A few days after the successful launch of NASA’s behemoth Curiosity rover with its Mars Science Laboratory instruments on November 26th 2011, a somewhat muted piece of news came out admitting that the strict biological planetary protection rules had not been adhered to quite as everyone expected. What this meant in practical terms was that the rover’s drill bits were not sealed up for launch with quite the same protocols for sterility as everyone had expected. Thus there is an added possibility that alien invaders from Earth are heading for Mars.
The reasons for trying to keep Curiosity and all of its bits and pieces effectively free of any Earthly biological microbial contamination are twofold. First, you don’t want to get to Mars, start sniffing for interesting organic chemistry and end up detecting someones nasal microbiome, or some other bits of our rich and soupy broth of organisms. That really messes up one’s ability to find martian life, whether extinct or extant. The second reason is that we don’t want to forward contaminate Mars, unleashing our alien fauna on what might be a pristine or ecologically fragile world. Even the landing site of Curiosity has been chosen to avoid any obvious water-ice deposits within 3 feet of the surface, for fear of contaminating the Martian hydrological system.
The minor breach in protocol for Curiosity’s drills is unlikely to spell impending biological apocalypse for Mars, but it does raise some fascinating questions, including whether humans have already contaminated Mars, and whether Nature has beaten us to it by hundreds of millions of years anyway.
… there is another route for biological alien invasion, one that has nothing to do with us, and which has been active for approximately 4 billion years. This is known as “impact transfer”, the ejection of material from a planetary surface during collisions with asteroids or comets, and its subsequent travel through space until sometimes falling into the gravity well of another planet or moon. The chain of events may go like this: a large kilometer scale asteroid hits the Earth’s continental surface at an oblique angle. During the moments of impact a “spall layer” of Earth’s crust can be accelerated to escape velocity, thrusting a mess of rocky particles and chunks up out of the atmosphere and into space.
Although this material experiences severe g-forces and temperature fluctuations, experimental studies indicate that conditions would certainly allow for hardy microbial or microscopic hitchhikers to be carried aloft.What happens next is a fascinating result of celestial mechanics. The range of velocities and trajectories of impact ejected material result in a multitude of pathways. Some of the spall will fall back to Earth promptly, some will fall back over weeks and months, some will enter orbital routes that carry it away for tens of thousands of years before it too returns to Earth a pathway that could put terrestrial chemical and biological material into “cold storage” during an episode of apocalyptic destruction on the homeworld. Other chunks go much further, entering what can be thought of as a very slow and very inefficient orbital conveyor belt. Some of this material can traverse interplanetary space only to be swept into the gravity well of another world or moon, including that of Mars, where it can rain down onto the planetary surface.Simulations of these impact ejecta “transfers” indicate that over billions of years collisions will have resulted in bits of Earth as well as other solid bodies being spread out across the solar system, even arriving at places as distant as Europa or Titan.
In these cases the ‘hit rate’ is low, perhaps one in ten million bits of Earth ejecta might ever make it to Europa or Titan over a million years, a smaller number would make the journey significantly faster. For Earth-to-Mars transfers the rate can be much higher, one in every thousand chunks of material ejected in a single impact event will make it to Mars in every million year interval after that initial dispersal into space. So over time the flux of transferred material adding up all impact events can be very significant. …