ALMA has explored the Solar System from its small cold distant bodies, to its planets and moons, and to the Sun, resulting in sensitive high resolution images of their surfaces and atmospheres.
ALMA’s sensitivity to millimeter radiation emitted by large and small Solar System bodies provides a measurement of their size and albedo while its high resolution enables imaging of surface properties. This was well demonstrated by the long baselines Science Verification data obtained for the Juno asteroid (see Figure 6.1). Juno is a well-known asteroid which is too irregular to be deemed a ‘dwarf planet’. Viikinkoski et al. (2015) found that despite Juno’s large size, global shape features may be modeled by impacts rather than gravitational processes.
Figure 6.1: Animation of the asteroid Juno as imaged by ALMA as part of the telescope's Long Baseline Campaign. Images were taken when Juno was approximately 295 million kilometers from Earth. Credit: ALMA (NRAO/ESO/NAOJ).
Viikinkoski et al. 2015, A&A, 581, L3
Comets are key objects for the study of the origin and evolution of icy materials in the Solar System. ALMA observed two bright comets in 2013 (C/2012 F6, Lemmon, and C/2012 S1, ISON; Cordiner et al. 2012) revealing the detailed distribution of HCN, HNC and H2CO in the coma. The data are found to be in agreement with models that predict the formation of these compounds, especially H2CO, in the coma at a few hundred to few thousand kilometers from the nucleus from unidentified precursor molecules. The different formation lengths observed in the two comets at different heliocentric distances are consistent with the effects of photolysis or thermal degradation. Methanol was imaged spatially and spectrally in C/2012 K1 (PanSTARRS; see Figure 6.2) showing isotropic, uniform outflow from the nucleus. The rotational temperature declines with distance, attributable to the adiabatic cooling of the outflowing gas, as well as radiative cooling of the molecular rotational levels. Models suggest a significant source of coma heating in addition to the photolytic sources needs to be considered.
Figure 6.2: An image and spectrum of the comet C/2012 K1 (PanSTARRS) in the J=7-6 line of methanol, showing several transitions of the molecule, Figure from Cordiner et al. (2017).
Cordiner et al. 2012, ApJ 792, L2
Cordiner et al. 2017, ApJ, in press (astroph 1701.08258)
Pluto is the most famous dwarf planet. ALMA provided high-precision measurements of Pluto’s location and orbit around the Sun to help the New Horizons spacecraft accurately home in on its target when it neared Pluto and its five known moons in July 2015. Thus guided by ALMA, New Horizons provided an enormous amount of data on Pluto and its tenuous atmosphere, consisting mostly of nitrogen and methane.
Lellouch et al. (2016) detected CO and HCN from Pluto using ALMA. From the CO profile, a thermal profile of the atmospheric temperature could be produced, showing that there is a well-marked temperature decrease above the 30-50 km stratopause; the temperature measured agrees with the New Horizons data. HCN is abundant throughout the atmosphere and is markedly supersaturated in the cold upper atmosphere.
Figure 6.3: Pluto as imaged by New Horizons with an ALMA HCN spectrum (Lellouch et al. 2016) superposed.
Lellouch et al. 2016, A&A, 588, A2
Planets and Moons with atmospheres
ALMA spectral line observations can probe the structure (wind, temperature, and pressure) and composition of solar system bodies. For example, several studies have used ALMA to observe Titan. From the atmospheric composition and tracking of atmospheric evolution, its photo- and thermo-chemistry can be monitored and the importance of outgassing and sublimation can be explored. Ethyl cyanide (C2H5CN) is one example of a complex molecule detected in Titan’s atmosphere (see Figure 6.4). It is thought to be produced through photochemistry in Titan’s upper atmosphere and shows seasonal variations in abundance, peaking on the autumn hemisphere. Other molecules, such as HC3N, CH3CN and CH3CCH were detected simultaneously (see Figure 6.5), peaking in the northern (spring) hemisphere. Over time, ALMA will be able to track changes in the molecular content of Titan’s atmosphere with its varying seasons. ALMA’s sensitivity allowed measurements of five isotopomers of HCN and four of CO to infer isotopic ratios (Molter et al. 2016; Serigano et al. 2016). The carbon and oxygen ratios are terrestrial but 15N is enhanced by a factor of 2.3 over terrestrial values. Deuterium enrichment in HCN is about twice that seen in methane in Titan.
