Mercury is not the Moon. After finally being fully revealed by the MESSENGER orbiter, there is much more to Mercury than meets the eye and still much mystery for future missions to solve. This roundup of facts and figures will make one a pro when talking about Mercury.
Mercury is the closest planet to the Sun. It orbits the Sun at an average distance of 36 million miles; it can get as close as 28.5 million or as far as 43.5 million miles from the Sun. At closest approach, the Sun appears ten times the size we see it in our sky. It is the smallest planet at 3030 miles in diameter and has no moon.
A day on Mercury is twice as long as its year. Mercury takes only 88 days to go around the Sun once and rotates once on its axis every 58.65 days relative to the stars. The combination of these two motions means that for any one spot on the surface, the time from noon to noon is 176 Earth days, which is exactly twice as long as its year.
The combination of its rotation and varying orbital speed in its very elliptical orbit causes a strange effect. At certain locations it is possible to see two sunrises or two sunsets because the Sun stops moving in the sky, reverses direction briefly, and then resumes its regular motion again. The night sky sparkles at certain times with three blazing stars: Venus, shining as bright as magnitude -8, Earth at magnitude -5, and a magnitude -2 Moon attending it, all bright enough to cast shadows.
Mercury can come as close as 48 million miles to Earth and can shine as bright as magnitude -2, but it is lost in the Sun’s glare most of the time or low on the horizon in deep twilight after sunset or before sunrise. Because Mercury orbits closer to the Sun than Earth it can go through a complete set of phases from full to crescent in the evening and then from crescent to full in the morning. Its size can range from 6 to 12 arcseconds across. Legend has it that Copernicus never saw Mercury.
Mercury orbits the Sun at an inclination of 7º, therefore it does not orbit in the same plane as Earth. If the alignment is close enough, Mercury can be seen as a tiny black dot crossing the Sun in a transit. These are rare with the last successful one seen here locally on May 9, 2016, and the next one locally occurring on May 7, 2049.
Early observations of Mercury were frustrating with speculation of towering mountains and raging dust storms. Percival Lowell even observed canals on Mercury along with Mars and Venus but were an optical illusion. This led to the mistake that Mercury always kept the same face towards the Sun. It was not until 1965 that Mercury’s rotation rate of 58.65 days was confirmed using the Arecibo radio telescope in Puerto Rico. The Hubble Space Telescope and James Webb Space Telescope can never image Mercury for fear that they might accidentally sweep across the sun and therefore damage the delicate optics.
Mercury is not the hottest planet, but it has the greatest extremes of temperature of any planet, over 1100ºF! Daytime temperatures can reach 800ºF when closest to the Sun, which is hot enough to melt lead, tin, and zinc, but still around 100ºF cooler than the average surface temperature on Venus! The temperature can plunge as low as -300ºF during the long night. The large fluctuation is due to the lack of an appreciable atmosphere.
Mariner 10 was the first spacecraft to visit Mercury when it flew by on March 29, 1974. It made two additional flybys on September 21, 1974, and March 16, 1975, photographing 45% of the surface. It revealed a moon-like world with thousands of craters, ridges, mountains, and a giant, multi-ringed, Texas sized basin called Caloris, one of the largest in the Solar System at nearly 800 miles across. The impact that created the giant bullseye basin caused seismic waves that converged on the opposite side of the planet resulting in a region with a chaotic jumble of rocks, mountains, ridges, and faults called “weird terrain”.
The highest point on Mercury is in a mountain range just south of the equator at 2.78 miles above average elevation among some of Mercury’s oldest terrain. The lowest point is at the floor of Rachmaninoff basin at 3.34 miles below average elevation that may contain recent volcanic deposits.
Mercury has an atmosphere that is a better vacuum than what can be produced on Earth, so the sky is always black. The atmosphere is composed of helium from the solar wind along with sodium and potassium released from the soil when it is bombarded by energetic particles from the Sun. Mercury has a weak magnetic field due to the unusually large iron core, comprising about 85% of the planet giving it the nickname “Iron Planet”. The magnetic field is about 1% as strong as Earth’s, but still strong enough to create a bow shock and a comet-like ion tail.
Mercury was suspected to have polar water ice. Back in the 1990’s, radar detected bright patches in some of the craters near Mercury’s poles that could be water ice. Impacting comets could have deposited water ice where it has been protected from the intense heat in the perpetual shadows within the polar craters since Mercury is not tilted on its axis. MESSENGER finally confirmed the ice in November 2012 while orbiting Mercury.
MESSENGER was the first fully dedicated Mercury orbiter. It orbited Mercury from March 17, 2011, until April 30, 2015, when it ran out of fuel. It orbited Mercury every 12 hours as close as 125 miles from the surface during the first year and flew within five miles of the surface in its final orbits before crashing. It was a Johns Hopkins University Applied Physics Laboratory mission costing a reasonable $450 million and came well within budget. MESSENGER was powered by solar panels and had a sunshield to protect its sensitive instruments from the intense heat and sunlight. It carried seven instruments and a radio science experiment. It successfully photographed all of Mercury in true color revealing an ash-colored world with hints of tan.
MESSENGER discovered a vast lava flood plain covering the north polar region that is nearly 60% as wide as the U.S. and up to two miles deep, making it the largest feature on Mercury. MESSENGER also detected a partially molten core overlain with a solid silicate crust and mantle. The core has a solid iron sulfide outer layer, a deep liquid core layer, and possibly a solid inner core. MESSENGER discovered a feature unique to Mercury known as hollows. These are found within several craters, along crater rims, and peppering central mountain peaks. They are up to a mile wide, 100 feet deep, and most likely pockets where volatile materials boiled away from the heat below and/or from the Sun and burst through the surface, giving it a sponge-like appearance. Mercury is also peppered with volcanic vents with deposits indicating they may have been active not long ago.
BepiColombo is the next mission to Mercury. It launched on October 20, 2018, and will arrive on December 5, 2025. It is a dual orbiter flying to Mercury as one unit that will separate upon arrival. The mission is overseen by the European Space Agency (ESA) in partner with the Japan Space Exploration Agency (JAXA) and named after Giuseppe (Bepi) Colombo, a brilliant mathematician and engineer from Italy. He discovered the 3:2 resonance of Mercury and the Sun and figured out that Mariner 10 could fly past Mercury multiple times using Venus’s gravity assist. The Mercury Planetary Orbiter (ESA) will study the geology of Mercury while the Mercury Magnetospheric Orbiter (JAXA) will study the environment surrounding Mercury. The mission was to include a small lander but was cancelled due to technological challenges and cost. Total mission cost is around US $1.9 billion classifying it as a flagship or cornerstone mission, the best of the best.
Mercury is a target for a small NASA lander possibly in 2045, eventually rovers, and a sample return mission will be undertaken as high temperature technology is proven. The list of fun and mysterious facts will only grow longer as our knowledge of Mercury increases.