Crowdsourcing the unknown: the satellite search for Genghis Khan.

Massively parallel collaboration and emergent knowledge generation is described through a large scale survey for archaeological anomalies within ultra-high resolution earth-sensing satellite imagery. Over 10K online volunteers contributed 30K hours (3.4 years), examined 6,000 km², and generated 2.3 million feature categorizations. Motivated by the search for Genghis Khan's tomb, participants were tasked with finding an archaeological enigma that lacks any historical description of its potential visual appearance. Without a pre-existing reference for validation we turn towards consensus, defined by kernel density estimation, to pool human perception for "out of the ordinary" features across a vast landscape. This consensus served as the training mechanism within a self-evolving feedback loop between a participant and the crowd, essential driving a collective reasoning engine for anomaly detection. The resulting map led a National Geographic expedition to confirm 55 archaeological sites across a vast landscape. A increased ground-truthed accuracy was observed in those participants exposed to the peer feedback loop over those whom worked in isolation, suggesting collective reasoning can emerge within networked groups to outperform the aggregate independent ability of individuals to define the unknown.

Interfacial shear strength in abalone nacre.

The shear strength of the interface between tiles of aragonite in the nacre of red abalone Haliotis rufescens was investigated through mechanical tensile and shear tests. Dog-bone shaped samples were used to determine the tensile strength of nacre when loaded parallel to the plane of growth; the mean strength was 65 MPa. Shear tests were conducted on a special fixture with a shear gap of 200 microm, approximately 100 microm narrower than the spacing between mesolayers. The shear strength is found to be 36.9+/-15.8 MPa with an average maximum shear strain of 0.3. Assuming the majority of failure occurs through tile pull-out and not through tile fracture, the tensile strength can be converted into a shear strength of 50.9 MPa. Three mechanisms of failure at the tile interfaces are discussed: fracture of mineral bridges, toughening due to friction created through nanoasperities, and toughening due to organic glue. An additional mechanism is fracture through individual tiles.

Structure and mechanical properties of crab exoskeletons.

The structure and mechanical properties of the exoskeleton (cuticle) of the sheep crab (Loxorhynchus grandis) were investigated. The crab exoskeleton is a natural composite consisting of highly mineralized chitin-protein fibers arranged in a twisted plywood or Bouligand pattern. There is a high density of pore canal tubules in the direction normal to the surface. These tubules have a dual function: to transport ions and nutrition and to stitch the structure together. Tensile tests in the longitudinal and normal to the surface directions were carried out on wet and dry specimens. Samples tested in the longitudinal direction showed a convex shape and no evidence of permanent deformation prior to failure, whereas samples tested in the normal orientation exhibited a concave shape. The results show that the composite is anisotropic in mechanical properties. Microindentation was performed to measure the hardness through the thickness. It was found that the exocuticle (outer layer) is two times harder than the endocuticle (inner layer). Fracture surfaces after testing were observed using scanning electron microscopy; the fracture mechanism is discussed.

Mechanical strength of abalone nacre: role of the soft organic layer.

The nacreous portion of the abalone shell is composed of calcium carbonate crystals interleaved with layers of viscoelastic proteins. The resulting structure yields unique mechanical properties. In this study, we focus on the thin viscoelastic layers between the tiles and on their role on the mechanical properties of the shell. Both SEM and AFM show that the thin (approximately 30 nm) organic layer is porous, containing holes with diameter of approximately 50 nm. These holes enable the formation of mineral bridges between adjacent tile layers. The mineral bridges play a pivotal role in growth and ensure the maintenance of the same crystallographic relationship through tile growth in the 'terraced cone' mode. The existence of mineral bridges is consistent with the difference between tensile and compressive strength of the abalone. Mechanical tests with loading applied perpendicular to the plane of the organic layers reveal a tensile strength lower than 10 MPa, whereas the compressive strength is approximately 300-500 MPa. These nanoscale bridges have, by virtue of their dimensions (50 nm diameter x 30 nm length), a strength that reaches their theoretical value. The calculated tensile strength based on the theoretical strength predicts a bridge density of approximately 2.25/microm(2). A major conclusion of this investigation is that the role of the organic layer is primarily to subdivide the CaCO(3) matrix into platelets with thickness of 0.5 microm. Its intrinsic effect in providing a glue between adjacent tiles may not be significant.

The growth of nacre in the abalone shell.

The process of mineral formation following periods of growth interruption (growth bands) is described. Flat pearl implantation as well as a new trepanning method are used to observe the transitory phases of calcium carbonate which nucleate and grow during this process. An initial random nucleation of the aragonite polymorph is observed followed by a transition towards spherulitic growth. During this transition the animal forms the structure of the shell through both mechanical and chemical actions. About 6 weeks after implantation a steady-state growth of aragonite tiles begins after shorter and more irregular tiles cover the outer surface of the spherulites. The growth rate of aragonitic spherulite during this transition period was calculated to be approximately 0.5 microm per day. An organic scaffolding is observed during the steady-state growth of tiled aragonite. Observations of mineral growth following the deposition of these membranes confirm the presence of mineral bridges originating from subsurface tiles and extending through the organic matrix, confirming the growth model proposed by Schäffer et al. [Schäffer TE, Ionescu-Zanetti C, Proksch R, Fritz M, Walters, DA, Almqvist N, et al. Does abalone nacre form by heteroepitaxial nucleation or by growth through mineral bridges? Chem Mater 1997;9:1731-40]. Field emission scanning electron microscopy of fractured deproteinated nacre shows the presence of mineral bridges existing between individual layers of tiles. Transmission electron microscopy provides further evidence of mineral bridges.