Beneath the surface: Exploring the composition of Philae’s comet, Part 1

When Philae, the lander accompanying the ESA’s Rosetta probe, successfully touched down on Comet 67P/Churyumov-Gerasimenko and became the first man-made spacecraft ever to pull off such a feat, it captured the public’s imagination – and scored some epic data in the process.

The landing, which took place on November 12, 2014, as well as the in-depth analysis of 67P/C-G that followed, are the focus of several new studies appearing in a special edition of the journal Science on Thursday. In one of those papers, a team of scientists led by Jens Biele of the German Aerospace Center (DLR) detailed the events shortly after Philae’s harrowing landing.

Analyzing Philae’s impact, bounces and eventual touchdown

During its descent, the lander was supposed to activate a cold gas system, which would push it to the surface of the comet, activating a pair of anchoring harpoons to attach it to the ground. As it turns out, however, neither system operated as expected, causing Philae to bounce off the soft designated landing area and ultimately come to rest on a harder surface elsewhere.

Biele’s team analyzed the exact dynamics of those bounces and the compressive strengths of the two different surfaces based on the trajectory of the lander’s bounces. They analyzed the layering and mechanical properties of the comet’s surface, marking the first time that researchers were able to conduct actual, direct observations of the surface.

Based on their analysis of the landing, the authors concluded that Philae’s feet initially came into contact with a soft granular surface known as Agilkia. This particular surface was approximately 0.82 feet (0.25 meters) thick on top, with a harder layer located beneath it.

This layering gave the surface a compressive strength of about one kilopascal. In comparison, the location where the lander finally came to rest, a region known as Abydos, was found to have a compression strength of two megapascals. This could help explain why only one of Philae’s legs was able to find a foothold when it finally came to rest.

Instruments reveal fractured surface, reflective rock structures

In a related study, Jean-Pierre Bibring from the French National Centre for Scientific Research’s Space Astrophysics Institute (CNRS IAS) and his colleagues analyzed the surface of 67P using a series of panoramic images captured by Philae’s Comet Infrared and Visible Analyzer instrument shortly after the lander’s initial bounce and final touchdown.

These images revealed that the comet possessed “a fractured surface with complex structure and a variety of grain scales and albedos” or reflective rock structures “possibly constituting pristine cometary material.” Their work provides new insight into the structure and composition of these cometary constituents, which could reveal the processes and ingredients that form comets, as well as how they evolved to become so diverse.

A third paper looked at descent images captured by the Rosetta Lander Imaging System (ROLIS) instrument to better understand the geography of 67P/C-G. According to the authors, the comet’s surface of the comet is “photometrically uniform” and “covered by regolith composed of debris and blocks ranging in size from centimeters to 5 meters” in size.

Philae’s landing spot covered with porous dust, ice layer

This study, which was led by Stefano Mottola of the DLR Institute of Planetary Research, found boulders on the comet’s surface surrounded by depressions similar to the wind tails found on Earth. Using models, the authors confirmed that these regions were caused by a phenomenon in which soil particles become displaced as the result of an impactor (also known as “splashing”).

Finally, the DLR’s Tilman Spohn and his colleagues analyzed data from Philae’s Multi Purpose Sensors for Surface and Subsurface Science (MUPUS) thermal and penetrating sensors to learn that the comet has a daytime surface temperature of between 90 and 130 degrees Kelvin, and that the surface of its final landing spot is covered with a compact and porous layer of dust and ice.

However, they also reported that the MUPUS thermal probe could not fully penetrate the near-surface layers because the ground in that area was resistant to penetration. More accurately, they believe that the surface had a more than 4 megapascals resistance to penetration equivalent to an approximately two megapascal uniaxial compressive strength.

(Image credit: ESA/Rosetta/Philae/ROLIS/DLR)