Mercury

Young Astronomers Blog, Volume 29, Number 12.

Mercury is the closest of our Solar System’s eight official planets to the Sun. It speeds around the Sun in just 88 days. Its orbit is the most elliptical of the eight planets (eccentricity .205), with a perihelion of 28.6 million miles (.3 AU) and an aphelian of 43.4 million miles (.44 AU).

Mercury’s orbit is rotating so that the orientation of its perihelion and aphelian shift by around 574 arc seconds every century. An arc second is only 1/3600^th of a degree and there are 360 degrees in a full circle (one orbit). So, doing a little math, it will take around 226 thousand years for Mercury’s perihelion to complete one full rotation.

Nevertheless, the perihelion is shifting and at a rate that for many years couldn’t be completely explained. In 1856, Urban Le Verrier first showed that Newton’s laws of gravity could not entirely account for the rate of Mercury’s orbital progression. The difference was only 43 arcseconds per century, but it was real. It was Einstein’s General Theory of Relativity that finally explained the discrepancy in 1915.

With the “demotion” of Pluto from the status of full-time planets, Mercury now has the distinction of being the smallest official planet (diameter of 3,032 miles, roughly 1/3 the diameter of the Earth). It is also smaller than Jupiter’s Moon Ganymede and Saturn’s moon Titan. Despite being the smallest planet, Mercury is the second densest planet with a core 2,250 miles in diameter (around 40% of its volume and ¾ of its diameter). It is thought that Mercury was once larger but was struck by a large object that knocked much of its surface off into space.

Mercury spins on its axis almost straight up and down with a tilt of only 1/30 degrees. So, on Mercury there are no seasons. There is just the same weather all year long. However, the daily temperature differences are huge. The surface temperature is 800^o on the sunlit side and -290^o F on the dark side. There might also be ice deposits deep in some of its polar craters where the surface is bathed in permanent shadows during the long sunlit day.

When viewed from space, Mercury’s surface resembles that of the Earth’s Moon. It is gray in color with hundreds of craters. The largest impact area is Caloris Basin, which is located in the mid northern hemisphere and is some 950 miles in diameter.

Mercury does not have an atmosphere but has what is described as an exosphere composed of material blasted off the surface by meteoroids and the solar wind.

For many years, astronomers thought that Mercury was tidally locked with the Sun, keeping one side always facing the Sun just as the Moon keeps one side always facing the Earth. In 1965, astronomers Gordon Pettengill and Rolf Dyce found that Mercury rotates every 58.6 days (its sideral rotational period relative to the stars). This means that it rotates three time during every two of its orbits. Because of this, a “day” (sunrise to sunset) on Mercury lasts 176 Earth days (two full Mercury years).

Mercury goes through a 115.9 day “synodic period” relative to the Earth. Mercury, like Venus and the Moon, has phases depending on its position relative to the Sun. Mercury will appear full when farthest from the Earth as its orbit passed behind the Sun (Superior Conjunction) and as a crescent at its near point to the Earth when its orbit passed in front of the Sun (Inferior Conjunction).

Mercury is visible from the Earth either low in the western horizon just after sunset or low in the eastern horizon just before sunrise. It only rises to a peak around 10^o to 20^o above the horizon when it is at a point called the Greatest Elongation. It also is relatively dim compared to the other planets with a magnitude more in line with the brightest stars. So, it is difficult to spot.

Like Venus, Mercury will transit across the face of the Sun from time-to-time. Unlike Venus, which transits only twice a century, Mercury’s transit occurs more often, although if you missed the last one in 2019, you will have to wait until 2032 for the next.

Only two spacecraft have visited Mercury, and a third on its way. One of the challenges is that Mercury is speeding around the Sun so fast that a spacecraft must move quickly to catch up with the planet. Then, to go into orbit, it must slow down substantially. This is difficult to do in one maneuver, so spacecraft perform multiple flybys of the planet.

• The first spacecraft to visit Mercury, Mariner 10, performed three flybys in 1974-75 without achieving orbit.
• MESSINGER performed three flybys in 2008 and 2009 before finally reaching orbit in 2011.
• The ESA and JAXX BepiColombo spacecraft will perform seven flybys from 2021 through 2025 before settling into orbit in late 2025.

Selected Sources and Further Reading

“All About Mercury.” NASA Science, Space Place. Updated April 14, 2020. https://spaceplace.nasa.gov/all-about-mercury/en/

“Mercury.” NASA Science, Solar System Exploration. Updated February 15, 2021. https://solarsystem.nasa.gov/planets/mercury/overview/

Bill Arnett. “Mercury Facts.” The Nine Planets. Updated January 4, 2021. http://nineplanets.org/mercury.html

“Your Guide to Mercury.” The Planetary Society. (Accessed May 10, 2021). http://www.planetary.org/explore/space-topics/mercury/

Fraser Cain. “Characteristics of Mercury.” Universe Today. April 1, 2012. https://www.universetoday.com/34571/characteristics-of-mercury/

Phil Plait. “Mercury: Crash Course Astronomy #13.” CrashCouse/YouTube. April 16, 2015. https://youtu.be/P3GkZe3nRQ0

Jeff Coffey. “Rotation of Mercury.” Universe Today. May 1, 2008. http://www.universetoday.com/14008/rotation-of-mercury/

Dominic Ford (Editor). “Mercury.” In-The-Sky.org. Updated March 17, 2019. https://in-the-sky.org/article.php?term=Mercury

“Seven Century Catalog of Mercury Transits: 101 CE to 2300 CE.” NASA Eclipse Web Site. (Accessed May 10, 2021). https://eclipse.gsfc.nasa.gov/transit/catalog/MercuryCatalog.html

“Mariner 10.” NASA Science, Solar System Exploration. Updated July 24, 2019. https://solarsystem.nasa.gov/missions/mariner-10/in-depth/

“Messenger.” NASA. Updated August 2, 2019. http://www.nasa.gov/mission_pages/messenger/main/

“BepiColombo.” NASA Science, Solar System Exploration. Updated July 28, 2019. https://solarsystem.nasa.gov/missions/bepicolombo/in-depth/