The sense of smell is essential in the life of every human being. Different smells evoke different emotions and release memories. But one thing is certain; the sense of smell is crucial for our emotional and physical life. For example, babies firstly recognize their parents through their scent.
Our sense of smell also develops as we grow. We learn to identify smells; some of them are pleasant to us; others are not. We love to smell flowers, perfumes, and even rain. All of those things are located on the Earth, but what about the rest of the Universe? Does space has any scent at all? If so, to what can it be compared?
In fact, the entire Universe is mostly made up of elements that we can encounter on Earth. So it seems natural to say that things outside our planet have a smell. But to say that something has a smell is not enough, you are probably curious how the Universe smells. Thanks to our knowledge of elements, compounds, and their scents, we are able to determine the smell of the space. In this article, with the help of our space travels, we are going to look at the substances that make up the Universe and compare them to those located on the Earth.
Does the Milky Way smell like chocolate?
If we think about space, what comes to our minds first? Of course: the Milky Way, or even more specifically, the Solar System, location of our planet Earth. The Earth is full of scents; some are pleasant, and some are not. And what about the smell of the Universe? In theory, we can all smell it. If you thought that the scent of the Universe is as pleasant as the scent of chocolate, you would be disappointed.
Our Universe mostly consists of hydrogen or helium, which themselves do not have any specific smell. The bad news does not stop here. Most of Earth’s neighbors are smelly types, and if one day, space tourism becomes popular, they would not be five-star locations. Dear readers, we would like to welcome you on the board of our spacecraft, take your place, fasten your seatbelts, and enjoy the ride.
The first stop is the closest planet to the Sun, Mercury. Do not be fooled by its name; a planet itself is not made out of Mercury. It mostly consists of iron (about 70%), and unlike Earth, it does not have a thick, defined atmosphere [[1]]. Instead, it has an exosphere, rich in first group elements such as sodium, potassium, and hydrogen. It also contains oxygen and helium.
Despite such a wide range of elements, if you decide to visit Mercury, you will not smell any scent. Continuing our cosmic journey, let’s visit the second planet. Venus, which was the Roman goddess of love and beauty, smells like rotten eggs. Beautiful names can be deceptive. The main reason hiding behind this awful smell are the clouds made of sulfuric acid, which cause sporadic acid rainfall – lookup out there an ordinary umbrella may not be enough, as the acid is strong enough to even damage ruggedized titanium spacecraft parts.
The next stop is the planet that inspires many movies and books, Mars. It is often referred to as Red Planet. The protagonist of countless cinematographic images has the stinks also with rotten eggs. Once again, the compounds accused of committing the crime are the sulfur-based compounds.
Moreover, Marsian sand smells like metal, due to the presence of iron oxide, commonly known as rust. The Red Planet is a smelly place, and to add to it; there’s even more metallic smell in its atmosphere, around 40 km above the surface, in the ozone layer [[2]]. The gas present there has a sharp, metallic scent. Do not even think that traveling to the outer Solar System will help you avoid bad smells.
Rotten eggs beyond Mars?
This might sound odd, but a large chunk of our Solar System smells somewhat akin to rotten eggs. This smell comes from the hydrogen sulfide, a chemical also found in trace amounts deep in Jupiter’s atmosphere.
Most of Jupiter’s atmosphere is helium and hydrogen, concentrated in its upper section. These gases are odourless, so you would not smell anything. However, the deeper you dive towards the core of this enormous planet, the more of the heavier, more smelly gasses you encounter. For example, parts of Jupiter’s atmosphere contain ammonia, so the planet has a terrible odour of a neglected public toilet.
If your sense of smell holds, we continue to move on. Next planet, Saturn is an exception to the smell of planets in the solar system: it is for a large part odourless, as it consists mostly of hydrogen and helium, and trace amounts of methane.
Saturn’s core is covered with rocky materials with a metallic smell, so it is not fully odourless, but rather neutral. Titan, one of the moons of Saturn, is also odourless. It is shrouded in a thick, opaque atmosphere made up of smell-less nitrogen, and methane. And it is not just the atmosphere of Titan that contains plenty of those chemicals: seas and lakes on Titan are made up of methane and odourless ethane as well.
Our journey is a bit like a tour of Europe, the cities of Europe in seven days. So it is time for the next stop. Further away from the Sun, we encounter Uranus and again an unpleasant surprise, you only smell the smell everywhere like rotten eggs. Unfortunately, not very pleasant again. That is because the upper atmosphere of Uranus contains hydrogen sulfide [[3]] and couple other chemicals, which make it stink like smelly gases after belly revolution. Hopefully, the next planet, Neptune, is rather neutral when it contains, while it is made up of odourless hydrogen, helium, and methane [[4]].
