Mercury, Venus, Earth, and Mars are examples of terrestrial planets. Mercury is virtually completely devoid of atmosphere. Venus is surrounded by a dense atmosphere composed primarily of carbon dioxide, which traps heat and raises surface temperatures. Sulfuric acid causes clouds to grow on Venus. Earth's atmosphere is made up of nitrogen (78%), oxygen (21%), argon (1.5%), and other gases.
Mars has less than 1% atmospheric pressure compared to Earth's 1014 mbar, but it used to have more like 100 km altitude cloud cover before many of its volcanoes erupted, contaminating the air with sulfur dioxide, which destroyed much of the original atmosphere.
Terrestrial planets consist of a solid core covered by a thick layer of ice or rock, which explains why they can retain their atmospheres for as long as they do. The only way for a terrestrial planet to lose its atmosphere is if the surrounding star gets hot enough to melt its ice or vaporize its water, thus removing the planet's primary shield against radiation.
The origin of terrestrial planets is not well understood. Some scientists think they might be formed when dwarf planets like Ceres or Pluto crash into each other, while others believe they must be created out of disk of gas and dust that collapses to form a single star or black hole.
Mercury is the only planet that lacks a true atmosphere. It does, however, contain an exosphere, which is a collection of gases caught by the solar wind as well as discharged from the planet's surface. Hydrogen, helium, and oxygen are among the gases. Venus has an extremely dense, heated carbon dioxide atmosphere. Although most of the atmosphere is gas, there is also water vapor in the air.
Earth's atmosphere is made up of nitrogen (78%), oxygen (21%), argon (1.9%), other gases.
Without Earth's atmosphere, would we be able to survive on Mars? Yes and no. While there would be less air pressure on Mars, there would still be enough atmospheric pressure to keep water liquid even if it was found in the polar ice caps. If human explorers were to travel there today, they would need equipment to protect them from the low air pressure and extreme cold temperatures.
Why is the Moon always dark when viewed from Earth? This is because the Moon is never fully illuminated by sunlight; instead, it gets light from the Sun in the form of rays called "rays." As these rays reach the lunar surface, they are constantly being refracted by molecules of water vapor in the moon's atmosphere. The result is that only about 15% of the incoming light makes it through this atmosphere and onto the lunar surface.
Mercury is the only planet in our solar system without a significant atmosphere. Venus has a dense atmosphere. Earth undoubtedly has one. Mars' atmosphere is quite thin. But all these planets are inside their star's habitable zone; they must have atmospheres to be able to support life as we know it.
Earth's atmosphere is made up of molecules that are either gases or liquids under standard conditions. It contains water vapor, carbon dioxide, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine. The presence of these elements explains why we can live on Earth. There are other planets in our galaxy that might be able to support life as we know it (if they had the right ingredients). However, most scientists believe that any world outside our solar system would be void of any form of organic matter.
The outer layer of Earth's atmosphere extends out into space for hundreds of miles, but its exact composition is unknown. We know that it prevents harmful particles from entering our planet's surface and destroys many types of incoming radiation. Without this protection Earth would be unable to sustain life.
So yes, mercury is the only planet that doesn't have an atmosphere.
Mercury, Venus, Earth, and Mars are the terrestrial planets. The planets Mercury, Venus, Earth, and Mars are known as terrestrials because their surfaces are compact and rocky, similar to Earth's terra firma. The term "terrestrial" also describes any planet that does not have large oceans covering most of its surface.
Terrestrial planets include our own Moon which has no atmosphere to speak of. It gets heated by the sun but otherwise would be cold and dark without any life-supporting gases such as oxygen or nitrogen. Living things require water and organic molecules such as amino acids and nucleic acids, and so they need planets with liquid water and some form of life. Our moon doesn't provide this kind of environment so it's reasonable to assume that if another planet identical to ours moon was discovered in the night sky, it would be called a "terrestrial planet."
The four terrestrial planets were part of the original planet lineup before the discovery of many other planets. Today, scientists think that perhaps there were more planets in the early universe than stars. If this is true, then some of these planets might have grown large enough for gas to seep out of their interiors, forming atmospheres similar to those of Earth and Venus. But since we know of only four terrestrial planets, this must be rare happenstance.
All of the planets in our solar system have atmospheres, however Mercury's atmosphere is incredibly thin and not particularly different from the vacuum of space. Our solar system's four largest planets—Jupiter, Saturn, Uranus, and Neptune—all have extremely thick, deep atmospheres.
These large planets formed where there was ice on their surfaces. As they moved around the sun over time, many of these ices sublimated (changed directly from a solid to a gas) or vaporized (turned into a gas). The gases then escaped into space. The remaining bodies were left with dense atmospheres made up of hydrogen and helium with some other gases too.
Earth's atmosphere is much thinner than those of the larger planets but it's still very important for life as we know it. Without an atmosphere, the surface of Earth would be completely exposed to radiation from the sun and other sources. Over time, this would cause all the water on Earth to evaporate and the planet would become a desert. However, because Earth's atmosphere protects us from most of these effects, we can live here in complete peace.
Deep-sea probes have discovered oxygen, carbon dioxide, and other gases in the atmospheres of other planets in our solar system. These results show that molecules in the atmosphere can move between planets and help explain how organisms could have evolved on Mars or Venus even though they are both dead planets.