Figure 6.4: ALMA images of Titan in the lines of C2H5CN (upper left), HC3N (lower left), CH3CN (upper right) and CH3CCH (lower right). Figure from Cordiner et al. (2015).
Figure 6.5: ALMA images of Titan (Cordiner et al. 2014) in the lines of HNC and HC3N showing the different distributions of these molecules across the surface of that moon (orange disks). Credit: NRAO/AUI/NSF; M. Cordiner (NASA) et al.
Venus’ upper atmosphere (70-150 km altitude) is a transition region characterized by a complex dynamics: strong retrograde zonal winds dominate the lower mesosphere while a solar-to-antisolar circulation is observed in the upper mesosphere/lower thermosphere. ALMA images of CO on Venus provided the mean center-disk thermal profile for interpretation of SO and SO2 images. The data show that up to about 90 km temperatures increase from the morning side towards the evening terminator, this trend is inverted above 90 km. SO exhibits a strong spatial and temporal variability with a mixing ratio ranging from 0 to 15 ppb. It presents also a clear cutoff around 89 km in the atmosphere (Encrenaz et al. 2015).
Figure 6.6: ALMA images of Venus obtain on 2011 November 2014, shown as a spectral map of the CO J=3-2 transition in the central core of the line over a spectral range of 10 MHz (Encrenaz et al. 2015). The diameter of the planet is 11 arcsec. The background image shows a radar view of Venus (without the clouds) from Magellan between 1990 and 1994, shown for reference only, as this does not reflect the hemisphere of Venus lying beneath the CO in the image.
Cordiner et al. 2014, ApJ, 795, 30
Cordiner et al. 2015, ApJ, 800, L14
Encrenaz et al. 2015, Planetary and Space Science, 113, 275
Molter et al. 2016, AJ, 152, 42
Serigano et al. 2016, ApJ, 821, L8
ALMA has developed the ability to image solar activity at longer wavelengths than observed with typical solar telescopes on Earth. These observations provide an important expansion of the range of observations that can be used to probe the physics of the Sun. Since the Sun is many billions of times brighter than the faint objects ALMA typically observes, the solar commissioning team had to developed special procedures to enable ALMA to safely image the Sun.
The result of this work is a series of images that demonstrates ALMA’s unique vision and ability to study the Sun on multiple scales (see Figure 6.7). Sunspots are transient features that occur in regions where the Sun’s magnetic field is extremely concentrated and powerful. They are lower in temperature than their surrounding regions, which is why they appear relatively dark in visible light. The ALMA image is essentially a map of temperature differences in a layer of the Sun’s atmosphere known as the chromosphere, which lies just above the visible surface of the Sun (the photosphere). The chromosphere is considerably hotter than the photosphere. Understanding the heating and dynamics of the chromosphere are key areas of research that will be addressed by ALMA. Observations at shorter wavelengths probe deeper into the solar chromosphere than longer wavelengths. Hence, observations at 1.3mm wavelength map a layer of the chromosphere that is closer to the visible surface of the Sun than 3mm wavelength observations.
Figure 6.7: ALMA images of the Sun (inset) compared to an image taken at a wavelength of 617.3 nanometers. In the latter, the light originates in the solar photosphere. The ALMA image of the same spot, taken at 1.3mm wavelength, probes material in the chromosphere, which lies above the photosphere. Credit: ALMA (ESO/NAOJ/NRAO); B. Saxton (NRAO/AUI/NSF)
Full-disc solar image: Filtergram taken in Fe I 617.3 nm spectral line with the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). Credit: NASA.