Surface Simulation

As of Update 24, Universe Sandbox simulates several surface properties of Planetary Body objects in two dimensions, to represent the variation in these properties across the surface of each object. The Surface view at the top of an object's View Panel can display a map representing any of these properties.

Properties
The following properties can be simulated:
 * Temperature represents the surface temperature at each point on the map. Temperature is displayed using the same units as Surface Temperature.
 * Elevation represents the height of each point on the map. Elevation is displayed using the same units as Radius.
 * Displacement represents the change in the height of each point of the map relative to the original elevation. Displacement is displayed using the same units as Radius.
 * Water Vapor Pressure represents the pressure of water vapor in each point on the map. Water Vapor Pressure is displayed using the same units as Surface Pressure.
 * Liquid Water Depth represents the height of the surface of the water above the sea floor in each point on the map. Water Depth is displayed using the same units as Radius.
 * Ice Thickness represents the thickness of the ice (solid water) layer in each point on the map. Ice Thickness is displayed using the same units as Radius.

Settings
The surface map for each of these surface properties can be displayed by using the Type setting in the object's View Panel.

Temperature
The Temperature layer represents the surface temperature of each point on the map of the object's surface. The temperature calculations consider the effects of incoming heat from sources like nearby stars, the effects of cooling, and the spread of heat across the surface of the object.

Elevation and Displacement
The Elevation and Displacement properties represent the height of every point on the surface map and the change in that height, respectively.

The Elevation map of certain known planets in the Solar System, including Mercury, Venus, Earth, and Mars, is stored in the Universe Sandbox database and based on actual data for these planets. The elevation maps of other objects, like exoplanets and randomly generated planets, are randomly generated.

The Displacement map of an object is empty at the beginning of a simulation. If the Elevation of the object's surface is changed, the Displacement map shows how much each grid cell has been changed from its original elevation. A positive value on the Displacement map indicates that the Elevation value of that cell has decreased, and a negative value indicates that the Elevation value of that cell has increased.

The Elevation map of an object can currently be changed via impacts, which will create craters that will decrease the elevation in some areas of the surface, and material is excavated, and increase the elevation in other areas, as debris is deposited on the surface.



Lapse Rate and Snow
Universe Sandbox includes a graphical representation of snow that will be present on mountain peaks above a certain elevation if water is present. The snow is not directly simulated, like ice and water are.

The resolution of the simulated surface of an object in Universe Sandbox is generally lower than the resolution of the heightmap used to display the object in a simulation. This is due to the performance demands of surface simulation. If Universe Sandbox used only the Temperature map to calculate where snow should be displayed on the surface, the distribution of snow would have a lower resolution than the rest of the graphical surface.

Instead, for each object, Universe Sandbox calculates how temperature changes with the elevation of the high-resolution heightmap, and uses this temperature to determine where snow is present. The relationship between temperature and elevation depends on the Lapse Rate of temperature, or the rate at which temperature changes with altitude. The Lapse Rate depends on the Surface Pressure, Surface Gravity, and Average Surface Temperature of the object.

Lapse Rate measures the change in temperature based on the change in elevation from sea level, and is displayed in the following units:


 * Degrees Celsius per meter (°C/m)
 * Degrees Celsius per kilometer (°C/km) = 0.001 °C/m

Material Phases
Universe Sandbox tracks three phases of water across the surface of the object. At every time step, at every point on the surface map, Universe Sandbox calculates how much of water will exist as a solid, liquid, or gas. The temperatures at which water changes between these phases (e.g., the Boiling Point or Freezing Point) depend on several properties of the object, including its Surface Pressure.

The amount of water that exists as a solid at a given point on the surface is represented by the Ice Thickness, the amount of liquid by the Liquid Water Depth, and the amount of gas by the Water Vapor Pressure. Water in the liquid or gas state can also move across the surface (see below).

Vapor Flow
The movement of water vapor across the surface of the object is simulated with a method similar to the one used to simulate heat diffusion. Vapor flows from areas of high pressure to areas of low pressure, so the vapor in a single point on the surface map of an object may spread or diffuse into nearby points, and the vapor in those points may spread to the first point. Universe Sandbox calculates this diffusion by using the FTCS method.

The water vapor pressure of each point on the surface map due to the water vapor present in that point is then updated based on the results of this diffusion calculation.

Liquid Flow
The movement of liquid water across the surface of the object is simulated with a method similar to the one used to simulate heat diffusion and vapor flow. The rate at which liquid flows in each direction is determined by the difference in elevation in nearby regions, as well as the Surface Gravity of the object.