Ground-based Mid-infrared Observations Research Articles

Saturn’s seasonal variability from four decades of ground-based mid-infrared observations

A multi-decade record of ground-based mid-infrared (7–25μm) images of Saturn is used to explore seasonal and non-seasonal variability in thermal emission over more than a Saturnian year (1984–2022). Thermal emission measured by 3-m and 8-m-class observatories (notably NASA’s Infrared Telescope Facility, Subaru, and ESO’s Very Large Telescope) compares favourably with synthetic images based on both Cassini-derived temperature records and the predictions of radiative climate models. We find that 8-m class facilities are capable of resolving thermal contrasts on the scale of Saturn’s belts, zones, polar hexagon, and polar cyclones, superimposed onto large-scale seasonal asymmetries. Seasonal changes in brightness temperatures of ∼30 K in the stratosphere and ∼10 K in the upper troposphere are observed, as the northern and southern polar stratospheric vortices (NPSV and SPSV) form in spring and dissipate in autumn. The timings of the first appearance of the warm polar vortices is successfully reproduced by radiative climate models, confirming them to be radiative phenomena, albeit entrained within sharp boundaries influenced by dynamics. Axisymmetric thermal bands (4–5 per hemisphere) display temperature gradients that are strongly correlated with Saturn’s zonal winds, indicating winds that decay in strength with altitude from the cloud-tops to the ∼1-mbar level, and implying meridional circulation cells in Saturn’s upper troposphere and stratosphere forming the system of cool zones and warm belts. Saturn’s thermal structure is largely repeatable from year to year (via comparison of infrared images in 1989 and 2018), with the exception of low-latitudes. Here we find evidence of inter-annual variations because the equatorial banding at 7.9μm is inconsistent with a ∼15-year period for Saturn’s equatorial stratospheric oscillation, i.e., it is not strictly semi-annual. Either the oscillation has a longer period closer to ∼20 years, or its progression is naturally variable and interrupted by tropospheric meteorology (e.g., storms). Finally, observations between 2017–2022 extend the legacy of the Cassini mission, revealing the continued warming of the NPSV during northern summer in line with predictions of radiative climate models.

“Slow-scanning” in Ground-based Mid-infrared Observations

Abstract Chopping observations with a tip-tilt secondary mirror have conventionally been used in ground-based mid-infrared observations. However, it is not practical for next generation large telescopes to have a large tip-tilt mirror that moves at a frequency larger than a few hertz. We propose an alternative observing method, a “slow-scanning” observation. Images are continuously captured as movie data, while the field of view is slowly moved. The signal from an astronomical object is extracted from the movie data by a low-rank and sparse matrix decomposition. The performance of the “slow-scanning” observation was tested in an experimental observation with Subaru/COMICS. The quality of a resultant image in the “slow-scanning” observation was as good as in a conventional chopping observation with COMICS, at least for a bright point-source object. The observational efficiency in the “slow-scanning” observation was better than that in the chopping observation. The results suggest that the “slow-scanning” observation can be a competitive method for the Subaru telescope and be of potential interest to other ground-based facilities to avoid chopping.

Itokawa: The power of ground-based mid-infrared observations

AbstractPre-encounter ground-based thermal observations of NEA 25143 Itokawa at 10μm led to a size prediction of 520(±50) × 270(±30) × 230(±20)m, corresponding to an effective diameter of DTPMeff= 318m (Müller et al.2005). This is in almost perfect agreement with the final in-situ results 535 × 294 × 209m (DHayabusaeff= 320m; Demura et al.2006). The corresponding radar value, based on the same shape model (Kaasalainen et al.2005), were about 20% too high: 594 × 320 × 288m (DRadareff= 379m; Ostro et al.2005). The very simple N-band observations revealed a surface which is dominated by bare rocks rather than a thick regolith layer. This prediction was nicely confirmed by the Hayabusa mission (e.g., Fujiwara et al.2006; Saito et al.2006). The ground-based measurements covered three different phase angles which enabled us to determine the thermal properties with unprecedented accuracy and in excellent agreement with the results from the touch-down measurements (Okada et al.2006; Yano et al.2006). These thermal values are also key ingredients for high precision Yarkovsky and YORP calculations (mainly the rotation slowing) for Itokawa (e.g., Vokrouhlický et al.2004; Vokrouhlický et al.2005). In addition to the above mentioned properties, our data allowed us to derive the surface albedo and to estimate the total mass. We believe that with our well-tested and calibrated radiometric techniques (Lagerros 1996,1997,1998; Müller & Lagerros 1998, 2002; Müller 2002) we have tools at hand to distinguish between monolithic, regolith-covered and rubble pile near-Earth objects by only using remote thermal observations. This project also emphasizes the high and so far not yet fully exploited potential of thermophysical modeling techniques for the NEA/NEO exploration.

A Mid-Infrared Study of the Class 0 Cluster in LDN 1448

We present ground-based mid-infrared observations of Class 0 protostars in LDN 1448. Of the five known protostars in this cloud, we detected two, L1448N:A and L1448C, at 12.5, 17.9, 20.8, and 24.5 μm, and a third, L1448 IRS 2, at 24.5 μm. We present high-resolution images of the detected sources and photometry or upper limits for all five Class 0 sources in this cloud. With these data we are able to augment existing spectral energy distributions for all five objects and place them on an evolutionary status diagram.

High-Resolution Mid-Infrared Observations of High Mass Protostellar Objects

This article presents a review of all high-resolution ground-based mid-infrared observations of high mass protostellar objects (HMPOs) to date. An introduction to the characteristics of HMPOs and their relationship to the hot molecular core phase of massive star formation is outlined. I explain why observations in the mid-infrared of these types of sources are important, and in particular why high-resolution observations are needed. A summary is given of the earliest mid-infrared detections of HMPOs, followed by a detailed account of the most recent observations.