But suppose for a moment that Mars once was alive. What does that have to do with Earth?
In 1984, an unusual meteorite was found in the Allan Hills region of Antarctica. Meteorites are relatively easy to find there; on the ice, a black rock sticks out pretty well. The dry air preserves the meteorites well too.
This particular meteorite, dubbed ALH84001, packed a huge surprise. The chemical composition of the rock itself was very similar to the known composition of the rocks on the surface of Mars. Inside it were small bubbles of gas, trapped within the rock when it formed. When the ratios of the various elements of the gases were measured, they matched the ratios found in the Martian atmosphere! Scientists checked this against the chemicals in other planets as well, but no other gas ratio in the solar system matched nearly as well as that of Mars.
Clearly, ALH84001 was an interplanetary interloper, a rock from the Red Planet. It wasn’t even the first discovered; many were found before their now more famous cousin. How these rocks got here is a matter of some irony.
The surface of Mars bears the marks of a violent history. Like every other planet in the solar system, Mars has been bombarded over its life by asteroids and comets. In contrast to Earth, with its thick atmosphere and watery surface, erosion works more slowly on Mars, so we still see the craters, scars from the ancient impacts.
When such a collision occurs, rock from the surface is launched into the air, of course. But some of that material can be given so much energy that it can actually leave the planet altogether and fly into space. From various studies of ALH84001, it has been found that it formed very early in the history of the solar system, cooling from volcanic lava some 4.5 billion years ago. It sat on or below the Martian surface for almost all the time since, and then suffered a terrible blow. An asteroid impact on Mars hurled the rock into space. It orbited the Sun for at least 16 million years, judging from the rate of cosmic-ray impacts on its surface. Its orbit got nudged this way and that every time it passed by its home planet, and eventually the orbit changed enough that it began to move closer to the Sun. Some 13,000 years ago, our planet got in the way. The rock fell to Earth, got lodged in the Antarctic ice, and sat patiently waiting to be found.
So the rocks from Mars that landed on Earth as meteorites were themselves launched into space by impacts from much larger rocks.60 This is a very violent way to get a rock into space, of course, and you’d expect that an impact like that would destroy any rocks, or certainly damage them considerably.
But recent studies have shown that that might not be the case. A low-angle impact, for example, can loft rocks relatively softly (though it’s not exactly the kind of ride for which you’d line up for tickets). There may be other factors involved as well, including pressure waves from the atmosphere and under the rock due to the impact that also combine to soften the blow.
However it worked, very small structures inside ALH84001 survived the ordeal. When the rock was examined by scientists, they found a wealth of intact structure inside, including some that indicated that the rock had been exposed to flowing water at some point in its past. When they took microscopic images, however, they got the shock of their lives.
Tiny wormlike formations were evident inside the Martian rock. They looked for all the world like fossilized bacteria! In 1996, a press conference was held, and the scientists who examined the rock announced what they had found, indicating that it could be, for the very first time in the history of the world, evidence of life outside the Earth. They were very careful to say that the evidence was not (pardon the expression) rock-solid, but very compelling. The “fossils” were actually the weakest of their evidence, and they took great pains to say that they might not be fossils at all. They might be natural formations caused by any number of processes.
A microphotograph of the “Mars rock” ALH84001 shows the presence of small, wormlike objects. While they look like primitive life-forms similar to those on Earth, their origin is unclear. They are far smaller than any similar terrestrial organism. The scale bar is 0.5 micron across; for comparison, a human hair is 50 microns in width.
DAVID MCKAY, JOHNSON SPACE CENTER
Of course, the press ran with the fossil interpretation; a picture, after all, is worth a thousand words, and will sell a million issues of a newspaper. It was quite the media sensation. Over the years, however, one by one, the evidence for life in the rock has come under fire. At the moment, the best one can say is that the evidence is interesting, perhaps even still compelling in some ways, but everyone agrees we will need far better sources before we can talk clearly about ancient life on Mars.
