Hollow Moon Planetology: Blue Moon - Atmosphere & Gravity
by Sharon Dornhoff Internal atmosphere
The interior of the Nearside is blanketed in a breathable
atmosphere similar to Mystara's, although slightly higher in carbon
dioxide than what PCs are used to. This difference is too small to
endanger life or affect a character's performance, but adventurers
from Mystara will find themselves breathing hard for the first few
days of their visit. This excess of CO2 results from Matera's
sparse vegetation -- in such a dim-lit world, the oxygen-generating
process of photosynthesis is less efficient -- and may seem quite
stuffy at first ... unless, of course, the PCs' ship is already
suffering from a fouled atmosphere, in which case the lunar air
seems fresh by comparison! Sulphur gases are also quite common in
the Hollow Moon, thanks to the large number of volcanoes; a faint
odour of rotten eggs prevails over most geologically-active regions
(which includes just about anywhere that's not the maria), although
it's not strong enough to inconvenience lupins' scent-recognition
or other olfactory abilities, let alone pose a danger to
anyone.
Another reason PCs will gasp for air, upon first entering the
Hollow Moon, is that the atmosphere within Matera varies in its
density from place to place. Whereas gravity from Matera causes the
Mystaran seas to periodically rise and fall as tides, as the
visible moon orbits the planet, Mystara's gravity always pulls its
strongest on the same region of Matera: the "midlands", which are
located on the equator at the very centre of the Nearside. The
farther away one moves from the midlands -- and hence, from the
part of the Hollow Moon closest to Mystara's attraction -- the
thinner the available air becomes. The altitude limit of the inner
moon's breathable atmosphere also varies, from nearly twice the
height of Mystara's in the midlands (37,500' high), to less than
half of Mystara's in the "rimlands" (9000'), along the
Nearside/Farside border. On the Farside rim's various peninsulas of
outjutting land -- which include the Redlands and the rugged
vicinity of Mare Orientale -- the internal atmosphere is thin as
the air on a high Mystaran mountaintop; in the midlands, the air is
thick enough to retain extra heat and provide a near-subtropical
warmth, despite the Hollow Moon's rather chilly (40 degrees F) mean
air temperature.
In spite of the fact that PCs may FEEL as if they are suffocating,
between the extra carbon dioxide and the rarefied air they'll
encounter near Mare Orientale, there's no less oxygen in the
Materan atmosphere than in Mystara's. After a few (Mystaran) days
-- specifically, 60 hours minus three times a character's
Constitution score -- visitors will become acclimatised to the
conditions, and cease to find the CO2 levels uncomfortable. The
rimlands' rarefied air will still leave them gasping if they've
spent long periods in the central Nearside, but that's no different
from moving from a lower to a higher altitude, on Mystara.
Internal gravity
Within the Hollow Moon, Earth-normal gravity prevails, drawing
creatures downward towards the lunar "shell", much as does the
Hollow World's internal gravity. In Matera, the gravitational pull
of Mystara has a lesser but significant effect, as well; although
it doesn't alter the fact that things fall to the ground, even in
the rimlands, it creates a "perpetual tide" in the midlands that
affects both air and seas. This gives the central Nearside both a
thicker atmosphere, and deeper waters, than would otherwise exist
within that region, were the gravity equal in all directions in the
HM setting. Within the rimlands, where Mystara's gravity pulls
perpendicular to Matera's own, plum bobs hang at a full four-degree
angle toward the central Nearside; a definite sense of "tilting"
can be plainly felt by characters in these regions, giving the
impression that one is ascending a slight slope as one moves in the
farward direction on level ground. (This drives AD&D dwarves
and gnomes bonkers, when they try to estimate the slope of an
underground passage. ;-D) Rimland architecture must take this
skewed gravity into account, so the lunar rim-dwellers build one
side of their homes' foundations a bit taller than the other to
compensate, not unlike hillside-residents on Mystara. This
"tilting" levels out as you move closer to the midlands, where
Mystara's gravitational pull is parallel to Matera's.
Much like Mystara's exterior, the inner atmosphere of the Hollow
Moon is bounded by a Skyshield. Its altitude, however, varies with
the actual depth of the lunar atmosphere, from nearly twice
(150,000') to less than half (36,000') the elevation of Mystara's
Skyshield. Vortigern's Vortex and tubular breaches do not occur, in
the Hollow Moon; however, the seasonal events called "circle tides"
offer aerial vessels the brief opportunity to lift off and cross
over the internal Skyshield*. The inner moon's Cordillera and Rook
Mountains, with their thick underlying bedrock and steep
5-mile-plus ascents, actually poke their highest peaks right up
through the Farside's lower-than-normal Skyshield! Piloting a ship
from Mare Orientale to the inhabited points beyond, therefore,
entails navigating along low-altitude valleys within these two
concentric mountain ranges, rather than sailing above them.
Fortunately, safe passes through the double ring of mountains are
easily found.
[* - The outer Matera, having no atmosphere to retain, also has no
Skyshield to inhibit ships' passage. If you're using Bruce Heard's
idea of giving all of Mystaraspace an Alphatian-created, breathable
atmosphere as of 1016 AC, then the Immortals -- leery of intruders
stumbling upon their extradimensional city of Pandius, on the outer
moon's Farside -- will set up a new lunar Skyshield at that time...
but it serves to keep air OUT, not in!]
Internal void
Beyond the Hollow Moon's Skyshield, the interior of Matera is
similar to outer space -- it lacks air, warmth, or protection from
UV radiation (which, as previously mentioned, is able to pass
through the crystal Firmament in abundance). Unlike the deadly-cold
temperatures encountered in open space, Matera's inner Void is only
a few degrees below freezing; within the moon's enclosed confines,
trapped volcanic heat and crystal-refracted sunlight keep
conditions moderate by comparison. The hazard of ultraviolet, on
the other hand, is all the more severe, for the dimness of the
Hollow Moon's environment may mislead explorers into foregoing all
precautions against it! (Note that even in campaigns which treat
outer space as being filled with breathable air, the interior of
Matera should STILL be an airless Void. Given that -- unlike the
Hollow World setting -- virtually every PC who visits Matera will
arrive in a Void-worthy ship, it's a necessary device for campaigns
to ensure such craft can't easily take short-cuts through the inner
moon's sunless core, bypassing huge stretches of territory and the
dangers and adventures they offer.) The whole of the internal Void
is subject to zero-G.
Although there are no stars in the Hollow Moon, the open region
beyond the Skyshield is home to a scattering of tiny, flashing
points of light, which occasionally move about. To natives of the
moon, these "Shimmerlights" are believed to be many things, from
distant fireflies to Immortal omens to benign spirits of the dead;
in reality, most are flecks of phosphorescent quartz or reflective
mica floating in the gravity-free void. They are most visible
during the darkest portion of the lunar "day", as this is when
light shines upward through the Nearside maria and can be reflected
back down to the setting's watching inhabitants.
While the peninsulas of land that protrude into the Farside have an
atmosphere that's about half as thick as Mystara's, even this
meagre air tapers away, by the time one moves about 200 miles past
the Nearside/Farside border. The central Farside, itself, is
totally devoid of an atmosphere. Hence, although it's entirely
possible to circumnavigate the Firmament by walking around the rim
of the moon's crystalline hemisphere, only a voidship or other
method of space-travel can carry anyone to its very centre.