@article{HammPelivanGrottetal.2020,
  author    = {Hamm, Maximilian and Pelivan, Ivanka and Grott, Matthias and de Wiljes, Jana},
  title     = {Thermophysical modelling and parameter estimation of small solar system bodies via data assimilation},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {496},
  journal   = {Monthly notices of the Royal Astronomical Society},
  number    = {3},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/staa1755},
  pages     = {2776 -- 2785},
  year      = {2020},
  abstract  = {Deriving thermophysical properties such as thermal inertia from thermal infrared observations provides useful insights into the structure of the surface material on planetary bodies. The estimation of these properties is usually done by fitting temperature variations calculated by thermophysical models to infrared observations. For multiple free model parameters, traditional methods such as least-squares fitting or Markov chain Monte Carlo methods become computationally too expensive. Consequently, the simultaneous estimation of several thermophysical parameters, together with their corresponding uncertainties and correlations, is often not computationally feasible and the analysis is usually reduced to fitting one or two parameters. Data assimilation (DA) methods have been shown to be robust while sufficiently accurate and computationally affordable even for a large number of parameters. This paper will introduce a standard sequential DA method, the ensemble square root filter, for thermophysical modelling of asteroid surfaces. This method is used to re-analyse infrared observations of the MARA instrument, which measured the diurnal temperature variation of a single boulder on the surface of near-Earth asteroid (162173) Ryugu. The thermal inertia is estimated to be 295 +/- 18 Jm(-2) K-1 s(-1/2), while all five free parameters of the initial analysis are varied and estimated simultaneously. Based on this thermal inertia estimate the thermal conductivity of the boulder is estimated to be between 0.07 and 0.12,Wm(-1) K-1 and the porosity to be between 0.30 and 0.52. For the first time in thermophysical parameter derivation, correlations and uncertainties of all free model parameters are incorporated in the estimation procedure that is more than 5000 times more efficient than a comparable parameter sweep.},
  language  = {en}
}
@article{GrottKnollenbergHammetal.2019,
  author    = {Grott, Matthias and Knollenberg, J. and Hamm, M. and Ogawa, K. and Jaumann, R. and Otto, Katharina Alexandra and Delbo, M. and Michel, Patrick and Biele, J. and Neumann, Wladimir and Knapmeyer, Martin and K{\"u}hrt, E. and Senshu, H. and Okada, T. and Helbert, Jorn and Maturilli, A. and M{\"u}ller, N. and Hagermann, A. and Sakatani, Naoya and Tanaka, S. and Arai, T. and Mottola, Stefano and Tachibana, Shogo and Pelivan, Ivanka and Drube, Line and Vincent, J-B and Yano, Hajime and Pilorget, C. and Matz, K. D. and Schmitz, N. and Koncz, A. and Schr{\"o}der, Stefan E. and Trauthan, F. and Schlotterer, Markus and Krause, C. and Ho, T-M and Moussi-Soffys, A.},
  title     = {Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu},
  series = {Nature astronomy},
  volume    = {3},
  journal   = {Nature astronomy},
  number    = {11},
  publisher = {Nature Publishing Group},
  address   = {London},
  issn      = {2397-3366},
  doi       = {10.1038/s41550-019-0832-x},
  pages     = {971 -- 976},
  year      = {2019},
  abstract  = {C-type asteroids are among the most pristine objects in the Solar System, but little is known about their interior structure and surface properties. Telescopic thermal infrared observations have so far been interpreted in terms of a regolith-covered surface with low thermal conductivity and particle sizes in the centimetre range. This includes observations of C-type asteroid (162173) Ryugu1,2,3. However, on arrival of the Hayabusa2 spacecraft at Ryugu, a regolith cover of sand- to pebble-sized particles was found to be absent4,5 (R.J. et al., manuscript in preparation). Rather, the surface is largely covered by cobbles and boulders, seemingly incompatible with the remote-sensing infrared observations. Here we report on in situ thermal infrared observations of a boulder on the C-type asteroid Ryugu. We found that the boulder's thermal inertia was much lower than anticipated based on laboratory measurements of meteorites, and that a surface covered by such low-conductivity boulders would be consistent with remote-sensing observations. Our results furthermore indicate high boulder porosities as well as a low tensile strength in the few hundred kilopascal range. The predicted low tensile strength confirms the suspected observational bias6 in our meteorite collections, as such asteroidal material would be too frail to survive atmospheric entry7},
  language  = {en}
}
@article{GrottKnollenbergHammetal.2019,
  author    = {Grott, Matthias and Knollenberg, J. and Hamm, M. and Ogawa, K. and Jaumann, R. and Otto, Katharina Alexandra and Delbo, M. and Michel, P. and Biele, J. and Neumann, W. and Knapmeyer, M. and Kuehrt, E. and Senshu, H. and Okada, T. and Helbert, J. and Maturilli, A. and M{\"u}ller, N. and Hagermann, A. and Sakatani, N. and Tanaka, S. and Arai, T. and Mottola, S. and Tachibana, S. and Pelivan, Ivanka and Drube, L. and Vincent, J-B and Yano, H. and Pilorget, C. and Matz, K. D. and Schmitz, N. and Koncz, A. and Schr{\"o}der, S. E. and Trauthan, F. and Schlotterer, M. and Krause, C. and Ho, T-M and Moussi-Soffys, A.},
  title     = {Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu},
  series = {Nature astronomy},
  volume    = {3},
  journal   = {Nature astronomy},
  number    = {11},
  publisher = {Nature Publishing Group},
  address   = {London},
  issn      = {2397-3366},
  doi       = {10.1038/s41550-019-0832-x},
  pages     = {971 -- 976},
  year      = {2019},
  abstract  = {C-type asteroids are among the most pristine objects in the Solar System, but little is known about their interior structure and surface properties. Telescopic thermal infrared observations have so far been interpreted in terms of a regolith-covered surface with low thermal conductivity and particle sizes in the centimetre range. This includes observations of C-type asteroid (162173) Ryugu1,2,3. However, on arrival of the Hayabusa2 spacecraft at Ryugu, a regolith cover of sand- to pebble-sized particles was found to be absent4,5 (R.J. et al., manuscript in preparation). Rather, the surface is largely covered by cobbles and boulders, seemingly incompatible with the remote-sensing infrared observations. Here we report on in situ thermal infrared observations of a boulder on the C-type asteroid Ryugu. We found that the boulder's thermal inertia was much lower than anticipated based on laboratory measurements of meteorites, and that a surface covered by such low-conductivity boulders would be consistent with remote-sensing observations. Our results furthermore indicate high boulder porosities as well as a low tensile strength in the few hundred kilopascal range. The predicted low tensile strength confirms the suspected observational bias6 in our meteorite collections, as such asteroidal material would be too frail to survive atmospheric entry7.},
  language  = {en}
}