Mercury mission to explore origin of Solar System

Is Mercury's core liquid or solid, and why -- on the smallest planet in our solar system -- is it so big? What can the planet closest to the Sun tell us about how our solar system came into being? An unmanned European-Japanese space mission, dubbed BepiColombo, blasted off early Saturday morning from French Guiana, to probe these and other mysteries. "BepiColombo is coming like a white knight with better and more precise data," said Alain Doressoundiram, an astronomer at the Paris Observatory. "To understand how Earth was formed, we need to understand how all rocky planets formed," including Venus and Mars, he told AFP. "Mercury stands apart and we don't know why." First, however, the suite of instruments on board the Ariane 5 rocket will have to travel seven years and nine million kilometres (5.6 million miles) to reach their destination. In a statement after the launch, ArianeGroup said the satellite had successfully escaped Earth's gravity field and was beginning its long journey where it will reach speeds of up to 40,000 kilometres (25,000 miles) an hour. According to Pierre Bousquet, an engineer at France's National Centre for Space Research and head of the French team contributing to the mission, Mercury is "abnormally small," leading to speculation that it survived a massive collision in its youth.

"A huge crater visible on its surface could be the scar left over from that encounter," Bousquet told AFP. Finding out if this is true is on BepiColombo's "to do" list.

- Going hot and cold -

This scenario would explain why Mercury's core accounts for a whopping 55 percent of its mass, compared to 30 percent for Earth.

Mercury is also the only rocky planet orbiting the Sun beside our own to have a magnetic field.

Magnetic fields are generated by a liquid core but given its size, Mercury's should have grown cold and solid by now, as did Mars.

This anomaly might be due to some feature of the core's composition, something BepiColombo's instruments will measure with much greater precision than has been possible so far.

On its surface, Mercury is a planet of extremes, vacillating between hot days of about 430 degrees Celsius (more than 800 degrees Fahrenheit) to super-frosty nights of minus 180C (minus 290F).

Those days and nights last nearly three Earth months each.

Earlier missions have detected evidence of ice in the deepest recesses of the planet's polar craters.

Scientists speculate that this may have accumulated from comets crashing onto Mercury's surface.

"If the presence of ice is confirmed, it means that some of those water samples date back nearly to the origin of the solar system," Doressoundiram said.

- Lashed by solar winds -

Mercury is 58 million kilometres (36 million miles) from the Sun, nearly three times closer than Earth.

"The planet is whipped by solar winds," a constant torrent of ionised particles bombarding the surface at 500 kilometres per second, said Bousquet.

The scientists will be able to study the impact of these winds -- 10 times stronger than the ones hitting Earth's atmosphere -- on Mercury's magnetic field.

The BepiColombo mission will deploy two spacecraft. The Mercury Planet Orbiter, built by ESA, will investigate planet's surface and interior composition.

The Mercury Magnetospheric Orbiter, made by the Japan Aerospace Exploration Agency, will study the region of space around the planet that is influenced by its magnetic field.

The mission will also look for tectonic activity, and seek to understand why spectroscopic observations show no iron even if it is thought to be one of the planet's major component elements.

Compared to Mars, Venus, and Saturn, Mercury has barely been explored. Only two spacecraft have ever paid it a visit.

NASA's Mariner 10 did three flybys in 1974 and 1975, providing the first up-close images. More than 30 years later, NASA's Messenger did the same, before settling into orbit around Mercury in 2011.

The new mission is named after Giuseppe (Bepi) Colombo, a brilliant Italian mathematician and engineer who first understood the relationship between Mercury's rotation and orbit.

BepiColombo: Two Orbiters Head to Mercury

Known since Antiquity, Mercury has not yet delivered all its secrets. The international mission BepiColombo, scheduled to launch in the coming days, will study the planet's surface and compare its magnetic field with that of the Earth. Apart from Earth, Mercury is the only terrestrial planet with its own magnetic field, and yet it has only been visited by two space missions so far. This is indeed no easy task: because it is so close to the Sun, a spacecraft that misses the Swift Planet's weak gravitational field will inevitably plunge towards the solar surface, heated to a fiery 5,500 C. The European and Japanese space agencies, ESA and JAXA, have therefore worked in close collaboration to ensure BepiColombo's success. The mission, which comprises two orbiters, is scheduled to launch from Kourou, French Guiana, on the night of 19-20 October aboard an Ariane 5 rocket. After a seven-year journey and two flybys of Venus to benefit from a gravity assist, it will then survey Mercury's surface, atmosphere, and magnetosphere for two years, until 2027. In the 1970s, during a mission mainly focused on Venus, the American spacecraft Mariner 10 carried out three flybys of Mercury. One of the researchers involved was a professor at the University of Padua, Italy, called Giuseppe "Bepi" Colombo. The new spacecraft, the very first collaboration between ESA and JAXA, was named after him.

