Robert Zubrin says that there are no contamination issues involved in colonizing Mars, because microbes get transferred between the planets all the time on meteorites. His ideas get a lot of publicity, and so did a paper earlier this year "The overprotection of Mars". However there is another possibility, that life on Mars might be interestingly different from Earth life.

Earth and Mars - could life have got transferred from Earth to Mars or from Mars to Earth on meteorites in the past? Could this process still be going on today?So, how easy is it for a microbe to travel on a meteorite from Earth to Mars or in the other direction? Is this something that happens often, and can many species do this? Or is this something rare and unusual, that perhaps never happened at all?

Much of this derives from the American National Research Council (NRC) study in 2009 which came to the opposite conclusion to Zubrin and this recent paper.

Tons of meteorites from Mars

We do receive many meteorites from Mars. You can tell by the small samples of Martian atmosphere trapped in the rocks, which match spectroscopic measurements from Earth, and even more convincingly, direct measurements of isotope ratios by landers on Mars. They couldn't come from any other place that we know about in our solar system.

You may be surprised to learn that we get roughly similar numbers of meteorites from Mars and from the Moon (more about this later). Over centuries we certainly get tons of materials from Mars.

Transfer of life by meteorite

This is another point that nearly everyone agrees on. There is increasing evidence of the hardiness of some microbes in their dormant states or as spores. It's thought that many could survive transfer to another planet on a meterorite.

Late heavy bombardment that formed the craters on the Moon - life might well have existed on Earth at this time. - artist impression by Tim Wether

However the best time for this to happen was during the "late heavy bombardment" - the impacts that caused the largest craters on the Moon. This was a time of impactors hundreds, even thousands of kilometers across. The scars of this time can be seen on many of the objects in the solar system. It's called "late" because it is surprisingly late in the formation of the solar system, but is long ago from our perspective, these events happened about four billion years ago.

Mars was much more hospitable then, probably with liquid water on the surface, and sometimes, oceans as well. Conditions were ideal for transfer of life. It also seems possible that there was life already on Earth, and that it could survive the bombardment, and abundant water on Mars, and much material exchanged between the two planets, .

If life did get transferred, then that means Mars had life in the past, introduced via meteorite transfer during the late heavy bombardment. This life might still be there today in surface habitats and there would seem to be a high chance that it is there deep underground.

Optimism for search for life on Mars

This gives grounds for a great deal of optimism that if we look for it for long enough and carefully enough, we may have a good chance to find this life eventually. This doesn't rule out independently evolved life on Mars. The two types of life might be able to coexist or occupy different habitats.

If we don't find any life on Mars, that would be a remarkable discovery too, for a planet habitable for so long. We could learn much from finding out what did happen there, and what pre-biotic processes happened on a planet so like Eartj.

It could also go the other way. Life on Earth could originate from Mars. Mars formed further out in a cooler part of the solar nebula, and so might have been habitable for life before Earth was. Also if Earth did have life in the very early solar system, it was surely completely sterilized by the impact that created the Moon, as this would have turned the entire surface into molten rock. Mars had many large impacts at the same time, which would be devastating for surface life, but these didn't melt the entire surface of Mars, and some life could survive these impacts deep underground.

What about the present day solar system?

This is where opinions differ. It is less certain that any life is transferred between the planets in the present solar system. It may happen but is likely to be a rare event. Also only some hardy microbes could ever make this transfer, most would not survive the vacuum, UV radiation and cosmic radiation.

So, let's look at this in more detail.

How often do meteorites leave Mars for Earth?

From our estimates of meteorite numbers, it is clear that Earth must receive many meteorites from Mars every year (most land in the sea or uninhabited areas). So how often do the meteorites leave Mars? You might guess perhaps many meteorites every year again. But this is one of those conundrum type puzzles where our first impressions easily lead us astray.

