But now it has been proven that Mercury is a world of contradictions and a dynamic planet that hides unexpected surprises. A thin atmosphere, a magnetic field, and a reservoir of volatile compounds still exist on this planet, and scientists often associate these features with planets that are larger and farther from the Sun.

“We expected Mercury to be a hot, baked object,” says Deborah Domingo, a senior scientist at the Institute for Planetary Sciences. He says the observations of the last few decades have changed the scientists’ belief about this planet. Evidence shows that Mercury is not just a dry mass of rock. One of the wonders of this planet is that it still has bubbles of ice.

So far, only two missions have reached Mercury. The third mission is Bepi Colombo (a joint mission of the European Space Agency and Japan Aerospace Exploration Agency), which is on its way and will reach its destination in late 2025.

While the few ground-based observations and space missions have not yielded much knowledge, they have helped clear up many early misunderstandings about Mercury’s mysteries. In the following, we mention some of the most amazing discoveries that have been made about Mercury.

Mercury may be small, but it is heavy. Although the diameter of Mercury is not much larger than the diameter of the Moon, its mass is more than four times that of the Moon. After Earth, Mercury is the densest planet in the solar system. The planet’s high density comes from the fact that it has a large iron core that makes up about 60% of the planet’s mass. In contrast, the Earth’s core contains only about 15% of the planet by volume.

The first mission to Mercury, Mariner 10, was carried out in 1973 and showed that the planet has a magnetic field. This discovery came as a surprise to the scientific community, who had assumed that such a small planet would quickly cool and harden and lack a magnetic field. The presence of the magnetosphere indicates that part of Mercury’s core is still churning.

Mercury’s magnetic field is almost 100 times weaker than Earth’s magnetic field. The weak magnetic activity means that the planet is at the end of its evolution to become a dead planet like Mars.

In the 2010s, the second Mercury mission showed that the planet’s magnetic field was unbalanced. The magnetic south pole is not located on the geographic south pole but is buried about a fifth of the way inside the planet.

Antonio Genova, an aerospace engineer who studies geodesy and geophysics at Rome’s Sapienza University, says the magnetic field offers insights into the planet’s interior and its history, showing how its internal rotation has slowed over billions of years.

Mercury has a thin atmosphere that cannot be considered a real atmosphere. Instead, scientists call this thin layer of gas the exosphere, where the gas is so thin that nothing like atmospheric pressure can be measured.

Astronomers in the 1980s detected atomic sodium, potassium, and calcium in Mercury’s exosphere, metals with strong emission signals visible from Earth with telescopes. These metallic elements are not usually considered as gases, but they make their way into the planet’s sky as a result of the impact of solar particles and meteorites on the surface of the planet.

Solar winds penetrate the resulting exosphere, and the interaction between gases and particles ejected from the Sun creates a 24 million km long glowing trail behind Mercury. The trail seasonally shortens and lengthens depending on the proximity of Mercury to the Sun. If you stand on Mercury and look up at the right time of year, Mercury’s long trail will appear as an orange glow in the sky.

A planet so close to the Sun shouldn’t have water or ice, or so researchers thought. But in the 1990s, scientists at Goldstone in California and the Arecibo Radio Telescope in Puerto Rico directed a stream of radar signals toward Mercury. They were amazed to see two bright reflective spots at the poles, which were probably ice deposits.

In 2012, the MESSENGER spacecraft confirmed that the ice in the north pole of Mercury is frozen water. Surface laser measurements identified carbon-rich material on the surface that insulates the underlying ice.

Mercury has been able to retain its water because ice-containing bubbles lie beneath its permanent shadows. This planet rotates completely vertically in relation to its orbit around the sun; This means that impact craters near the poles have interiors that never see the light of day. The temperature inside these gaps is minus 170 degrees Celsius, which is close to the temperature at which nitrogen gas liquefies. “It’s cold enough there for the ice to be stable over geological timescales,” says Sean Solomon, a former planetary scientist at Columbia University and principal investigator of the MESSENGER mission.

Like many other rocky planets, the water on Mercury probably came from asteroids that landed on land. This water is hidden inside the craters of Mercury, which has not changed since the early times.

On other terrestrial planets in the solar system, geological processes such as climate circulation have spread the ejected water across the planet. Mercury’s poles are probably the best source if scientists want to sample intact ancient ice in the solar system, Solomon says.