During the brief flybys, Mariner 10 was able to map half of Mercury and detect its magnetic field. Although it is much weaker than Earth's, it shows that the core of the planet is still active. Mariner 10 also confirmed the presence of an exosphere, an extremely tenuous atmosphere extending to very high altitudes.

Many years later, NASA launched the MESSENGER spacecraft. Placed in orbit around Mercury in March 2011, it crashed onto its surface in April 2015 when it ran out of fuel. It confirmed Mariner 10's observations and carried out further mapping and surveys of the surface. In particular, MESSENGER discovered evidence not only of volcanic activity and plate tectonics, but also of water ice: due to Mercury's extremely small axial tilt, no direct sunlight ever reaches the bottom of impact craters at the poles.

"Although MESSENGER carried a magnetometer and equipment to measure ions and energetic particles, the spacecraft's main mission was to survey the planet, its thin atmosphere and its surface," explains Dominique Delcourt, CNRS senior researcher and director of the LPC2E, in charge of the ion mass spectrometer on board BepiColombo's Japanese-designed orbiter, MMO. "In the presence of an intrinsic magnetic field, a magnetic cavity forms in space. This is called the magnetosphere, where many particle transport and acceleration processes take place."

One Mission, Two Orbiters

The BepiColombo mission comprises two orbiters carrying a science payload of nearly 100 kilograms. The first one, MPO (Bepi), will be dedicated to fully mapping the planet and studying its surface, internal structure, and exosphere, while the second, MMO (renamed Mio), will study its magnetic environment. Once at their destination, Mio will be released first, followed by Bepi, which will be placed into the lowest orbit ever achieved around Mercury.

Delcourt is upbeat: "This wider array of instruments will enable us to not only make new discoveries, but also review MESSENGER's data. By combining observations from both orbiters, we will also be in a position to perform what you might call stereoscopic measurements, something that was previously impossible."

The MMO orbiter will complete one rotation in just four seconds, enabling its instruments to point in all directions in space in search of neutral or ionized particles and electromagnetic waves. With a higher resolution than the MESSENGER instrument, the MSA ion spectrometer, developed at the LPP in collaboration with Japanese and German teams, can distinguish between heavy atoms only one atomic mass unit apart, such as potassium and calcium.

"These measurements will enable us to characterize ejected planetary material," says Delcourt. "As a result of meteorite bombardment and the solar wind, matter is ejected from the surface of Mercury. It can then be ionized by the Sun's ultraviolet radiation, and transported and accelerated around the planet." By studying these ions, it will be possible to analyze the composition of the surface without having to land on it.

A Model Magnetic Field

The magnetic field of Mercury is also an interesting generic model. Observing a magnetosphere that is smaller than ours should improve our understanding of the behavior of both neutral and ionized matter in space.

At such a short distance from the Sun, the density of the solar wind means that it has a greater impact on the planet. Another interesting factor is that Mercury's highly elliptical orbit causes significant cyclical variations in this exposure. As a result, BepiColombo's various scientific instruments are likely to be kept extremely busy. Six of them were designed with the participation of eight CNRS laboratories, including the LPC2E, the IAS, IPGP, the Research Institute in Astrophysics and Planetology (IRAP), the LAM], LATMOS, LESIA, and LPP.

At the LATMOS, Eric Quemerais is the lead scientist for PHEBUS. This ultraviolet spectrometer scans frequencies ranging from 50 to 320 nanometers, as well as a few lines used to detect calcium and potassium.

Analyzing Mercury's Surface

"We cover a wider spectral range that includes elements that were invisible to MESSENGER, such as helium, sulfur, ionized calcium, dihydrogen, etc.," Quemerais explains. "Thanks to a better signal-to-noise ratio, we also have an improved detection limit."

And whereas the American spectrometer was aligned with its probe, PHEBUS has an independent pointing mechanism. This allows it to choose its direction and provides better spatial and temporal coverage in orbit. "The exosphere gives an idea of the composition of Mercury's surface and of its outermost layers," Quemerais adds.

"For instance, we know that we will detect calcium and sodium, but we also expect to find magnesium, potassium, and oxygen, whose presence has not yet been systematically confirmed."

Another advantage of ultraviolet light is that it reflects off ice in a different way, which means that PHEBUS will be able to spot any water ice present. "This technique has already been employed on the Moon," says Quemerais. "We will use these changes in the amount of reflected light to map Mercury's two poles." Because of its specific orbit, selected so that it could survey the North Pole, MESSENGER was only able to map half the planet.

BepiColombo thus promises to provide the scientific community with a wealth of new data. In June 2020 in Orleans (central France), Delcourt will be organizing the next important conference dedicated to the Swift Planet.

