Inner Planets
The planets of the inner Solar System are our closest neighbors. We're packed pretty close together...compared with the rest of the Solar System. All of these planets are small and are considered rocky in composition. But they are all so very different. We'll take a look at each one and then examine why Earth, alone, has life...that we know of.
Mercury
The first planet in the Solar System is also the smallest. Mercury is a little bigger than the Moon but smaller than at least two other moons in the Solar System, but it is the densest planet. Being the first planet makes it the fastest in its orbit around the Sun. It is always on the inside lane of the track so it has less distance to travel. It takes about 88 Earth days to go around the Sun, but it rotates very slowly. One day on Mercury lasts 176 Earth days*; meaning that it takes two Mercury years to have one Mercury day. If you lived on Mercury (I don't recommend it because you'd melt for half of the day and freeze for the other half), you'd celebrate two birthdays every day: one at night and one during the day. Mercury does have an atmosphere...sometimes. When it does, it is a very thin layer of hydrogen and helium from the Sun. But, then, the solar wind strips it away and later replaces it. Mercury has no moons. * - I have seen different lengths reported for Mercury's day. This is the one I have seen most often. Also, click on the picture to get more information about Mercury (and you'll notice that the day length is different, too). |
Venus
Often called Earth's twin sister, Venus is the beautiful, but evil twin. Slightly smaller than Earth, the pretty clouds hide a surface that is drastically different than here on Earth. First, the clouds are close to 5 times thicker than Earth's atmosphere. Made predominantly of carbon dioxide, the clouds also contain sulfuric acid. The clouds are so thick and bright (it's the brightest object in the night sky after the Moon) that very little sunlight gets to the surface, but when it does, it stays. Carbon dioxide is great at holding in heat. At approximately 870 degrees on the surface all the time, this makes Venus the hottest planet in the Solar System. There is no let up. Also, all those clouds weigh a lot. If the extreme heat and sulfuric acid doesn't get you, then the extreme pressure would. Oh, did I mention that it might rain liquid metal due to its high heat? Venus is another oddball of rotation vs. revolution. It goes around the Sun slightly faster than it rotates. Oh, and it rotates backward. We call this retrograde rotation. Whereas all other planets rotate from east to west (meaning the Sun rises in the east and sets in the west), Venus rotates west to east. We don't know why, but it does. Finally, it doesn't have any moons either. It and Mercury are the only two planets without moons. |
Earth
The third planet from the Sun is also the biggest of the inner planets. It's also (duh) the only planet with life...that we know of. Why is this? We'll take a closer look at that later when we discuss the Goldilocks Zone. For now, we can be content with knowing that it's the only planet with liquid water on its surface and an abundance of oxygen in its atmosphere. It also has only one moon. One day is 23 hours and 56 minutes (we tend to round up to 24 hours) and the year is 365.25 days (hello leap year). |
Mars
The red planet is the last of the inner planets. It's the second smallest planet in the Solar System, but seems to offer the best chance for us to leave Earth. It's day is similar to Earth's but it's year is obviously longer. Mars has an atmosphere that is predominantly carbon dioxide, just like Venus. But, unlike Venus, it is very cold on the planet. The average temperature is similar to the worst day in Antarctica. It also has the biggest volcano in the Solar System. At nearly 3 times the size of Mount Everest, Olympus Mons dominates the Martian surface. But, without a doubt, the most striking and recognizable feature is the reddish surface. Caused by large amounts of oxidized iron, it's essentially rust. And, Mars has two moons: Phobos and Deimos. We know from the many rovers and landers that have gone to Mars that it once had liquid water flowing on its surface. What happened to it? Does that mean that it had life? We don't yet know the answers to these questions. |
Greenhouse Effect
The greenhouse effect is the name given to the observation that certain gases trap heat or absorb heat radiating from the surface, preventing it from escaping into space. This causes the surface to stay warm more than it would otherwise, like in a greenhouse. For the Earth, this is a good thing. Without the small amount of carbon dioxide causing the greenhouse effect, our planet would be too cold to support life...at least, our life. But on Venus, there's too much and it has turned the surface into a nightmarish world. On the other hand, Mars has a mostly carbon dioxide atmosphere as well. But it's atmosphere is thin and it's really far from the Sun. That might be why it's so much colder. It's thought that we could possibly end up like Venus, or at least close to it, if we continue to pollute the air with carbon dioxide from burning fossil fuels. Hopefully that never happens, but we do know that while the greenhouse effect is a good thing for us right now, it might hurt us in the future.