Summarizing, the planets outside of our Solar System are mostly odourless. For example, planet HD 209458b (what an interesting name! Its nickname is also Osiris) is also known to contain hydrogen and methane in its atmosphere [[5]]. A point has to be made through: detection of chemicals such as sulphur in exoplanetary atmospheres is a very hard feat, so there might be smelly planets out there, but we might not have detected those yet.
Towards Pluto and Beyond
Feeling exhausted? But you are surely curious about the planets more distant from Earth. Objects far out in the Solar System receive very little sunlight, which means they are mostly frozen lumps of ice [[6]]. For instance, Pluto’s surface is mostly made out of frozen nitrogen in ice form [[7]].
So, what is the smell of the air on Pluto? Firstly, there is not much air to work with, forasmuch pressure on that planet is just 0.022 Pa, while on Earth, it is about 100.000 Pa [[8]]. It means that you probably would not smell anything due to very low atmospheric density. You would suffocate instead. But if you were to somehow concentrate the chemicals of Pluto’s atmosphere, then you would smell, surprise here, nothing, because methane, nitrogen, and carbon monoxide present Pluto are smell-less [[9]].
Figure 1. Image of Arrakoth obtained by the New Horizons spacecraft. Credits: NASA/JPL [10]
But what lies beyond Pluto? Meet Arrakoth, the only Kuiper-belt object that has been explored in relative detail. A photograph is shown in Figure 1.
By planetary standards, Arrakoth is a small asteroid (with a mean diameter of just 18 km [[11]]), on the edge of the Solar System. It is 46 times further away from the Sun than the Earth, which means that very little sunlight reaches this remote clump of ice and rock. That means you would not get to smell any atmosphere: all gasses that should be around the asteroid are frozen and in an icy form.
And the surface of Arrakoth is composed of methanol, hydrogen cyanide, water ice, and organic compounds [[12]]. Here is what you would smell if you were to de-freeze a sample of Arrakoth: methanol smells like alcohol, while hydrogen cyanide smells somewhat like almonds. That is a relatively pleasant smell for something so far away from Earth.
The smells of interstellar space
What if we extend our scent journey beyond the Solar System? Even there outside of our Solar System, the smells are not too pleasant. There is plenty of ammonia in the interstellar space, in objects such as Orion A or W43 [[13]]. It has to be noted its smell of ammonia is hardly pleasant. We do not even recommend trying to smell ammonia, as it can irritate your nose and your throat.
Most of the nebulae, that is, clouds of gas in the interstellar space, contain an ionized form of sulphur. If you thought we are done with our journey through smelly sulfur, wait until you hear about a nebula so smelly that it was literally dubbed by the scientists “Rotten egg nebula”, previously known as Calabash nebula [[14]] (see, Figure 2).
Figure 2: Image of Rotten Eggs nebula obtained by the Hubble Space Telescope. Image Credits: NASA/JPL [15]
Of course, in the grand scale of things, the Universe is odourless, i. e. it is mostly just a huge, empty void. Even the areas which are not empty are mostly filled by odourless hydrogen and helium. Those two simple atoms make up 99.9% of the Universe [[16]]. So wherever you go, you will not have to do with a lot of odour, unless you are quite lucky to land in places filled with the sweet smell of a good rum and raspberries.
Alcohol in space?
If someone is a fan of cold drinks, and they enjoy the smell of high volume alcohol, some parts of the interstellar medium may be just the right fit for them. A team of researchers in 2009 found a large region of interstellar space near the centre of our galaxy, which contains ethyl formate, a compound that smells like rum. This chemical is also responsible for the taste of raspberries [[17]]. Yummy! A bit like pirates of the Caribbean.
Sagittarius B2 is the right place to enjoy the rum and raspberry aroma. But there is more places like this: W3OH maser in our galaxy is a giant cloud of pure, gaseous alcohol [[18],[19]]. After travelling 6500 light-years, one would reach a group of alcohol clouds, each with a diameter of 3-4x the orbit of Pluto. The clouds smell like typical spirit alcohol. But it is not yet the time to fill your shot glasses: the nebula is made out ofmethyl alcohol, which is poisonous to humans.
This alcohol is however well chilled, down to almost -230°C. can be widely found in space because it can act as a Maser: a specific emitter of radio signals, that can be easily detected with radio telescopes. Ethanol can also be detected using those tools, but it’s much less abundant making it harder to detect. Yet, it has been detected in object W51M as early as in 1988 [[20]], proving that in space you can not only smell the alcohol. The concentration of alcohol particles in the Universe is in order of 1 in a million, because of how empty space is. It is mostly a vaccum with very little molecules.
Can we finally have a refreshing smell?
Nothing quite beats the smell of the ocean. Ah, the smell of summer holidays by the sea. It turns out, Earth is not the only place in Solar System where you can experience this smell, but it is definitely the most accessible one.
