Could meteorites carrying bacterial stowaways transfer life from one planet to another? The chances against it aren’t as long as scientists once thought. A theory known as “pansperma” …
Every million years or so, a comet or asteroid smashes into Mars fast enough to blast thousands of tons of Mars rocks into space.
That’s the theory on how several meteorites from Mars have reached Earth. Brett Gladman is an expert on solar system dynamics at the University of British Columbia . . .
Brett Gladman: You might think it’s a small chance but in fact … something like 5 to 10 percent of the material that would be blasted off Mars would actually eventually find its way to Earth.
Scientists aren’t sure if there’s ever been life on Mars. But if there ever was, it’s conceivable that life hitched a ride on these martian rock fragments and made their way to our world. Gladman also wonders if the reverse is possible — could earthly life reach other planets? He’s now using a supercomputer to model what happens when bits of Earth are ejected into space after asteroid or comet collisions. Meanwhile, other scientists have shown that earthly life appears to be extremely hardy.
Brett Gladman: Bacteria are able to resist vacuum and even being nearly exposed directly to hostile radiation for at least a timescale of years. Inside big, well shielded boulders, they may very well be able to last for hundreds to thousands of years.
At least every million years, a mile-wide comet or asteroid crashes into Mars with enough speed to blast about 10 million small boulders of martian rock into space. The pieces can range from fist-sized chunks to boulders the size of trucks. Brett Gladman calculates the dynamics of these giant impacts to determine how much material is cast off of planets and how often. He then calculates the paths these rocks take through the solar system to see where they may land.
Gladman’s calculations take into account the influence of the sun and all the planets to find the travel itineraries of thousands of space rocks over millions and millions of years. This takes months of computer time, and is possible only because of recent advances in speed and calculating power of modern computers.
According to the calculations, an asteroid between a half mile and a mile across (1- to 2-kilometers in diameter) striking Mars would produce a spew of particles of average size from 1 to 2 feet in diameter. “And there’d be roughly 10 million particles of that size lobbed off the planet,” Gladman says, “Of which, roughly 5 to 10 percent — so a few million particles — would actually end up eventually reaching the Earth over 10 or 20 million years following the impact.”
Some rocks take millions of years to reach Earth, but in theory the shortest possible trip puts a piece of martian rock on Earth in just a few years. “If you do the numbers, it’s extremely likely that over the history of the solar system, an impact on Mars has ended up putting something on the earth in just a few decades,” Gladman says.
The odds for pieces of other planets reaching Earth are not very good, however. Rocks have an easy time leaving Mars because of its low gravity (one third that of Earth gravity) and thin atmosphere (less than one percent Earth’s). Add gravity and the wind resistance of thick air, and rocks fall back to a planet much more frequently. Venus’ thick carbon dioxide atmosphere make it all but impossible for a rock to be blasted from it’s surface, Gladman says.
And except for Pluto, Jupiter and the planets beyond do not have solid surfaces, although there are slim chances that pieces of the solid moons of Jupiter or Saturn could be cast into space.
Scientists are now wondering if rocks from Earth could ever be launched into space to make it to other planets. Given Earth‘s relatively thick atmosphere and high gravity, the velocity required for a rock to escape Earth is much greater than is necessary to lob one off Mars. It would require an impact of amazing speed and extremely high energy — an impact which has a very good chance of entirely melting the rock.
Modeling the dynamics of the interaction between the atmosphere and escaping objects is extremely difficult, Gladman says, but it is rather simple to see where an object may go after it escapes Earth. Gladman is now working on a project to find out what happens to material — if any — which escapes Earth after large impacts. “It’s very computer intensive, but we can do the calculations to figure where the material would go and how long it would take.”
Gladman’s computers will be running these calculations for most of 2003. The outcome may help answer the question of whether Earth could have been sending living organisms into space throughout its history.
- Meteorites from Mars – Johnson Space Center
- The depate over the “Mars Life” meteorite — Lunar and Planetrary Institute
- Jet Propulsion Laboratory info on Mars Meteorites
Excerpts from interview with Brett Gladman:
– We can ask the question should we be worried about contaminating other planets in the solar system by landing spacecraft on them that might be carrying bacteria from us? … And if it turns out that it’s relatively easy to get material inside rocks naturally landing on other places in the solar system, they may very well already have been contaminated by life from Earth.
-“You might think it’s a small chance but in fact at least half of the stuff that gets lobbed off the moon eventually ends up hitting the earth. And something like 5 to 10 percent of the material that would be blasted off Mars would actually eventually find its way to Earth.”
– “… and so the conditions, although extreme, are within the bounds that we know bacteria can resist.”
– “But if you do the numbers, it’s extremely likely that over the history of the solar system, an impact on Mars has ended up putting something on the earth in just a few decades.”
– “… for an impactor coming in, an asteroid the size of a km or two, ahh, The impact would produce a spew of particles where their average size would be something like 10 cm to a meter, or something like a foot or two and there’d be roughly, 10 million particles of that size lobbed off the planet. Of which, roughly, ahhh, 10 percent, five to ten percent so a few million particles, would actually end up eventually reaching the Earth over 10 or 20 million years following the impact.”
-“The surprising result is that the bacteria are tremendously hardy organisms and are able to resist extremes of acceleration and heat and even total absence of air for periods of — at least — years.” There have been several experiments with putting bacterial plates on spacecraft. (**note — this is not Gladman’s work and he is not a biologist)
– “Bacteria are able to resist vacuum and even being nearly exposed directly to hostile radiation for at least a time scale of years. Inside big, well shielded boulders, they may very well be able to last for hundreds to thousands of years.”
– “… for objects coming in from other planetary systems they’ve had to travel thru the galaxy for much longer time, so any possibility of life that might be hiding inside rocks which carried them off of their parent planet have a much more difficult time of surviving the hostile intergalactic radiation.”
– “I am actually right now involved on a project trying to calculate the probability and time scales for material that escapes from the Earth — if any does after large impacts — to be distributed amongst other bodies in the solar system.”
– “It’s very difficult to constrain if you can get thru the atmosphere it’s not actually very hard to do the calculations — its very computer intensive — but we can do the calculations to figure where the material would go and how long it would take.”
His newest project has a couple of interesting implications. For example, we worry about spacecraft contaminating other worlds. Maybe we don’t need to worry. Maybe it’s easy for life from Earth to get to other parts of the solar system. Maybe other worlds are already contaminated by earthly life. But it’s hard to blast material off Earth and get it past our atmosphere. We do know that some material can leave the atmosphere, but that’s been shocked and melted into liquid form (tektites). It’s not clear that you could get a piece of rock that life could survive in to be launched beyond our atmosphere.
– “We can ask the question should we be worried about contaminating other planets in the solar system by landing spacecraft on them that might be carrying bacteria from us and thus contaminate the biosphere of another body in the solar system? And if it turns out that it’s relatively easy to get material inside rocks naturally landing on other places in the solar system, they may very well already have been contaminated by life from Earth.”
– “As yet we have no proof that one can blast intact low shock rocks off the surface of our planet to get past the barrier of the atmosphere.”
– “There are meteorites that are called tektites, which are melted glasses which we’re absolutely sure are Earth rocks that were melted in a large impact into liquid form and launched through the atmosphere, clearly going up through the atmosphere and then coming back and so we know impacts which are capable of getting material above the atmosphere exist, but this is highly shocked and melted material in which nothing could possibly survive. So we’re still looking for our first turine meteorite as we call material that comes back to us from the Earth itself.”