While the InSight seismometer patiently waits for the next large earthquake to illuminate its interior and determine the structure of the crust and core, the two scientists have built a new compositional model of Mars.
Takashi Yoshizaki of Tohoku University and Bill McDonough of Tohoku University and the University of Maryland used rocks from Mars and measurements from orbiting satellites to predict depth to the core-mantle boundary, about 1800 km below the surface, and suggested that the core contains moderate amounts of sulfur. oxygen and hydrogen as light elements.
“Knowledge of the composition and internal structure of rocky planets tells about the conditions of formation, about how and when the core was separated from the mantle, as well as about the timing and amount of crust extracted from the mantle,” comments Yoshizaki.
Early astronomers used the relative distances and orbital periods of the planets and their moons to determine the size, mass, and density of these bodies. Modern spacecraft in orbit provide detailed information about the shape and density of the planet, but the density distribution inside remains unknown. The seismic profile of the planet provides this insight. When an earthquake rocks a planet, sound waves travel through its interior at a speed controlled by its internal composition and temperature. Strong density contrasts, such as rocks and metals, cause sound waves to react in different ways, revealing the depth of the core-mantle boundary and details of the likely composition of these layers.
By the end of the 19th century, scientists hypothesized about a metal core inside the Earth, but it wasn't until 1914 that seismologists demonstrated its existence at a depth of 2,900 km. Seismologists have uncovered the structure of the inner space of the planet, which helps to locate sources and understand the nature of earthquakes. Four lunar seismometers installed by Apollo astronauts have determined the structure of the Moon's core-mantle-crust. Mars, the second planet to be explored, received its first seismometer from the InSight mission in mid-2018.
Compositional models for a planet are developed by combining data from surface rocks, physical observations, and chondritic meteorites, the primitive building blocks of planets. These meteorites are mixtures of rock and metal, composed of solid particles that have accumulated at the beginning of the solar nebula. These particles are composed of various proportions of oxides of magnesium, silicon, iron and alloys of iron and nickel.
Yoshizaki adds: "We found that the core of Mars is only 1/6 of its mass, while for Earth it is 1/3 of its mass." These data are consistent with the fact that Mars has more oxygen atoms than Earth, a smaller core, and a rusty red surface. They also found higher volatiles on Mars than on Earth, such as sulfur and potassium, but fewer than chondritic meteorites.
NASA's InSight seismometer will directly test this new Mars model as it measures depth to the core-mantle boundary. Such compositional models for Mars and Earth provide clues about the origin and nature of the planets and their habitability.