Terrestrial Planets

The terrestrial planets of our solar system include Mercury, Venus, Earth, and Mars. Terrestrial means “Earth-like.” These planets are similar in many ways; notably, they all have solid surfaces on which someone could stand. Nevertheless, their differences tell us a great deal about what makes each one unique. Let’s compare their atmospheres, interiors, surfaces, and magnetic fields to understand them better.

Mercury: A Planet of Extremes

Mercury is a planet of extreme temperatures. It gets up to 425 degrees Celsius on the side facing the Sun but is a cold -150 degrees Celsius in the shade. These extreme temperatures are what we see on worlds with very thin atmospheres, and indeed, Mercury’s atmosphere is almost nonexistent.

Mercury’s surface is dominated by cratering. This is what happens when an impactor such as a meteorite hits a surface.

Mercury has some very big impact craters, including the Caloris Basin, which covers over 2% of the surface of the planet. There are no active volcanoes or noticeable quakes on Mercury. The planet seems at first glance to be geologically dead, though magma still lurks deep below its crust.

One surface feature unique to Mercury is the cliffs on its surface called scarps. These were formed when the planet cooled so fast that it shrank and cracked.

Mercury’s core, just like the core of every planet (including terrestrial and giant planets), formed through a process called differentiation. This is the same process you might have seen at work in your salad dressing, where the oil rises to the top and the seasonings sink to the bottom. Dense things sink and light things float, both in salad dressings and in the interior of a planet. A planet is molten all the way through when it forms, and the light minerals float while the dense metals sink. When the planet later cools to be a solid, these layers get frozen into place.

The average density of Mercury is higher than the other terrestrial planets. Since its surface is made of the same light minerals as the other terrestrial planets, this implies its core is bigger than expected. In fact, Mercury may have an inner iron core almost as large as the Earth’s core. If it looks like the Earth on the inside, why is it so small? One hypothesis is that an impact—maybe even the same one that formed the Caloris Basin—caused much of Mercury’s crust to vaporize. This vaporized crust probably fell into the Sun.

Venus: A Fiery Runaway Greenhouse

The second planet from the Sun is Venus. Venus is incredibly hot at about 450 degrees Celsius. The surface pressure is about one hundred times as strong as Earth’s. Standing on the surface of Venus would feel like being almost a thousand meters under water on Earth. Despite these differences, Venus is similar to the Earth in size. While we can’t see the surface because it is completely covered in clouds, it has been mapped using radar.

Its surface contains volcanic objects called coronae or “pancake dome” volcanoes. They are like volcanoes that are oozing instead of erupting, and accordingly, we believe there to be rivers of lava on the surface. Very few impact craters are visible to radar, and we have sent few spacecraft to Venus because we have not yet found a way to engineer them to survive very long in the unforgiving environment.

Venus may be the victim of what we call a runaway greenhouse effect from evaporated oceans and volcanic activity. The volcanic coronae put carbon dioxide into the atmosphere, which was already mostly carbon dioxide, with clouds of sulfuric acid. Being close to the Sun, Venus’s oceans (if it did in fact have any) may have begun to evaporate. Both water vapor and carbon dioxide are potent greenhouse gasses, so together they would have contributed to heating the atmosphere even more. This would cause more water vapor to evaporate, which would increase the atmospheric temperature. This would have continued until the entire ocean evaporated. Sunlight separated that water vapor into oxygen and hydrogen, and the hydrogen floated away. The leftover oxygen is a main component in Venus’s sulfuric acid clouds. But rest assured, it’s not possible for the Earth to get warm enough to experience a runaway greenhouse effect of this magnitude. The Earth is too far from the Sun to ever be warm enough to completely evaporate the oceans.

Earth: Our Home

The third planet from the Sun is Earth. Obviously, we know much more about this planet than any of the others, but there are a few aspects of Earth you may not have considered. Unlike Mercury’s cratered landscape and Venus’s lava terrain, Earth’s surface is dominated by erosion due to water and wind, as well as by plate tectonics. The Earth’s crust is divided into plates that bump against each other, causing earthquakes. Plate tectonics appear to be completely unique to Earth, as no other planet has a crust made of plates that we have observed so far. Looking around the Earth, we do see thousands of impact craters, but they are often disguised as lakes or bays. These get erased relatively quickly due to erosion.

