Can Viruses Travel Between Planets? (Script) (Patreon)

We are generally not big fans of viruses right now, but we sure are thinking about them a lot. That’s right, even astrophysicists are pondering these bizarre little critters. In fact astrovirology, although very new, is actually an emerging subfield of astrobiology. And that’s because it turns out viruses don’t just influence organisms - they’re incredibly important on a planetary scales. Perhaps an interplanetary scales.

There are far more viruses on Earth than all cellular organisms combined, and so they drastically affect our entire biosphere. And it’s increasingly believed that they are critical factors in the process of evolution, perhaps even in pre-cellular days. That’s right, viruses or their ancestors may have played a role in the origin of life on earth, and may be needed to initiate life on any planet. Oh, and astrovirology is also concerned with the possibility that viruses can travel between worlds. 

I’m going to start with this absolute statement, because you know how the internet can be. The new coronavirus - Sars-CoV-2, the virus that causes COVID-19 … did absolutely not fall to earth from space. I’ll come back to how we can be 100% confident of that, a bit later. 

First up, let’s talk viruses generally. But let me preface this by stating the obvious - astrophysics channels are NOT the best place to learn virology. I want to do this review to highlight the details of viruses that make them important on a planetary, and even interplanetary scales.

Viruses sit on the blurred line between living and not. They contain genetic material - either single-strand RNA or double-strand DNA - and when not active inside an infected cell, they surround themselves in a protein shell - a capsid. We often think of a “virus” as this bundle of genetic material within a shell, but the term for that bundle is a virion, and this is more of a transport mechanism for the genetic material. The term virus better describes the entire microbal lifecycle, and inside an infected cell, a virus has no shell - it IS its genetic material. 

Unlike cells, viruses do not metabolize - so they don’t produce their own energy, and they can’t reproduce without coopting the reproduction machinery of a cell - hence the dispute over whether they can be called “alive”. One thing viruses do share in common with life is the ability to evolve. Their extreme numbers and reproduction rate mean rapid evolution. As well as resulting in the vast array of existing viruses, this means some viruses produce new strains every year. That’s why you need a new flu shot each season.

But viruses aren’t just masters of their own evolution - it seems they may be an essential driver of the evolution of cell-based life. Because viruses insert themselves into DNA to reproduce, they are actually able to transfer genes between organisms. Viruses are the original genetic engineers. And some of your own most useful genes were left in your DNA by a virus that infected an ancestor. 

As well as driving cellular evolution, viruses may have been critical in the formation of cells in the first place. For a long time it was believed that viruses evolved as escapees from DNA-based cells, but more and more virologists think that viruses may predate cells. RNA-based viruses in particular may have emerged from the pre-cellular RNA world, which would make them our most distant cousins on the tree of life. Or tree of ?life? And pushing their gene-transferring powers that far back means they may have massively accelerated the evolution that led to the development from RNA-based pseudo-life to the first true DNA based cellular life. Understanding viruses may be the key to understanding the origin of life. 

The extreme abundance of viruses on Earth, and their critical role in evolution and possibly even abiogenesis, suggests that we probably should be thinking about whether there are viruses on other worlds. 

One of the most promising prospects for our first detection of alien life is to find signatures of its chemical activity in the atmospheres of alien worlds. We’ve talked about how this is done before - but in short, when an alien world passes in front of its home star, we can see signatures frequencies plucked from the starlight due to it passing through that planet’s atmosphere. Cellular life massively alters the atmosphere because it excretes gases - oxygen, methane, nitrous oxide… as it metabolized. But viruses don’t metabolize, so won’t leave direct atmospheric biosignatures. But what about rocks that fall to Earth? Well because they’re so tiny, they aren’t likely to leave distinguishable fossils like bacteria can. Nor do they leave very distinct biosignatures. That means the remnants of dead viruses are unlikely to be found in meteorites from other worlds. So the evidence of alien viruses is unlikely to come to us. We’ll probably have to go out to look for it.

The first stop in our search for life is always Mars. The red planet seems pretty dead these days, but it’s conceivable that an ecosystem of extremely hardy microbes lives beneath the Martian surface, And some viruses are among the hardiest of all microbes, so it might pay to look for the signatures of viruses as we continue to explore Mars. 

Perhap more promising are the ocean moons of the gas giants. Jupiter’s Europa and Saturn’s Enceladus these are known to harbour vast oceans beneath their icy surfaces; oceans kept warm by the tidal flexing caused by their parent gas giants. If those oceans contain microbial life, then they may also contain viruses. Both Europa and Enceladus spray geysers of ocean water into space that can be captured by probes. Again, specialized instruments on future missions could detect viral material. 

