Astrophysics

One objective of a lander mission to Jupiter's icy moon Europa is to detect liquid water within 30 km as well as characterizing the subsurface ocean. In order to satisfy this objective, water within the ice shell must also be identified. Inductive electromagnetic (EM) methods are optimal for water detection on Europa because even a small fraction of dissolved salts will make water orders of magnitude more electrically conductive than the ice shell. Compared to induction studies by the Galileo spacecraft, measurements of higher-frequency ambient EM fields are necessary to resolve the shallower depths of intrashell water. Although these fields have been mostly characterized by prior missions, their unknown source structures and plasma properties do not allow EM sounding using a single surface magnetometer or the orbit-to-surface magnetic transfer function, respectively. Instead, broadband EM sounding can be accomplished from a single surface station using the magnetotelluric (MT) method, which measures horizontal electric fields as well as the three-component magnetic field. We have developed a prototype Europa Magnetotelluric Sounder (EMS) to meet the measurement requirements in the relevant thermal, vacuum, and radiation environment. EMS comprises central electronics, a fluxgate magnetometer on a mast, and three ballistically deployed electrodes to measure differences in surface electric potential. In this paper, we describe EMS development and testing as well as providing supporting information on the concept of operations and calculations on water detectability. EMS can uniquely determine the occurrence of intrashell water on Europa, providing important constraints on habitability.

GJ9827 is a bright star hosting a planetary system with three transiting planets. As a multi-planet system with planets that sprawl within the boundaries of the radius gap between terrestrial and gaseous planets, GJ9827 is an optimal target to study the evolution of the atmospheres of close-in planets with a common evolutionary history and their dependence from stellar irradiation. Here, we report on the Hubble Space Telescope (HST) and CARMENES transit observations of GJ9827 planets b and d. We performed a stellar and interstellar medium characterization from the ultraviolet HST spectra, obtaining fluxes for Ly-alpha and MgII of F(Ly-alpha) = (5.42+0.96-0.75) X 10^{-13} erg cm^{-2} s^{-1} and F(MgII) = (5.64 +- 0.24) X 10^{-14} erg cm^{-2} s^{-1}. We also investigated a possible absorption signature in Ly-alpha in the atmosphere of GJ9827b during a transit event from HST spectra, as well as H-alpha and HeI signature for the atmosphere of GJ9827b and d from CARMENES spectra. We found no evidence of an extended atmosphere in either of the planets. This result is also supported by our analytical estimations of mass-loss based on the measured radiation fields for all the three planets of this system, which led to a mass-loss rate of 0.4, 0.3, and 0.1 planetary masses per Gyr, for GJ9827b, c, and d respectively. These values indicate that the planets could have lost their volatiles quickly in their evolution and probably do not retain an atmosphere at the current stage.

Subsequent to the Moon's formation, late accretion to the terrestrial planets strongly modified the physical and chemical nature of silicate crusts and mantles. This alteration came in the form of melting through impacts, as well as the belated addition of volatiles and the highly siderophile elements (HSEs). Current debate centres on whether the lunar HSE record is representative of its whole late accretion history or alternatively that these were only retained in the mantle and crust after a particular time, and if so, when. Here we employ improved Monte Carlo impact simulations of late accretion onto the Moon and Mars and present an updated chronology based on new dynamical simulations of leftover planetesimals and the E-belt. We take into account the inefficient retention of colliding material. We compute the crater and basin densities on the Moon and Mars, the largest objects to strike these planets and the amount of material they accreted. Outputs are used to infer the mass in leftover planetesimals at a particular time period, which is then compared to the lunar HSE abundance. From this estimate we calculate a preferred lunar HSE retention age of ca. 4450 Ma which means that the modelled lunar mantle HSE abundances trace almost all of lunar late accretion. Based on our results, the surface ages of the lunar highlands are at least 4370 Ma. We find that the mass of leftover planetesimals with diameters Di<300 km at 4500 Ma that best fits the crater chronology is approximately 2x10^{-3} Earth mass (ME) while the mass of the E-belt was fixed at 4.5x10^{-4} ME. We also find that a leftover planetesimal mass in excess of 0.01 ME results in a lunar HSE retention age younger than major episodes of lunar differentiation and crust formation, which in turn violates geochemical constraints for the timing and intensity of late accretion to the Earth (Mojzsis et al., 2019).

