Solar System and Planetary Science

Radio observations provide unique probes of the thermal and non-thermal processes of Solar system objects. Recent ALMA observations of Jupiter’s moon Callisto demonstrate the power of such observations to penetrate surface properties in unprecedented detail. Callisto is particularly interesting for its very quiescent, ancient surface (> 4 billion years), and a liquid water sub-surface ocean, with no evidence for crustal recycling. Surface structure is therefore predominantly exogenic in origin (i.e. impacts and dust infall). Thermal observations of Callisto’s leading and trailing hemispheres were obtained at 0.87 mm to 3 mm, with resolutions of 0.09” to 0.24” (420–1100 km). Callisto’s millimeter emissivities are high, 0.85–0.97, compared to 0.75–0.85 for Europa and Ganymede. The surface thermal modeling requires either two thermal inertia components or a variable electrical skin depth. Residuals from the global best-fit models show thermal anomalies, including impact craters with brightness temperatures that are locally 3–5 K colder than surrounding regions, such as the giant Valhalla impact basin and a suite of large craters, such as Lofn, appearing as cold anomalies.

Figure: ALMA images of Callisto after subtracting mean model disk emission (in K), at 0.09’’ to 0.24’’ resolution. The images are scaled such that the horizontal and vertical axes span -3500 km to +3500 km (Cordiner et al. arXiv:2508.05925).

Comets originating in the Oort cloud provide our most direct view of conditions in the nascent solar system. Recent ALMA observations of the Halley-type comet Pons-Brooks indicate outgassing of both H₂0 and HDO directly from the nucleus. The D/H ratio of 1.7x10^-4 is consistent with the D/H ratio in the Earth’s oceans, suggesting a common heritage between the Oort cloud’s water ice reservoir, and the water that was delivered to the young Earth during the early history of the solar system.

Figure: ALMA emission maps of H2O and HDO in comet 12P/Pons-Brooks at 1” resolution. Insets show the spectra (Camarca et al. arXiv:2507.12671).

Solar prominences (PR) are common features on the solar surface, corresponding to magnetic loops anchored in the photosphere, and extending an order of 100,000 km into the corona, forming on timescales of days and persisting for weeks. They play an important role in space weather, both participating in heating the corona and powering coronal mass ejections. Their physical origin remains a puzzle. ALMA single dish maps have recently been combined with broader radio frequency measurements to probe the hot thermal gas emission and absorption from a solar prominence. The density in the prominence was found to be about 100 times higher than the quiescent disk, and the temperature 150 times lower, and evidence was found for departures from hydrostatic equilibrium. The results suggest stability is most likely maintained by a dynamically dominant magnetic field of order 10 G.

Figure: Left: Hα image of solar prominence taken with Cerro Tololo optical telescope on December17, 2015, with PR boundaries (red contours). Right: Concurrent ALMA Band 3 total power image. The blue circle is the ALMA Band 3 single dish beam size (Matkovic et al. arXiv:2509.08605).