Formation of the moon

According to the giant impact hypothesis, a planet the size of Mars crashed into the Earth 30 million years after the formation of the solar system. This dislodged material from the Earth, forming a disk around Earth, which eventually coalesced and formed the moon. (Figure 5‑6 and Figure 5‑7).

The impact of “Thea” ejected material from the outer mantle of the Earth into orbit around the Earth. Scientists think that Thea did not directly hit the earth but hit it with a glancing blow that dislodged the surface layers of the earth and sent them into orbit around the earth. According to the hypothesis, the dislodged matter eventually coalesced and formed the moon. The strongest evidence for this hypothesis is that the moon is only 1-3% iron while the Earth is 30% iron, mostly in the core. The earth’s outer mantle had a low fraction of iron at the time of the impact because iron would have already sunk by the process of differentiation to the center of the molten Earth. Thus, only the lighter elements would have been in the outer mantle when Thea struck the Earth. Simulations indicate that 40% of the mass of the moon would have come from the impactor (Thea), and 60% from the original Earth. Another indication of the origin of the moon in the earth's outer mantle is that the moon and outer mantle of the Earth have similar isotopes.

The moon has high and low areas (Figure 5‑8). The low areas are darker and smoother, and are the result of lava flows from volcanic activity that contained darker iron-rich basalt that filled in the largest craters. The oldest rocks on the moon are in the lighter “Highlands.” The number and density of craters on the Highlands leads to the conclusion that the moon Highlands hardened before a period of intensive meteorite bombardment early in the history of the solar system (The Late Heavy Bombardment, 3.9 billion years ago). Unlike earth, craters are stable on the moon because of the lack of weathering. Astronauts collected rock samples on the moon, which revealed that many of the craters formed (solidified) 3.9 billion years ago. This was the first evidence of the Late Heavy Bombardment.

There are several ways in which the moon benefited life on Earth. For example, the ancient moon might have increased the abundance of lighter metals in the Earth’s crust. When it formed, the moon was much closer to the Earth and probably caused a molten condition at the Earth’s surface by causing huge deformations (1 km) due to gravitational changes as it passed across the Earth’s surface. This may have aided the process of differentiation in which lighter elements such as silicates moved from the core of the Earth to the Earth’s surface, and heavier elements such as uranium, iron, and nickel moved to the core. During the first 200 million years of the Hadean Eon, planetesimal impacts, and radioactive decay of highly radioactive isotopes such as aluminum 27 also contributed to heating and melting Earth’s surface.

The moon's tides possibly contributed to life's evolutionary transitions. Tides might have provided heating and cooling, drying and wetting, and shear stress processes that might have aided in protocell formation. During the Mid Archaean Eon, ancient structures of blue-green algae and other organisms grew on beaches and benefitted from intermittent immersion and exposure to the atmosphere. Lunar tides probably contributed to the movement of plants, insects, amphibians from the sea to land during the Phanerozoic Eon.

Most planetary scientists think that the moon has stabilized the Earth’s angle of obliquity, which is the tilt of Earth’s axis.[1] Without the moon, the Earth’s angle of obliquity might have varied from 0 to 85⁰. With the moon’s stabilizing effect, the Earth’s angle of obliquity only varies by 1.3⁰ from the mean value of 23.3⁰.[2] The stability of the Earth’s angle of obliquity within the last hundreds of millions of years has been a key factor in climate stability for the development of complex and interdependent ecosystems.

References

[1] Jacques Laskar, J. F. Joutel, and P. Robutel. Stabilization of the Earth’s Obliquity by the moon. Nature 361 (1993): 615-617.

[2] Laskar, Stabilization, 615.

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Moon orbiting the earth. Credit: NASA/EPIC