Mark Stephen Bentley

Aggregate dust particles at comet 67P/Churyumov–Gerasimenko

Comets are thought to preserve almost pristine dust particles, thus providing a unique sample of the properties of the early solar nebula. The microscopic properties of this dust played a key part in particle aggregation during the formation of the Solar System. Cometary dust was previously considered to comprise irregular, fluffy agglomerates on the basis of interpretations of remote observations in the visible and infrared and the study of chondritic porous interplanetary dust particles that were thought, but not proved, to originate in comets. Although the dust returned by an earlier mission has provided detailed mineralogy of particles from comet 81P/Wild, the fine-grained aggregate component was strongly modified during collection. Here we report in situ measurements of dust particles at comet 67P/Churyumov–Gerasimenko. The particles are aggregates of smaller, elongated grains, with structures at distinct sizes indicating hierarchical aggregation. Topographic images of selected dust particles with sizes of one micrometre to a few tens of micrometres show a variety of morphologies, including compact single grains and large porous aggregate particles, similar to chondritic porous interplanetary dust particles. The measured grain elongations are similar to the value inferred for interstellar dust and support the idea that such grains could represent a fraction of the building blocks of comets. In the subsequent growth phase, hierarchical agglomeration could be a dominant process and would produce aggregates that stick more easily at higher masses and velocities than homogeneous dust particles. The presence of hierarchical dust aggregates in the near-surface of the nucleus of comet 67P also provides a mechanism for lowering the tensile strength of the dust layer and aiding dust release.

The Castalia mission to Main Belt Comet 133P/Elst-Pizarro

We describe Castalia, a proposed mission to rendezvous with a Main Belt Comet (MBC), 133P/Elst-Pizarro. MBCs are a recently discovered population of apparently icy bodies within the main asteroid belt between Mars and Jupiter, which may represent the remnants of the population which supplied the early Earth with water. Castalia will perform the first exploration of this population by characterising 133P in detail, solving the puzzle of the MBC’s activity, and making the first in situ measurements of water in the asteroid belt. In many ways a successor to ESA’s highly successful Rosetta mission, Castalia will allow direct comparison between very different classes of comet, including measuring critical isotope ratios, plasma and dust properties. It will also feature the first radar system to visit a minor body, mapping the ice in the interior. Castalia was proposed, in slightly different versions, to the ESA M4 and M5 calls within the Cosmic Vision programme. We describe the science motivation for the mission, the measurements required to achieve the scientific goals, and the proposed instrument payload and spacecraft to achieve these.

MIDAS: Lessons learned from the first spaceborne atomic force microscope

Abstract The Micro-Imaging Dust Analysis System (MIDAS) atomic force microscope (AFM) onboard the Rosetta orbiter was the first such instrument launched into space in 2004. Designed only a few years after the technique was invented, MIDAS is currently orbiting comet 67P Churyumov–Gerasimenko and producing the highest resolution 3D images of cometary dust ever made in situ . After more than a year of continuous operation much experience has been gained with this novel instrument. Coupled with operations of the Flight Spare and advances in terrestrial AFM a set of “lessons learned” has been produced, cumulating in recommendations for future spaceborne atomic force microscopes. The majority of the design could be reused as-is, or with incremental upgrades to include more modern components (e.g. the processor). Key additional recommendations are to incorporate an optical microscope to aid the search for particles and image registration, to include a variety of cantilevers (with different spring constants) and a variety of tip geometries.

The footprint of cometary dust analogues – I. Laboratory experiments of low-velocity impacts and comparison with Rosetta data

Cometary dust provides a unique window on dust growth mechanisms duringthe onset of planet formation. Measurements by the Rosetta spacecraftshow that the dust in the coma of comet 67P/Churyumov-Gerasimenko has agranular structure at size scales from sub-μmup to several hundredsof μm, indicating hierarchical growth took place across these sizescales. However, these dust particles may have been modified duringtheir collection by the spacecraft instruments. Here, we present theresults of laboratory experiments that simulate the impact of dust onthe collection surfaces of the COSIMA (Cometary Secondary Ion MassAnaylzer) and MIDAS (Micro-Imaging Dust Analysis System) instrumentsonboard the Rosetta spacecraft. We map the size and structure of thefootprints left by the dust particles as a function of their initialsize (up to several hundred μm) and velocity (up to 6 ms-1). We find that in most collisions, only part of the dustparticle is left on the target; velocity is the main driver of theappearance of these deposits. A boundary between sticking/bouncing andfragmentation as an outcome of the particle-target collision is found atv ˜ 2 m s-1. For velocities below this value, particleseither stick or leave a single deposit on the target plate, or bounce,leaving a shallow footprint of monomers. At velocities >2 ms-1and sizes >80 μm, particles fragment upon collision,transferring up to 50 per cent of their mass in a rubble-pile-likedeposit on the target plate. The amount of mass transferred increaseswith the impact velocity. The morphologies of the deposits arequalitatively similar to those found by the COSIMA instrument.

Morphology of Cometary Dust at the Nanometre Scale Detected with MIDAS

Prior to the European Space Agency’s Rosetta mission, which is revolutionizing our understanding of comets, investigation of cometary dust was primarily carried out by remote observation and brief in-situ (fly-by/impact) measurements and sample return missions. Cometary dust is often considered to comprise low density (porous) aggregates with complex shapes. Evidence for this comes from i) interplanetary dust particles (IDPs) collected in the Earth’s stratosphere, some of which are likely from comets, ii) analysis of polarimetric observations and iii) the (highly modified) dust collected by the Stardust mission. The latter provided the first opportunity to investigate cometary dust from a known source on Earth. Although the finest fraction of the particles were heavily modified, Stardust nonetheless showed large compositional and structural heterogeneities – most collected dust grains being composed of many different sub-grains [1,2]. Rosetta offers the opportunity to further characterize highly pristine cometary dust, but extends this to the smallest particles.