MERCURY: The atmosphere on the smallest planet in our solar system

Mercury is a planet without much of atmosphere and its surface full of craters tells us about the great cosmic bombardment a.k.a. the Late Heavy Bombardment which is a hypothesized event thought to have occured approximatelly 3.9 billion years ago. It’s not easy to find information about the early life of this globe, because Mercury is simply the most “shy” planet. It is hard to inspect closely even with the modern technology available today. Back in the days when astronomers didn’t have anything else available but only the telescopes instaled on Earth, it was really extremelly dificult to discover something about Mercury, because of it’s position – it is the closest planet to the sun and in order to see it from Earth, we must always look in the direction of Sun. Hiding shyly in the “skirts” of the Sun, Mercury is just difficult to be seen. Being so close to the solar light, it only appears in sight from time to time for short periods.

Since it is the planet closest to the Sun, being strongly influenced by its gravity, Mercury is very fast, holding the record of being the fastest planet in orbit. It rotates around the sun in only 88 days while the Earth needs 365 days to do the same. From our position in the solar system, namely from Earth, we can see the planet Mercury appearing alternately on one side and the other of the sun, while moving in orbit. For about a month the planet is seen as an evening light observable close to the horizon in the faint light of the dusk. It disappears for a month in the blinding light of the sun and then is seen as a morning lightbulb before dawn for another month. After this, it disappears again for a month behind the Sun, before returning to the starting point. The completion of this cycle takes 116 days. The visibility cycle of Mercury from Earth of course depends on both the orbit of Mercury and that of the Earth – this is why the visibility period of Mercury is different from its orbital period.

Mercury, like all its other 3 the rocky siblings (namely Venus, Earth and Mars), was formed of molten rock. A few million years later, as the young planet began to cool, its crust solidified and its journey around the Sun transformed from being part of a swirling cloud into a clearly defined passage: an orbit. The path the infant Mercury travelled, however, was most probably far removed from the course it now holds. The young Mercury was born not as the closest planet to the Sun but at a much greater distance, far beyond the orbit of Venus, beyond Earth perhaps even beyond Mars. This was a planet that came into being in the mildest region of the Solar System. It was far enough away from the Sun to allow volatile elements like sulphur (S), Potassium (K) and Phosphorus (P) to be folded into its 1st rocks without being vaporized away by the heat of the Sun, but maybe near enough for its surface to be warmed, perhaps even just the right amount for liquid water to settle on its surface. This may well have been a planet big enough to hold an atmosphere, a watery world upon which all the ingredients of life could well have existed. Mercury, it seems, really did have its own moment in the sun, but these hopeful beginnings were not last. One possible theory is that Mercury didn’t form where it is today, but much closer to the other planets, maybe even outside Venus or Earth, or somewhere in between. Then because of interactions with Jupiter, Earth, Venus and so on, it got put into a chaotic path that pushed it farther into the Sun. But before talking about it’s surface (which I will do in another article), for now let’s take a look at its atmosphere.

The atmosphere of Mercury

Mercury has an atmosphere but it is very thin, it’s trillion times thiner than the Earth’s which is why it is called an exosphere. Some astronomers believe Mercury once had a thick atmosphere like Earth but because the planet is small its gravity could not stop the atmosphere being blown away by the solar wind. The gases that remain include hydrogen (H), oxygen (O), helium (He), water vapour, sodium (Na) and potassium (K).

Since Mercury only has something like 38% of Earth’s gravity it becomes challenging for the planet to hold on to its atmosphere and this is also because the planet’s mass is too small for an atmosphere to persist. This becomes even more challenging because solar winds from the “nearby” Sun constantly buffet and erode it. However, the very same solar winds, micrometeorites dust, and radioactive decay also partially replenish the gases escaping into space. Mercury’s atmosphere is a fragile one and has challenges retaining heat, culminating in the freezing temperatures on the side of the planet in shade.Mercury is very close to the Sun, so daytime temperatures are extremely high reaching 430°C. The escape velocity is less than half that of Earth,- it takes a velocity of only 15300km/h to escape from Mercury, while to escape from Earth you must have a velocity of 40300km/h – , hence hot, light elements in the atmosphere, such as helium (He), quickly fly off into space. All the atmospheric gases therefore need constant replenishment. Mercury’s atmosphere was analysed by an ultraviolet spectrometer onboard the Mariner 10 spacecraft in 1974. Oxygen (O), helium (He) and hydrogen (H) were detected in this way and subsequently atmospheric sodium (Na), potassium (K) and calcium (Ca) have been detected by Earth-based telescopes. The hydrogen and helium are captured from solar wind of gas that is constantly escaping from the Sun. The other elements originate from the planet’s surface and are intermittently kicked up into the tenous atmosphere by the impact of ions from Mercury’s magnetosphere and micrometeorite particles from the Solar System dust cloud. The atmospheric gases are much denser on the cold night-side of the planet than on the hot day-side as the molecules have less energy to escape.

Oxygen (O) is the most abundant gas on Mercury, followed by sodium (Na), hydrogen (H) and helium (He). However, loss and regeneration of gases is continuous and the atmospheric composition can vary drastically over time.

