Surface of the Planets

Astronomy is the study of space. The prefix ‘ast’ means star. The word astronomy literally means ‘the study of the stars’. However, eventually it morphed into the study of almost everything in space. Some of the more complex topics I will be covering are: quasars, pulsars, neutron stars, magnetars, black holes, unnovas, supernovas, matter, antimatter, strange matter, and dark matter. Some simpler topics are: nebula, star types, double star systems, the different types of galaxies, the planets and their moons, the sun, the five exoplanets, and the asteroid and kuiper belts. You will find that most modern astronomers are practically obsessed with the search for life on other worlds, so I will try not to spend too much time on that. Astronomy has always been one of the most complex branches of science, but for a long time it wasn’t considered science. In early times people were killed for suggesting the earth revolved around the sun rather than vise versa. Today that is common knowledge, as I hope the topics I cover will become to whomever may read them.

The Planets

The planets (along with the sun) are probably the most known of all the topics you will read about. Their names, in order from the sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was considered a planet until 2006, when Mike “I Killed Pluto” Brown, discovered Pluto did not have the necessary requirements a planet must. Now, Pluto will be covered in the five exoplanets. The planets can, and most often will be, split into two groups: The inner planets, and the outer ones. The inner planets all have few or no moons and no rings, are all small, and rocky. The outer planets, having much more matter to collect in their formation, are all considerably larger than any of the inner planets. They are also known as the gas giants, because they are all composed of gas, and in some cases, ice with a small rocky core. All of them have rings and many moons. Together, the inner and outer planets, make our Solar System.

Mercury

Mercury is the first planet from the sun. Because of this most people think it will be the hottest, but because of it’s thin atmosphere, the planet is unable to hold on to heat as well as it’s neighbor, Vernus. That being said, Mercury is by no means a cold or even mild planet. It’s mean, or average surface temperature is 130?. That would be 266?. In the day the temperature on Mercury reaches a sweltering 700?. In the night it can plunge down to a frigid -310?. Mercury was named after the Roman god Mercury, the god of messengers. Mercury is the smallest of the planets, just barely making the cut as a planet. It is 4878 kilometers in diameter. Not only is it the smallest, but it is also the least massive, having a mass of only 0.05 earths. We consider where Mercury sits, a mere 48 million kilometers from the sun as close, yet entire solar systems have been discovered with all the planets inside that distance. Mercury orbits around the sun at an unbelievable rate, making it’s entire year just 88 earth days. While it’s year is short, the same cannot be said for it’s day. It’s day would be 58.6 earth days. If you were standing on the surface of Mercury (which is highly inadvisable) you would feel almost no gravitational pull, because the mean surface gravity of Mercury is 0.38 of Earth’s. That is only slightly more than that of the moon. Mercury is tilted on its axis at 2 degrees, giving it the smallest tilt of all the planets and only about 1/12 of Earth’s tilt. Mercury is one of the closest planets to Earth, only 4.3 light minutes away. Mercury is one of the only two planets in the solar system, along with Venus to not have a moon. It is composed almost entirely of it’s iron core, surrounded by a mantle and thin crust of silicate rock.

We had little of this information until very recently. In fact, for a long time Mercury was the least studied planet in our solar system. The speed at which it orbited, and it’s closeness to the sun, just made it even more difficult to collect data of any kind from this tiny planet. Mariner 10 had made three high-speed flybys in 1974, but returned with little more than images of half of the surface. This changed in 2011, when NASA’s MESSENGER probe became the first to orbit the planet. MESSENGER was able to map the entire surface, changing our view of the innermost planet. MESSENGER showed Mercury’s active past, and raised many questions about it’s seemingly simple geography. Mercury looks much like our moon. Small, cratered, and flood basins filled with solidified lava. The Mariner flybys showed us that Mercury is surprisingly heavy and quite dense considering it’s small size, thanks to its iron core, which accounts for 85% of the planet’s diameter. The possibly most impressive of all MESSENGER’s discoveries is the confirmation in 2012 that ice and carbon-based ‘organic’ elements and chemicals reside in the forever shadowed craters of Mercury’s north and south poles.

