinctions within groups where the timescale of larval development is tightly constrained. The cheapness of carbonate skeletons compared to organic materials also helps explain the widespread evolutionary convergence upon calcification within the metazoa