Comparing Earth and Mars
Earth and Mars are both terrestrial planets, meaning they both are earth-like, i.e., they are rocky. These two planets will serve as baseline data for my students to use as a support in identifying what is needed for a planet to be considered habitable. Scientists use Earth all the time because it is the planet we live on. There are two theories about how these planets could have formed, the first is that they are formed by collisions among planetary embryos in the protoplanetary disk close to the sun, the second is that they formed by the accretion sunward-drifting small pebbles “pebbles” from the outer parts of the solar system. 7 Scientists use Earth as a reference because we have plenty of information to use. Our science curriculum starts off the beginning of the 7th grade school year having students explore the possibility of there being life on the planet Mars. We will start with our planet first and then delve into Mars’ data.
Earth
The Earth is a good starting point because we are most familiar with our home planet. We know that Earth has the Sun as its star, which provides us with heat and light. The Earth has an atmosphere and it contains water. We are far enough away from the sun to not be burned by radiation. Being a terrestrial planet enables us to stand on our planet and to support vegetation which is needed to provide food for the inhabitants. The resources that we have are plentiful and provide us with the essentials to sustain life.
Mars
The planet Mars is also a terrestrial planet, however, it does not have the same qualities that Earth has. There is some evidence that Mars was once covered with water, and that there is water in the polar regions. There are many popular tales of life on Mars, but no life has been detected as yet.
Atmosphere
The atmosphere of a planet needs to be considered when discussing habitability. A planet’s atmosphere may have the right amount of essential gasses, or they may be deficient or excessive. We will take a look at these elements or gasses that can be found in the atmospheres of many planets, these are Hydrogen (H2), Oxygen (O2), Nitrogen (N2), Argon (Ar), Methane (CH4), Carbon Dioxide (CO2). Some planetary atmospheres, such that of Venus also have sulfur dioxide (SO2) and with clouds of sulfuric acid (H2SO4) others can contain ammonia.
Earth’s Atmosphere
Let us start with our home planet Earth. Earth has a thick atmosphere that is composed of various amounts of different gasses, the most abundant ones being Nitrogen and Oxygen. There are also some trace elements in our atmosphere as well as dust and water vapor. The total mass of Earth’s atmosphere is about 5x1018 The troposphere is the layer that is closest to Earth’s surface. Temperature is also controlled by this part of the atmosphere. The troposphere is wetter than any of the other layers because it contains a lot of moisture.
The next layer is the stratosphere which is the layer that is directly above the troposphere. The stratosphere is vitally important because it serves as a shield from the ultraviolet rays of the sun. The stratosphere also provides us with warmth because of the presence of greenhouse gasses, if it were not for the greenhouse gasses the Earth would be cold. Directly above the stratosphere is the mesosphere which has a decrease in temperature as the altitude increases. This temperature profile is quite different from that of the stratosphere. Meteors and other space objects will burn up in this part of the atmosphere despite its thinness. As we go above the mesosphere, we will enter the thermosphere, a very hot part of the atmosphere where the temperature can reach an astounding 2000 degrees Fahrenheit. As you ascend further up, you will emerge into the last segment of the atmosphere, the exosphere, which has gasses escaping into the outer space realm. We have just taken a journey more than 10,000 km from the surface of the Earth directly to outer space.
The Earth’s atmosphere has a high amount of nitrogen in it. Nitrogen comprises 78% of the atmosphere. Nitrogen in its natural state does not do anything in the atmosphere. Nitrogen has to be combined with other elements to become a nitrate. Nitrogen is a very necessary nutrient for humans because it is used by amino acids which are the building blocks of proteins.
Oxygen is the 2nd most abundant gas found in the atmosphere, at a breathtaking 21%. Oxygen is needed by all living organisms on Earth.
Carbon dioxide is also found in Earth’s atmosphere approximately 0.04%. This gas is needed to warm the atmosphere. Without the warming effects of gases, the Earth’s temperature will decrease drastically.
The next element is Argon, which makes up about 0.94% of Earth’s atmosphere. Methane makes up a small percentage of volume in the atmosphere, 0.00017%. The methane gas helps the planet to stay warm and comfortable however, too much methane would contribute to global warming. The following elements and particles such as dust and water vapor are also in the atmosphere but the percentages are very small. (See Table 2 below)
Table 2-: Earth’s Atmospheric Layers
| Atmosphere Layers | Description |
|
Exosphere |
Outer Space, which is 6200 miles thick Spaceships and satellites are in this sphere. |
|
Thermosphere |
This layer is above the mesosphere and extends about 90 km to 500-100 km above our planet. X-ray and UV radiation from the sun is absorbed in this sphere. The aurora lights can be seen from this sphere. |
|
Mesosphere |
This layer lies directly above the stratosphere: it lies about 50 to 85 km above Earth. Temperatures decrease in this layer which is quite different in the stratosphere. The sky changes color from blue to black because of the lack of molecules. Rockets and meteorites can be found in this layer. |
|
Stratosphere |
Ozone can be found in this layer. It is vitally important because it absorbs ultraviolet radiation from the sun. Temperatures rise as you go higher in the stratosphere. Weather balloons and jet aircraft fly in this zone because of the thinnest of the air. The stratosphere is about 30 miles (km) above the troposphere. |
|
Troposphere |
Clouds are found in this layer and nearly all weather is found here as well. The troposphere extends to about 10 km into the air. It is the thickest part in the atmosphere. Aircraft can be found in this layer as well as mountains. |
Credit: nationalgeographic.org
Source: National Geographic Society
Mars’ Atmosphere
Mars’ atmosphere is very thin in comparison to the Earth’s atmosphere. The primary gas that comprises Mars’ atmosphere is CO2. This gas can be found in surprisingly large amounts in Mars’ atmosphere, 95% CO2. Oxygen is less abundant, about 1%. The excessive amounts of CO2 and a lack of oxygen would kill humans, assuming that the thin atmosphere did not do so in the first place. Since there is not any plant life that we know of, photosynthesis is not taking place and therefore there are not any plants or vegetation to use as food.
Mars is the farthest terrestrial planet from the Sun which makes it colder than the Earth. The thinness of the atmosphere is also a factor which gives Mars its cold climate. Mars also has dust storms, dust devils, and snow, which is made up of frozen CO2. The dust storms are always a part of Mars’ atmosphere, which is also the biggest dust storm in the solar system.
Mars' atmosphere can also be divided in layers, and they are the same as that of Earth. Mars does not have a stratosphere. The bottom layer is the troposphere, which is twice as large as Earth’s troposphere. The next layer is the mesosphere, which is very cold. The next layer is the exosphere, which is extremely hot and leads to outer space.
Mars does experience seasons, but they are different from Earth’s because they last considerably longer because Mars takes 687 days to revolve around the sun. Taking a look below you can see that there is a substantial difference in the average number of days each planet experiences weather in each of those seasons. (See Table 3 Below)
Table 3: Comparison Chart for Seasons on Mars and Earth
| Mars | Days per Year | Earth | Days per Year |
| Spring | 194 | Spring | 93 |
| Summer | 178 | Summer | 93 |
| Fall | 142 | Fall | 90 |
| Winter | 154 | Winter | 89 |
Credit: Mars.nasa.gov/all-about-mars/facts/mars-years/
Mars’ resources are vastly different from our planet. There have been several visits from different spacecraft and rovers -- Curiosity, Perseverance, and the Ingenuity -- hoping to discover significant information to share with the world. Rocks found on the surface enabled scientists to get insight into the changes that the terrain had endured over millions of years. It was discovered that Mars had once been a wet planet.
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