Venus

Venus was named after the Roman goddess of beauty, which is ironic considering no form of life could survive for more than a millisecond on this planet. First the pressure would crush you, the average temperature is hot enough to melt lead, and it rains acid. Venus is the second planet from the sun, has no moons and is even hotter than the first planet from the sun, Mercury. Venus is the first planet to have weather systems. Mercury has too thin of an atmosphere to have any possible weather systems, but Venus, with it’s incredibly thick atmosphere and abundance of greenhouse gasses, is host to many forms of weather. The first probes able to make flybys and landings found that the clouds on Venus race around the planet at a rapid speed, making their way around the entire planet in just a day or two. Venus is sometimes thought of as Earth’s twin, because of the similar sizes. It ‘s mass is very close to Earth’s, exactly 0.82 earths, which is the closest size to Earth in all the planets. It is only 2.3 light minutes from earth, making it Earth’s closest neighbor. Venus’ structure is also very similar to Earth’s. Both planets have a thin silicate crust, a large mantle of rock, and an iron and nickel core. The gravity surface gravity would be 0.9 of Earth’s, but the pressure would crush you into a grease spot instantly. A year on Venus would be 225 Earth days, making it longer than a day, which is 243 Earth days. The diameter of Venus is 12,104 kilometers, and the average surface temperature is 464? and 876?. Venus is sometimes known as the Evening or Morning Star, and oftentimes, the only thing that outshines it is the Moon. This is because of it’s incredibly thick atmosphere. It’s atmosphere of carbon dioxide and yellow-white clouds reflect 70% of all light, making it one of the most reflective objects in our solar system.

For centuries, astronomers wondered and guessed about what those inpenatatable clouds hid. They thought Venus might be host to forms of life. Others thought Venus might be a jungle planet. Only 26 million miles between Earth and Venus’ orbit wouldn’t make the planet completely uninhabitable right? Wrong. The first probe to fly by measure the temperature and relayed back that it’s average temperature was a scorching 464?. The later probes that were able to make it through the clouds told us the atmosphere was an over of almost completely carbon dioxide, the clouds were sulphuric acid, and a surface of baked volcanic rock. The discoveries these probes made crushed everything astronomers had previously thought about Venus. These probes also found another thing astronomers were not expecting. They thought, due to the high wind speeds, and rapid movement of clouds on Venus, the planet would be rotating very fast. What they found was quite the opposite. Venus rotates at such a slow pace it’s day is longer than its year. Even more surprising was the fact that Venus, not only rotated extremely slowly, but also seemed to be rotating backwards. Venus has been rotating the opposite direction of all the other planets.