Crater chronologies are a fundamental tool to assess relative and absolute ages of planetary surfaces when direct radiometric dating is not available. Martian crater chronologies are derived from lunar crater spatial densities on terrains with known radiometric ages, and thus they critically depend on the extrapolation Moon to Mars. This extrapolation requires knowledge of the time evolution of the impact flux, including contributions from various impactor populations, factors that are not trivially connected to the dynamical evolution of the early Solar System. In this paper, we will present a new martian crater chronology based on current dynamical models, and consider the main sources of uncertainties. The new martian crater chronology is discussed using two interesting applications: Jezero crater's dark terrain (relevant to the NASA Mars 2020 mission) and the southern heavily cratered highlands. [abridged]

In the so-called "global polytropic model", we assume planetary systems in hydrostatic equilibrium and solve the Lane--Emden equation in the complex plane. We thus find polytropic spherical shells providing hosting orbits to planets. On the basis of this model, we develop a numerical method which has three versions. In its three-dimensional version, the method is effective for systems with substantial uncertainties in the observed host star radius, and in the orbit of a particular planet (compared to the uncertainties in the orbits of the other planets); the method uses as fixed entry values the observed orbits of the remaining planets. In its two-dimensional version, the method is effective for systems with substantial uncertainty in the host star radius; in this case, the method uses as fixed entry values the observed orbits of the planets. The one-dimensional version was previously developed and applied to several systems; in this version, the observed values of the host star radius and of the planetary orbits are taken as fixed entry values. Our method can compute optimum values for the polytropic index of the global polytropic model which simulates the exoplanetary system, for the orbits of the planets, and (excluding the one-dimensional version) for the host star radius.

Asteroid pairs, two objects that are not gravitationally bound to one another, but share a common origin, have been discovered in the Main belt and Hungaria populations. Such pairs are of major interest, as the study of their evolution under a variety of dynamical influences can indicate the time since the pair was created. To date, no asteroid pairs have been found in the Jovian Trojans, despite the presence of several binaries and collisional families in the population. The search for pairs in the Jovian Trojan population is of particular interest, given the importance of the Trojans as tracers of planetary migration during the Solar system's youth. Here we report a discovery of the first pair, (258656) 2002~ES 76 and 2013~CC 41 , in the Jovian Trojans. The two objects are approximately the same size and are located very close to the L4 Lagrange point. Using numerical integrations, we find that the pair is at least 360 ~Myr old, though its age could be as high as several Gyrs. The existence of the (258656) 2002~ES 76 --2013~CC 41 pair implies there could be many such pairs scattered through the Trojan population. Our preferred formation mechanism for the newly discovered pair is through the dissociation of an ancient binary system, triggered by a sub-catastrophic impact, but we can not rule out rotation fission of a single object driven by YORP torques. A by-product of our work is an up-to-date catalog of Jovian Trojan proper elements, which we have made available for further studies.

Pebbles of millimeter sizes are abundant in protoplanetary discs around young stars. Chondrules inside primitive meteorites - formed by melting of dust aggregate pebbles or in impacts between planetesimals - have similar sizes. The role of pebble accretion for terrestrial planet formation is nevertheless unclear. Here we present a model where inwards-drifting pebbles feed the growth of terrestrial planets. The masses and orbits of Venus, Earth, Theia (which later collided with the Earth to form the Moon) and Mars are all consistent with pebble accretion onto protoplanets that formed around Mars' orbit and migrated to their final positions while growing. The isotopic compositions of Earth and Mars are matched qualitatively by accretion of two generations of pebbles, carrying distinct isotopic signatures. Finally, we show that the water and carbon budget of Earth can be delivered by pebbles from the early generation before the gas envelope became hot enough to vaporise volatiles.

