Pr f J Gp2 R r2

In comparing this result with more sophisticated models of the Earth, we find that it underestimates the central pressure by a factor of about 2. (Among other things, the assumption of constant density is an oversimplification.)

Earth or Moon. The radius of the core is about 1800 km, leaving a 700 km mantle. The core radius is 72% of that of the planet.

A surprising result of the Mariner 10 studies was the discovery of a weak magnetic field around Mercury. This field is about 1% the strength of the Earth's field. It is surprising since Mercury's low mass would suggest that the core is not hot enough to be molten. Also, the rotation is so slow that the core is not stirred up very much. There is also radioactivity coming from near the surface, suggesting a differentiated interior. This differentiation would have also required a period of melting of the interior.

The surface plains may have resulted from volcanic flooding, suggesting an active past. Mercury also has scarps, unlike the Moon. These scarps may have resulted from a contraction of the surface, something that would have also required a molten history for the planet.

The interior structure of Venus is believed to be very similar to that of the Earth. There are some composition differences. We think that the core of Venus formed later than that of the Earth. The lithosphere is also about twice the thickness of that of the Earth. Venus also has no measurable magnetic field. This may also be an effect of the planet's slower rotation.

The density of Mars is 3 g/cm3, much lower than that of the Earth. This suggests that the core cannot be very large. It is about 1200 km in radius, meaning that it is only about 40% of the planetary radius. The core is probably a combination of iron and iron sulfide. If it is all iron, it is probably even smaller than currently estimated. We have already said that the existence of large volcanoes indicates that there are no plate tectonics, also arguing for a cooler interior than the Earth.

24.3.2 Results

Even though Mercury's surface looks like that of the Moon, Mercury's density is higher. We estimate that an iron core makes up about 70% of Mercury's mass. This is a greater percentage than for either the Earth or the Moon. This is because Mercury formed under higher temperature conditions (being closer to the Sun) than did the

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