Of all the planets in the solar system, Mercury has the thinnest atmosphere, thinner than even Mars. Several components are constantly replenished by the solar wind blowing off of the nearby sun.
Mercury is the smallest and least massive of the eight planets. Its low surface gravity make holding on to an atmosphere in the best of circumstances a challenge.
But Mercury isn't ideally located for an atmosphere. Orbiting only a few million miles from the sun, the rocky planet is constantly bombarded by solar weather. The fast-moving winds blowing off the star constantly bombard Mercury, sending plasma down to the surface. As such, the planet holds onto only a very thin atmosphere, with many particles actually stemming from the sun.
The makeup of Mercury's negligible atmosphere is:
- Oxygen: 42 percent
- Sodium: 29 percent
- Hydrogen: 22 percent
- Helium: 6 percent
- Potassium: 0.5 percent
- With possible traces of argon, carbon dioxide, water, nitrogen, xenon, krypton, neon, calcium, and magnesium
The hydrogen and helium atoms likely come from the sun, streaming in on the solar wind and spreading out through the planet's atmosphere. Impacting comets and meteorites likely brought in the water vapor and other elements. Others may come from the radioactive decay of the planet's crust. Eventually, these gases are caught by the solar wind and carried off of the planet.
Climate and weather
With virtually no atmosphere, Mercury feels very little in terms of traditional weather. It does feel the presence of solar weather, with the constant ebb and flow of the solar wind bombarding its surface.
The lack of atmosphere also contributes to the planet's wild temperature extremes. On other planets, the atmosphere functions as a blanket, helping to redistribute heat somewhat. But on Mercury, the thin atmosphere does nothing to stabilize the incoming solar rays—and because the distance to Mercury from the sun is so small, the day side of the planet feels the heat keenly, while the night side, turned from the sun, only registers the cold. Mercury's lack of atmosphere means that it is not the hottest planet; Venus, with its runaway global warming, has that honor.
The temperature of Mercury varies from day to night, but the planet only changes slight during its seasons. The planet stands essentially straight up and down in relation to its orbit, with no tilt to put one hemisphere closer than the other.
However, the planet does boast the most eccentric orbit of all the other planets (Pluto's orbit is more eccentric, but alas, it is only a dwarf planet). As such, Mercury does experience some temperature variations over the course of its short year.
— Nola Taylor Redd, SPACE.com Contributor