Mercury again challenged scientists’ expectations when the MESSENGER spacecraft detected volatiles in the burning world of Mercury. Volatile substances are chemicals that can change state between solid and gas phases in a short temperature change.

Mercury has already been proven to contain water, but the MESSENGER mission identified other elements such as sulfur, potassium, and chlorine that evaporate easily at relatively high temperatures. These volatile substances are spread all over the surface of the planet.

Due to its size, Mercury has higher amounts of volatiles than other Earth-like planets in the solar system, which are farther from the Sun and therefore much colder. Where the volatiles come from and how Mercury has preserved them is still a matter of debate among scientists.

Some researchers think the volatiles came from beneath the surface in recent history, while others think chemicals from Mercury’s embryonic days remained on its surface.

The presence of volatiles on Mercury raises questions. For example, if the planets that are close to their stars have volatile materials, especially water, could these regions be habitable? According to Domingo, Mercury shows that planets close to the Sun should not be ignored.

Mercury has irregular depressions on its surface. Mariner 10 first revealed them in 1975. Messenger then recorded high-resolution images of these areas. The depressions range from a few meters to more than 1.6 kilometers in width, and their depth reaches 36 meters.

Scientists believe that the holes may have been created by the escape of volatile substances. Since an atmosphereless Mercury has no wind or rain to batter the Earth, surface features such as craters can form as a result of other processes, such as the leakage of volatiles from land into space.

Craters are relatively young formations, averaging about 100,000 years old compared to the four-billion-year-old impact craters on Mercury. Scientists think the holes are still forming. These holes have only been seen on Mercury. It seems that other objects in the solar system do not have such effects.

In recent years, scientists have also identified other structures on Mercury: irregular ridges that cover a large portion of its surface. Some researchers believe that these uneven terrains are caused by the turbulent flow of fugitives from the depths of the planet. Other scientists think the bumps were caused by the impact of an asteroid.

Mercury’s topography provides clues that volcanoes once spewed lava onto the planet’s surface. The MESSENGER spacecraft clearly showed the bright plains scattered across the surface of Mercury. Lava accumulated on older craters and ridges flattened to form plains. Researchers think that an active volcano on Mercury became dormant between 1 billion and 3.5 billion years ago as the planet cooled and contracted, blocking magma escape routes.

Mercury also shows signs of explosive volcanic activity. Irregular pits several kilometers long and more than three kilometers deep point to ancient pyroclastic volcanoes that have destroyed themselves. Around the pits, there are sediments that, according to researchers, were released as a result of volcanic explosions. These types of volcanic explosions are probably caused by volatile substances underground. When these buried chemicals come to the surface, their volume increases. Finally, the increase in gas pressure causes the volcano to explode.

BPI Colombo scientists hope to learn more about where the volatiles on Mercury come from. Mapping volatiles on the planet’s surface provides clues about how they got there. The origin of volatile substances is one of the main topics of space exploration.

Indeed, gaseous moons exist! Although they are not in the solar system. Although more than 5,500 extrasolar planets have been discovered so far, only two possible extrasolar moons have been identified and the existence of none of them has been definitively confirmed yet. The strange thing about these two exosolar moons is that they are gas giants orbiting larger gas giants. Of course, as we shall see, they are the exceptions that prove the rule.

To understand why gas moons do not exist, at least in the solar system, it is better to learn how gas giant planets form.

There are two scenarios for the formation of a gas giant planet: the bottom-up scenario and the top-down scenario.

The bottom-up, or “core accretion” scenario, explains how the gas giant planets of the Solar System formed.

If we could go back 4.5 billion years, we would see a young Sun surrounded by a disk of gas and dust. All planets are formed from this protoplanetary disk. They first formed as rocky bodies and grew larger by collecting dust, pebbles, and surrounding asteroids. Some of them only grew to the size of Mars or Venus, but others continued to grow and became giant rocky bodies with about 10 times the mass of Earth.

When planets grow to such large sizes, their gravity is strong enough to pull huge amounts of gas from the protoplanetary disk. How much gas they stole and how big they grew depended on their gravity and the amount of gas available. But in the end, our solar system was left with four gas-giant planets, Jupiter and Saturn, and the ice giants Uranus and Neptune.

NASA’s Juno mission to Jupiter has detected gravity from a large, rocky, yet diffuse core about ten times the mass of Earth at the center of Jupiter, helping to find evidence for the core accretion model.

In the bottom-up model, gaseous worlds, just like stars, form directly from the disintegrating mass of gas in the nebula. However, there is a minimum amount of mass that can produce this process.