"Of course, BepiColombo won't have reached its destination by then, but we will nonetheless be able to make use of MESSENGER's data," he explains. No doubt the scientists at that time will have their sights set on Venus, which Bepi Colombo will be about to swing past, propelled on its way to Mercury, its final destination.

Strofio will measure Mercury's exosphere on BepiColombo mission

The European Space Agency's BepiColombo spacecraft will launch towards Mercury carrying a unique payload designed and built at Southwest Research Institute: an instrument called Strofio, which will study Mercury's tenuous exosphere. Part of the SERENA suite of instruments, Strofio's measurements will help us better understand the planet's surface and the history of the smallest rocky planet orbiting close to the Sun. "An exosphere is different from an atmosphere," said Dr. Stefano Livi, an Institute Scientist who leads the Strofio experiment, funded by NASA's Discovery Mission of Opportunity program. "Mercury doesn't have enough gravity to hold onto a proper atmosphere. Instead, it is surrounded by a thin, collision-free particle environment. Particles escape from Mercury's surface and briefly populate this exosphere before they return to the surface or drift away into interplanetary space." Mercury's proximity to the Sun makes it difficult to observe from Earth. It is also challenging for spacecraft to reach and to survive in the harsh environment. The BepiColombo mission includes two spacecraft - ESA's Mercury Planetary Orbiter and the Japan Aerospace Exploration Agency's (JAXA) Mercury Magnetospheric Orbiter (MMO) - that will study Mercury and help us learn about the formation of our solar system. From aboard MMO, Strofio will study how Mercury's exosphere and magnetosphere interact with each other and the planet's surface.

To understand these interactions, Strofio must identify the particles escaping from Mercury's surface. Because the exosphere is so thin, sampling particles is particularly challenging - in fact, the amount of particles is so sparse that the environment can't be easily mimicked in a typical vacuum chamber.

Scientists had to tackle this problem while developing and evaluating the super-sensitive Strofio instrument. Facilities at the University of Bern in Switzerland could create the necessary parameters and over a course of a few months allowed SwRI to analyze and demonstrate how the experiment will work in the Mercury environment.

"Strofio is novel in its ability to detect the rare, static particle population in Mercury's exosphere," Livi said. "We had to rethink and retool typical spectrometer designs."

Every particle captured is analyzed in a rotating field. When and where each particle gets to the detector determines the mass and composition. Strofio uses detection algorithm tools to enhance the instrument's sensitivity and improve identification. The instrument also had to meet strict mass and configuration limits associated with the spacecraft. The basketball-sized instrument weighs just over 7 pounds.

"Strofio is like one of my children off on its way to Mercury," Livi said. "It's exciting. I'm looking forward to 2025 when we'll 'see' the chemical particles coming off the planet. In addition to answering many questions, I'm also expecting some surprises."

BepiColombo is the first time ESA and JAXA have joined forces on a major space science mission. NASA frequently collaborates on ESA missions, including the Rosetta mission to Comet 67P and the JUpiter ICy moons Explorer (JUICE) mission, set to launch in 2022.

New estimates of Mercury's thin, dense crust

Mercury is small, fast and close to the sun, making the rocky world challenging to visit. Only one probe has ever orbited the planet and collected enough data to tell scientists about the chemistry and landscape of Mercury's surface. Learning about what is beneath the surface, however, requires careful estimation. After the probe's mission ended in 2015, planetary scientists estimated Mercury's crust was roughly 22 miles thick. One University of Arizona scientist disagrees.Using the most recent mathematical formulas, Lunar and Planetary Laboratory associate staff scientist Michael Sori estimates that the Mercurial crust is just 16 miles thick and is denser than aluminum. His study, "A Thin, Dense Crust for Mercury," will be published May 1 in Earth and Planetary Science Letters and is currently available online. Sori determined the density of Mercury's crust using data collected by the Mercury Surface, Space Environment and Geochemistry Ranging (MESSENGER) spacecraft. He created his estimate using a formula developed by Isamu Matsuyama, a professor in the Lunar and Planetary Laboratory, and University of California Berkeley scientist Douglas Hemingway. Sori's estimate supports the theory that Mercury's crust formed largely through volcanic activity. Understanding how the crust was formed may allow scientists to understand the formation of the entire oddly structured planet.

"Of the terrestrial planets, Mercury has the biggest core relative to its size," Sori said.

Mercury's core is believed to occupy 60 percent of the planet's entire volume. For comparison, Earth's core takes up roughly 15 percent of its volume. Why is Mercury's core so large?

"Maybe it formed closer to a normal planet and maybe a lot of the crust and mantle got stripped away by giant impacts," Sori said. "Another idea is that maybe, when you're forming so close to the sun, the solar winds blow away a lot of the rock and you get a large core size very early on. There's not an answer that everyone agrees to yet."

