We report the discovery of two new planets: a 1.94 M-Jup planet in a 1.8 yr orbit of HD 5319, and a 2.51M(Jup) planet in a 1.1 yr orbit of HD 75898. The measured eccentricities are 0.12 for HD 5319b and 0.10 for HD 75898b, and Markov chain Monte Carlo simulations based on the derived orbital parameters indicate that the radial velocities of both stars are consistent with circular planet orbits. With low eccentricity and 1 AU < a < 2 AU, our new planets have orbits similar to terrestrial planets in the solar system. The radial velocity residuals of both stars have significant trends, likely arising from substellar or low-mass stellar companions.

Long-term geodynamo evolution is expected to respond to inner core growth and changing patterns of mantle convection. Three geomagnetic superchrons, during which Earth's magnetic field maintained a near-constant polarity state through tens of Myr, are known from the bio/magnetostratigraphic record of Phanerozoic time, perhaps timed according to supercontinental episodicity. Some geodynamo simulations incorporating a much smaller inner core, as would have characterized Proterozoic time, produce field reversals at a much lower rate. Here we compile polarity ratios of site means within a quality filtered global Proterozoic paleomagnetic database, according to recent plate kinematic models. Various smoothing parameters, optimized to successfully identify the known Phanerozoic superchrons, indicate 3-10 possible Proterozoic superchrons during the 1300 Myr interval studied. Proterozoic geodynamo evolution thus appears to indicate a relatively narrow range of reversal behavior through the last two billion years, implying either remarkable stability of core dynamics over this time or insensitivity of reversal rate to core evolution. (C) 2016 Elsevier B.V. All rights reserved.

Earth's climate, mantle, and core interact over geologic time scales. Climate influences whether plate tectonics can take place on a planet, with cool climates being favorable for plate tectonics because they enhance stresses in the lithosphere, suppress plate boundary annealing, and promote hydration and weakening of the lithosphere. Plate tectonics plays a vital role in the long-term carbon cycle, which helps to maintain a temperate climate. Plate tectonics provides long-term cooling of the core, which is vital for generating a magnetic field, and the magnetic field is capable of shielding atmospheric volatiles from the solar wind. Coupling between climate, mantle, and core can potentially explain the divergent evolution of Earth and Venus. As Venus lies too close to the sun for liquid water to exist, there is no long-term carbon cycle and thus an extremely hot climate. Therefore, plate tectonics cannot operate and a long-lived core dynamo cannot be sustained due to insufficient core cooling. On planets within the habitable zone where liquid water is possible, a wide range of evolutionary scenarios can take place depending on initial atmospheric composition, bulk volatile content, or the timing of when plate tectonics initiates, among other factors. Many of these evolutionary trajectories would render the planet uninhabitable. However, there is still significant uncertainty over the nature of the coupling between climate, mantle, and core. Future work is needed to constrain potential evolutionary scenarios and the likelihood of an Earth-like evolution.

The paleomagnetic record indicates the geodynamo has been active over much of Earth history with surprisingly little trend in paleointensity. Variability, however, is expected from models that predict a sharp increase in intensity following inner core nucleation (ICN) and implied by Neoproterozoic anomalies that hint at a highly variable field over several hundred million years. Here we demonstrate with a suite of numerical dynamos driven by a new thermal evolution model that the geodynamo could have transitioned from a multipolar to dipolar regime around 1.7Ga, then to a weak-field dynamo around 1.0Ga, and finally to a strong-field dipole following ICN around 650Ma that is maintained to the present day. The occurrence of a weak-field geodynamo in the Neoproterozoic may be consistent with the observed anomalous apparent polar wander paths and reversal behavior. Recovery to a dipolar geodynamo in the Phanerozoic could be a signature of inner core nucleation. Index terms: 1507, 1560, and 1521.

The origin of Earth's ancient magnetic field is an outstanding problem. It has recently been proposed that exsolution of MgO from the core may provide sufficient energy to drive an early geodynamo. Here we present new experiments on Mg partitioning between iron-rich liquids and silicate/oxide melts. Our results indicate that Mg partitioning depends strongly on the oxygen content in the iron-rich liquid, in contrast to previous findings that it depends only on temperature. Consequently, MgO exsolution during core cooling is drastically reduced and insufficient to drive an early geodynamo alone. Using the new experimental data, our thermal model predicts inner core nucleation at similar to 850 Ma and a nearly constant paleointensity.

The paleomagnetic record is central to our understanding of the history of the Earth. The orientation and intensity of magnetic minerals preserved in ancient rocks indicate the geodynamo has been alive since at least the Archean and possibly the Hadean. A paleomagnetic signature of the initial solidification of the inner core, arguably the singular most important event in core history, however, has remained elusive. In pursuit of this signature we investigate the assumption that the field is a geocentric axial dipole (GAD) over long time scales. We study a suite of numerical dynamo simulations from a paleomagnetic perspective to explore how long the field should be time-averaged to obtain stable paleomagnetic pole directions and intensities. We find that running averages over 20 - 40 kyr are needed to obtain stable paleomagnetic poles with alpha(95) <10 degrees, and over 40 - 120 kyr for alpha(95) <5 degrees, depending on the variability of the field. We find that models with higher heat flux and more frequent polarity reversals require longer time averages, and that obtaining stable intensities requires longer time averaging than obtaining stable directions. Running averages of local field intensity and inclination produce reliable estimates of the underlying dipole moment when reversal frequency is low. However, when heat flux and reversal frequency are increased we find that local observations tend to underestimate virtual dipole moment (VDM) by up to 50% and overestimate virtual axial dipolemoment (VADM) by up to 150%. A latitudinal dependence is found where VDM underestimates the true dipole moment more at low latitudes, while VADM overestimates the true axial dipole moment more at high latitudes. The cause for these observed intensity biases appears to be a contamination of the time averaged field by non-GAD terms, which grows with reversal frequency. We derive a scaling law connecting reversal frequency and site paleolatitude to paleointensity bias (ratio of observed to the true value). Finally we apply this adjustment to the PINT paleointensity record. These biases produce little change to the overall trend of a relatively flat but scattered intensity over the last 3.5 Ga. A more careful intensity adjustment applied during periods when the reversal frequency is known could reveal previously obscured features in the paleointensity record.

In a metal, as in Earth's core, the thermal and electrical conductivities are assumed to be correlated. In a planetary dynamo this implies a contradiction: that both electrical conductivity, which makes it easier to induce current and magnetic field, and conductive heat transport, which hinders thermal convection, should increase simultaneously. Here we show that this contradiction implies that the magnetic induction rate peaks at a particular value of electrical and thermal conductivity and derive the low- and high-conductivity limits for thermal dynamo action. A dynamo regime diagram is derived as a function of electrical conductivity and temperature for Earth's core that identifies four distinct dynamo regimes: no dynamo, thermal dynamo, compositional dynamo, and thermocompositional dynamo. Estimates for the temperature-dependent electrical conductivity of the core imply that the geodynamo may have come close to its high-conductivity "no dynamo" limit prior to inner core nucleation, consistent with recent paleomagnetic observations.