THERE ARE THOSE WHO BELIEVE LIFE HERE BEGAN OUT THERE . . .
But this brings up an intriguing possibility: if life did originate on Mars first, it could have been brought here by the same mechanism that brought us ALH84001. Is it possible that life on Earth actually came from Mars?
At first blush, this sounds like a dumb idea. Earth is flourishing with life—it’s actually quite hard to find any place on the planet that isn’t infected with it—but Mars is quite dead. Still, the steps needed to seed life on Earth are at the very least possible: life may have originated there first; a plausible mechanism exists for it to have gotten here; and conditions here eventually were good enough for that life to take hold.
The idea that life was brought to Earth from space is called panspermia. It’s a fascinating topic, fraught with one simple problem: how do you prove it?
Honestly, I’m not sure you can.61 But it’s very hard to rule it out. How do you perform experiments to test it? Re-creating conditions that haven’t existed in billions of years can be tough, and even then it doesn’t prove things one way or another because of uncertainties inherent in the experiments. But experiments along those lines can steer thinking in directions that may lead to further progress; in science, a good experiment is worth a thousand suppositions.
An interesting test would be to look for fossilized microbes on Mars that have a chemical tie to early life on Earth. A clear example of RNA-based or DNA-based fossil bacteria would be incredibly compelling evidence in favor of panspermia—either life started on Mars and came here, or both Mars and Earth were seeded from some third source.62
Until such evidence is found, we can only conjecture.
Still, in principle, it’s possible to examine the processes involved with panspermia.
The step after getting the life-laden rock off Mars (or some other body) is the journey here. ALH84001 spent at least 16 million years in space, and possibly more, where it was exposed to the hard vacuum of space, bombarded by high-energy subatomic particles, and bathed in killer ultraviolet rays from the Sun.
Anything that could survive that would have to be pretty tough.
Microbes can be tough hombres. Some bacteria can form protective spores around themselves, shielding them from the ravages of heat, cold, drought, and radiation. One type of bacterium—Deinococcus radiodurans—can survive intense doses of radiation, hundreds of times what’s needed to kill a human. It is rather like a computer with multiple file backups: it has many copies of its DNA that it can use in case some get destroyed by radiation, and the tools it uses to repair its own DNA (every cell nucleus has a repair kit) appear to be extremely adept at dealing with extreme conditions.
Of course, it would also help if any microscopic stowaway on board a meteoroid were gift-wrapped carefully too. A rock dislodged by an asteroid impact and flung into space would be irradiated by all manner of destructive sources. But if the rock were large enough, it might protect any microscopic cargo. Cosmic rays may not penetrate very far into the surface, for example. Other disruptive influences like ultraviolet light from the Sun, particle irradiation from the solar wind, and the odd solar flare or coronal mass ejection would have a hard time getting deep into the rock as well. Some early experiments in lofting bacteria samples into space and exposing them to the environmental hazards there indicate that some microbes could survive for a period of tim
e in space.
If some Martian protovirus or bacterium were to wend its way deep into a rock that got blasted off Mars, then there is some chance—small, but finite—that it could survive the journey.
It would also have to survive the journey through our atmosphere. But again, if the rock were large enough only the outer layers would burn off as it plummeted through the Earth’s air. If the meteoroid disintegrated into smaller pieces above the ground, the individual impacts wouldn’t be so jarring to its biological stowaway either. A small rock would simply plop down, and if it fell into water or mud, which seeped into cracks, the microbe might suddenly find itself getting, in a literal sense, food delivery.
It’s important to note that Mars isn’t the only source of potential life. Comets are giant balls of rocks and ice that orbit the Sun, and are known to contain rather complex organic compounds, some of which are precursors (or have at least basic chemicals needed) for life. It’s possible that comets impacting the young Earth brought much of the water to our planet, and brought these chemicals as well. A meteorite that fell in Australia in the 1960s was also found to have amino acids in it, including glycine and alanine, both of which are commonly found in animal proteins. Even giant clouds of gas and dust in space are found to be rich sources of complex organic compounds. One study done by scientists even showed that DNA or RNA from bacteria, if shielded well, could actually survive being blown by the solar wind to another star.63 That work is pretty speculative, but it shows in principle that transport across large distances, even vast ones, is theoretically possible, even if very unlikely.
