Solar System and Planetary Science

Figure-1-1-2-1.pngALMA and the VLA have been used to determine the abundance and distribution of metals on the surfaces of the asteroid Kalliope and its moon Linus, with observations at 1.3, 9, and 20 mm down to 25 mas resolution (30 km on the surface). Kalliope is an M-type asteroid, with a high concentration of metals, such as iron. The mean thermal inertia of Kalliope is about 116 J m-2 s-0.5 K-1, and the surface radio emissivity about 0.6. Kalliope’s millimeter wavelength emission is suppressed compared to its centimeter wavelength emission, and it is also depolarized. The radio emissivity for Linus is higher, indicating a less metal-rich surface composition. Spatial structure in the emissivity is observed on Kalliope, with a region in the northern hemisphere with a high dielectric constant, suggesting enhanced metal content. These results are consistent with a scenario in which Linus formed from reaggregated ejecta from an impact onto a differentiated Kalliope (differentiation meaning elemental separation during a melting phase driven by radioactive elements), leaving Kalliope with a higher and more structured surface metal distribution than Linus. The low emissivity and lack of polarization suggest a regolith composition where iron is in the form of metallic grains and constitutes ∼25% of the surface composition.

Left: images of one rotational/orbital phase of the asteroid Kalliope and its moon Linus at 1.3mm with ALMA at 30 km (25mas) resolution, with units of arcsec (upper) and km (lower). Right: derived physical surface quantities for the regolith in a full surface projection (de Kleer et al. 2024, arXiv:2409:12364).

The VLA and IRAM telescopes have been used to study the centimeter and millimeter brightness distribution across Saturn’s synchronous moon Iapetus. In the optical, Figure-1-1-2-2.pngIapetus has the most dramatic difference of albedo for two hemispheres of any object in the Solar system, with the albedo on the leading hemisphere (direction of orbit), much lower than on the trailing hemisphere. This optical albedo difference is thought to be due to an accumulation of dark material on the leading hemisphere, while the trailing hemisphere remains mostly icy and bright. The centimeter and millimeter observations are sensitive to thermal emission from the subsurface, but they also show a major difference between leading and trailing hemispheres in radio emissivity, or brightness temperature. The results imply complex variations in structure and/or composition with depth on the leading side, while the trailing side emissivity is low at all observed frequencies, indicating efficient scattering processes on subsurface structures, as observed on Saturn’s other icy moons. These radio observations also involve Saturn’s retrograde moon Phoebe. Iapetus’ prograde orbit is within the inner edge of the large but faint outermost ‘Phoebe ring’ around Saturn, thought to be formed by material ejected by meteoric impacts on the surface of Phoebe. The radio observations show a high radio emissivity for the surface material of Phoebe, similar to the leading side of Iapetus, supporting the theory that Phoebe is the source of the optically dark material accumulated on the leading hemisphere of Iapetus.

Right: optical image of Saturn’s synchronous moon Iapetus indicating the measurement regions of radio brightness temperature. Bottom: the brightness temperature spectrum in these regions (Bonnefoy et al. 2024, Icarus 411, 115950), showing the large difference in radio emissivity of the leading (red) vs trailing (blue) hemispheres.  

Figure-1-1-2-3.pngMagnetic activity on M-dwarf stars has become topical in the search for life in other planetary systems, since many M-dwarfs host potentially habitable planets, but strong stellar storms may inhibit development of life. The VLA and ALMA have observed the nearby (2.7 pc) binary M-dwarf system BL and UV Ceti, from 1 GHz to 100 GHz. The stars have similar masses, spectral types, and rapid rotation rates, but UV Ceti has much stronger magnetic activity, showing radio flares similar to Solar coronal flares, auroral-like emission analogous to planetary magnetospheres, and slowly varying persistent emission. These phenomena suggest both small-scale field reconnection activity, and larger scale auroral currents in a global magnetic field. The persistent emission mechanism may parallel that seen for gyrosynchrotron emission from Jupiter’s radiation belts, corresponding to electrons trapped in a large scale radiation belt associated with the star.

Left: VLA light curves for UV Ceti on two days. Stokes I is shown with filled circles and Stokes V with open diamonds (Plant et al. 2024, 2406.17280). Flaring, time varying, and persistent emission can be seen.

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