Astronomers did not understand. How they could have got everything they had previously thought to be true about Venus so wrong? They decided to map the entire planet. Doing that would require the use of a new technology - radar. The first space probes equipped with radar was sent to orbit Venus in the 1970s. However, the radar was so poor astronomers were not able to get more than a vague image of the surface. Plains, with a few highlands, and a mountain range called Maxwell Montes near the equator. In 1989, however, a new probe with a far more sophisticated radar, Magellan, came in to the orbit of Venus. The Magellan space probe was able to measure the height of the surface of the planet, provide information about the slope, surface roughness, and composition of the planet below it. The mission of Magellan revealed much about the planet’s surface, and in more detail than ever before. The most impressive discovery was possibly the extreme number of volcanoes and volcanic features on its surface. Other probes had, of course found Earth-like volcanoes on the surface of Venus, but had also found bizarre features that did not resemble anything we had on Earth. Magellan discovered that almost all of these were caused by volcanoes. Pancake domes are one of the best examples. They were named after their unique circular bump shape. Lava from under the surface on Venus Pushes up and tries to break through. The lava is too viscous to run too far from the vent it came from, it piles up around the edges. This presents a question though. Magellan found no evidence of tectonic plates, which is what causes volcanoes to rise on Earth. With no tectonic plates, how could Venus have any volcanoes, active or otherwise? Another piece of evidence from Magellan bings us some of the answer. Since Magellan was the first advanced probe to visit Venus, astronomers did not know about how many craters there were. After Magellan came, they were shocked at how surprisingly few craters Venus had. Even with it’s incredibly thick atmosphere, there should be many more craters than there are. With the surfaced mapped, astronomers had to admit that 90% of Venus’ surface was surprisingly young, a mere 6 million years. It appears almost the entire planet was resurfaced by volcanic activity. And that is exactly what happened. With the absence of tectonic plates, there is no slow and steady volcanic activity, like there is on Earth. Without any way to come out, the pressure of lava beneath the surface builds. Then every few hundred million years the pressure becomes too great, and the lava spills through, giving the entire planet a new surface. That is the best current model of Venus, but it is not complete, and may not be 100% accurate. This model raises the question of how much ‘background’ volcanic activity there is between these major eruptions. These volcanoes could be powered by hot spots in the mantle of the planet, similar to Earth’s. It would be strange if the the most volcanic world in our solar system had no current volcanic activity.

With the surface of Venus mapped the attention of astronomers was turned to its atmosphere. In 2006 the Venus Express arrived. It’s main job was to discover the role the ‘greenhouse effect’ played in the planet’s superheated atmosphere. The greenhouse effect is when an abundance of carbon dioxide and other greenhouse gasses, like methane, is present in a planet’s atmosphere. This comes from the great amount of volcanoes on the surface of Venus. The volcanoes constantly expel more greenhouse gasses, causing the entire planet to turn into a world completely inhospitable for any form of life. The amount of greenhouse gasses allows sunlight and heat in, but it traps it inside the atmosphere. The heat in the atmosphere can never leave, much like a greenhouse would trap heat for the plants to thrive. However, with the entire planet in this superheated greenhouse there is no way for plants to live, much less thrive. Venus is the closest planet to our home, Earth, but it is the last planet in the solar system you would ever want to live on.

Earth

Earth, our home planet. The watery planet. The planet of life. Earth is the third planet, the largest of the inner planets, and is the first from the sun to have a moon. Many people think that because they live on this planet they know everything about it. In many cases that is not true. Earth is composed much like Venus, with a thin silicate crust, a large mantle of molten rock, a outer core of molten nickel and iron, and an inner core of the same, but solid. Earth has a diameter of 12,756 kilometers, making it the largest of the inner planets. As you know, our day length is 24 hours. Interestingly, most people are unaware that instead of being, 365 days, our year is actually 365.25. This is why every four years we have a leap year, a 366 day year with the extra day becoming the last day of February. Earth is tilted on its axis at 23.45 degrees, which it what gives it its seasons. The mean surface temperature is 15? or 59?. Our planet orbits around the sun at an average distance of 93 million miles, or 150 million kilometers, putting it at just the right place to form a world hospitable for life. Like most of the other planets in our solar system, the tilt of Earth’s axis is relative to its orbit. The tilt always points the same direction in space, regardless of position in its orbit. Because of this, different parts of the Earth receive different amounts of sunlight throughout the year, causing the seasons.