The single-lined spectroscopic binary ν Octantis provided evidence of the first conjectured circumstellar planet demanding an orbit retrograde to the stellar orbits. The planet-like behaviour is now based on 1437 radial velocities (RVs) acquired from 2001 to 2013. ν Oct's semimajor axis is only 2.6 AU with the candidate planet orbiting ν Oct A about midway between. These details seriously challenge our understanding of planet formation and our decisive modelling of orbit reconfiguration and stability scenarios. However, all non-planetary explanations are also inconsistent with numerous qualitative and quantitative tests including previous spectroscopic studies of bisectors and line-depth ratios, photometry from Hipparcos and the more recent space missions TESS and GAIA (whose increased parallax classifies ν Oct A closer still to a subgiant ~ K1 IV). We conducted the first large survey of ν Oct A's chromosphere: 198 Ca II H-line and 1160 H α indices using spectra from a previous RV campaign (2009-2013). We also acquired 135 spectra (2018-2020) primarily used for additional line-depth ratios, which are extremely sensitive to the photosphere's temperature. We found no significant RV-correlated variability. Our line-depth ratios indicate temperature variations of only ± 4 K, as achieved previously. Our atypical Ca II analysis models the indices in terms of S/N and includes covariance significantly in their errors. The H α indices have a quasi-periodic variability which we demonstrate is due to telluric lines. Our new evidence provides further multiple arguments realistically only in favor of the planet.

We report the discovery and characterization of two transiting planets around the bright M1 V star LP 961-53 (TOI-776, J = 8.5 mag, M = 0.54+-0.03 Msun) detected during Sector 10 observations of the Transiting Exoplanet Survey Satellite (TESS). Combining the TESS photometry with HARPS radial velocities, as well as ground-based follow-up transit observations from MEarth and LCOGT telescopes, we measured for the inner planet, TOI-776 b, a period of 8.25 d, a radius of 1.85+-0.13 Re, and a mass of 4.0+-0.9 Me; and for the outer planet, TOI-776 c, a period of 15.66 d, a radius of 2.02+-0.14 Re, and a mass of 5.3+-1.8 Me. The Doppler data shows one additional signal, with a period of 34 d, associated with the rotational period of the star. The analysis of fifteen years of ground-based photometric monitoring data and the inspection of different spectral line indicators confirm this assumption. The bulk densities of TOI-776 b and c allow for a wide range of possible interior and atmospheric compositions. However, both planets have retained a significant atmosphere, with slightly different envelope mass fractions. Thanks to their location near the radius gap for M dwarfs, we can start to explore the mechanism(s) responsible for the radius valley emergence around low-mass stars as compared to solar-like stars. While a larger sample of well-characterized planets in this parameter space is still needed to draw firm conclusions, we tentatively estimate that the stellar mass below which thermally-driven mass loss is no longer the main formation pathway for sculpting the radius valley is between 0.63 and 0.54 Msun. Due to the brightness of the star, the TOI-776 system is also an excellent target for the James Webb Space Telescope, providing a remarkable laboratory to break the degeneracy in planetary interior models and to test formation and evolution theories of small planets around low-mass stars.

Temperatures of the outer planet thermospheres exceed those predicted by solar heating alone by several hundred degrees. Enough energy is deposited at auroral regions to heat the entire thermosphere, but models predict that equatorward distribution is inhibited by strong Coriolis forces and ion drag. A better understanding of auroral energy deposition and circulation are critical to solving this so-called energy crisis. Stellar occultations observed by the Ultraviolet Imaging Spectrograph instrument during the Cassini Grand Finale were designed to map the thermosphere from pole to pole. We analyze these observations, together with earlier observations from 2016 and 2017, to create a two-dimensional map of densities and temperatures in Saturns thermosphere as a function of latitude and depth. The observed temperatures at auroral latitudes are cooler and peak at higher altitudes and lower latitudes than predicted by models, leading to a shallower meridional pressure gradient. Under modified geostrophy, we infer slower westward zonal winds that extend to lower latitudes than predicted, supporting equatorward flow from approximately 70 to 30 degrees latitude in both hemispheres. We also show evidence of atmospheric waves in the data that can contribute to equatorward redistribution of energy through zonal drag.