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To the human eye, Mercury may resemble an opaque gray orb, but this enhanced color image from NASA’s Messenger probe tells a completely different story. Stripes of iridescent blue, sand-colored plains, and delicate strands of greyish white create an ethereal and colorful view of the innermost planet in our Solar System. Credit: NASA / JHU Applied Physics Laboratory / Carnegie Inst. Washington
Mercury is a desert world that scientists until recently considered uninteresting. POTHowever, the Mariner and MESSENGER missions revealed that there is much more to the smallest and most intimate planet in the Solar System than meets the eye. Although temperatures on its surface rise to 450 ° C, there appears to be water ice on Mercury. The planet also appears to have an inner core too large for its size and a surprising chemical composition. Here are the top five mysteries of Mercury that the European-Japanese BepiColombo mission could solve.
1. Where did Mercury form?
Only slightly larger than the Moon, Mercury approaches the Sun in an elliptical orbit every 88 days. At its closest point, the planet reaches only a third of the Earth-Sun distance. Have you always been in this place? Scientists are not so sure.
Data from NASA’s MESSENGER spacecraft, which orbited Mercury between 2011 and 2015, revealed that there is too much of the volatile chemical element potassium, compared to the more stable radioactive thorium, in the material on Mercury’s surface.
“Potassium evaporates very quickly in a warm environment, while thorium survives even at very high temperatures,” says Johannes Benkhoff, ESA’s BepiColombo project scientist. “Therefore, planets that formed closer to the Sun generally have more thorium compared to potassium. The proportion of these elements was measured on Earth, Mars, the moon and Venus and it appears to be correlated with the temperature at which the bodies are believed to have formed. But on Mercury we see much more potassium than we would expect. “
In fact, the ratio of potassium to thorium on Mercury is comparable to that of Mars, which is much farther from the Sun. Johannes admits that no existing planetary formation model can adequately explain this deviation. Therefore, scientists began to study the possibility that Mercury has formed farther from the Sun, almost as far as Mars, and has been pushed closer to the star by a collision with another large body. A powerful impact could also explain why Mercury has such a large inner core and a relatively thin outer shelf.
There is too much potassium compared to thorium on Mercury’s surface than scientists would expect considering how close to the Sun the planet is. Credit: ESA
Mercury’s nucleus, approximately 4000 km in diameter, is within the planet’s diameter of less than 5000 km, which represents more than 55% of the planet’s volume. In comparison, Earth is about 12,700 km in diameter, but its core is only 1,200 km in diameter.
“One theory is that this major impact in the past, in addition to possibly pushing Mercury to where it is today, also removed most of the material from the crust and left behind the dense core with only a thin outer shell,” says Johannes.
Some even suggest that ancient Mercury may have been the mysterious body believed to have hit Earth about 4.5 billion years ago, a collision that some theories say created a large amount of debris that led to the formation of the Moon.
How much light can BepiColombo shed on the mystery of the formation of Mercury? Johannes says instruments such as the MERTIS radiometer and the thermal infrared spectrometer, the MIXS Imaging X-ray spectrometer, and the MGNS neutron gamma-ray spectrometer will provide a new level of understanding of the mineralogical and elemental composition of Mercury’s surface. Orbiting closer to the planet than its predecessor MESSENGER, ESA’s Mercury Planetary Orbiter (MPO), one of the two orbiters comprising the BepiColmbo mission, will image Mercury’s surface with higher resolution and also provide better coverage of the hemisphere. south of the planet compared to MESSENGER.
2. Is there really water in Mercury?
A mosaic of images taken by NASA’s MESSENGER mission, which orbited around Mercury between 2011 and 2015. The image shows what appear to be water ice deposits within the craters around Mercury’s North Pole. Credit: NASA
With temperatures on its surface reaching 450 ° C, one would not expect to find water on Mercury, let alone ice. Surprisingly, when MESSENGER examined some of the craters around the planet’s poles, he saw what looked like light reflected by a mass of water ice.
“We have strong indications that there could be water ice in these craters, but it has not been directly detected,” says Johannes. “With the instruments we have at MPO, we hope to be able to not only measure the water content directly and confirm if there really is water, but also try to find out how much there is.”
The notion of water ice on the burned planet is not so absurd, Johannes adds. Mercury rotates around an axis that is perpendicular to its orbital plane. Therefore, the planet is not tilted like Earth. As a result, the rays of the Sun, which seems three times larger than on Earth, never reach the interior of the polar craters, allowing them to remain constantly frozen.
Johannes hopes that with the ability of MPO’s instruments to identify the precise elemental composition of Mercury’s surface, scientists may even have an idea where this ice actually comes from. Scientists think that the ice probably does not come directly from Mercury. Its origin, however, is another mystery. Comets are the most likely source of water on Earth, but many are not believed to have hit Mercury in the past.
