Matthew Addicoat, Nottingham Trent University

The universe is flooded with billions of chemicals, each a tiny pinprick of potential. And we’ve only identified 1% of them. Scientists believe undiscovered chemical compounds could help remove greenhouse gases, or trigger a medical breakthrough much like penicillin did.

But let’s just get this out there first: it’s not that chemists aren’t curious. Since Russian chemist Dmitri Mendeleev invented the periodic table of elements in 1869, which is basically a chemist’s box of Lego, scientists have been discovering the chemicals that helped define the modern world. We needed nuclear fusion (firing atoms at each other at the speed of light) to make the last handful of elements. Element 117, tennessine, was synthesised in 2010 in this way.

But to understand the full scale of the chemical universe, you need to understand chemical compounds too. Some occur naturally – water, of course, is made of hydrogen and oxygen. Others, such as nylon, were discovered in lab experiments and are manufactured in factories.

Elements are made of one type of atom, and atoms are made of even tinier particles including electrons and protons. All chemical compounds are made of two or more atoms. Although it’s possible there are undiscovered elements left to find, it’s unlikely. So, how many chemical compounds can we make with the 118 different sorts of element Lego blocks we currently know?

Big numbers

We can start by making all the two-atom compounds. There are lots of these: N₂ (nitrogen) and O₂ (oxygen) together make up 99% of our air. It would probably take a chemist about a year to make one compound and there are 6,903 two-atom compounds in theory. So that’s a village of chemists working a year just to make every possible two-atom compound.

There about 1.6 million three-atom compounds like H₂0 (water) and C0₂ (carbon dioxide), which is the population of Birmingham and Edinburgh combined. Once we reach four- and five-atom compounds, we would need everyone on Earth to make three compounds each. And to make all these chemical compounds, we’d also need to recycle all the materials in the universe several times over.

But this is a simplification, of course. Things such as the structure of a compound and its stability can make it more complex and difficult to make.

The biggest chemical compound that has been made so far was made in 2009 and has nearly 3 million atoms. We’re not sure what it does yet, but similar compounds are used to protect cancer drugs in the body until they get to the right place.

But wait, chemistry has rules!

Surely not all those compounds are possible?

It’s true there are rules – but they are kind of bendy, which creates more possibilities for chemical compounds.

Even the solitary “noble gases” (including neon, argon and xenon and helium), which tend to not bind with anything, sometimes form compounds. Argon hydride, ArH⁺ does not exist naturally on Earth but has been found in space. Scientists have been able to make synthetic versions in laboratories that replicate deep space conditions. So, if you include extreme environments in your calculations, the number of possible compounds increases.

Carbon normally likes being attached to between one and four other atoms, but very occasionally, for short periods of time, five is possible. Imagine a bus with a maximum capacity of four. The bus is at the stop, and people are getting on and off; while people are moving, briefly, you can have more than four people actually on the bus.

Some chemists spend their entire careers trying to make compounds that, according to the chemistry rulebook, shouldn’t exist. Sometimes they are successful.

Another question scientists have to grapple with is whether the compound they want can only exist in space or extreme environments – think of the immense heat and pressure found at hydrothermal vents, which are like geysers but on the ocean floor.

How scientists search for new compounds

Often the answer is to search for compounds that are related to ones that are already known. There are two main ways to do this. One is taking a known compound and changing it a bit – by adding, deleting or swapping some atoms. Another is taking a known chemical reaction and using new starting materials. This is when the method of creation is the same but the products may be quite different. Both of these methods are ways of searching for known unknowns.

Coming back to Lego, it’s like making a house, then a slightly different house, or buying new bricks and adding a second storey. A lot of chemists spend their careers exploring one of these chemical houses.

But how would we search for truly new chemistry – that is, unknown unknowns?

One way chemists learn about new compounds is to look at the natural world. Penicillin was found this way in 1928, when Alexander Fleming observed that mould in his petri dishes prevented the growth of bacteria.

Over a decade later, in 1939, Howard Florey worked out how to grow penicillin in useful amounts, still using mould. But it took even longer, until 1945, for Dorothy Crowfoot Hodgkin to identify penicillin’s chemical structure.

That’s important because part of penicillin’s structure contains atoms arranged in a square, which is an unusual chemical arrangement that few chemists would guess, and is difficult to make. Understanding penicillin’s structure meant we knew what it looked like and could search for its chemical cousins. If you’re allergic to penicillin and have needed an alternative antibiotic, you have Crowfoot Hodgkin to thank.

Nowadays, it’s a lot easier to determine the structure of new compounds. The X-ray technique that Crowfoot Hodgkin invented on her way to identifying penicillin’s structure is still used worldwide to study compounds. And the same MRI technique that hospitals use to diagnose disease can also be used on chemical compounds to work out their structure.

But even if a chemist guessed a completely new structure unrelated to any compound known on Earth, they’d still have to make it, which is the hard part. Figuring out that a chemical compound could exist does not tell you how it’s structured or what conditions you need to make it.

For many useful compounds, like penicillin, it’s easier and cheaper to “grow” and extract them from moulds, plants or insects. Thus the scientists searching for new chemistry still often look for inspiration in the tiniest corners of the world around us.

