Articles in 2020

High-spatial-resolution images of the bright points at Occator crater on Ceres, taken during the second extended Dawn mission, allowed reconstruction of the chronology of their formation. The area experienced extensive cryovolcanism less than nine million years ago that lasted several million years, indicating recent geological activity.

An analysis of the relation between a star’s initial mass and its final mass (as a white dwarf) reveals a kink in the initial mass range 1.65–2.10 M_⊙. This kink appears to correspond to the minimum mass required for carbon star formation in the Milky Way at solar metallicity.

Analysis of a catalogue of accreted stars by their radial and prograde motions has identified a 200-plus-member coherent stellar stream (called Nyx) that is likely to be the remnant of a dwarf galaxy that merged with the Milky Way.

Two sources of variability are reported in extreme horizontal branch (EHB) stars found in globular clusters, both related to the action of weak magnetic fields: large surface spots and very energetic flares. EHB stars in clusters can thus be linked to EHB field stars, and beyond, to other stars with radiative envelopes.

Different plasma modes in various Galactic environments are identified on the basis of a synchrotron polarization analysis. These results open up the study of interstellar turbulence and demonstrate its importance in all relevant processes including cosmic ray transport and star formation.

Simulations show that turbulent heating at the solar surface randomly evaporates material to the corona, naturally reproducing the formation of long, fibril-like threads within solar filaments in the solar corona and their counterstreamings.

Without an intrinsic magnetic field, Mars’s magnetosphere is induced by direct interaction between its atmosphere and the solar wind. The mapping of the associated current system, obtained by the MAVEN spacecraft, unveils its convective-driven nature and displays various structural differences compared to Earth.

Galactic close encounters can induce gravitational effects in the participants. Here Ruiz-Lara et al. have reconstructed the star formation history of the region of our Galaxy close to the Sun, finding that three recent visits of the neighbouring Sagittarius dwarf galaxy have resulted in well-defined episodes of star formation.

A zirconium-based crystal (baddeleyite) found embedded in a sample brought to Earth by Apollo 17 provides evidence of large-scale impact bombardment of the Moon about 4.33 Gyr ago, when the baddeleyite grain was formed. This result points to the importance of impacts in the early evolution of planetary crusts.

Escherichia coli bacteria and yeast cultures (representative prokaryotes and eukaryotes) have been tested under laboratory conditions in a 100% H₂ atmosphere. They can reproduce normally, with lower growth rates, producing a range of biosignature gases. Exoplanets with a H₂-dominated atmosphere might thus not be totally hostile to some forms of life.

Methane ice has been presumed to form via the sequential hydrogenation of carbon atoms on dust grains for many years, but now Qasim et al. have performed the experiment, with and without the presence of water. Methane forms more rapidly in the polar ice phase.

Laboratory experiments show that the inclusion of even small quantities of sulfur in the atmospheres of exoplanets at 800 K significantly increases photochemical complexity, both in the vapour and in the solid phase: many sulfur gas products are created (including potential biosignature gases) and the production of organic haze particles increases threefold.

The European Southern Observatory trialled a distributed peer-review system—augmented by automated reviewer assignment—for its telescope time allocation process, finding that it worked as well as the standard process but resulted in a smaller burden on reviewers.

A magnetohydrodynamic model of the interaction between the solar wind and the interstellar medium shows that our heliosphere has a round shape, not a tail-like shape as usually assumed. The model reproduces the observations from Cassini, New Horizons and the two Voyager probes.

Parsons et al. calculate masses and radii for both members of a double white dwarf system, where the less-massive member is exhibiting non-radial g-mode pulsations, making it a prime target for asteroseismic analyses.

HD 74423 is an unusual binary star system containing two λ Boötis pulsators. TESS photometry reveals that one of the pair is pulsating along its tidal axis—but only in one hemisphere. Such an odd arrangement provides an interesting laboratory in which to study stellar pulsations and tidal distortion.

The combination of electromagnetic and gravitational-wave observations of binary neutron-star merger GW170817 with systematic sets of neutron-star equations of state has produced a tightly constrained radius of 11 km for a 1.4 M_⊙ neutron star. This constraint suggests that a neutron star–black hole merger is unlikely to produce an electromagnetic counterpart.

Radio observations of black hole MAXI J1820+070’s 2018 outburst captured an isolated radio flare that the authors connect with the launch of bipolar relativistic ejecta. Following the oncoming ejecta for more than half a year reveals that black hole jet energetics have been systematically underestimated.

Observations of the Faraday rotation towards pulsars in the halo globular cluster 47 Tucanae have been used to constrain the magnetic field strength in the Galactic halo, finding that it is unexpectedly strong.

Juno’s microwave radiometer data could measure the water concentration in the deep atmosphere of Jupiter (0.7 to 30 bar) at the equator: \(2.7^{+2.4}_{-1.7}\) times the solar O/H abundance, with a thermal vertical structure compatible with a moist adiabat.