“Comets in this region are quite rare and generally end up in the Sun due to their strong gravity,” says Johannes. “Ice can come from asteroids that have collided with Mercury throughout its evolution. Thanks to the low temperatures in the shaded craters, the ice may have survived there for tens of millions of years. “
Although BepiColombo will not provide a definitive answer, its exhaustive measurements of the polar areas may provide some clues as to the origin of Mercury ice.
3. Is Mercury alive or dead?
Small dents, or holes, in the Kertész crater of Mercury. These previously unknown geological features were discovered by NASA’s MESSENGER mission, and their origin remains a mystery. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington
Unlikely to harbor life, with a dry and apparently dead surface, Mercury has always been a loser in exploration of the Solar System. However, when the MESSENGER spacecraft finally looked closely at the planet’s surface, it discovered that there could be more to Mercury than might be expected.
The mission found strange geological features, unknown from other planets, that dot the areas in and around some of Mercury’s craters. These dents on the surface, or holes, as scientists call them, appear to be caused by evaporation of the material inside Mercury.
“The interesting thing is that these gaps appear to be fairly recent,” says Johannes. “There appears to be a volatile material coming out of Mercury’s outer shell and sublimating into the surrounding space, leaving behind these strange characteristics.”
Since BepiColombo will begin its study of Mercury ten years after the end of the MESSENGER mission, scientists hope they can find evidence of changes in the gaps, either growing or shrinking. That would mean that Mercury is still an active and living planet, and not a dead world like the Moon.
“If we show that these gaps are changing, that would be one of the most fantastic results we could get with BepiColombo,” says Johannes. “The process that drives the creation of these gaps is totally unknown. It can be caused by heat or by solar particles that bombard the planet’s surface. It’s a whole new thing and everyone is hoping for more data.
4. Why is Mercury so dark?
With its dusty, crater-covered surface, Mercury might look quite similar to Earth’s natural satellite, the Moon. At least at first glance. On closer inspection, and for reasons scientists still don’t understand, Mercury appears much darker. The planet reflects only about two thirds of the light that the material collected from the Moon.
The MERTIS thermal infrared spectrometer aboard the MPO will create a detailed map of the distribution of minerals on Mercury’s surface. By providing better accuracy and the resolution of the elemental composition compared to the MESSENGER, MERTIS, and other MPO instruments data will help answer the question of why Mercury is so dark.
“There are several explanations for why Mercury is as dark as it is,” says Johannes. “The material on its surface may be similar to what we can see on other planets, but the extreme heat on Mercury makes those materials appear darker.” There is also the possibility that what we see on the surface is graphite, which is also very dark. A graphite rich layer could have formed inside the planet while it cooled. Some of this material may have been brought to the surface during further evolution. “
5. How does Mercury have a magnetic field?
Scientists believe that Mercury’s oversized core must be partially molten to explain the planet’s magnetic field. Credit: ESA / Mercury surface: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington
Not many planets have a magnetic field. Among the rocky planets of the inner Solar System, only Mercury and Earth have one. Mars used to have a magnetic field in the past and lost it. Mercury seems too small to have one. However, it still does, even though it is one hundred times weaker than Earth’s magnetic field. Scientists wonder what holds this magnetic field despite the odds against it.
Earth’s magnetic field is generated by the rapid spin of its liquid iron core. As for Mercury, scientists used to think that the nucleus, due to the small size of the planet, must have cooled and solidified since the planet’s formation. Is it really the case?
“The core of Mercury must be partially molten to explain this magnetism,” says Johannes. “We can also measure the tides on Mercury’s surface, suggesting that there must be liquid inside the planet. As Mercury orbits the Sun and interacts with its gravity, we expect a lump to form and change its size as it moves around the Sun. “
At its largest, this bulge, by some estimates, can be up to 14 meters high. By following Mercury along its journey around the Sun, which takes the planet from about 46 million km to about 70 million km from the Sun, BepiColombo will be able to make precise measurements of changes in the lump. The data will help scientists better estimate the size of the inner liquid core.
Mercury’s magnetic field also appears to be offset 400 kilometers to the north and not centered in the middle of the planet like Earth’s.
The two orbiters comprising the BepiColombo mission, ESA’s MPO and the Japan Aerospace Exploration Agency’s Magnetospheric Mercury (Mio) Orbiter (JAXA), will study Mercury’s magnetic field in more detail than any spacecraft before and shed light on these puzzling questions. The two orbiters will travel through different areas of Mercury’s magnetosphere and on different time scales. They will simultaneously measure how the magnetic field changes over time and in space, and will try to explain how the proximity of the Sun and interaction with the powerful solar wind affect the magnetic field.
Understanding Mercury’s magnetic field in greater detail will also help astronomers better understand what is going on inside the mysterious planet.
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