Matthew Addicoat, Senior Lecturer in Functional Materials, Nottingham Trent University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Gareth Dorrian, University of Birmingham

It’s been 30 years since a group of scientists led by Carl Sagan found evidence for life on Earth using data from instruments on board the Nasa Galileo robotic spacecraft. Yes, you read that correctly. Among his many pearls of wisdom, Sagan was famous for saying that science is more than a body of knowledge – it is a way of thinking.

In other words, how humans go about the business of discovering new knowledge is at least as important as the knowledge itself. In this vein, the study was an example of a “control experiment” – a critical part of the scientific method. This can involve asking whether a given study or method of analysis is capable of finding evidence for something we already know.

Suppose one were to fly past Earth in an alien spacecraft with the same instruments on board as Galileo had. If we knew nothing else about Earth, would we be able to unambiguously detect life here, using nothing but these instruments (which wouldn’t be optimised to find it)? If not, what would that say about our ability to detect life anywhere else?

Galileo launched in October 1989 on a six-year flight to Jupiter. However, Galileo had to first make several orbits of the inner Solar System, making close flybys of Earth and Venus, in order to pick up enough speed to reach Jupiter.

In the mid-2000s, scientists took samples of dirt from the Mars-like environment of Chile’s Atacama desert on Earth, which is known to contain microbial life. They then used similar experiments as those used on the NASA Viking spacecraft (which aimed to detect life on Mars when they landed there in the 1970s) to see if life could be found in Atacama.

They failed – the implication being that had the Viking spacecraft landed on Earth in the Atacama Desert, and performed the same experiments as they did on Mars, they might well have missed signatures for life, even though it is known to be present.

Galileo results

Galileo was kitted out with a variety of instruments designed to study the atmosphere and space environment of Jupiter and its moons. These included imaging cameras, spectrometers (which break down light by wavelength) and a radio experiment.

Importantly, the authors of the study did not presume any characteristics of life on Earth ab initio (from the beginning), but attempted to derive their conclusions just from the data. The near infra-red mapping spectrometer (NIMS) instrument detected gaseous water distributed throughout the terrestrial atmosphere, ice at the poles and large expanses of liquid water “of oceanic dimensions”. It also recorded temperatures ranging from -30°C to +18°C.

Evidence for life? Not yet. The study concluded that the detection of liquid water and a water weather system was a necessary, but not sufficient argument.

NIMS also detected high concentrations of oxygen and methane in the Earth’s atmosphere, as compared to other known planets. Both of these are highly reactive gases that would rapidly react with other chemicals and dissipate in a short period of time. The only way for such concentrations of these species to be upheld were if they were continuously replenished by some means – again suggesting, but not proving, life. Other instruments on the spacecraft detected the presence of an ozone layer, shielding the surface from damaging UV radiation from the Sun.

One might imagine that a simple look through the camera might be enough to spot life. But the images showed oceans, deserts, clouds, ice and darker regions in South America which, only with prior knowledge, we know of course to be rain forests. However, once combined with more spectrometry, a distinct absorption of red light was found to overlay the darker regions, which the study concluded was “strongly suggestive” of light being absorbed by photosynthetic plant life. No minerals were known to absorb light in exactly this fashion.

The highest resolution images taken, as dictated by the flyby geometry, were of the deserts of central Australia and the ice sheets of Antarctica. Hence none of the images taken showed cities or clear examples of agriculture. The spacecraft also flew by the planet at closest approach during the daytime, so lights from cities at night were not visible either.

Of greater interest though was Galileo’s plasma wave radio experiment. The cosmos is full of natural radio emission, however most of it is broadband. That is to say, the emission from a given natural source occurs across many frequencies. Artificial radio sources, by contrast, are produced in a narrow band: an everyday example is the meticulous tuning of an analogue radio required to find a station amidst the static.

An example of natural radio emission from aurora in Saturn’s atmosphere can be heard below. The frequency changes rapidly – unlike a radio station.

Galileo detected consistent narrowband radio emission from Earth at fixed frequencies. The study concluded this could only have come from a technological civilisation, and would only be detectable within the last century. If our alien spacecraft had made the same flyby of Earth at any time in the few billion years prior to the 20th century then it would have seen no definitive evidence of a civilisation on Earth at all.

It is perhaps no surprise then that, as yet, no evidence for extra-terrestrial life has been found. Even a spacecraft flying within a few thousand kilometres of human civilisation on Earth is not guaranteed to detect it. Control experiments like this are therefore critical in informing the search for life elsewhere.

In the present era, humanity has now discovered over 5,000 planets around other stars, and we have even detected the presence of water in the atmospheres of some planets. Sagan’s experiment shows this is not enough by itself.

A strong case for life elsewhere will likely require a combination of mutually supporting evidence, such as light absorption by photosynthesis-like processes, narrowband radio emission, modest temperatures and weather and chemical traces in the atmosphere which are hard to explain by non-biological means. As we move into the era of instruments such as the James Webb space telescope, Sagan’s experiment remains as informative now as it was 30 years ago.

Gareth Dorrian, Post Doctoral Research Fellow in Space Science, University of Birmingham

This article is republished from The Conversation under a Creative Commons license. Read the original article.