OK, all this talk of viruses hitching rides into space brings us to our last, and most sci-fi-horror-movie relevant point. Can viruses from one planet infect another?  There are two parts to this question: can a virion survive an interplanetary journey? And then can it actually do anything when it arrives?

The answer to the first part is definitely sort of maybe. Like I said, viruses can be carried by tiny water droplets. Earth’s upper atmosphere is thick with bacteria and viruses - mostly swept into the atmosphere from the oceans by seaspray. From there they can travel across the globe, but also perhaps be lifted into space. Very light droplets can be lifted to the very edge of an atmosphere with the help of the planet’s magnetic field and then swept into interplanetary or even interstellar space by the radiation from the star.  This is radiopanspermia, and it’s a compelling way to transport viruses because the mechanism is especially effective for very small water droplets - of the kind that viruses tend to occupy. In fact it would be surprising if there was not quite a lot of viral and other microbial material riding the solar wind into interstellar space. So yeah, stars sneeze, you might want to maintain 6 light years distance.

Viruses can also be launched into space embedded in rocks that are ejected in asteroid or comet impacts. This is lithopanspermia, and again - virus-ridden rocks have definitely left the Earth before, and probably impacted other bodies in our solar system. Earth certainly gets hit by rocks from Mars - so if Mars ever had viruses then some of that material may have reached Earth. Virus-bearing rocks from worlds beyond our solar system are possible, but those viruses would need to remain viable for 100s of thousands to millions of years. And it's the ability of viruses to survive long spaceflight that becomes the real question.

Outer space is deadly - it’s a freezing cold vacuum bathed in intense radiation.Virions are a bit like seeds or spores - dormant until they come into contact with their prey. And like seeds or spores, some virions are remarkably resilient. Take the tobacco mosaic virus - it can actually be crystalized, protecting it from desiccation, cold, and vacuum. In fact that particular virus has survived being bathed in the equivalent of 250 years of cosmic rays, simulated by an intense shower of energetic protons. Polioviruses and bacteriophages have stayed infectious after space-like environments of high-altitude balloon and rocket rides. Some bacteriophages have also shown resistance to UV exposure in experiments on the international space station, however their viability after that exposure hasn’t been tested. 

Some viruses do have the remarkable ability to sort of reassemble their genetic code after irradiation by UV or cosmic rays. Even if most individuals within an infecting population have damaged genes, as long as the full, correct virus genome is represented across the population within a given infected cell, the viral genome can be reconstructed as the virus replicates itself. 

All of that said, UV radiation from stars is probably our best hope for obliterating space viruses. Beyond a certain point, a damaged genome renders a virus non-functional and can’t be reconstructed. It’s likely UV exposure would destroy  most viruses trying to hitch rides via radiopanspermia - and that’s good, because that’s the only known way to really spread LOTS of viruses between planetary systems. Lithopanspermia can spread vastly fewer microbes, but it’s more reliable because a barrier of rock or ice will shield against ultraviolet light and cosmic rays. 

OK, so we’ve established that there’s at least a remote chance of interplanetary or interstellar viruses making their way to earth. Or Earth viruses going back the other way. But  

are such alien lurgies really a possible danger? In order for a virus to infect a cell, a virus needs to bind with receptors on the cell’s surface and then integrate its genome into the cell’s DNA. Different viruses have evolved to infect a vast array of different cells, and some even infect other viruses. But one thing they all have in common is that viruses all infect life as we know it. At the very minimum, a cell needs to be DNA or RNA-based to be susceptible to earth viruses. Would alien viruses have evolved to attack the same genetic structure? That would only be the case if DNA/RNA based life was the norm for extraterrestrial life. And we have no idea whether that’s the case. Perhaps DNA is the only possible basis for complex life, or perhaps panspermia seeded DNA-based life and DNA-preying viruses across the galaxy. But it’s very likely that an alien virus that made its way to earth would find have absolutely no way to attack the cells it finds here.

Let’s get to the last pressing question, which is also the easiest. Could any of the past recent pandemics have resulted from alien viruses? … No. We can trace the origins of viruses by their genes. Take the SARS-CoV-2 virus - it shares the overwhelming majority of its genes with coronaviruses found in populations of horseshoe bats - and the philogenics - the evolutionary path - is consistent with it evolving out of that population. Same with other epidemic-causing viruses like ebola and spanish flu. It’s never aliens if there’s a sensible earthly explanation. But like I said - this is an astrophysics show, so I’ll add some links for more expert analysis of the virology in the description. In the meantime, please stay safe from the current very terrestrial virus outbreak, and at least appreciate this: we are now MUCH more aware of the importance and danger a hidden world that has always lived alongside us, driven our evolution, and perhaps predated life itself - the world of viruses, our currently not-so-favourite, but still fascinating fellow denizens of space time.