The greenhouse effect is the name given to the observation that certain gases trap heat or absorb heat radiating from the surface, preventing it from escaping into space. This causes the surface to stay warm more than it would otherwise, like in a greenhouse. For the Earth, this is a good thing. Without the small amount of carbon dioxide causing the greenhouse effect, our planet would be too cold to support life...at least, our life. But on Venus, there's too much and it has turned the surface into a nightmarish world. On the other hand, Mars has a mostly carbon dioxide atmosphere as well. But it's atmosphere is thin and it's really far from the Sun. That might be why it's so much colder. It's thought that we could possibly end up like Venus, or at least close to it, if we continue to pollute the air with carbon dioxide from burning fossil fuels. Hopefully that never happens, but we do know that while the greenhouse effect is a good thing for us right now, it might hurt us in the future.
Goldilocks Zone
"This place in the Solar System is just right." Just right for what though? And why is it just right? And what am I talking about? Well, I'm glad you asked those questions. Earth has liquid water, but Venus and Mars don't. Venus is really (really, really, really) hot and Mars is cold. Sound familiar? To scientists, it did. They dubbed this set of circumstances the Goldilocks Zone after the golden-haired intruder of the bears' house. It's theorized that there is a certain distance from a star for which the temperature is just right for liquid water to exist. If the star is big, the zone is farther away. If the star is small, the zone is closer. Makes sense: Venus is too close to the Sun, therefore it's too hot; Mars is too far away from the Sun, therefore it's too cold; and Earth is in the middle...just right. Perfect! That explains so much! Oh...if it were only that simple. Look at the paragraph above. Without the greenhouse effect, Earth would be too cold for liquid water, too. So, does that mean that the Goldilocks Zone is a bust? Not exactly. While the climates for the given planets are caused by the greenhouse effect, if Earth were closer or farther from the Sun, it wouldn't have liquid water under the same conditions. So, looking for an exoplanet (a planet not in our Solar System) in the Goldilocks Zone for its star is not a fool's errand. It's one of many possible characteristics that might indicate potentially habitable planets for life. Also, in about 1.75 billion years, the Earth will be on inside edge of the habitable zone for our Solar System and all of the surface water will boil off...ending life as we know it. But, I assure you, we won't be here when that happens. So, no need to worry.
"This place in the Solar System is just right." Just right for what though? And why is it just right? And what am I talking about? Well, I'm glad you asked those questions. Earth has liquid water, but Venus and Mars don't. Venus is really (really, really, really) hot and Mars is cold. Sound familiar? To scientists, it did. They dubbed this set of circumstances the Goldilocks Zone after the golden-haired intruder of the bears' house. It's theorized that there is a certain distance from a star for which the temperature is just right for liquid water to exist. If the star is big, the zone is farther away. If the star is small, the zone is closer. Makes sense: Venus is too close to the Sun, therefore it's too hot; Mars is too far away from the Sun, therefore it's too cold; and Earth is in the middle...just right. Perfect! That explains so much! Oh...if it were only that simple. Look at the paragraph above. Without the greenhouse effect, Earth would be too cold for liquid water, too. So, does that mean that the Goldilocks Zone is a bust? Not exactly. While the climates for the given planets are caused by the greenhouse effect, if Earth were closer or farther from the Sun, it wouldn't have liquid water under the same conditions. So, looking for an exoplanet (a planet not in our Solar System) in the Goldilocks Zone for its star is not a fool's errand. It's one of many possible characteristics that might indicate potentially habitable planets for life. Also, in about 1.75 billion years, the Earth will be on inside edge of the habitable zone for our Solar System and all of the surface water will boil off...ending life as we know it. But, I assure you, we won't be here when that happens. So, no need to worry.