While it rotates so fast, Mercury has a tail. Mercury’s sodium and hydrogen tails MESSENGER’s ultraviolet and visible spectrometer, part of the M.A.S.C.S. instrument (Mercury Atmospheric and Surface Composition Spectrometer), mapped the tail of Mercury as the spacecraft approached for its first flyby of the planet. The plots below show the intensity of the emission of certain wavelengths of light, which is related to the abundance of neutral atoms in Mercury’s exosphere.

Mercury’s sodium tail = Pressure exerted by sunlight pushes sodium atoms away from Mercury, forming a “tail” of some 35.000km long. Mercury and the Sun are off to the left in this false-colored view of Mercury’s sodium tail. This plot indicates the intensity of light at 589 nanometers, the orange color being familiar from sodium lamps used widely in street lights. The intense emission of sodium atoms at this wavelength makes them very easy to detect. Emissions from this tail have previously been observed with Earth-based telescopes, but this image from a spectrometer on board MESSENGER is the most detailed image yet.

Likewise for hydrogen. The plot image shows Lyman-alpha emission at 121.6 nm associated with neutral hydrogen in the near vicinity of Mercury. This is the first detection of hydrogen tail emission at Mercury and the first time that neutral hydrogen and sodium atoms have been observed in the tail simultaneously.

However in case of hydrogen, this emission is about 100 times less intense than the sodium emission. As with the sodium emission, discovering the true spatial distribution requires more analysis. The similar asymmetries in hydrogen, derived from the solar wind, and the much heavier sodium nonetheless suggest that solar-wind interactions with Mercury’s magnetosphere have played a strong role in supplying tail material at the time of MESSENGER’s flyby.

Temporary atmosphere = Thin clouds of sodium suddenly appear over some regions of Mercury and then just as quickly disappear, as seen below in these false-color observations made by the Kitt Peak Solar Observatory, USA. The cloud might be produced by meteorite impacts – the freshly cratered surface releases sodium vapour when it is next heated by sunlight. Another possibility is that ionized particles actually hit Mercury’s surface and release sodium from the regolith.

With such atmosphere as weak as it is, Mercury defends itself from the gusts of the solar wind with the help of its magnetic field – just as the Earth’s magnetic field protects the surface and atmosphere of our planet. But Mercury’s magnetic field is not strong enough to deflect solar particles during the periods of “maximum sunspots”. The sun has lots of sunspots and uses its own magnetic field to attack the planet with furious, spinning arms like the blades of a windmill. In such moments, the solar wind is strong enough to overcome Mercury’s magnetic field and reach its surface.

Mercury is a feeble planet that lives in the proximity of the Sun, which treats it mercilessly. The heat of the sun pours unimpeded on the surface of the planet, so that its temperature is “everywhere” very high. Consequently, Mercury lost all the atmosphere it had initially, when it was formed. But in the meantime it managed to acquire a new atmosphere. Given that Mercury’s atmosphere is so rarefied, there is no blanket of air to even out the temperature on the surface of the planet. Thus, during the day the temperatures vary between -180°C and 430°C at the equator. During the night, the bare rocks quickly lose the accumulated heat, so the temperature can drop to even -200°C. Because the distance between Mercury and the Sun changes so much as the planet moves in its eccentric orbit, the amount of light and heat projected onto the planet can increase or decrease by a factor of more than 2, so that at any latitude, the temperature at a certain moment of the day also varies enormously.

Common metals, such as lead (Pb) and tin (Sn), would melt at Mercury’s equator, as would plastics do as well. Obviously this would be fatal for electrical equipment, which have components insulated with plastic and cables soldered with tin alloy; of course is not excluded to discover a solution to this problem, but so far no solution has been found. Even the space probes that reach the orbit of Mercury have difficulties in managing the solar heat, although they can reorient themselves in space in such a way as to position themselves behind the sunshields. Spaceships that can land (that is, landers) or vehicles that can move on the ground (rover-type vehicles) would not be able to operate on Mercury. Sometimes due to comets crashing into Mercury’s surface, its atmosphere contains a lot of steam. Comets have a large amount of water in their composition, which evaporates upon impact with Mercury. Thus, the vapors cover the entire planet for a short time. Some craters near the poles are so deep that their bottoms never see direct sunlight and therefore make an exception to the statement that all Mercury’s surface is hot during the day. At the bottom of these craters, the temperature never rises above -160°C. For this reason, part of the water from comets condenses and gives rise to patches of ice that can reach a thickness of several meters and that have an indefinite lifespan. The ice was first detected by the way it reflects radar waves, and the existence of these patches of ice was confirmed by the Messenger probe in 2008. Astronomers were surprised to discover that water can survive as ice on the planet closest to Sun, meaning the hottest of the planets. A completelly unexpected discovery.

GEOMAGNETIC STORMS – While much smaller than Earth, Mercury actually has its own geomagnetic storms like the Earth’s various auroras. A recent study found that the planet has a ring current consisting of charged particles that flows laterally around the planet and excludes the poles. This ring is then able to generate geomagnetic storms at the poles too. The processes are in fact quite similar to here on Earth. The main differences are the size of the planet and Mercury has a weak magnetic field and virtually no atmosphere. However, unlike on Earth, these auroras are only visible in the X-ray and gamma-ray spectrums, not as visible light.

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