<P> The force exerted by Earth's gravity can be used to calculate its mass . Astronomers can also calculate Earth's mass by observing the motion of orbiting satellites . Earth's average density can be determined through gravimetric experiments, which have historically involved pendulums . </P> <P> The mass of Earth is about 7024600000000000000 ♠ 6 × 10 kg . </P> <P> The structure of Earth can be defined in two ways: by mechanical properties such as rheology, or chemically . Mechanically, it can be divided into lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core . Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core . The geologic component layers of Earth are at the following depths below the surface: </P> <Table> <Tr> <Th_colspan="2"> Depth </Th> <Th> Layer </Th> </Tr> <Tr> <Th> Kilometres </Th> <Th> Miles </Th> </Tr> <Tr> <Td> 0--60 </Td> <Td> 0--37 </Td> <Td> Lithosphere (locally varies between 5 and 200 km) </Td> </Tr> <Tr> <Td> 0--35 </Td> <Td> 0--22 </Td> <Td>... Crust (locally varies between 5 and 70 km) </Td> </Tr> <Tr> <Td> 35--60 </Td> <Td> 22--37 </Td> <Td>... Uppermost part of mantle </Td> </Tr> <Tr> <Td> 35--2,890 </Td> <Td> 22--1,790 </Td> <Td> Mantle </Td> </Tr> <Tr> <Td> 210 - 270 </Td> <Td> 130 - 168 </Td> <Td>... Upper mesosphere (upper mantle) </Td> </Tr> <Tr> <Td> 660--2,890 </Td> <Td> 410--1,790 </Td> <Td>... Lower mesosphere (lower mantle) </Td> </Tr> <Tr> <Td> 2,890--5,150 </Td> <Td> 1,790--3,160 </Td> <Td> Outer core </Td> </Tr> <Tr> <Td> 5,150--6,360 </Td> <Td> 3,160--3,954 </Td> <Td> Inner core </Td> </Tr> </Table>

What do you know about the structure of the earth explain