Surface roughness of asteroid (162173) Ryugu and comet 67P/Churyumov–Gerasimenko inferred fromin situobservations

Abstract

Alteration processes on asteroid and comet surfaces, such as thermal fracturing, (micrometeorite) impacts or volatile outgassing, are complex mechanisms that form diverse surface morphologies and roughness on various scales. These mechanisms and their interaction may differ on the surfaces of different bodies. Asteroid Ryugu and comet 67P/Churyumov-Gerasimenko, both, have been visited by landers which imaged the surfaces in high spatial resolution. We investigate the surface morphology and roughness of Ryugu and 67P/Churyumov-Gerasimenko based on high resolution in situ images of 0.2 mm and 0.8 mm pixel resolution over an approximately 25 cm and 80 cm wide scene, respectively. To maintain comparability and reproducibility, we introduce a method to extract surface roughness descriptors (fractal dimension, Hurst exponent, joint roughness coefficient, root mean square slope, hemispherical crater density, small scale roughness parameter and Hapke mean slope angle) from in situ planetary images illuminated by LEDs. We validate our method and choose adequate parameters for an analysis of the roughness of the surfaces. We also derive the roughness descriptors from 3-dimensional shape models of Ryugu and orbiter camera images and show that the higher spatially resolved images results in a higher roughness. We find that 67P/Churyumov-Gerasimenko is up to 6% rougher than Ryugu depending on the descriptor used and attribute this difference to the different intrinsic properties of the materials imaged and the erosive processes altering them. On 67P/Churyumov-Gerasimenko sublimation appears to be the main cause for roughness, while on Ryugu micrometeoroid bombardment as well as thermal fatigue and solar weathering may play a significant role in shaping the surface.

The MASCOT lander aboard Hayabusa2: The in-situ exploration of NEA (162173) Ryugu

“…Deduced from the MicrOmega measurement (Bibring et al, this issue), it was most probable that a prominent local depression in the foreground was beneath MASCOT. This thesis is supported by the observations of MASCam (Jaumann et al 2019, Otto et. al 2021, and see also section 5) describing that the surface of Ryugu was rougher than expected and covered with boulders of few tenth cm sizes.…”

“…Especially the images of MASCam at MP2 reveal a boulder with a surface texture of cauliflower-like undulations of ~1 cm scale and 2-3 mm scale. This topographic surface roughness in the sub-millimetre scale can be quantified by surface roughness parameters allowing to quantify and compare Ryugu's surface with that of other asteroids and comets (Otto et al 2021).…”

“…Next to the type 1 boulders, MASCam could also detect the brighter type 2 boulder, which are smooth and displays several thin and parallel layers with some exhibiting distinctive striped patterns observed by ONC(Sugita et al 2019). The observation of MASCam suggested that the planar slabs with smooth surface are due to elongated, non-denditric fractures(Otto et al 2021). MASCam observed boulders with sizes from centimeters to a few tens of meters.…”

The influence of geologic sample surfaces on TIR spectroscopy

“…Another example is the investigation of asteroids like the near-Earth Ryugu asteroid that orbits the sun. The work of Otto et al (2021) investigates the asteroids surface morphology and surface roughness and links it to complex alteration processes such as (micro)meteorite impacts or volatile outgassing. By studying asteroids like Ryugu scientists hope to gain important insights into the origins of the solar system and the formation of planets.…”