The Arcanum mission is a proposed L-class mother-daughter spacecraft configuration for the Neptunian system, the mass and volume of which have been maximised to highlight the wide-ranging science the next generation of launch vehicles will enable. The spacecraft is designed to address a long-neglected but high-value region of the outer Solar System, showing that current advances make such a mission more feasible than ever before. This paper adds to a series on Arcanum and specifically provides progress on the study of areas identified as critical weaknesses by the 2013–2022 decadal survey and areas relevant to the recently published Voyage 2050 recommendations to the European Space Agency (ESA).

While a thick vein of scepticism marked Enlightenment thinkers’ studies, such investigations cannot be divorced from their concurrent quest to merge the wondrous and the rational. Perhaps nowhere is this more apparent than in philosophers’ investigations of merpeople. Examining European gentlemen’s debates over mermaids and tritons illuminate their willingness to embrace wonder in their larger quest to understand the origins of humankind. Naturalists utilized a wide range of methodologies to critically study these seemingly wondrous creatures and, in turn, assert the reality of merpeople as evidence of humanity’s aquatic roots. As with other creatures they encountered in their global travels, European philosophers utilized various theories—including those of racial, biological, taxonomical, and geographic difference—to understand merpeople’s place in the natural world. By the second half of the eighteenth century, certain thinkers integrated merpeople into their explanation of humanity’s origins, thus bringing this phenomenon full circle.

The state of knowledge about the structure and composition of icy satellite interiors has been significantly extended by combining direct measurements from spacecraft, laboratory experiments, and theoretical modeling. Interior models of icy bodies will certainly benefit from future missions to the outer solar system, providing new and improved constraints on the surface chemistry, bulk composition and degree of internal differentiation, possible heterogeneities in radial mass distribution, the presence and extent of liquid reservoirs, and the amount of tidal heating for each target body. Here we summarize geophysical constraints on the interior structure and composition of selected Jovian and Saturnian icy satellites and investigate conditions under which potentially habitable liquid water reservoirs could be maintained. Future geophysical exploration which includes gravitational and magnetic field sounding from low-altitude orbit and close flyby, combined with altimetry data and in-situ monitoring of tidally-induced surface distortion and time-variable magnetic fields, would impose important constraints on the interiors of outer planet satellites.