If you stop and think about it some more, though, for your meteorite to leave Mars it has to be ejected with escape velocity. Clearly this requires a fairly large impact, a micrometeorite couldn't do it. It turns out that you need an imapct that should be easily visible from orbit about Mars. This has sparked a search of our images of the planet, to try to find these source craters for the Maritan meteorite.

Impacts on Mars and rayed craters

Zunil crater, a strong candidate for origins of some of ourMartian impactsYou can do theoretical modelling of the craters to guide this search, and also fire pellets at targets and scale the results up to Mars impacts. It turns out that you need an oblique impact powerful enough to form a crater about 10 km in diameter, or a direct hit causing an impact crater about 100 km across. The impactor would be about 1 km across, and you would get impacts like that roughly every one or two million years. The search has turned up some interesting rayed craters on Mars that might perhaps be the sources of Martian meteorites.. Here is a detailed study of the formation of the young Zumil rayed crater, including impact simulation of a 1 km impactor at an oblique angle. They found that some of the material from this impact would eventually reach Earth. It is a strong candidate for a source for some of our Martian meteorites..

You can also look at the cosmic radiation ages of the Martian meteorites we have on Earth. This tells you how long they were exposed to cosmic radiation during the crossing from Mars to Earth (while geological age shows the age since the rock first formed). Thee youngest meteorite, EET 79001, is from 730,000 years ago. Then there is a cluster of meteorites all from 1.2 million years ago, and there are other clusters every one or two million years going back about 20 million years.

So, theoretical predictions and the cosmic radiation ages are in agreement. It's that all the Martian meteorites we receive on Earth were sent here by large impact events on Mars roughly every one or two million years.

Long journey from Mars to Earth

Typically, the material takes a long time to reach Earth. If a large 1 km across meteorite hits Mars today, at an oblique angle, and creates a rayed crater like Zunil crater, we can expect the first meteiorits to arrrive on Earth about a century from now. At first only a few rocks will arrive, then they build up to larger numbers after about 16,000 years. The numbers keep building up for a few million years, then die away, and are nearly all exhausted within about 20 million years.

It is no surprise this takes so long. The meteorites are ejected with slighlty over the escape velocity, and go into a similar orbit to Mars. After several flybys of Mars,some hit Mars again, but most end up being diverted elsewhere in the solar system. Some will go past Earth and some of those end up in Earth crossing orbits. Some of those eventually impact Earth, a similar number hit Venus. Most take at least thousands of years to get here, and they continue to arrive for about 20 million years. Over these longer periods of time the meteorites tail off again rather rapidly, because they get lost into the sun or Jupiter. (see The Exchange of Impact Ejecta Between Terrestrial Planets )

BTW this also helps explain the puzzle of the small number of meteories from the Moon. If there is a big impact on the Moon today then for some thousands of years we will get many lunar meteorites on Earth. Eventually these will be exhausted, because Earth is such a big target for a lunar meteorite in an Earth crossing orbit. Others hit the sun or Jupiter, and before long on a geological timescale they are nearly all gone, and we are back to the present situation again.

The most recent meteorites to leave Mars for Earth

The EETA 79001 martian meteorite, with a young cosmic radiation age of 730,000 years Photo courtesy of Johnson Space CenterThe last time a meteorite left Mars for Earth might well be about 730,000 years ago, the cosmic radiation exposure age of our youngest Martian meteorite.

This is long enough ago so that even the radio resistant radiodurans, tough enough to survive in reactor cooling ponds, wouldn't be able to survive the cosmic radiation dose if anywhere near the surface of the rock. Cosmic radiation is the highliy energetic radiation from elsewhere in our galaxy, and even from other galaxies. It penetrates meters into rocks. Here for instance is a study of Mars cosmic radiation exposure - after 450,000 years there would be no viable life left in dormant state to a depth of 2 m.

So, you would need a rock several meters in diameter, for microbes in its interior to survive passage to Earth for hundreds of thousands of years.

