The landing(s) of Philae and inferences about comet surface mechanical properties

Abstract

The Philae lander, part of the Rosetta mission to investigate comet 67P/Churyumov-Gerasimenko, was delivered to the cometary surface in November 2014. Here we report the precise circumstances of the multiple landings of Philae, including the bouncing trajectory and rebound parameters, based on engineering data in conjunction with operational instrument data. These data also provide information on the mechanical properties (strength and layering) of the comet surface. The first touchdown site, Agilkia, appears to have a granular soft surface (with a compressive strength of 1 kilopascal) at least ~20 cm thick, possibly on top of a more rigid layer. The final landing site, Abydos, has a hard surface.

Characterization of the Abydos region through OSIRIS high-resolution images in support of CIVA measurements

“…Philae first touched down at 15:34 UT at the selected Agilkia "J" landing site on the head of the nucleus of 67P, where it failed to anchor at the surface. Then, it bounced three times for an additional two-hour flight (Biele et al 2015) before finally landing at a site later named Abydos, about 1 km away from the first touchdown. During the lander descent the comparison between high-resolution images acquired by the ROsetta Lander Imaging System (ROLIS) onboard Philae (Mottola et al 2007(Mottola et al , 2015 with those obtained by the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS, Keller et al 2007) allowed the identification of the first touchdown point.…”

Decay of COSAC and Ptolemy mass spectra at comet 67P/Churyumov-Gerasimenko

“…For instance, data from the Surface Electrical Sounding and Acoustic Monitoring Experiment (SESAME) on Philae and Philae's landing gear indicated that Agilkia apparently has a soft and granular surface (∼ 20 cm in thickness) on top of a more rigid, icy subsurface . In contrast, Abydos was found to consist mostly of rigid, icy material (Biele et al 2015;Spohn et al 2015).…”

“…On 12 November, when Philae impacted on the solarilluminated Agilkia, it excavated ∼ 0.4 m 3 of regolith from 67P's top surface layer (Biele et al 2015). De Sanctis et al (2015) discussed in detail a temperature gradient that exists between the top layer of the surface, which is reported to have a temperature between about 180 K and 230 K during daytime whereas the temperature of the subsurface, at a depth of 4 cm is, even during daytime, only about 130 K. Therefore, we infer that Philae's excavation would have suddenly exposed the colder subsurface to the higher temperatures of the solar insolated surface and that this process would have rapidly caused the sublimation of volatile species from dust grains present in the subsurface.…”

“…From the circumstances of the first touchdown it is believed that COSAC collected with Article number, page 1 of 7 arXiv:1702.00192v1 [astro-ph.EP] 1 Feb 2017 its twin vent tubes -located at the bottom of Philae pointing toward the cometary surface (Figure 1, left panel) -a tiny amount of the cometary regolith that was raised in a dust cloud during touchdown and later sublimed inside the instrument (Goesmann et al 2015). A dust cloud was indeed recorded by the Rosetta navigation camera close to Agilkia a few minutes after touchdown, approximately 0.4 m 3 of material was excavated (Biele et al 2015).…”

“…On 12 November 2014 Philae successfully landed on its target comet 67P/Churyumov-Gerasimenko (hereafter 67P) at a heliocentric distance of 2.99 AU (Biele et al 2015). Philae carries two evolved gas analyzers on board: The Cometary Sampling and Composition (COSAC) experiment designed for in situ analysis of organic molecules on 67P (Goesmann et al 2007) and the gas chromatograph/mass spectrometer Ptolemy optimised to measure ratios of stable isotopes (Wright et al 2007).…”