Planetary Differentiation and the Development of the Earth-Moon System

After their initial formation, the planetesimals and protoplanets of the Solar System were characterized by a relatively homogeneous composition. However, they soon started to heat up and differentiate. This heat was generated mainly from two sources: further collisions with other celestial objects and the decay of radioactive elements. In planetesimals and protoplanets whose temperatures rose sufficiently to cause melting, denser metallic materials sank towards the center, while lighter rocky materials remained behind at the surface. During this process of differentiation, these objects developed a layered structure consisting of a core, mantle and crust, after which they eventually cooled and became mostly solidified. Under the effects of gravity, the irregular shapes of the protoplanets were molded into spheres.

Even after differentiation had taken place, the young planets were continuously bombarded by meteorites and as a result, their surfaces became scarred with large amounts of impact craters. Approximately 4.53 billion years ago, an especially intense collision occurred between the Earth and a small planet known as Theia. This collision caused Theia and a large part of the Earth to disintegrate, melt and be blasted into space, after which a debris ring formed around the Earth. The Moon was ultimately formed from the materials in this ring of debris. Scientists know this because Moon rocks have an age of roughly 4.53 billion years and because they are very similar in composition to rocks from Earth’s mantle.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: Earth as seen from the surface of the Moon by the Lunar Reconnaissance Orbiter. Credit: NASA/Goddard/Arizona State University.

Zooplankton migrate under influence of moonlight in dark Arctic winter

New research performed by biologists from different universities has shown that zooplankton migrate vertically through the waters of the Arctic Ocean by using moonlight during dark winter nights. By studying acoustic data recorded by moored instruments, the scientists found that the vertical migration patterns of zooplankton in winter are driven by lunar illumination across the entire Arctic, in fjord, shelf, slope and open sea environments. During the Arctic winter, zooplankton shift their known diel vertical migration (DVM) periods from a solar day (24 hours) to a lunar day (24.8 hours) when the moon rises above the horizon. In addition, mass sinking of zooplankton from the surface waters to a depth of approximately 50 meters occurs during periods of full moon (every 29.5 days). The scientists suggest that lunar vertical migration (LVM) may enable zooplankton to avoid visual predators, such as carnivorous zooplankters, fish and birds, which use moonlight to hunt during the polar night. The discovery of LVM in the Arctic indicates that light-mediated patterns of biological migration may occur even without the presence of sunlight and has important implications for the exchange of carbon between the surface waters and deeper waters during the Arctic winter.

Journal reference: Last, K. S., Hobbs, L., Berge, J., Brierley, A. S. & Cottier, F. (2016). Moonlight Drives Ocean-Scale Mass Vertical Migration of Zooplankton during the Arctic Winter. Current Biology, 26, 1-8.

Image: Icebergs in the Arctic near Cape York, Greenland. Credit: Brocken Inaglory, Wikimedia Commons.

The Moon

The Moon is the celestial body that orbits the Earth. It has a mean radius of ~ 1737 km, a mean distance to Earth of ~ 385 thousand km and an orbital period of ~ 27.3 days. Earth’s satellite is thought to have formed approximately 4.5 billion years ago as a result of the impact between the proto-Earth and a celestial body called Theia, not long after the formation of the Solar System. The Moon has a differentiated structure and consists of a crust, mantle and core and its surface is scarred with many impact craters.

Information source: NASA

Image: Full Moon as seen from Earth. Credit: Gregory H. Revera, Wikimedia Commons.