Estimating sizes and distances on planetary surfaces is a difficult task on spacecraft imagery because of the lack of familiar landmarks of known sizes, the ubiquity of scale-independent landforms (craters, cracks, cones, dunes), and also due to the different radius of planets and satellites.
To estimate the extent of features and distinct regions is therefore almost impossible on a planetary surface without a scale. A map that the student is familiar with greatly helps estimating sizes and distances since it projects the known environment onto the planetary surface, virtually placing the student into a known 3D environment. The basic goal, however, is not just to add a sense of sizes to a map view, but to help the virtual exploration in the environment of another planetary surface. Ultimately, by exploring a lifeless planet in imagination with a sense of the real scale, the unique characteristics of our own, well-accustomed terrestrial environments will be emphasized.
A few unusual phenomena that will likely be encountered in viewing orbital images or planetary maps are: the inability to discriminate sizes of simple craters and the abundance of craters; problems in viewing craters and hills in inverted relief, the large sizes of canyons, troughs, volcanoes, lava flows; the large sizes of terrains with undifferentiated (similar, or repetitive) relief features in general (without vegetation and man-made features that would otherwise segment an undifferentiated geologic substrate).
Students should also focus to deliminate single features by identifying boundaries where a terrain changes. These are the contact lines of geologic units or features, and even think about their origin, stratigraphic relations (which was produced first, and which cut into that or covered that subsequently) or relief changes. This is a basic requirement to be able to identify and discriminate standalone “features” on the landscape.
How the App works
The outline of a chosen country, U.S. state or a standard 100 km radius Mars Exploration Zone can be displayed and moved on the surface of another planet or moon (currently, Mars, Venus, the Moon, Io, Titan and Jupiter), keeping its original size. This comparison can be done either on the surface of a rotatable globe model or on a 2D map in Web Mercator projection.
Select and display maps: The user can select thematic raster maps from eight planetary bodies. Scale and legend is included in the maps in rasterized form.
- Zoom in-out: mouse scrolling or +/- button on screen
- Position of the globe within the screen: Shift+drag mouse
- Rotate globe: drag mouse
- Change center of perspective: ctrs+shift+drag mouse
- Start/end line measurement: mouse click
- Reset distance measurement tool: turn the tool off and on
**Country overlay: The user can overlay the vector outline of any country or U.S. State or a standard 100-km-radius Exploration Zone on any embedded planetary body. The outline can be displayed and moved on the surface, keeping its original size, in both 2D and 3D views. When drawing the country outlines, the radius differences of the bodies are taken in account.
Projection: The maps have two views: a 2D flat map in Web Mercator projection, and a 3D rotatable virtual globe model (no surface relief is shown in 3D). These two different view modes offer various uses. The flat version can also be used to demonstrate the high area distortions of the Mercator projection e.g. by moving Greenland to lower latitudes (Fig. 3). However, due to the properties of Mercator projection (poles are projected to infinite distance), polar regions cannot be explored effectively in the flat view, only on the 3D globe.
The virtual globe mode is a useful tool to demonstrate the size differences of the planetary bodies. For instance, Australia on the Moon would cover almost an entire hemisphere (Fig. 4).
Distance measurement: This tool is used to draw a polyline, calculates and displays distance and travel time between the two end points, in various units that includes the maximum speed of a walking astronaut, an automatic rover, a human-driven rover and a car on a blacktop freeway.
Coordinate display: It shows the position (IAU geographic coordinate) of the cursor. Earth and Moon coordinates are displayed in the ±180 longitude system, Mars and Venus in the 0-360° Eastern longitudes system and the other bodies in the 0-360° Western longitudes system.
Colors: The user can determine the color of the measurement polyline and the country outline.
Screenshot: Using the screenshot feature built into the App, users can save the current view as a png image file that includes any traverses the user created but excludes the user interface), providing a printable background map for further activities. The user can continue working on this map in an external image processing application where they can add settlements, roads, regions of scientific interests etc., learning the concepts of planetary physical geography, toponymy, cartography, mission and city planning. These aspects are discussed in Chapter 4 in detail.
Place names: Place names are rasterized or “burnt into pixels”, so that we could use the full spectrum fonts can provide, including font faces, sizes and styles, to distinguish places of different type, size and landscape hierarchy level. This function is not available in nomenclature vector layers that are not feature polygon-linked, as is the case for the majority of planetary map platforms. The major disadvantage of the rasterized nomenclature, however, is that it is not scale-dependent: by zooming into the view, the names become disproportionally large and place names cannot be searched for.
Info (about): This window contains information on the Tool, including links to references and online tutorials.
The Mercury globe is a green-to-red color hillshade MESSENGER Global DEM topographic map. Mercury is characterized by densely cratered terrains and less cratered smooth volcanic plains. Some large basins resemble those on the Moon. Wrinkle ridges (long ridges) occur in all parts of the planet, likely resulted from the cooling of the crust.
The Venus view is a monochromatic colorized mosaic of Magellan radar images in which bright tones show rough areas (trough systems, tectonically deformed tesserae, and lava flows), medium tones show lava plains, and dark tones represent smooth, dust covered regions.