Mars: The Red Planet’s Mysteries

The final terrestrial planet in our solar system is Mars. The Martian surface averages about -80 degrees Celsius, but summertime at the equator can get to 20 degrees Celsius. This is a livable temperature on the Earth! However, Mars is not like the Earth. The atmosphere on Mars is extremely thin—more than 100 times thinner than Earth’s atmosphere. Even though sometimes the temperature is above freezing, the atmospheric pressure is so low that liquids freeze anyway, since liquids freeze more easily at low pressure. Mars’s atmosphere is 95% carbon dioxide, but it has not experienced a runaway greenhouse effect like Venus. Why not? First, it is farther from the Sun so it was never warm enough to have large amounts of evaporating water. Also, Mars is relatively small. For example, the surface gravity on Mars is almost a third of the Earth’s. Mars’s light atmosphere is partially due to its low gravity, since atmospheres are caused by gas molecules attracted to planets by gravity. Because Mars has so little gravity, it is not able to hold its atmosphere against the Sun’s onslaught. These combined factors would make it impossible for Mars to experience a runaway greenhouse effect.

In spite of this, Mars does have weather and seasons. Different latitudes have different temperatures, but the atmospheric circulation on Mars is much different than the Earth’s. Mars’s higher mountains and lack of oceans mean that its terrain disrupts the flow of winds much more than the mountains do on Earth. Nevertheless, some of Mars’s weather is similar to the Earth’s. Mars has dust devils and dust storms. These are not very destructive, though. On Mars, a one-hundred-mile-per-hour wind would feel like a one-mile-per-hour wind—barely a breeze. Mars’s winds can only pick up fine dust.

Mars has a fascinating surface. Like the Earth, the surface of Mars is dominated by erosion. We do see volcanoes on Mars, including Olympus Mons, the largest volcano in the solar system at about the size of Texas. Impact craters near its summit imply that this volcano is no longer active. Current Martian landers and orbiters are trying to discover if Mars has magma.

Mars also has a giant rift valley called Vallis Marineris that’s as long as the United States is wide. This canyon is about three times the depth of Earth’s Grand Canyon. Unlike the Grand Canyon, Vallis Marineris might have been formed by tectonic plates. However, there is no evidence of plates on Mars aside from this one location where the surface was pulled apart.

There is also evidence of past water on Mars. The temperature is currently too cold for liquid water, but rovers have found rocks on Mars that only form in the presence of water. Mars also contains dried riverbeds and runoff channels.

There is one additional aspect of these planets that has not yet been discussed, and that is their magnetic fields.

The three things you need to have a magnetic field are

• liquid interior,
• iron core, and
• fast rotation.

Scientists believe that these three elements interact with each other in a way which forms what is called a “core dynamo,” creating a planetary magnetic field.

The Earth has all three of these things, and accordingly, we have a strong magnetic field. You probably take the Earth’s magnetic field for granted. You may have even benefited from it when using a compass to navigate. The magnetic field protects our atmosphere and our bodies from the harmful radiation of the Sun. While most of the terrestrial planets have magnetic fields, they are not all the same.

Mercury has a very weak magnetic field, despite its large core. There are several theories as to why the field is so much weaker than Earths, one of which is that its core may only have a small amount of liquid.

Venus has lava on its surface, so it no doubt has a molten core. Nonetheless, Venus does not have a magnetic field because it rotates very slowly. One day on Venus is 116 Earth days.

Mars does not have a magnetic field, but astronomers believe that it used to. It rotates quickly enough, taking only 25 hours to rotate, similar to Earth’s 24 hours. The inactive volcanoes on the surface indicate that there was once a molten interior, and the core is believed to be made of iron. These pieces of evidence, along with the crystal structures of Mars rocks indicate that there was a magnetic field on Mars similar to the Earth’s. So where did Mars’s magnetic field go? One theory posits that the planet’s density caused it to cool more quickly than Earth, and that as it cooled, the mantle solidified around the iron core, causing the action which creates a magnetic field to stop. When Mars lost its magnetic field, it began to lose its atmosphere to the Sun’s harmful radiation. This created catastrophic climate change on Mars. The planet might have once had an atmosphere very suitable for life, but once it lost its magnetic field and began to lose its atmosphere, there was no way to save the planet. If anything was alive on Mars, it would have had to find a way to shield itself from the Sun’s radiation, or else perish. Our rovers are still searching for hidden life, perhaps underground, or the fossil remains of life that didn’t survive this transition. So far, no evidence of life has been found anywhere besides the Earth, but Mars is considered one of the best possible candidates in our solar system.

That’s all for this video. Thanks for watching and happy studying!