Marrow

Marrow is a solitary moon in a retrograde orbit around 50 Persei VI, a Saturn-sized gas giant. It was well known to the public prior to the Human-Covenant War, infamous for its strange properties and the many business ventures that failed to exploit it.

Characteristics
There are three properties that make Marrow unique in all of known space. The first is its incredible density. Although it is smaller than Luna, Marrow has a mean density of 22.5 g/cm³, around four times that of Earth, giving it a surface gravity of 1.09 g. The composition of its core is unknown, but its outer crust is almost entirely composed of accretionite, a sedimentary rock formed from accumulated meteorites and space dust, and which mostly matches the composition of asteroids found in the 50 Persei system. The final oddity is its retrograde orbit. Although the orbit is unusually stable, Marrow is clearly an extrasolar capture that fell into 50 Persei VI's gravity well many millions of years ago. Because of its unusually strong gravity, Marrow must have ejected any other moons from their orbits. Indeed, Marrow may be coated in rubble from its half-sibling moons. Accordingly, Marrow was named for the antagonist of the 2447 horror film Shepard's Flock, a ghost child that was accepted into an orphanage, only to kill all the other orphans one by one.

Scientists and entrepreneurs alike have speculated about the composition of Marrow's core ever since it was first discovered. Some propose that the core is a massive chunk of heavy metals thrown out by a supernova, while others estimate that it is a smaller blob of elements from the island of stability, and fringe theorists hold that it must be a machine of alien origin.

A few have put their money where their mouth is. Several mining ventures were organized to drill to the core. Many prospectors were convinced that it had a motherlode of valuable metals at its heart, although none of them had the evidence to prove that it existed, let alone was easily recoverable. The larger mining companies weren't interested, so all the drilling on Marrow's surface were wildcat operations, all of which failed.

While Marrow's favorable climate and atmosphere made mining operations easier than on other worlds, its accretionite crust played hell on drilling equipment, and supplies were expensive to ship in due to its remote location. One by one the ventures failed, with the most successful operation stopping just 45 meters shy of a ten kilometer depth. No end to the accretionite crust was found, though it changes composition around the 700 meter mark and exhibits astro-minerals uncommon to the 50 Persei system. After the Human-Covenant War, it was speculated that Covenant excavation equipment may be powerful enough to reach its core.

Marrow's comfortable gravitational pull, thin atmosphere, warm climate, and protection from radiation by 50 Persei VI's VARB meant that it has seen sporadic human habitation after 2539. This has seen it identified as an ideal spot for a new colony, if only the ownership disputes can be resolved.