One additional place where you can feel like at the ocean side would be Enceladus, but there is a catch: You must either break through at least 2 kilometres of ice on the surface of this moon of Saturn [[21]], or you must fly through one of the large clouds of water expelled by geysers around the South pole of Enceladus.
Beware though: Enceladus has no dense atmosphere, which means you would suffocate, and probably also freeze there. If you are a fan of the seaside, you could also visit one of Jupiter’s moons, Europa. Just like in the 2013 sci-fi movie “Europa Report”, you’d need to crack through miles of ice, but below it, fresh smell of ocean awaits (and if you stick with the movie, lethal, alien, deep-sea creatures trying to rip you apart).
Oh, and if you are longing for fresh smells, we also invite you to check out most of the comets in the Solar System: their coma, that is, a shell of gas surrounding the icy core of the comet, is mostly made up of water (up to 90%) [[22]]. But even with comets, there’s a twist to the smells: the water on the surface of the comet 67P/Churyumov-Gerasimenko, one of few comets to have been visited by spacecraft, was found to contain formaldehyde [[23]], which has a strong, pickle-like odour. Some describe it as “suffocating”. That is quite a smelly mix.
What about smells inside of spacecraft?
Have you ever wondered what is the smell of the interior of a spacecraft? Different smells escort us during travelling through the space for the whole time. Opinions of astronauts are not yet fully decisive, but descriptions range from the smell of burnt gunpowder or smoke [[24]] to “pleasant”, at least when it comes to smells on-board of the International Space Station, the largest orbital facility as of now.
Recently, Chris Cassidy, an astronaut aboard the International Space Station commented on the smell of the brand new SpaceX Dragon spacecraft, stating that Elon Musk’s feat of engineering has the smell of a “brand new car” [[25]]. But since ISS is a large facility, there is plenty of different smells in various situations. For example, in the US section of the ISS, there is a cookie baking oven [[26]], which means that sometimes, the US laboratory smells like freshly made cookies!
And if you are wondering about the smell inside of the famous “Eagle” lander of Apollo 11, the first manned mission to land on the moon, it was the smell of spent gunpowder. This is best evidenced by a statement from one of Apollo 17 astronauts: about 7 minutes after returning from his first spacewalk on the Moon, astronaut Harrison Schmitt commented on the fact that their spacecraft now smells of the lunar regolith, that is, the dust from the Moon’s surface, which smells like spent gunpowder [[27]]. And once again mentioning the ISS, there’s a specific, sharp, “funny” ozone-like smell to spacesuits after they are brought back inside from a spacewalk [[28]].
But wait, how do we know the smell of things if we have not visited them?
The problem of probing what space is made out of is a very old one. In 1842, a philosopher Auguste Comte said that humans will never know what stars are made out of: “We can never know anything of their chemical or mineralogical structure; and, much less, that of organized beings living on their surface” [[29]].
However, astronomers have found a nice trick, called spectroscopy: they decided to analyze the light from stars and other objects in space, looking for so-called absorption/emission lines. Those are footprints that correspond to various chemicals. That means that by analysis of the light or radio waves from astronomical objects we can see what they are made out of.
We just need to compare the light from stars/planets with calibration samples of light reflected or transmitted through different chemicals in laboratories on Earth. In fact, most of the objects described in this article had their composition checked by spectroscopy, be it in visible light, ultraviolet, infrared or radio waves.
Okay, but can I actually smell space for myself?
The short answer is, yes, to some extent you can. For example, you could buy yourself a specific type of a meteorite: Carbonaceous Chondrites are types of meteorites which contain volatile compounds. Those compounds give off a relatively weak but existent organic odour [[30]]. But if you can’t afford a meteorite, you could always buy yourself a space-themed perfume: several companies have developed perfumes that re-create the smells described by the astronauts on International Space Station. For just 39$ you can get yourself “Eau De Space”.
Conclusion
The Universe is mostly odourless, as over 99% of the matter in the Universe is composed of hydrogen and helium. Some places in space contain high concentrations of chemicals which have smells, mostly unpleasant ones, due to sulphur or ammonia. That means a large portion of space smells like rotten eggs.
On the other hand, insome places you can just be lucky enough to smell rum, raspberries or fresh seawater. Hopefully, when astronauts reach Mars in the next few years, they bring some air fresheners with them, or they will have to deal with the stink of rotten eggs on the Red Planet. It will be a while before humanity ventures light-years deep into interstellar space where the smell of raspberries and rum greets us.
On our way to interstellar space, we will probably manage to also catch a glimpse of Kuiper Belt Objects with their almond smell. And although the practical implications of knowing the smells of objects in the universe are relatively low, it is still an interesting way to look at the chemical composition of planets, comets, asteroids or nebulae. Well, that is the smell of the cosmos, at least the parts of it which we know about.
Article written by Karol Masztalerz, Ewa Klejman, Agnieszka Pregowska, Magdalena Osial / IPPT PAN
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