NASA's major announcement on December 2nd is old news by now, but I decided to write something about it anyway. It was heralded as changing the fate of Astrobiology, but it fizzled out from the news surprisingly quickly I thought. Maybe it's because it didn't capture people's imagination, or most people didn't really understand what the big deal was. Maybe it's because it wasn't really a big deal... I'll try to explain it, and you can make up your own mind.
I'll start by explaining why arsenic is a poison, and what that has to do with life in other planets. Arsenic is a notorious poison: Napoleon is probably the most famous person to have died from it. The reason why arsenic is a poison is because of it's chemical nature. Arsenic is similar to phosphorus, and since phosphorus is vital to life (it's one of the building blocks of DNA and proteins), arsenic goes where phosphorus should be, and the whole living machine breaks down.
As an analogy, think of a bicycle chain. It's made up of maybe a hundred "rings", that fit with each other, the chain fits onto the bicycle gears, and the whole bicycle works. Let's say the rings of the chain are like phosphorus. Now imagine someone comes along and changes one of the rings in the chain with one that's different (like arsenic), maybe just a bit wider on the sides. It still fits with the other rings, so the chain is not broken, but it has a different shape in the middle, with bits sticking out left and right. When the chain passes over the cogs of the gears, this different ring gets stuck, so the whole chain gets stuck, so the whole bicycle doesn't work.
Getting back to chemistry, if you look at the periodic table, arsenic (As) is just below phosphorus (P), meaning that it has the same number of free electrons, but with a bigger nucleus. Which means it can make the same number of bonds. Phosphorus is a major part of DNA and proteins, and even the cell membrane lipids. So phosphorus is important to life. When arsenic comes along, it fits into most of the places where phosphorus can fit, but not all, so once you have arsenic instead of phosphorus in DNA and other molecules, things get stuck and the whole organism doesn't work; like the bicycle chain getting stuck on the gears when one ring is changed.
So, based on this property of arsenic, there was a theory around that maybe there could be life forms out there that used arsenic instead of phosphorus, the same way that life forms based on silicon instead of carbon have been proposed. Essentially, imagine you designed a bicycle (organism) with gears and cogs (enzymes) that fit the weird chain rings (arsenic). You would ultimately have a working bicycle, the parts would just look a bit different than normal.
Ok, after this lengthy introduction, we can get back to NASA's announcement. What the scientists set out to do was find one of these weird life forms that use arsenic instead of phosphorus. And they found some cells that arguably can at least survive with arsenic in their DNA and proteins. Although there is a lot of debate about the methods and the interpretation of the results, this finding supports the theory that there could be life forms out there that use arsenic instead of phosphorus!
This does affect the astrobiology field quite a lot. When we send spaceships to look for life on other planets, they're not just looking around to see what's there, they're trying to detect specific things. So when we setup the whole mission to look for "life as we know it" with DNA, proteins, and lipids with the same structure as what we find on Earth, we are necessarily restricting the search. This means we may be missing other interesting things, that are slightly different to what we know, but are still evidence of life. So, should we be looking for organisms with arsenic in their DNA? Maybe... it's still a bit early to tell, but the whole arsenic story should remind us that we don't know everything, and looking for something that we're not used to seeing is tricky. To borrow a more eloquent expression from Marcel Proust, "the voyage of discovery consists not in seeking new landscapes but in having new eyes". How much should we expand our search? What exotic molecules and organisms might we find, and how do we look for them if we can't even imagine them?
I'll start by explaining why arsenic is a poison, and what that has to do with life in other planets. Arsenic is a notorious poison: Napoleon is probably the most famous person to have died from it. The reason why arsenic is a poison is because of it's chemical nature. Arsenic is similar to phosphorus, and since phosphorus is vital to life (it's one of the building blocks of DNA and proteins), arsenic goes where phosphorus should be, and the whole living machine breaks down.
As an analogy, think of a bicycle chain. It's made up of maybe a hundred "rings", that fit with each other, the chain fits onto the bicycle gears, and the whole bicycle works. Let's say the rings of the chain are like phosphorus. Now imagine someone comes along and changes one of the rings in the chain with one that's different (like arsenic), maybe just a bit wider on the sides. It still fits with the other rings, so the chain is not broken, but it has a different shape in the middle, with bits sticking out left and right. When the chain passes over the cogs of the gears, this different ring gets stuck, so the whole chain gets stuck, so the whole bicycle doesn't work.
Getting back to chemistry, if you look at the periodic table, arsenic (As) is just below phosphorus (P), meaning that it has the same number of free electrons, but with a bigger nucleus. Which means it can make the same number of bonds. Phosphorus is a major part of DNA and proteins, and even the cell membrane lipids. So phosphorus is important to life. When arsenic comes along, it fits into most of the places where phosphorus can fit, but not all, so once you have arsenic instead of phosphorus in DNA and other molecules, things get stuck and the whole organism doesn't work; like the bicycle chain getting stuck on the gears when one ring is changed.
So, based on this property of arsenic, there was a theory around that maybe there could be life forms out there that used arsenic instead of phosphorus, the same way that life forms based on silicon instead of carbon have been proposed. Essentially, imagine you designed a bicycle (organism) with gears and cogs (enzymes) that fit the weird chain rings (arsenic). You would ultimately have a working bicycle, the parts would just look a bit different than normal.
Ok, after this lengthy introduction, we can get back to NASA's announcement. What the scientists set out to do was find one of these weird life forms that use arsenic instead of phosphorus. And they found some cells that arguably can at least survive with arsenic in their DNA and proteins. Although there is a lot of debate about the methods and the interpretation of the results, this finding supports the theory that there could be life forms out there that use arsenic instead of phosphorus!
This does affect the astrobiology field quite a lot. When we send spaceships to look for life on other planets, they're not just looking around to see what's there, they're trying to detect specific things. So when we setup the whole mission to look for "life as we know it" with DNA, proteins, and lipids with the same structure as what we find on Earth, we are necessarily restricting the search. This means we may be missing other interesting things, that are slightly different to what we know, but are still evidence of life. So, should we be looking for organisms with arsenic in their DNA? Maybe... it's still a bit early to tell, but the whole arsenic story should remind us that we don't know everything, and looking for something that we're not used to seeing is tricky. To borrow a more eloquent expression from Marcel Proust, "the voyage of discovery consists not in seeking new landscapes but in having new eyes". How much should we expand our search? What exotic molecules and organisms might we find, and how do we look for them if we can't even imagine them?
