I. Scientific rationale, goals and timeliness

I.1. The impact process

The collisional history of a small body is at the origin of its current properties. Therefore, characterizing the impact response of a body as a function of its physical/material properties can help us make reasonable assumptions regarding the internal and surface properties of target candidates of space missions (rendezvous, sample return, deflection). The scales of the phenomena that are involved in planetary and small-body impacts are by far much larger than those reached in laboratory experiments. While those experiments remain crucial to understanding the impact process at small scales (centimeter-size; see e.g. Nakamura and Fujiwara 1991, Icarus 92, 132; Arakawa et al. 2002, Icarus 158, 516), extrapolations by 15 orders of magnitude in mass are necessary to achieve ranges that are relevant to asteroids and planetesimals. In addition to scaling laws (see e.g. Holsapple 1994, PSS 42, 1067), numerical simulations have been developed to approach this problem more completely. In the 1990s, a hydrodynamical code based on the Smoothed Particle Hydrodynamics method (SPH), and that included a model of brittle failure, was developed by Benz and Asphaug (1994, Icarus 107, 98). Recently, a model of fragmentation of porous material was introduced into this code (Jutzi et al. 2008, Icarus 198, 242) and was successfully matched against laboratory experiments on pumice targets (Jutzi et al. 2009, Icarus 201, 802). However, further tests with other materials are required to check the full validity of the numerical models. Therefore, interactions between experimental and numerical experts are required to determine the measurements that are needed to perform validity tests. This should allow the development of new campaigns of impact experiments on different porous and non-porous target materials, covering a wide range of porosity, density, other material properties and impact speeds, and a comparison of the outcome of these experiments with the results of impact simulations. Such a project is thus a multidisciplinary collaboration involving different kinds of expertise. Moreover, disciplines and fields with interest in the process of rock and porous material fragmentation would benefit from the results of this study. In parallel, we will continue our exploration of the impact process at large scales. In the size range adapted to small Solar System bodies (>100 meters), the role of gravity can strongly influence the collisional outcome. Indeed, ejected fragments produced by the fragmentation process interact gravitationally during their ejection, a crucial process neglected by earlier studies. Reaccumulation can occur when relative velocity magnitudes between fragments are below their mutual escape speeds, leading to a distribution of large aggregated remnants. This was demonstrated by Michel et al. (2001, Science 294, 1696) who successfully simulated for the first time the formation of asteroid families from the disruption of large parent bodies. We will continue this exploration. In particular the increase in realism of the simulations of the gravitational phase during which fragments interact and reaccumulate will allow us to better determine their resulting shape and rotational properties, which can help in the interpretation of the images provided by space missions.

We need to point out that members of the team who are experts in numerical simulations are at the origin of state-of-the-art models of fragmentation of both porous and non-porous bodies and of the gravitational phase. Those who are experts in laboratory experiments have already provided the most detailed data from experiments that have allowed validation of numerical models against real data. Bringing together these two communities on a regular basis will certainly lead to fruitful work.

