Atacama Large Aperture Submillimeter Telescope (AtLAST) Science: Planetary and Cometary Atmospheres

Abstract

The study of planets and small bodies within our Solar System is fundamental for understanding the formation and evolution of the Earth and other planets. Compositional and meteorological studies of the giant planets provide a foundation for understanding the nature of the most commonly observed exoplanets, while spectroscopic observations of the atmospheres of terrestrial planets, moons, and comets provide insights into the past and present-day habitability of planetary environments, and the availability of the chemical ingredients for life. While prior and existing (sub)millimeter observations have led to major advances in these areas, progress is hindered by limitations in the dynamic range, spatial and temporal coverage, as well as sensitivity of existing telescopes and interferometers. Here, we summarize some of the key planetary science use cases that factor into the design of the Atacama Large Aperture Submillimeter Telescope (AtLAST), a proposed 50-m class single dish facility: (1) to more fully characterize planetary wind fields and atmospheric thermal structures, (2) to measure the compositions of icy moon atmospheres and plumes, (3) to obtain detections of new, astrobiologically relevant gases and perform isotopic surveys of comets, and (4) to perform synergistic, temporally-resolved measurements in support of dedicated interplanetary space missions. The improved spatial coverage (several arcminutes), resolution (~ 1.2″ - 12″), bandwidth (several tens of GHz), dynamic range (~ 10 ⁵) and sensitivity (~ 1 mK km s ^-1) required by these science cases would enable new insights into the chemistry and physics of planetary environments, the origins of prebiotic molecules and the habitability of planetary systems in general.

Keywords: Comets; Instrumentation; Planetary atmospheres; Planets; Spectral imaging; Spectral lines; Submillimeter.

Figure 1.

Angular diameters of major Solar System bodies and Saturn’s largest moon, Titan. The vertical extents of the coloured bars for each body represent their range of angular sizes due to differing geocentric distances throughout the year. The second y-axis shows the angular resolution a 50 m single-dish facility would achieve for a range of representative frequencies.

Figure 2.

HST (left) and longitude-smeared ALMA (center, right) observations of Jupiter (reproduced with permission from de Pater et al. ). The ALMA observations show data after the subtraction of a limb-darkened disk model, enabling the high contrast (≲ 10 K) differences in brightness temperature between Jupiter’s zonal structures to be easily observed. Processed in this way, the lack of short-spacing data (large scale flux) – an inherent feature of interferometric observations – is not apparent.

Figure 3.

(Left) Jupiter zonal wind velocities derived from ALMA observations of HCN at ~ 1′′ resolution (color map); ultraviolet auroral emission at the south pole is also shown , (reproduced with permission from 54). (Center) Zonal wind speeds as a function of latitude in Saturn’s stratosphere from Benmahi et al. , compared to winds from Cassini imaging. (Right) Comparison of ALMA wind speed measurements of Neptune from CO and HCN emission lines compared to Voyager cloud tracking measurements (reproduced with permission from 53).

Figure 4.

Figure reproduced with permission from Cavalié et al. , showing Herschel/HIFI data of the CO ( J = 8 − 7) spectrum on Uranus compared to internal, cometary, and steady external infall source models (left), and the corresponding vertical profiles (right). The comparison of Herschel data to the CO source models allows for the inference of an external source of Uranus’s CO.

Figure 5.

Spectral line map of CO J = 1 − 0 absorption towards Mars, obtained using the IRAM Plateau de Bure Interferometer with a beam size ~ 7′′ (adapted from ; overlaid on an optical map of the Martian surface from the NASA Solar System Simulator; https://space.jpl.nasa.gov/). Black lines: Observations. Red lines: Line profile expected from GCM predictions. Green lines: Fit of observations with retrieved thermal profiles.

Figure 6.

Contour maps of molecular line emission from in comets S1/ISON and F6/Lemmon using ALMA with a beam size of ~ 0.5′′. Contour intervals in each map are 20% of the peak flux (the lowest, 20% contour has been omitted from panel ( c) for clarity). On panel ( b), white dashed arrows indicate HNC streams/jets. The peak position of the simultaneously observed) 0.9 mm continuum is indicated with a white ‘+’. See for further details. Multi-beam mapping studies with AtLAST would dramatically improve our knowledge of cometary compositions and gas production processes on larger coma scales (up to several arcminutes).

Figure 7.

Broadband spectral model of a comet at 1 au from the Sun and Earth observed using a 50 m diameter telescope in the 1 mm band. We assume a typical cometary gas production rate of Q(H ₂O) = 10 ²⁹ s ⁻¹, spherically symmetric outflow velocity of 0.8 km s ⁻¹ and rotational temperature of 60 K. Molecular abundances are the average of previously observed cometary values ^, .

Figure 8.

Left: Simulated HDO line strengths in a typical (moderately bright; Q(H ₂O) = 10 ²⁹ s ⁻¹) comet at 1 au from the Earth and Sun, with average cometary HDO/H ₂O ratio , using a diffraction-limited 50 m telescope beam size. The strongest HDO line in AtLAST’s frequency range is highlighted, which is much stronger than the cometary HDO lines observed previously in the (sub)millimeter band. Right: Simulated HDO 894 GHz map and spectral extract from the central pixel. The 1.4′′ AtLAST beam FWHM is shown lower left.