When a large mass of gas contracts under its own gravity, it heats up because the gas is compressed into a smaller and therefore denser volume. But when the gas is hot, it tends to expand, so to maintain the contraction, the mass of gas must remove excess heat. As a result, we often see the collapse of gas clouds that glow as thermal infrared energy.

The radiation of enough heat so that the gas can cool and still decay depends on the dust’s opacity, temperature, and density, and the process becomes more inefficient in smaller objects; So, at a mass about three times that of Jupiter, it cannot generate enough heat to continue disintegrating. The smaller the volume, the cloudier and denser the dust becomes, and the process of radiating the excess heat due to gravitational contraction becomes increasingly inefficient. Therefore, an object smaller than three times the mass of Jupiter cannot form during the top-down process.

Most of the moons of our solar system, like their parent planets, were formed by the process of core accretion from the bottom up in disks of residual material that surrounded their parent planets. Since the planets had already collected most of the available material, there was not enough material left to form a moon with enough mass to have enough gravity to hold a large amount of gas. In fact, only one moon in the solar system even has an atmosphere, and that is Saturn’s largest moon, Titan. Similarly, the top-down process could not occur because there was not enough gas left.

According to the explanations given, in the solar system, gaseous moons cannot be formed through the two conventional processes of producing gaseous universes. However, there are wonders in our cosmic neighborhood that are formed in a different way.

In the case of Earth, the Moon is likely formed from material blasted from Earth following a massive collision with a Mars-sized protoplanet. These remnants formed a ring that created the moon’s core through accretion. But could the impact of a gas giant planet eject enough gas to form a gas moon?

Unfortunately no. Rocky planets can experience such collisions, but remember when Comet Shoemaker-Levy 9 hit Jupiter in 1994 and disappeared, Jesse Christiansen of the California Institute of Technology told Space.com. Gas giants devour everything. Anything that hits a gas giant just becomes part of the gas giant instead of throwing debris into space.

Another strange case is the trapped moons. For example, the moons of Mars, Phobos, and Deimos, are trapped by the red planet’s gravity. Saturn’s outermost moon Phoebe is a captured comet mass, and Neptune’s moon Triton is a Kuiper Belt mass that was trapped by Neptune’s gravity millions of years ago. They did not form around the planet, but formed on their own in space and then drifted until they were finally trapped by a planet’s gravity.

Now, the question arises, can a smaller gas planet be captured by a larger gas planet? After all, gaseous worlds can reach up to 12 times the size of Jupiter, so in principle, they could trap a gaseous world the size of Neptune.

It seems possible for smaller gaseous bodies to be captured by larger gaseous planets. “It’s possible that there are (gas) moons around the size of Neptune around giant exoplanets,” Christiansen said.

The two possible exomoons mentioned at the beginning of the article (Kepler 1625b-i and 1708b-i) are both gas giants in their own right but appear to be originally moons of larger gas giants. “Both of these are candidates,” Christiansen says. “We see something in the data that is consistent with the moon, but other phenomena could also explain it.”

Assuming that Kepler 1625b-i is a real moon, it has a mass 19 times that of Earth (about 6% of the mass of Jupiter), is similar in mass to Neptune, and accompanies a gas planet with a mass 30 times the mass of Earth and a diameter equal to half that of Jupiter. Kepler 1708b-i is probably less massive than Kepler 1625b-i, has a diameter about five times that of Earth (about half the diameter of Kepler 1625b-i), and orbits a gas planet 4.6 times the size of Jupiter.

“They challenge a lot of theories,” says Christiansen. “It’s hard to find a way for moons to form, so they must be trapped.” Being trapped makes them virtually like captured moons in the solar system. Like planets, they form from accretion cores in the disk and are then captured as they migrate towards their star.

Migration appears to be a common process in young planetary systems. Migration is the astronomers’ explanation for objects known as “hot Jupiters,” which are gas giants very close to their star but may have originally formed further away.

The extrasolar moons Kepler 1625b-i and Kepler 1708b-i were captured by larger planets as they migrated in front of them. However, they are probably not true moons, but rather examples of binary planets rather than extrasolar moons.

A binary planet exists when both worlds orbit the center of mass between them, rather than one orbiting the other. In our solar system, we have a double planet in the form of Pluto and its largest companion, Charon.

So, gas moons exist somehow, but nature has to cheat to make them!