The Moon topographic map is based on the Lunar Reconnaissance Orbiter Camera Wide Angle Camera (LROC WAC) DEM data. The Moon is a relatively small body, with two distinct terrains colored according to their different altitude range blue and yellow: lowland plains (maria) and highlands, respectively. This topographic difference coincides with the albedo difference visible to the naked eye: volcanic (basaltic) maria are dark and megabreccia-dominated anorthositic highlands are bright. This difference is partly due to the different materials and the different roughness of the two terrain types: basalt is darker and these plains are also smoother, while anorthosite is brighter and highlands are also densely cratered. Prominent features of the map are the rings of multiring basins usually with a mare plain at their centers. The enormous, ancient South Pole-Aitken Basin is also evident in the relief map but remains hidden in the photomosaic view.
Mars has two views: one topographic and one albedo. The topographic color hillshade map of Mars is based on MOLA gridded DEM and displays the lowlands and basins in white to yellow, highlands in brown, high shield volcanoes in dark brown. The albedo globe shows the permanent, bright ice caps, medium-toned dust covered regions and dust-free dark areas that may be covered with basaltic sand. The albedo map is produced from the Mars Global Surveyor Mars Orbiter Camera (MGS MOC) photomosaic and shows both albedo and topographic place names.
The map of Io is a false color Galileo–Voyager photomosaic in which the most distinct features are the red, sulfur-rich plume deposits of active volcanic centers (symbolized by red asterisks) and dark lava-filled calderas. The yellow coloration of the surface is caused by sulfur coating while the brightest areas are covered by a volatile, sulfur dioxide frost, deposited from volcanic degassing. Most irregular patterns represent lava flows, while the over 100 mountain blocks are most apparent in the south polar region where they are shown casting long shadows in this mosaic. Mountain peaks are indicated by black triangles and peak heights are displayed in meters. These symbols are taken from terrestrial maps and are not usual parts of planetary maps.
The map of Jupiter shows the cloud bands with nomenclature, in which any country can be easily placed into the white ovals and the Great Red Spot. The map background is the color image of Jupiter produced by the Hubble Space Telescope OPAL Program. Since Jupiter has no solid surface, this map only shows the size of the planet and its atmospheric features (clouds, cyclones) relative to others. NOT SUITABLE FOR LANDING – NO SOLID SURFACE.
The composite (infrared+radar) Titan globe shows the Cassini infrared (ISS) view of the satellite that shows dark equatorial dunes, dark polar liquid methane filled lakes and bright terrains made of rocks of H2O ice. Stripes of Cassini radar images give a higher resolution view in the north polar regions.
In addition to the above listed maps, Google’s photomosaics of Mars (THEMIS daytime thermal infrared mosaic at 100 m/px raster data resolution) and the Moon (Clementine albedo mosaic, 100 m/px resolution) are also embedded into the App without any modification, and provide the highest resolution background maps.
The maps have somewhat different themes. This difference is partly due to the differences in planetary mission designs, instruments and surface conditions: for instance, the atmosphere of Venus and Titan is opaque in visible wavelength and therefore an optical image mosaic of the surface is not available for this body.
Photomosaic maps show the surface at a particular wavelength(s)and solar incidence angle. Low-sun mosaics are composed of images taken when the Sun is near the horizon. These images emphasize relief by showing long shadows. High-sun images show albedo, or reflectivity, of the surface that may represent differing composition, grain size, or surface roughness. Albedo features may or may not correspond to topographic features (relief). These maps are usually monochromatic (greyscale), and may show the surface in visible, near infrared or thermal infrared wavelengths. Mosaics of images taken at different wavelengths may be combined into a color image. These are usually false color mosaics and include visible and infrared bands. Examples: Mars, Moon, Io
Imaging radar maps show the radar reflectance properties of the planetary surface. A radar instrument onboard the orbiting spacecraft “illuminates” the surface by cm-scale radar pulses emitted by the spacecraft’s transmitter. A portion of this electromagnetic radiation is reflected from the surface and is received by the antenna the radar instrument. The radar echo or radar return depends on the roughness of the surface relative to the radar wavelength used for the mapping. Typically, dusty or sandy (particles less than a centimeter) areas do not reflect cm-scale radar waves and remain dark and rocky surfaces (particles greater than a cm) produce strong radar return signal and are represented as bright areas on radar maps. Radar maps are typically displayed in monochromatic (greyscale) form. Examples: Venus, Titan (partially).
Topographic maps show surface elevation and relief. Although the initial topographic dataset (digital elevation model, DEM) has no associated colors and could be displayed in greyscale shades, these maps are typically displayed in a custom-made color ramp (color coding) combined with hill-shading that enhance the sense of relief. This visualization provides the most familiar cartographic picture of a surface. The color ramp for topographic maps of different bodies could be similar, however, we chose different color ramps to reflect a color-related mental association of the particular body. These colors are symbolic and do not correspond to the real color of the body, neither follow the color ramp of terrestrial physical geographic maps where colors are related to the presence of water or are symbolic, simplified representations of zonal vegetation. Students should familiarize with the color ramp before studying the map (rasterized legends are available in the maps). Planetary topographic data may be obtained from radar altimetry, laser altimetry or stereo imaging etc., techniques. Examples: Mercury, Mars, Moon.
Merged themes. A map product may combine different themes. Relief can be shown by colors or shadows (either in a low-sun photomosaic or by producing hillshading from a digital elevation model). Surface material / compositional units may be emphasized in albedo maps, high-sun (local noon) or nighttime infrared photomosaics taken when shadows are equally absent. While noon images show the reflectance of surface materials, nighttime thermal infrared images show thermal inertia properties.