Death from the Skies! (25 page)

 

But four billion years ago it was a different story. Did ancient Martian oceans teem with simple life-forms, bacteria, protozoans? We don’t know, and we may never know. Still, future probes may yet find fossils in ancient Martian rocks.

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.

 

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.

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 time 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.

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.