The surface of the Earth is two-thirds water. The conditions here allow it to be found in its liquid form. We are at the right distance from the sun and we have a good atmosphere that keeps water from evaporating into space. The other third of the surface of Earth is land, in most cases, the highest points on the Earth’s thin crust. The crust of the Earth is slowly changing, due to the tectonic plates. The Earth’s surface is broken up into smaller pieces that drift around on the sea of molten rock, that is the mantle of the Earth. We call these pieces of rock, dirt and water the tectonic plates. The tectonics slam together, causing earthquakes, or sometimes mountains and volcanoes to rise. The two main forces that are constantly reshaping the surface of our planet are, tectonic, earth and fire, and erosion, air and water. While tectonics may give rise to mountains, even the greatest mountain chain will eventually be reduced to sand by the other factor of erosion. Earth, while being different from the other inner planets in many more ways, is also unique in the fact that it is the only inner planet with fully developed tectonic plates. The Earth’s crust is divided into seven main plates, plus many smaller ones. Below the crust lays a mantle of ever-churning molten rock, which forms currents that carry heat away from interior and closer to the surface. The boundary between the mantle and crust is a much more fluid layer called the asthenosphere. The tectonic plates float on top of this layer, constantly changing their direction because of the churning currents of the mantle below it. When the plates meet there are several ways they can move. One is drifting away from each other. When this happens they create a gap, from which molten rock seeps through and forms a new part of the crust. This is what forms the rift valleys. If the plates move sideways past each other they form earthquake zones. They jam against each other, then one of them slips, creating an earthquake. The last way them can move is slamming head-on into each other. When this happens with ocean and continental plates, usually the ocean plate is the one to yield because of its thinness. This forces the ocean plate down beneath the other, melting, and in doing so, releasing vast amounts of heat into the upper mantle. This heat may cause volcanic chains to form, such as ‘The Ring of Fire’ in the Pacific Ocean. Lastly, is two continental plates slamming into each other. When this happens, one is usually forced underneath the other. This results in a crumple zone where mountain chains are pushed up between the colliding plates. A collision like this is what is responsible for the Himalayas, the greatest mountain range today.

Tectonics may build mountain ranges, but erosion, wind and water, are constantly tearing them down. About 300 million years ago the Appalachians were the greatest mountain range, but now they are just a shadow of their former glory. Why does Earth have such a powerful and developed tectonic system, when the other inner planets have none? Part of the answer is simply the size of Earth. Being the largest of the inner planets, it has the hottest core, and the most energy to drive tectonics. However water may also be part of the reason. Carried into the upper mantle by oceanic plates that have been pushed under others, it could possibly help to moisturize the asthenosphere. The weather altering the density of rocks might keep the continents from becoming locked together.

The water in Earth’s atmosphere and surface can only survive because of the unique position Earth holds in the solar system. We have a stable, close to circular orbit around our star, a relatively calm one. Our fast rotation rate makes sure one side of our planet does not boil, while the other freezes, and Earth’s tilt ensures that the poles both get a reasonable amount of sunlight each year, which prevents them from expanding towards the equator. Even with all of this, water could still not exist on Earth if not for the atmosphere. Without one all water would boil off into space, leaving a dry inhospitable place. Fortunately, at the same time the atmosphere keeps the water on Earth, the water keeps the atmosphere’s temperature cool enough to prevent gas molecules from escaping. The chemical makeup of our atmosphere has changed drastically over time, though the pressure remains close to the same. It is thought that earlier in history, Earth had an atmosphere rich in hydrogen compounds like methane. Many things have played a role in changing our atmosphere over time, including our complex water cycle.

Earth’s water is always being transformed between its three states. Solid, liquid, and gas. Water in the ocean evaporates and becomes vapor in the atmosphere, where it re-condenses and forms clouds, then falls back to the Earth as precipitation, most often rain. Where water fall to the ground, it softens the land, and provides water to support life. It also weathers rock, turning them into carbonate minerals, and absorbing carbon dioxide from the atmosphere. Closer to the poles, rain can fall in the form of snow, rebuilding the polar caps as they constantly melt into the ocean surrounding them. Because they both are fluid, the oceans and the atmosphere are constantly moving and circulating, carrying heat away from regions around the equator and towards the poles. In the atmosphere, the rising and falling pattern of the air forms ‘Hadley cells’ between the tropics and the poles. Hadley cells are where air from the tropics rises and travels towards the poles, then sinks back down around mid latitude usually close to about 30° north or south.

One question is where all the water on the Earth comes from. Although our region of the solar nebula was most likely rich with water and ice, the heat during the formation of the Earth should have expelled any of this, sending boiling off into the vacuum of space.

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