The neural crest (NC) is a major contributor to the vertebrate craniofacial skeleton, detailed in model organisms through embryological and genetic approaches, most notably in chick and mouse. Despite many similarities between these rather distant species, there are also distinct differences in the contribution of the NC, particularly to the calvariae of the skull. Lack of information about other vertebrate groups precludes an understanding of the evolutionary significance of these differences. Study of zebrafish craniofacial development has contributed substantially to understanding of cartilage and bone formation in teleosts, but there is currently little information on NC contribution to the zebrafish skeleton. Here, we employ a two–transgene system based on Cre recombinase to genetically label NC in the zebrafish. We demonstrate NC contribution to cells in the cranial ganglia and peripheral nervous system known to be NC–derived, as well as to a subset of myocardial cells. The indelible labeling also enables us to determine NC contribution to late–forming bones, including the calvariae. We confirm suspected NC origin of cartilage and bones of the viscerocranium, including cartilages such as the hyosymplectic and its replacement bones (hymandibula and symplectic) and membranous bones such as the opercle. The cleithrum develops at the border of NC and mesoderm, and as an ancestral component of the pectoral girdle was predicted to be a hybrid bone composed of both NC and mesoderm tissues. However, we find no evidence of a NC contribution to the cleithrum. Similarly, in the vault of the skull, the parietal bones and the caudal portion of the frontal bones show no evidence of NC contribution. We also determine a NC origin for caudal fin lepidotrichia; the presumption is that these are derived from trunk NC, demonstrating that these cells have the ability to form bone during normal vertebrate development.

Experiments examining the production of Enceladus ice grain analogues and the characterization of their impact phenomena are reported. These measurements make use of a unique single particle acceleratorthe aerosol impact spectrometer (AIS)to extend studies of the impact dynamics of ice grains down to the 0.1–10 μm diameter range relevant to orbital sampling of Enceladus ice grains. Laboratory generation of Enceladus plume grains followed by an examination of their impact dynamics is required to support the interpretation of ice grain orbital sampling in a potential flyby mission concept. In the work reported here, the AIS was used to inject charged water droplets produced in an electrospray ionization source through an aerodynamic lens and into vacuum such that they freeze within 50–200 μs. The ice grains were then accelerated to a controlled final velocity using a linear accelerator (LINAC). The capability of the LINAC to achieve hypervelocity speeds is explored here. The AIS was equipped with the tapered image charge detector, a multielement image charge detector composed of three charge-sensitive rings and the collision analysis target, providing angle-resolved measurements that revealed impact phenomena including rebound, sticking, and fragmentation for ice grain impacts on a molybdenum target. The velocity-dependent trends of these impact phenomena are reported for impacts ranging from 20 to 900 m/s.

The impact dynamics of ice grains on surfaces as a function of velocity, including particle breakup and impact ionization, are of intrinsic interest and are critical for the design of future probes to study the ice grain plume around Enceladus. Measurements of the scattering dynamics of ∼700 nm diameter pure water ice grains upon 0.2−2.4 km/s impact with a metal target are reported here. Production of these Enceladus plume grain analogues and their subsequent acceleration to controlled final velocities were performed with an aerosol impact spectrometer. The particle impact and various impact behaviors, including rebound, sticking, particle fragmentation, and impact ionization, were characterized as a function of velocity with an angle-resolved image-charge particle detector. The probability of rebound, sticking, and particle fragmentation was the highest below 400 m/s, between 400 and 800 m/s, and above 800 m/s, respectively. Impact ionization was also observed for impact velocities above 1000 m/s.

Mass spectrometry (MS) using impact-induced ionization is an effective means of detecting organic molecules entrained within ice grains. Recent work has demonstrated that the optimal speed for volatilizing and ionizing biomolecules of interest in ice grains is 4-6 km/s. Above this speed range, the biomolecules start to fragment; below this threshold, the energy is not sufficient to promote ionization. A spacecraft with an impact plate and mass spectrometer could therefore be used to perform astrobiology investigations at Enceladus or possibly Europa, without the cost of resources to slow down and/or land. We recommend support of further experimental and theoretical work, in particular a focus on impacts of neutral ice grains, in order to gain a deeper understanding of the physical and chemical mechanisms that occur during hypervelocity impact. We also recommend investment in developing the next generation of MS techniques for hypervelocity sampling, as well as producing/generating mass spectral libraries for future missions to use in the identification of organics and biosignatures.

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