Incidentally, it should be noted that a comet need not directly impact the Earth to transfer any contents. When a comet gets near the Sun, the frozen material sublimates (turns directly to a gas) and leaves the comet, forming the long tail. If the Earth sweeps through a comet’s tail, the cometary materials can mix with the Earth’s atmosphere. It’s still a somewhat violent process, since the velocities are so high, but in principle the comet stuff can reach the Earth relatively intact.
Let’s also be careful here and state explicitly that all of these are the building blocks of life, and not life itself. But the fact remains that the components needed for life as we know it not only exist in space but are relatively abundant—and these sources made it down to the ground intact enough to be studied by scientists. It’s entirely possible that space is simply buzzing with life, and it’s also possible that this is where terrestrial life got its start. If true, actually proving it would be one of the most colossal and fundamentally profound discoveries ever made.
But it also could mean trouble. If life exists out there as microbes, and some of them came to Earth now, could there be a less than happy ending? It’s all well and good for the surviving bacterium, and if panspermia is true, we owe our existence to space bugs. But that was more than three billion years ago.
What would happen if this event were to repeat itself today? We’ve all seen the movies The Blob and The Andromeda Strain. Could an interplanetary infection invade us, wiping out humanity (or mutating us into horrible nasty gooey things)?
To be honest, probably not. Life here is pretty tough, and anything coming down from space will have an uphill battle trying to take us over. It’s my opinion that they won’t be able to win. But the fight itself depends on what type of gooey thing is on the prowl.
VIRAL ADVERTISING
Viruses, for example, are a favorite in science-fiction scenarios. There may be millions of types of viruses on Earth; we have only scratched the surface in investigating their diversity. Most are actually extremely simple structures: they are a snippet of DNA code wrapped in a protein shell called the capsid. They cannot reproduce on their own; instead, they invade a cell, inject their DNA into the cell’s nucleus, and then urge it to copy the viral DNA. Viruses are the stealthy ninjas of the submicroscopic world, sneakily invading the factory of the cell and turning it against itself.64
When enough viruses are created, they burst out of the cell, rupturing and destroying it. They then scurry off, seeking out more cells to subvert. If the body cannot fight off the infection, and the virus is virulent enough, the host can be killed. The tissue of the body literally liquefies.
Nasty.
There are other types of viruses too. Some use RNA, not DNA. Others attack bacteria and not tissue cells. And not to make you uncomfortable or anything, but your body is currently brimming with these viruses. Most are completely harmless. Some do cause a variety of issues—for example, they can throw off your body’s ability to regulate its systems, resulting in illnesses from mild to severe—but most don’t kill. They have to attack in ferocious numbers to do that, or be particularly virulent, like Marburg (which has a mortality rate of about 25 percent) and the more famous Ebola (with its truly terrifying 80 to 90 percent mortality rate).
The structural simplicity of viruses is both a blessing and a curse when it comes to an invasion from space.
Because they are such simple structures, viruses are resistant to many of the problems a more complicated microbe might have with exposure to space. Prolonged periods of vacuum, low temperatures, and even some radiation may not prove an obstacle to them. Embedded deep in a rock, they could fall to Earth intact, only to be opened like a cursed pharaoh’s tomb by a hapless scientist.
But if they got under his skin, they might starve to death.
That’s because viruses are generally adapted to attacking one specific kind of organism. A virus that can infect a plant can’t harm a butterfly, and one that is adapted to attacking bacteria (called bacteriophages) can’t hurt a human. Viruses are too simple to change radically, and the DNA or RNA snippet in the virus is like a key to a lock. A car key won’t work in a house door.
