How did the solar system originate? According to the nebular hypothesis, the Sun and its planets formed when a cloud of gases and dust, known as the solar nebula, condensed about 4.5 billion years ago. The terrestrial inner planets, including Earth, differ from the giant outer planets in their composition.
How did Earth form and develop over time? Earth probably grew by the accretion of colliding planetesimals. Soon after it formed, it was struck by a large body about the size of Mars. Matter ejected into space from both Earth and the impacting body reassembled to form the Moon. The impact generated enough heat to melt most of what remained of Earth. Radioactivity and gravitational energy also contributed to this early heating and melting. Heavy matter, rich in iron, sank toward Earth’s center to form the core, and lighter matter floated upward to form the crust. Still lighter gases formed Earth’s atmosphere and oceans. In this way, Earth was transformed into a differentiated planet with distinct layers.
What are some major events in the early history of the solar system? The age of the solar system, as determined from isotopic dating of meteorites, is about 4.56 billion years. Earth and the other terrestrial planets had formed within about 10 million years. The impact that formed the Moon occurred about 4.51 billion years ago. Minerals as old as 4.4 billion years have survived in Earth’s crust. The Late Heavy Bombardment, which peaked about 3.9 billion years ago, marks the end of the Hadean eon on Earth.
How can planetary surfaces be dated? Rocks returned from the surface of the Moon by the Apollo missions have been dated using isotopic methods. The lunar highlands show ages from 4.4 billion to about 4.0 billion years. The lunar maria show ages from 4.0 billion to 3.2 billion years. These isotopic ages allowed geologists to calibrate the relative time scale they had developed by crater counting.
Do other planets have plate tectonic systems? Venus is the only planet, other than Earth, that has features indicating tectonic activity resulting from mantle convection. But Venus does not appear to have thick lithospheric plates. Instead, it has a thin crust of frozen lava that breaks up into flakes or crumples like a rug as it is pushed and stretched by vigorous convection currents. This process, which geologists refer to as flake tectonics, may have occurred on Earth when it was younger and hotter.
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How have Mars and the other planets been explored? Four types of spacecraft have been used to explore Mars and other planets. During a flyby, a spacecraft comes close to a planet only once. An orbiter circles a planet, making remote observations of its surface and interior. A lander can actually touch down on the surface of a planet to make local observations. A rover can leave the landing site and travel up to several kilometers to investigate new terrains.
Does water exist on Mars? Today, water is present on Mars only as ice caps at the Martian poles and as permafrost. In the past, it may have been present as a liquid, as geologic evidence shows that it ran across the surface to carve stream channels and deposit sediments in meandering streams. It also accumulated in shallow lakes or seas, where it evaporated and precipitated a variety of chemical sediments, including sulfate minerals.
How do we use light in exploring stars and the solar system? In some cases, we can use enhanced photographs from telescopes, which may reveal surface features of distant objects. In other cases, we can use information from the spectrum of light, which varies depending on the composition of the object that produces or reflects that light.
Is our solar system unique? We have evidence of more than 300 planets that circle other stars. In several cases, there is more than one planet in these solar systems. Because these new planets lie outside our solar system, they are called exoplanets.