Far-infrared photometric observations of the outer planets and satellites with Herschel-PACS

TL;DR

This paper presents Herschel PACS photometric observations of outer planets and satellites, demonstrating high calibration accuracy, model agreement, and potential for cross-calibration with other instruments, while identifying some model offsets.

Contribution

It provides a comprehensive calibration and validation of Herschel PACS measurements for solar system objects, enhancing calibration standards and understanding of planetary models.

Findings

Flux measurements agree within 5% with models for Uranus, Neptune, Titan.

Callisto and Ganymede are brighter than model predictions by 4-21%.

PACS photometer shows linear response over four orders of magnitude.

Abstract

We present all Herschel PACS photometer observations of Mars, Saturn, Uranus, Neptune, Callisto, Ganymede, and Titan. All measurements were carefully inspected for quality problems, were reduced in a (semi-)standard way, and were calibrated. The derived flux densities are tied to the standard PACS photometer response calibration, which is based on repeated measurements of five fiducial stars. The overall absolute flux uncertainty is dominated by the estimated 5% model uncertainty of the stellar models in the PACS wavelength range between 60 and 210 micron. A comparison with the corresponding planet and satellite models shows excellent agreement for Uranus, Neptune, and Titan, well within the specified 5%. Callisto is brighter than our model predictions by about 4-8%, Ganymede by about 14-21%. We discuss possible reasons for the model offsets. The measurements of these very bright point-like sources, together with observations of stars and asteroids, show the high reliability of the PACS photometer observations and the linear behavior of the PACS bolometer source fluxes over more than four orders of magnitude (from mJy levels up to more than 1000 Jy). Our results show the great potential of using the observed solar system targets for cross-calibration purposes with other ground-based, airborne, and space-based instruments and projects. At the same time, the PACS results will lead to improved model solutions for future calibration applications.