This topic is closely tied to several others; consider checking out YSS topics Birth of Worlds, Volcanism, and Magnetopheres.
When they first form, planets are extremely hot in their interiors. The internal heat comes from a variety of sources: heat from accretion (from the energy of the colliding "accreting" materials that formed the planets, heat from the planet's core formation (from the gravitational energy as molten iron dropped to the center of a planet), radiogenic heat (from the decay of radioactive elements inside the planet), and tidal heating (when an object is repeatedly flexed by the gravitational pull of another).
This accumulation of heat was sufficient to cause at least partial melting of each of the planets, some of the asteroids, and many of the moons in our solar system. Their interiors differentiated, with heavier materials sinking to the center and lighter materials floating to the surface, forming cores, mantles and crusts in the terrestrial bodies, and layers of different densities within the gas giants.
Since formation, the planets, moons and asteroids all have been cooling. In general, the smaller bodies have cooled faster than the larger ones, much like small cupcakes cool faster than large cakes. The escaping heat from this process of planetary cooling can drive interior convection, tectonic and plate tectonic activity, volcanism, and the formation of a global magnetic field. As the planets cool, volcanic activity ceases, the magnetic field is lost, and atmospheres and oceans weather away.
Mars offers one example of a planet that has moved from youth into old age. Features on Mars resembling dry riverbeds, and the discovery of minerals that form in the presence of water, indicate a young Mars with a thicker atmosphere that was warm enough for liquid water to flow on the surface. However, today, the surface of Mars is very different. Its atmosphere, mostly carbon dioxide, is thin, with a surface pressure about 1/100th that of Earth's. It is cold and no water flows across the surface; water and carbon-dioxide ice are frozen in the ground of Mars and at its poles. These changes have been driven by the cooling of Mars' interior.
All planets in our solar system are constantly blasted by the solar wind, a thin stream of electrically charged gas that continuously blows from the sun's surface into space. On Earth, our planet's global magnetic field shields our atmosphere by diverting most of the solar wind around it. The solar wind's electrically charged particles, ions and electrons, have difficulty crossing magnetic fields. Earth's magnetic field is generated by electric currents in its molten iron outer core. Mars' global magnetic field is no longer active, so there is no protection of its surface from the solar wind. Mars lost its magnetic field in its youth billions of years ago as its interior cooled and flow of metallic liquid in its core ceased. Once this field disappeared, Mars' atmosphere was exposed to the solar wind and gradually stripped away. Alternatively, an ancient asteroid bombardment may have blasted large amounts of the Martian atmosphere into space. However, huge Martian volcanoes that erupted after the impacts during Mars' youth, like Olympus Mons, should have replenished the Martian atmosphere by venting massive amounts of gas from the planet's interior.
Today, the interior of Mars has cooled to the point where it is no longer volcanically active at its surface. Volcanic activity brings gases from the interior of a terrestrial planet to its surface. This outgassing from volcanos builds and maintains planetary atmospheres. Because Mars no longer produces gases for its atmosphere, and no longer has a global magnetic field to protect its atmosphere from being stripped by the solar wind, Mars has a very thin atmosphere.
Studies have suggested that several billion years ago the Venusian atmosphere was much more like Earth's than it is now, and it may have had substantial quantities of liquid water on the surface. However, Venus' proximity to the sun and the amount of carbon dioxide in its early atmosphere led to a "runaway greenhouse effect" in which heat became trapped at Venus' surface and the temperatures rose even higher. This would have evaporated any liquid water, which would have trapped even more heat. Eventually, the water molecules would have broken up into hydrogen and oxygen, and the hydrogen would have escaped into space. Further heating would have evaporated carbon dioxide out of the very rocks at Venus' crust, thickening the atmosphere to its present dense levels.
Like Mars, Venus doesn't have a magnetic field, which is surprising given that Venus is similar to Earth in size, and was expected to contain at least a partially liquid iron core and a resulting magnetosphere. Some scientists formerly suspected that Venus' extraordinarily slow rotation may have hindered the necessary flow of current to produce the magnetic field, but simulations have now shown that its slow rotation is sufficient. The current best explanation is that Venus lacks the necessary convection in its core. Scientists are actively investigating this topic.
The interiors of Mercury and the Moon have cooled significantly. As they cooled, they shrank, leading to lines of cliffs (known as scarps) that formed as the surface contracted and buckled. In spite of its cooler internal temperatures, Mercury does still have a global magnetic field. Initially scientists debated whether this magnetism was a fossil remnant of an earlier magnetosphere or perhaps if the core was divided into a thin liquid outer core and a larger solid inner core. Mercury has a surprisingly large iron core for its size, and it was recently discovered to be at least partly liquid, helping to explain its magnetic field. This large liquid core is an active topic of scientific investigation.
Gas giants are also cooling, primarily through convection. Their interiors are incredibly hot, and this convection can generate massive storms in their atmospheres. Because of this heat, the gas giants emit more thermal (far infrared) energy than they receive from the sun. Other processes occurring in their interiors can slow this cooling. For instance, in both Jupiter's and Saturn's interiors, helium may be forming into droplets and raining down into its metallic layer, generating additional heat. This process is an area of active scientific study.
Evolving Worlds Videos
Act 1: The Telescopic Era
Act 2: The Early Spacecraft Era
Act 3: The Modern Era Part 1
Act 3: The Modern Era Part 2
Resources and Questions
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