Closest Planet To The Sun

What is the closest planet to the Sun? Mercury – it is the closest planet to the Sun with a minimum distance of 28.5 million miles and a maximum distance of 43.5 million miles. Mercury makes a complete revolution around the Sun every 87.97 days which is shorter than any of the other planets in the Solar System. Its orbit has a high eccentricity of .20563, the most of any planet(the Earth has an eccentricity of .0167) and Mercury is tidally locked with the Sun in 3 to 2 ratio spin-orbit resonance. This simply means that for every 3 rotations Mercury makes on its axis, it makes 2 revolutions around the Sun. Because of this unusual 3 to 2 spin-orbit resonance, a day on Mercury is about 176 Earth days long!

Since Mercury is so close to the Sun, it can be quite a bit more difficult to view than the other major planets. As an inferior planet to Earth, which in astronomy means that Mercury’s orbit is inside the Earth’s, it is never more than 28 degrees away from the Sun as viewed from the Earth. Because of this, Mercury can only be seen close to the eastern or western horizons during the early morning or early evening hours. When it is seen, it often appears in the sky as a bright star-like object and will show a range of phases when observed through the telescope, similar to Venus.

One would think that being the closest planet to the Sun, Mercury would also be the hottest planet, but that is not the case. Venus although further from the Sun than Mercury, is actually the hottest planet, although by a margin of only around 90 degrees – Mercury can reach a temperature of about 800 degrees Fahrenheit close to its equator during the day, while Venus can reach a maximum temperature of about 890 degrees Fahrenheit. Venus is hotter than Mercury because of its very thick atmosphere and the resultant runaway greenhouse effect since this atmosphere is approximately 96 percent carbon dioxide.

What is the size of Mercury compared to the Earth? Well, Mercury is the smallest planet in the Solar System and its mass is only about 5.5 percent that of the Earth – this means that the Earth weighs about 18.18 times as much as Mercury! The diameter of Mercury is 3031.67 miles compared to 7926.41 miles for Earth at the equator; this is about 38.25 percent of the Earths – if the Earth were the size of a tennis ball, Mercury would be the size of a ping pong ball. The surface gravity on Mercury is about .38g, roughly 38 percent of the Earths.

The surface of Mercury is the least understood of the four terrestrial planets(Mercury, Venus, Earth, and Mars) since most of the data has come from the 1975 flyby of Mariner 10 as well as Earth-based observations. The Hubble Space Telescope is unable to take images of Mercury since it is too close to the Sun and would be far too risky – any such attempt could cause very serious damage to the Hubble Telescope from the brightness of the Sun.

From what we do know, the surface of Mercury has heavy cratering(contributed to by the fact that Mercury has almost no atmosphere) and extensive mare-like plains very much like the Moon, which seems to indicate that it has not been geologically active for billions of years. Most of the bombardment of Mercury’s surface was by asteroids, meteors, and comets about 4.6 billion years ago during and following its formation. This heavy stage of bombardment, which covered the entire surface of Mercury, is believed to have come to an end about 3.8 billion years ago, with more sporadic collisions with Mercury’s surface since then.

Also, during this period of heavy bombardment, the surface of Mercury had many active volcanoes which released magma into basins on its surface, such as the Caloris Basin – one of the largest impact basins in the Solar System, which then produced smooth plains similar to the maria on the Moon. When the Caloris Basin was filled with magma, it became a large plain called Caloris Planitia – this huge plain within the Caloris Basin is surrounded by a ring of mountains, known as Caloris Montes, which are about 1.2 miles high.

Something that is unusual about the surface of Mercury is the fact that it has a large number of compression folds, also called rupes, which crisscrosses the plains and some craters. These rupes are believed to have occurred as Mercury’s interior cooled over millions and billions of years, and are somewhat similar to compression folds on the Moon. Some of these can reach lengths up to about 620 miles with heights up to about 1.86 miles.

There is some smaller rupes in the range of a couple of miles long and around 30 to 100 or more feet high which probably formed in the last 50 million years. These probably indicate that significant geological activity on Mercury is still going on.

Mercury is too small and hot for it to retain any significant atmosphere; however, there is some evidence of a very tenuous exosphere which is bound very close to the surface of Mercury. This extremely thin exosphere has a pressure of only about .005 picobars and is not stable; due in part to Mercury’s relatively low gravity, molecules are constantly being lost into space and also replenished by the intense solar wind. The exosphere on the surface of Mercury consists of hydrogen, helium, oxygen, sodium, potassium, calcium molecules, and some others.

As mentioned previously, temperatures on the daylight side of Mercury can reach around 800 degrees Fahrenheit, while on the night side temperatures can be as low as -280 degrees Fahrenheit. Despite Mercurys extremely thin atmosphere, there is some evidence that ice may exist, not only on the night side but also on the daylight side in deep craters that remain in the shadow where the sunlight never reaches and bitterly cold temperatures around -275 degrees Fahrenheit exist.

Mercury has a significant and global magnetic field despite its small size and very slow 59-day rotational period, due to the 3 to 2 spin-orbit resonance factor. The 1975 flyby of Mariner 10 indicated a dipolar and stable magnetic field that is about 1.1 percent the strength of Earths. This magnetic field is probably generated in a manner which is similar to Earths – circulation of an iron-rich liquid core.

The magnetic field of Mercury, although much weaker than Earths, is still strong enough to deflect the solar wind around the planet which creates a magnetosphere. This magnetosphere, in turn, is strong enough to trap some solar plasma from the solar wind which contributes to some weathering effects and helps to replenish the very thin exosphere near the surface of Mercury. This low energy plasma was detected on the nightside of Mercury by Mariner 10 in 1975.

Only two spacecraft have visited Mercury so far – Mariner 10 with its 1974 to 1975 flyby and MESSENGER, launched by NASA in August 2004, which orbited Mercury from 2011 to 2015.

Mariner 10 flew by Mercury 3 times and mapped almost half of its surface; this resulted in the discovery of Mercury’s very thin exosphere near its surface and the magnetosphere which surrounded the planet.

MESSENGER orbited Mercury over 4000 times in the four years it was there until finally, it ran out of fuel, its orbit decayed, and it crashed into the planet ending its mission. During its time orbiting Mercury MESSENGER was able to map 100 percent of the surface of Mercury and made a number of significant discoveries including ice on Mercury, the fact that Mercury has gradually shrunk over many eons, conclusive evidence of volcanoes, bright unexplained depressions called hollows, and a strange magnetic field which is asymmetrical – stronger in the northern hemisphere than in the southern hemisphere – with no explanation as to why this is the case.

The BepiColumbo space probe is a joint venture between the European Space Agency and the Japan Aerospace Exploration Agency to Mercury and was launched on the 20th of October 2018. It is expected to enter orbit around Mercury in early December 2025 and will have three components – the Mercury Transfer Module for propulsion, the Mercury Planetary Orbiter, and the Mercury Magnetospheric Orbiter. A comprehensive study of Mercury by this joint mission is planned which includes Mercury’s magnetic field and magnetosphere, and the interior and surface structure of the planet.