Mohawk Valley Astronomical Society

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Iron and Ice

by Perry Pezzolanella

At first glance Mercury looks like the Earth’s Moon and is often considered a boring world, but exploration during the last decades has revealed otherwise. Mercury is difficult to see since it never strays far from the bright twilight glow, hardly visible for more than an hour after sunset or before sunrise and can never be seen against a truly dark sky. It is so fleeting that the Greeks called it Hermes, after the messenger of the gods. The Romans changed the name to Mercurius and now the closest planet to the Sun is known as Mercury.

Mercury orbits the Sun at an average distance of 36 million miles in only 88 days and can come as close as 48 million miles to Earth, but the turbulent atmosphere near Earth’s horizon makes it difficult to see surface detail even with the largest telescope. The Hubble Space Telescope cannot be pointed at Mercury due to the risk of stray sunlight damaging the sensitive optics, and the same is true for the James Webb Telescope. Knowledge of Mercury as late as 1970 was scarce. The rotation period of 58.65 days was determined by radar observation and the diameter of 3030 miles was accurately known. Observations suggested that Mercury was like the Moon with craters as it reflected sunlight the same way but had a large iron core. It was not known if Mercury had a magnetic field, an atmosphere, clouds, dunes, mountains, canyons, volcanoes, basins, or widespread craters. A spacecraft was needed, and it was discovered in 1962 that Earth, Venus, and Mercury would be aligned perfectly in 1970 and 1973 for a Mercury flyby using Venus as a gravity assist. If this opportunity was lost, another chance would not occur until the mid-1980’s. It was too good to pass up, so Congress approved a mission for a 1973 launch, and Mariner 10 was born.

Mariner 10 blasted off on time on November 3, 1973, on an adventure almost leading to disaster with a series of problems with the power system and high-gain antenna that nearly ended the mission, but Mariner 10 successfully made a midcourse correction on January 21 and flew within 10,000 miles of Venus on February 5, 1974, providing a wealth of new data and beautiful ultraviolet images of swirling clouds. History was made on March 29, 1974, when Mariner 10 flew within 350 miles of Mercury revealing a heavily cratered surface including a giant impact basin as large as Texas, large expanses of smooth, lightly cratered plains, and long cliffs up to two miles high running for hundreds of miles. Mariner 10 discovered that Mercury has a very thin atmosphere of helium, almost a vacuum, and a magnetic field originating from inside the planet, proving a large iron core. The surface is covered with a light, porous insulating layer of dust like the Moon’s regolith along with rocky outcrops and temperature ranges from -300ºF to 800ºF. Mariner 10 flew past Mercury two more times on September 21, 1974, and March 16, 1975, before it ran out of attitude control gas ending a successful mission. Only 45% of Mercury was photographed raising more questions. An orbiting spacecraft was needed.

Innovations in technology led to building lighter spacecraft that use less fuel, which made it possible to insert a spacecraft into orbit around Mercury. The discovery of possible water ice deposits in the permanently shadowed north polar craters during the 1990s fueled the urge to return to Mercury and stay. In July 1999 MESSENGER (for Mercury Surface, Space, Environment, Geochemistry, and Ranging) was approved and on August 3, 2004, it was launched on a 4.9-billion-mile journey to Mercury. MESSENGER required two flybys of Earth, two flybys of Venus, and three flybys of Mercury to slow it down enough to equal Mercury’s orbital velocity and be captured into orbit. MESSENGER finally entered orbit on March 17, 2011, at 9 P.M. EDT and orbited it every 12 hours as close as 125 miles from the surface during the first year allowing complete mapping of the geology, minerals, and magnetic field. Studies of the gravity field refined the diameter of the iron core upwards to 85% of the planet. Parts of the core were found to be partially molten and it is now theorized that Mercury has a solid silicate crust and mantle overlying a solid iron sulfide outer core layer, a deep liquid core layer, and possibly a solid inner core. Another unique feature exclusive to Mercury is the hollows found within several craters, along rims, and peppering central peaks. These are up to a mile wide, 100 feet deep, and are most likely pockets where volatile material boiled from heat below and/or from the Sun and bust through the surface, giving it a sponge like appearance. MESSENGER discovered a vast lava flood plain covering the north polar region that is as large as 60% of the U.S. and up to two miles deep making it the largest feature on Mercury. The biggest discovery came in late November 2012 when MESSENGER’S neutron spectrometer confirmed evidence for the presence of both water ice and organic compounds in the permanently shadowed craters of Mercury’s north pole. It transmitted the first true-colored images of Mercury as an ash-colored world with a touch of tan. MESSENGER was running out of fuel as 2015 arrived and therefore was programmed to fly as close as five miles above Mercury’s surface during its final orbits crashing on April 30, 2015, ending over four years of intense and rewarding exploration.

The European Space Agency’s (ESA) mission to Mercury was a big one with the launch of BepiColombo on October 20, 2018, and it is well along its journey with an arrival date of December 5, 2025. The mission is comprised of two spacecraft: the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter, known as Mio (Japanese for waterway or fairway). Overseen by ESA, this $1.3 billion mission is a joint mission between ESA and the Japan Aerospace Exploration Agency (JAXA). BepiColombo is designed to study the composition, geophysics, atmosphere, magnetosphere, and history of Mercury. The objectives are to: investigate the origin and evolution of a planet close to a star; study Mercury as a planet, its form, interior structure, geology, composition, and craters; examine the dynamics and composition of its atmosphere and magnetosphere along with the origin; investigate the composition and origin of the polar deposits; and perform a test of Einstein’s theory of general relativity. ESA’s Mercury Planetary Orbiter (MPO) carries a sophisticated payload of 11 experiments and instruments including cameras, spectrometers (infrared, ultraviolet, x-ray, -ray, neutron), radiometer, laser altimeter, magnetometer, particle analyzers, Ka-band transponder, and accelerometer. JAXA’s Mercury Magnetospheric Orbiter (Mio) carries a payload of five advanced scientific experiments that will utilize a magnetometer, ion spectrometer, electron energy analyzer, cold and energetic plasma detectors, plasma wave analyzer, and imager. The spacecraft will remain as one unit during its journey to Mercury and then separate into two orbiters. It will use ion and chemical propulsion along with eight gravity assists: Earth (once), Venus (twice), and Mercury (six times).

BepiColombo is named after Professor Giuseppe (Bepi) Colombo (1920-84) from the University of Padua, Italy, a mathematician and engineer of astonishing imagination. He was the first to see that an unsuspected resonance is responsible for Mercury’s habit of rotating three times on its axis for every two revolutions it makes around the Sun. He also suggested to NASA how to use a gravity-assist flyby of Venus to place Mariner 10 in a solar orbit that would allow it to fly by Mercury three times in 1974-5. ESA named the mission in 1999 in honor of his achievements, a fitting tribute for this cornerstone mission.

Beyond BepiColombo are plans to land on Mercury with a small lander possibly as soon as 2045. It would land just before sunset, take panoramic pictures and do daylight science, but primarily work during the nearly 3-month long night to escape the fierce heat that will destroy it not long after sunrise. Emerging high-temperature technology may make it possible to survive longer. A lander was to be flown with BepiColombo, but proving to be too expensive was cancelled. Rovers will also be possible for exploring interesting areas such as polar ice, the hollows, or volcanic vents. Ultimately a sample return mission will be undertaken to unlock the mysteries of this strange world Mercury, a world of polar ice, a huge iron core and ancient volcanic plains eternally baking in the Sun.