MODELING EARTH'S CRUST, MANTLE, AND CORE WITH GOOGLE MARS AND GOOGLE MOON
The equatorial radii of Earth (6,378 km), Mars (3,397 km), and Moon (1,738 km) are near-integer ratios. The Moonʼs radius is approx. half (51%) that of Mars which is approx. half (53%) that of the Earth. Furthermore, the radius of Earthʼs outer core (3,500 km) is within 3% of the radius of Mars. Consequently we can use the surface of the Google Mars virtual globe to represent the core-mantle boundary. The Martian 3-D terrain is turned off and a plain, lightly raised JPEG image overlay covers the whole martian surface imagery. At the core-mantle boundary depth of 2,900 km, the Earthʼs black-body temperature is white-hot, however white is not a suitable color for modeling, therefore we use red or gray overlays to convey temperature or metallicity respectively.
The advantage of switching to the Martian virtual globe is that the users can fly to placemarks with text balloons, image files, virtual specimens, and COLLADA models representing crustal and mantle features. Although under the planet's surface, these placemarks are above the virtual globeʼs surface thus avoiding the difficulties of sub-surface rendering and touring. Also, the scale of the ruler tool is valid.
The radius of the Earthʼs inner core (1,216 km) is significantly less than that of the Moon, nevertheless for qualitative visualization purposes, the same technique may be adopted using a draped Google Moon virtual globe to represent the Earthʼs inner core.
Other planets and moons may also be visualized in Google Earth. Venus is almost the same size as the Earth (95% by radius) and is estimated to have a core of similar size. The core radius of Mercury and the surface radii of the Galilean moons Io and Europa are all close enough to be represented by the Google Moon virtual globe with corresponding ground overlays.