There is next to zero chance of viable martian microbes arriving on Earth on the surface of these meteorites or in the interior of the smaller present day meteorites from Mars. They have been travelling in interplanetary space for too long.

So, many of those many tons of meteorites arrive on Earth from Mars are thoroughly sterilized by cosmic radiation and have no viable dormant life.

Life could still get to Earth if it is deep inside large Martian meteorites, meters across. The best chance for life to get to Earth however is almost immediately after the impact on Mars.

The most recent meteorite to leave Earth for Mars

Our rovers haven't yet found any meteorites from Earth on Mars, But theoretically, it is thought that a meteorite of about 10 km across impacting on the Earth would be large enough to send debris with escape velocity through the Earth atmosphere. It would create a crater about 100 km in diameter. The meteorite that helped to bring an end to the Cretacious (dinosaur) era sent meteoritic debris all over the surface of the Earth, creating the distinctive Iridium layer of the Cretaceous–Paleogene boundary. This size of impact might well be large enough to send debris into space with escape velocity.

Artist impression of asteroid impact that lead to extinction of the dinosaurs. This painting by Donald E. Davis depicts an asteroid slamming into tropical, shallow seas of the sulfur-rich Yucatan Peninsula in what is today southeast Mexico

Artist's impression of the Chicxulub impact by Don Davis

All the known large impact craters on Earth are tens of millions of years old. It seems unlikely that there is any viable Earth life currently landing on Mars, unless inside really large rocks say 15 or 20 meters in diameter. Also over those timescales, the natural radioactivity of the rock itself can be an issue.

Most of the material from Earth will be sterilized by the time it reaches Mars, as for the journey the other way around. There would be a short "window of opportunity" after the origiunal impact. The first few millennia after the impact would be the best time for Earth life to get transferred to Mars.

How easy is it for a microbe to travel on a meteorite

Well first, it turns out that the material that reaches escape velocity comes from the outside edge of the spreading crater. This material is only lightly shocked, and not melted. Many microbes could survive this. Their best chance is in "rayed craters" which form if a meteorite hits Mars at a shallow angle. Like the Tycho crater on the moon these would send material at great distances over the surface of Mars and also quite probably into orbit.

The microbe also has to survive in a vacuum, or at least, without new supplies of air or water, for the time it takes to get to Mars. They also have to survive cosmic radiation damage over that time period.

Many microbes can go into astonishingly hardy dormant states or spores. Many can survive millennia and even millions of years in such a state with no nourishment at all. Cosmic radiation damage can be repaired when the spore revives if it is not too extreme. But in inteplanetary space they don't have the protection of the Earth's atmosphere and so would be unable to survive long periods of cosmic radiation damage.

Impact and re-entry

The microbes then have to survive the impact on Mars, and again experiments and modelling suggest that many could survive that too.

Atmospheric re-entry is generally not a problem as only the surface of the meteorite heats up and is ablated. This would however be an issue for any microbes that survive only on the surface of the rock.

Certainly there are many microbes that could survive the journey, though many others could not.

It's a similar story the other way around from Mars to Earth, the journey would be a similar one. A Martian meteorite which hits the Earth will get more heating of the meteorite in the atmosphere, but this only affects the outer skin of the meteorite. The impact on Earth would be gentle enough so that many microbes could survive it no problem.

How likely is it that the metorite will contain life, and find somewhere habitable

The main challenge for a microbe from Mars is to get to Earth. Once it gets here, there is a chance of finding a decent habitat, An obligate anaerobe (i.e. if oxygen is poisonous for it) would have a harder time finding somewhere hospitable on a random spot on the Earth. A microbe able to tolerate oxygen has a good chance.

A microbe from Earth to Mars has a good chance to reach Mars (a few of the many microbes in the meteorite) then the main challenge is to find a suitable habitat on Mars when it lands.

How likely is it that a Martian meteorite had life on it when it left Mars?