I.2. The dynamics of granular material

Because the surfaces of most celestial solid bodies are covered with granular material, in the form of fine regolith or gravels and pebbles, it is important to characterize how such material responds to various stresses. In particular, the presence or relative absence of gravitational acceleration on granular flow is of importance for understanding the geology of small bodies and planets, and to clarify the environments that may be encountered during planetary exploration. Bodies with low surface gravity can be very sensitive to processes that appear irrelevant in the case of larger planetary bodies. For instance, seismic vibration induced by small impacts can occur throughout a small body and can be at the origin of motion of its granular surface. Such a mechanism has been proposed to explain the lack of very small craters both on Eros (Richardson et al. 2004, Science 306, 1526) and Itokawa (Michel et al. 2009, Icarus 200, 503). However, the cratering process on a small body’s surface that consists of granular material is not fully understood. Improving our knowledge of this process is not only important for our understanding of the surface geology of small bodies but also for estimating the surface age of small bodies on the basis of crater counting methods associated with estimates of impactor flux. We must still consider the possibility that it is difficult to make craters on the granular surface of small bodies, due to some processes such as armoring effects by pebbles and/or boulders. Could the number of small crater features on small bodies be small in the first place because of their granular surface? To answer this question, we need to clarify the impact cratering process on granular material under micro-gravity. On the other hand, small craters may not survive long on such small body surfaces because of the regolith motion as a result of seismic vibrations. However, the efficiency of these vibrations to generate motion, and the way the regolith will move, are poorly known and require better understanding of granular dynamics in the low-gravity environment. Moreover, understanding how granular materials, as a function of their properties (angle of friction, size distribution of their components, etc.), react to different kinds of stresses is of great interest for the design of landers and sampling devices of space missions. Then, granular matter may strengthen under a variety of conditions (Losert et al. 2000, PRL 85, 1428). Flow in response to stress can be described by continuum models under certain conditions (Savage 1998, Journal of Fluid Mech. 377, 1; Goddard 2004, Proceedings of the Royal So. of London A430, 105; Losert et al. 2000, Physic. Review E 61, 4060). However, many processes cannot be captured using classical continuum approaches. For example, highly stressed granular materials exhibit shear localization during failure (Mueth et al. 2000, Nature 406, 385) but this localization cannot be described accurately by existing continuum models (Kamrin et al. 2007, Physic. Review E 75). We have also identified situations in which the
material under shear stress weakens in other directions (Toiya et al. 2004, PRL 83, 088001-1; Falk et al. 2008, arXiv:0802.0485).

Our team includes specialists of the granular dynamics involved both in laboratory experiments and numerical models. As for the impact process, we aim to enhance existing numerical models and compare them with experiments performed both at micro-g and under Earth-gravity. Some team members and their students have already started such experiments, and the analysis as well as the preparation of future experiments will be discussed during the workshops. Moreover, once the numerical models show their ability to reproduce those experiments, they can be used to study the dynamics of granular materials under various gravitational environments that can be encountered in the Solar System and that cannot be addressed by experiments.

I.3. Timeliness

Rendezvous and sample-return missions to small bodies and planets are planned or under way by the main space agencies (ESA, NASA, JAXA). Some of them, such as the Rosetta mission devoted to in-situ characterization of a comet in 2014, already involve the interaction of an instrument (a lander) with the explored surface, and others will require the design of an efficient sampling device to collect material from the surface of either a small body or a planet (e.g. Mars) for in-situ analysis or return to Earth. To prepare those missions and to interpret their data, a better understanding of the physical properties of solid celestial bodies, in particular the smaller ones, and their evolution under various kinds of stresses is required. The stresses that are considered here are those resulting from the impact process, and those contributing to the dynamics of granular materials on those bodies’ surfaces (e.g. avalanches, seismic vibrations, etc.).

II. Expected outputs

The outputs of these interdisciplinary studies will consist of a larger database of experiments of impacts on solid targets covering a wide range of properties, and of the dynamics of granular materials. It will also consist of the improvement of numerical models aimed at characterizing the response of solid bodies and granular materials to impacts and other stresses. Scientific results will be announced during conferences and will be submitted for publications in leading scientific journals. Given the promising interactions and motivation of our team members, and the past record of our publications, we expect to submit several papers during the two-year ISSI activity detailing the outcome of our investigations.

III. Added value provided by ISSI for the implementation of the Team activity

ISSI will provide the best possible stimulating environment for interdisciplinary discussions among scientists spread over Europe, US and Japan who have common scientific interests. ISSI will provide an environment in which collaborative work could progress between team members who otherwise lack opportunities to meet and discuss on a systematic basis. It also offers great visibility to the creation and activity of an International Team devoted to those topics.

IV. Schedule of the project

We plan to organize 3 workshops over the 2-year ISSI project. We suggest having our first meeting this summer (middle of July 2010). Workshops will last 5 days, including probably a Saturday and/or a Sunday to minimize return fees and also to facilitate the presence of team members that have overly busy working weeks. A second meeting will take place in early fall 2011. Depending on progress, the third meeting, which will take place at the end of the 2-year schedule, may include other experts who will join at their own expense to discuss our results and provide long-term perspectives for the whole community.