Velnara, frozen moon of Karvtal

This is a write up of Velnara, the rock and icy moon of Karvtal , in the Galen Alpha planetary system, a setting for the Tyranny of the Daleks scenario, using the Second Edition of the Doctor Who Roleplaying Game from Cubicle 7.

The near  side of Velnara, the rocky and ice moon of Karvtal

Mass : 0.009 x Earth (0.731 x Luna)

Diameter: 3,161 kilometres

Density: 3.25 grams per cubic centimetre

Average temperature: -63.8 degrees Celsius (337 Kelvin) (minimum -117 degrees Celsius, maximum -49.7 degrees Celsius)

Rotational Period: 27.2 days

Orbital Period: 27.2 days (tidally locked)

Surface gravity : 0.146 g

Semi-Major Axis of orbit round Karvtal : 375,405 kilometres

Eccentricity of orbit : 0.00609

Inclination of orbit: 5.06 degrees

Obliquity : 1.04 degrees

Approximate escape velocity : 2.13 kilometres per second

Overview

Velnara is a cold, ancient rocky moon orbiting Karvtal, the third planet of the Galen Alpha system. Roughly similar in dimensions to Earth’s Moon it is a substantial planetary body, possessing a differentiated interior, a rocky surface, extensive impact cratering, deposits of water ice, and a thin but genuine atmosphere.

It has a thin atmosphere, estimated at approximately one fifth of Earth’s surface atmospheric pressure at ground level.  As such it has weather, winds, clouds and first, while remaining a harsh and inhospitable environment.

Velnara is tidally locked to Karvtal. Its rotational period exactly matches its orbital period around Karvtal of 27.2 days, meaning that the same hemisphere of Velnara always faces its parent planet. Karvtal consequently remains almost stationary in Velnara’s sky, while the stars and Galen Alpha appear to move across the heavens over the course of its orbit.

Velnara’s orbit around Karvtal is remarkably circular. Its modest obliquity contributes to a relatively stable seasonal environment.  From space it appears predominantly rocky with pale and blue-grey areas of exposed or near-surface ice scattered across its surface.  Its landscape is dominated by ancient highlands, impact craters, basaltic plains, fractured terrain and deposits of frozen volatiles. 

The icy regions are not distributed in broad, continuous bands, but occur irregularly across the surface, particularly in polar environments, ancient impact basins, recent landslides,  and permanently shaded regions. It is these deposits that give Velnara its characteristic scattered blue-grey appearance when seen from space. 

Velnara's orbit around Karvtal

Internal structure

Velnara is a differentiated rocky world as the heavier materials have generally migrated towards the core, while lighter silicate materials form the mantle and crust.

A core composed mostly of iron, nickel and sulphur has a radius of approximately 230 kilometres. Some residual internal heat permits limited geological activity deep within the moon.

A thick silicate mantel extending a further 1,300 kilometres is comprised mostly of magnesium and iron-rich silicate minerals.  Whilst the mantle is predominantly solid, temperatures deep within it allow for some limited convection. It is therefore not entirely solid.

Velnara’s orbit around Karvtal

A rocky, silicate crust some fifty kilometres covers Velnara, a relatively thick layering of basalt and other minerals.  The crust records a very long history of impacts. Ancient impact structures have been repeatedly battered by subsequent collisions, producing heavily fragmented regions and extensive deposits of pulverised rock.

Large areas of the surface therefore consist of regolith, a loose mixture of dust, rock fragments, impact debris and chemically altered material.

Regolith and near-surface ice

The uppermost surface consists of a variable layer of regolith, generally several metres thick although some areas are deeper. Mixed in with the regolith are significant quantities of frozen volatiles, including water ice, hydrated materials, frozen carbon dioxide, trapped atmospheric gases and carbon-bearing compounds and other material deposited by ancient comets and asteroids.

At these temperatures, ice is frozen hard almost like rock, and forms sheets, hills, valleys and low lying mountains. Ancient volcanoes from when Velnara had more geological activity hundreds of millions, if not billions, of years ago have left tunnels and channels in the ice.

At Velnara’s extremely low temperatures, carbon dioxide in the atmosphere can freeze onto the surface under suitable conditions, leaving a short-lived frost.

The atmosphere

Velnara possesses a thin but genuine atmosphere rather than merely an exosphere.  It is around 0.2 bar at the surface, that is 20% of Earth’s surface pressure. It is thicker than Mars’, but far thinner than Earth’s.

The low gravity means the atmosphere is vertically extended, reaching up to forty kilometres or so compared with roughly eight kilometres for Earth.

The atmosphere is composed of 77% nitrogen, 11% argon, 7% carbon dioxide, 3% oxygen, approximately 1% of water vapour and less than 1% trace gases.

Weather and climate

The thin atmosphere is sufficient to produce genuine weather, including winds, high-altitude clouds, ground-level haze and localised snow-like precipitation.

The low gravity means that dust and fine ice particles can remain suspended in the atmosphere relatively easily.  Large storms can lift enormous quantities of fine regolith into the atmosphere, producing broad, slowly moving dust veils visible from orbit. Its atmospheric circulation helps moderate temperature differences between regions.

Velnara’s two faces

Just as with Earth’s Moon, Velnara has two distinct faces, one that is always turned towards Karvtal, the other looking away. This is due to tidal locking in its orbit.

The far side, entirely unseen from the surface of Karvtal, is lighter and more rugged than the Karvtal-facing side.  The highlands are more dominant, and impact craters far more numerous. Less icy-deposits are visible on the far side.  To anyone on the surface of Karvtal, they only ever see the same side of Velnara. 

Given its size, it would look roughly the same size in the sky as Earth’s Moon does from the surface of Earth, albeit ever so slightly smaller.  Velnara’s ice deposits make it shine brightly in Karvtal’s sky, giving it a somewhat brighter, slightly bluish appearance compared to Earth’s Moon, with surface features on Velnara distinctly apparent in the Karvtal sky.