So even though any hypothetical space-borne virus might survive all the way into the lab of a scientist, it’s incredibly unlikely that it would swarm and multiply and turn us all into raging zombies.65
So in reality, viruses aren’t a big threat. They would find us completely incompatible for their purposes, and would quickly die out.66
Score one for life on Earth.
BUG IN THE SYSTEM
Interplanetary bacteria are another horror movie staple, and while they have some advantages as invaders over viruses, they’re also unlikely to be much trouble to us Earthbound creatures.
Unlike viruses, which are programmed to fit certain types of cells or proteins, bacteria are less choosy. And while viruses use our own cells’ machinery against us, bacteria consider us more as a flophouse. Like an unwanted guest, they can eat your food, mess up the place, and, of course, overstay their welcome.
The main difference between a virus and a bacterium is complexity. Bacteria are cells in their own right, and are considered alive. They can ingest food, excrete waste, and reproduce on their own. Give them a warm, wet environment with the nutrients they need, and they’ll do all three of these functions with abandon.
Our bodies make excellent sites for what bacteria need. Our bodies are loaded with bacteria, as with viruses. Bacteria are in your gut, in your skin, and living on your eyelashes. They’re everywhere, and in fact it’s been estimated that there are ten bacteria in your body for every single human cell!
You’re outnumbered.
The vast majority of bacteria inside you are benign. They either don’t do anything harmful or exist in numbers too small to do any damage. Many are beneficial to us; without them we’d die. They help us digest our food, for example; they also create vitamins and boost our immunity to more harmful types of bacteria. They even help us digest milk.
An excellent example of such a bacterium is Escherichia coli, more commonly known as E. coli. This little ovoid bug lives in huge numbers inside your intestines, and has the underappreciated job of helping you process your waste matter.67 Normally, they live happily in your gut, doing whatever it is they do. But that’s not always the case. E. coli eats and poops too, and some strains of the bacterium exude a toxic chem
ical brew. In low doses your body can handle it. But if you get too much in your system, it can make you quite ill. Food poisoning, for example, can be caused by eating food that has been contaminated by E. coli. If the infection is bad enough, it can be fatal. E. coli can also get out of your intestines (through a hole or herniated region) and into your abdomen, causing peritonitis.
The list of possible problems bacteria can invoke is lengthy (diarrhea, vomiting, nerve damage, cramps, fever . . . you get the picture), but usually we live in an uneasy truce with the bugs inside us.
The bacteria inside us have, of course, evolved along with us so they can maintain this symbiotic relationship. A hypothetical bacterium that evolved on Mars, say, or some other planet would not enjoy this luxury. Still, the effect of an alien bacterium on us really depends on what it needs, and, pardon the expression, what it excretes.
If all it needs is a warm place with water and some nutrients, then any port in a storm, as they say. Your intestines will look just as good as any other place. And if the bacterium multiplies, and the colony emits a toxin, then that can be trouble.
But is that likely?
In reality, almost certainly not. The very complexity that makes bacteria more versatile and therefore more adaptable than viruses is also their Achilles’ heel: it makes them more fragile. Their internal machinery is unlikely to survive the journey through space, and their entry into our atmosphere.
Moreover, the conditions alien bacteria need to survive would make Earth look pretty unfriendly. The chemistry of the surface of Mars, for example, is very different from Earth’s. Its thin atmosphere means it’s hit by a high level of UV light from the Sun. There is very little if any water on most of the surface, and some readings indicate rather high levels of hydrogen peroxide, a chemical that tends to destroy terrestrial bacteria (which is why it’s used to clean wounds, though it should be noted that it is produced by some forms of life on Earth—notably the bombardier beetle, which uses it to ward off predators). Any bacterium that evolved to survive on Mars would most likely find Earth to be a very difficult environment—too wet, too hot, too alien.
Death From the Skies!: These Are the Ways the World Will End... Page 18