Sori's work may help point scientists in the right direction. Already, it has solved a problem regarding the rocks in Mercury's crust.

Mercury's Mysterious Rocks

When the planets and Earth's moon formed, their crusts were born from their mantles, the layer between a planet's core and crust that oozes and flows over the course of millions of years. The volume of a planet's crust represents the percentage of mantle that was turned into rocks.

Before Sori's study, estimates of the thickness of Mercury's crust led scientists to believe 11 percent of the planet's original mantle had been turned into rocks in the crust. For the Earth's moon - the celestial body closest in size to Mercury - the number is lower, near 7 percent.

"The two bodies formed their crusts in very different ways, so it wasn't necessarily alarming that they didn't have the exact same percentage of rocks in their crust," Sori said.

The moon's crust formed when less dense minerals floated to the surface of an ocean of liquid rock that became the body's mantle. At the top of the magma ocean, the moon's buoyant minerals cooled and hardened into a "flotation crust." Eons of volcanic eruptions coated Mercury's surface and created its "magmatic crust."

Explaining why Mercury created more rocks than the moon did was a scientific mystery no one had solved. Now, the case can be closed, as Sori's study places the percentage of rocks in Mercury's crust at 7 percent. Mercury is no better than the moon at making rocks.

Sori solved the mystery by estimating the crust's depth and density, which meant he had to find out what kind of isostasy supported Mercury's crust.

Determining Density and Depth

The most natural shape for a planetary body to take is a smooth sphere, where all points on the surface are an equal distance from the planet's core. Isostasy describes how mountains, valleys and hills are supported and kept from flattening into smooth plains.

There are two main types isostasy: Pratt and Airy. Both focus on balancing the masses of equally sized slices of the planet. If the mass in one slice is much greater than the mass in a slice next to it, the planet's mantle will ooze, shifting the crust on top of it until the masses of every slice are equal.

Pratt isostasy states that a planet's crust varies in density. A slice of the planet that contains a mountain has the same mass as a slice that contains flat land, because the crust that makes the mountain is less dense than the crust that makes flat land. In all points of the planet, the bottom of the crust floats evenly on the mantle.

Until Sori completed his study, no scientist had explained why Pratt isostasy would or wouldn't support Mercury's landscape. To test it, Sori needed to relate the planet's density to its topography. Scientists had already constructed a topographic map of Mercury using data from MESSENGER, but a map of density didn't exist. So Sori made his own using MESSENGER's data about the elements found on Mercury's surface.

"We know what minerals usually form rocks, and we know what elements each of these minerals contain. We can intelligently divide all the chemical abundances into a list of minerals," Sori said of the process he used to determine the location and abundance of minerals on the surface. "We know the densities of each of these minerals. We add them all up, and we get a map of density."

Sori then compared his density map with the topographic map. If Pratt isostasy could explain Mercury's landscape, Sori expected to find high-density minerals in craters and low-density minerals in mountains; however, he found no such relationship. On Mercury, minerals of high and low density are found in mountains and craters alike.

With Pratt isostasy disproven, Sori considered Airy isostasy, which has been used to make estimates of Mercury's crustal thickness. Airy isostasy states that the depth of a planet's crust varies depending on the topography.

"If you see a mountain on the surface, it can be supported by a root beneath it," Sori said, likening it to an iceberg floating on water.

The tip of an iceberg is supported by a mass of ice that protrudes deep underwater. The iceberg contains the same mass as the water it displaces. Similarly, a mountain and its root will contain the same mass as the mantle material being displaced. In craters, the crust is thin, and the mantle is closer to the surface. A wedge of the planet containing a mountain would have the same mass as a wedge containing a crater.

"These arguments work in two dimensions, but when you account for spherical geometry, the formula doesn't exactly work out," Sori said.

The formula recently developed by Matsuyama and Hemingway, though, does work for spherical bodies like planets. Instead of balancing the masses of the crust and mantle, the formula balances the pressure the crust exerts on the mantle, providing a more accurate estimate of crustal thickness.

Sori used his estimates of the crust's density and Hemingway and Matsuyama's formula to find the crust's thickness. Sori is confident his estimate of Mercury's crustal thickness in its northern hemisphere will not be disproven, even if new data about Mercury is collected. He does not share this confidence about Mercury's crustal density.

MESSENGER collected much more data on the northern hemisphere than the southern, and Sori predicts the average density of the planet's surface will change when density data is collected over the entire planet. He already sees the need for a follow-up study in the future.

The next mission to Mercury will arrive at the planet in 2025. In the meantime, scientists will continue to use MESSENGER data and mathematical formulas to learn everything they can about the first rock from the sun.