Pedagoguery

The origin of life remains a mystery. However, some aspects of it are becoming clearer, and with recent evidence that Mars was once warmer and wetter than it currently is, the inevitable question is did life emerge on Mars? Tied to that question is the intriguing possibility that life on Earth is descended from travelers from Mars.

Let us begin at the beginning. Mars, by virtue of its greater distance from the sun, probably cooled and solidified before Earth. Because of this, Mars probably developed oceans before Earth. So, life had the opportunity to evolve on Mars before it had that opportunity on Earth. While there is no guarantee that it actually happened, we are speculating, so let us assume that it did. Given that speculation, life on Mars probably got a head start of as much as a few tens of millions of years. During this time period, there was still a considerable amount of debris left over from the formation of the inner solar system. In addition, Jupiter was still throwing the icy leftovers from the outer solar system inwards, so there was ample opportunity for both planets to get hit, which happened with some frequency. Mars, once again by virtue of its position, got hit more often. And this is where things get interesting.

At this point, life on Mars had probably gotten no more advanced than simple bacteria. In order to shelter from solar radiation, some bacteria probably sheltered within rocks. We find similar organisms on Earth today. We also know that when a large meteorite hits a planet, particularly at a low angle, rocks can be blasted off the planet into solar orbit. We know this for a fact because we have found Martian rocks on Earth. What if one of those rocks contained some Martian bacteria? Could it have survived the journey to Earth? The answer appears to be possibly.

There are three obstacles to be overcome by an organism during the journey. The first is surviving the ejection from Mars into space. The second is surviving the space journey itself, which could last anywhere from less than a year to more than 15 million years. And the final obstacle is surviving entry into Earth's atmosphere and the landing. The first obstacle is surprisingly easy to overcome. The rock that contains the bacteria has to be far enough from the site of the impact not to endure too much shock heating, but close enough to be thrown clear. We have evidence from the Martian rocks that have ended up here that it can be done. The most famous Martian meteorite, ALH84001 showed no minimal of shock heating. So we know that the possibility exists for life to survive that far.

Life is resilient, but can it survive 15 million years in deep space? Certain bacteria on Earth have the ability to enter a spore state when conditions become too adverse. In this state, they can survive for millions of years. The problem with deep space, however, is radiation. There are several types of radiation to be watchful of. The first is solar ultraviolet. If the bacteria are in the interior of rocks, however, this does not present much of a problem, since the ultraviolet radiation will not penetrate very far into the rock, only a few microns. The real problem for deeper-dwelling microbes is high energy radiation like x-rays and gamma rays, as well as charged particles. The sun is the source of many of these, but cosmic rays are a big source of charged particles. All of these can penetrate deeper into the rock than ultraviolet. In addition, these types of radiation will also generate showers of secondary radiation when they hit an atom within material, so to a point, adding more shielding actually increases the radiation risk. Only a very large meteorite, one that is at least two meters in diameter, could completely shield organisms within itself. However, large rocks tend to take longer making the trip between Mars and Earth, while fist-sized rocks can sometimes make it less than a year. The shorter the journey, the lower the overall radiation dose, and the higher the chances for survival. Some bacteria can survive incredibly high doses of radiation. A species called Deinococcus radiodurans can actually survive inside nuclear reactors, for instance. The bottom line is that without further studies, we cannot rule out the possibility that microbes could survive the journey, particularly if it was a short one, on the order of decades or less.

What about entry into Earth's atmosphere? Dust particles have been shown to decelerate fairly gently in Earth's upper atmosphere. Meteorites, however, experience significant friction, and the outer layers typically melt. The heat pulse only has time to travel a few millimeters into the interior of the meteorite, so organisms deeper inside will typically be unscathed. Studies done on Martian meteorites have found that many known single-celled could survive the temperature ranges found.

So, the bottom line. Are we Martians? Possibly, but more study, both here and on Mars, is needed to find out.

Next issue: Is spacetime really a kind of fluid?