Solar System -
Earth's Moon
The Moon is very large compared to the size of the Earth, being over 27% its size, although less than 1.25% its mass. It is also the fifth largest moon in the whole Solar System. At its closest, the Moon is 363,104 km (c. 227,000 miles) from Earth, and at its furthest, 405,696 km (c. 353,500 miles), averaging out at 384,399 km (c. 240,250 miles), and it is slowly moving about 2½ cms further away from Earth each year. Its average diameter is 1,737.10 km with an equatorial diameter of 1,738.14 km and a polar diameter of 1,735.97 km. Its surface gravity is around 16.5% that of earth. It is in synchronous orbit around the Earth, meaning it takes 27.321582 days both to complete one orbit, and to revolve once about its axis, thus always presenting the same face to us. The Moon moderates the Earth's climate by reducing the "wobble" as the Earth revolves on its axis, and it creates the tides in the Earth's seas and oceans. Similar to the Earth, the Moon has a core, a mantle and an outer crust.
The inner core has a radius of about 240 km, and comprises mainly iron. A 90 km thick shell of liquid iron surrounds this inner shell, with a 150 km layer of semi-molten iron around that.
Above the core, the mantle reaches up to the crust. The composition is not known but probably comprises minerals comprising magnesium, iron, silicon and oxygen.
Surmounting the mantle, the crust varies in thickness from about 70 ms on the near-side facing the Earth, to about 150 kilometers on the far-side. It comprises oxygen, silicon, magnesium, iron, calcium and aluminum, with small amounts of titanium, uranium, thorium, potassium and hydrogen.
There are several theories for how the Moon formed, but most have flaws; for example, capturing a passing object. This would reuire the Earth to have had an incredibly extended atmosphere in order to dissipate the energy of the object Most planetary astronomers agree that a body, perhaps half the diameter of the Earth, collided with it, and the material thrown off into space by the impact accreted to form the Moon. This solution also has some issues that require the vaporized material of the Earth and Moon to mix post the impact to equalize their composition. The best estimate of when the Moon formed is 4.527 ± 0.010 billion years ago, though there is some evidence that it occurred, possibly, 50 million years later than this.
Like other rocky objects in the Solar System, the Moon has distinct crust, mantle and core. The core is very small, occupying a diameter of less than 700 km (435 miles), about 20% the diameter of the Moon. Earth's core, for example, occupies nearly 55% of the Earth's diameter. The Moon's core is probably predominantly iron with small quantities of impurities like sulphur and nickel, and it is at least partly molten.
The Moon is, of course, covered in craters and maria or "seas". The largest, the South Pole-Aitken basin is about 2,240 km (1,400 miles) in diameter, and is the largest crater known in the Solar System. It is also very deep at about 13 km (~ 8 miles) below the surrounding terrain. Maria are the result of volcanic eruptions, and can form when when lava floods a crater created by an impact. Mare Imbrium has a diameter of 1123 km (700 miles), and it is the largest mare in an impact basin.
The Core
The Mantle
Above the core, the mantle reaches up to the crust. The composition is not known but probably comprises minerals comprising magnesium, iron, silicon and oxygen.
The Crust
Surmounting the mantle, the crust varies in thickness from about 70 ms on the near-side facing the Earth, to about 150 kilometers on the far-side. It comprises oxygen, silicon, magnesium, iron, calcium and aluminum, with small amounts of titanium, uranium, thorium, potassium and hydrogen.
There are several theories for how the Moon formed, but most have flaws; for example, capturing a passing object. This would reuire the Earth to have had an incredibly extended atmosphere in order to dissipate the energy of the object Most planetary astronomers agree that a body, perhaps half the diameter of the Earth, collided with it, and the material thrown off into space by the impact accreted to form the Moon. This solution also has some issues that require the vaporized material of the Earth and Moon to mix post the impact to equalize their composition. The best estimate of when the Moon formed is 4.527 ± 0.010 billion years ago, though there is some evidence that it occurred, possibly, 50 million years later than this.
Like other rocky objects in the Solar System, the Moon has distinct crust, mantle and core. The core is very small, occupying a diameter of less than 700 km (435 miles), about 20% the diameter of the Moon. Earth's core, for example, occupies nearly 55% of the Earth's diameter. The Moon's core is probably predominantly iron with small quantities of impurities like sulphur and nickel, and it is at least partly molten.
The Moon is, of course, covered in craters and maria or "seas". The largest, the South Pole-Aitken basin is about 2,240 km (1,400 miles) in diameter, and is the largest crater known in the Solar System. It is also very deep at about 13 km (~ 8 miles) below the surrounding terrain. Maria are the result of volcanic eruptions, and can form when when lava floods a crater created by an impact. Mare Imbrium has a diameter of 1123 km (700 miles), and it is the largest mare in an impact basin.