Let's look at a meteorite from Mars first. Nearly all the recent Martian meteorites, ones that left Mars in the last 10 or 20 million years, are volcanic rocks, though some have salts in them, which is promising. On Earth, volcanic (igneous) rocks do contain microbes, living in miniature fissures filled with ground water. But on Mars it is harder for life to colonize igneous rocks, because of the thin atmosphere which dries out the surface. It's thought that most near surface Martian igneous rocks will be either completely dry (if right on the surface) or too cold to support life.

Fragile habitats on Mars

In my article "Might there be microbes on the surface of Mars" I talked about some of these recent ideas about possible habitats on Mars. There are many possible habitats known now, including several new suggestions in the last decade or so. But most seem unlikely candidates for material ejected from Mars at escape velocity.

First, they are nearly all surface or near surface habitats, at most a few cms below the surface of Mars. The cosmic radiation studies of the Martian meteorites received to date show that the meteorites originate from at least some meters below the surface where temperatures on Mars are typically -50C or so, unlikely to have life.

So the first question is, could these fragile surface habitats be ejected in the same way as the sub surface meteorites in our collections?

I don't kow of any detailed study of this question, so will just pose some questions to think about. Let's look at some of the main possibilities.

There may be life in the soils in deposits of deliquescing salts, and perchlorates, a cm or so below the surface. Would these salts be ejected all the way through the atmosphere at escape velocity, or just spread out in the atmosphere as dust?

There may also be life in thin melt layers in the polar regions below ice sheets. The top of the soil and rock would be heated up by the solid state greenhouse effect of dry ice or water ice, and thin layers of water would form (as they do on Earth), possibly habitable..

These are both thin, fragile, surface habitats.

Warm seasonal flows on Mars (Newton crater) Unlike the dry gullies, these are thought to involve some form of liquid flow, this is one of the top candidates for a possible habitat for life on Mars.Other habitats like the warm seasonal flows and the Martian geysers only occur in a few special locations, so it would be rare for a meteorite to hit them.

Rocks with salt deposits in them seem ideal since the salts can deliquesce and take up water from the atmosphere - such habitats exist on Earth. Some Martian meteorites have these salts, so is not impossible that there could be life in a Martian meteorite. But the interior of a rock is likely to be very cold on Mars, and dry, and not such a good place for life to inhabit as it is on Earth. The life may only exist close to the surface of the rocks.

Deep habitats on Mars

The best place for life on Mars may be in rock strata deep enough for water to be liquid. However this is a very deep habitat. It is about 10 km below the surface in the equatorial regions, and about 15 km below the surface at the poles, the approximate depth where liquid water is stable due to combination of geothermal heat and high pressures. If there is water down there, then it could potentially have abundant microbial life. Salty solutions would be liquid at shallower levels, but still kilometers deep. However none of these habitats are close enough to the surface to be excavated by a typical large meteorite.

There may also be life in the rocks below but closer to the surface around geothermal hot spots. Mars has been geologically active in the geologically recent past and so must have hot spots not too far from the surface. But Mars is far less geologically active than the Earth. There are no known currently erupting volcanoes. It is not inert, but it is only marginally active, and no geothermal hot spots have yet been identifired from orbit. So, these spots would be rare, if they exist, and hard for a meteorite to excavate them. It might well be harder for a meteorite to hit a hot spot on Mars than to hit a volcano on Earth.

Uninhabited habitats

Then there is the other issue, that some habitats on Mars may be uninhabited. On Earth any habitat suitable for life gets inhabited quickly. On Mars, then conditions are far harder for life to spread. The air is so thin, the UV radiation and cosmic radiation don't help, and the life is likely to be sparse and may be slowly metabolising. The dust storms may help spread life. Still it seems quite possible that some of these habitats may be uninhabited. So the meteor impact also has to land not just on a habitat but an inhabited habitat.

All this would seem to suggest that most meteorites that arrive on Earth from Mars probably never had present day Martian life in them.

What about travel the other direction, from Earth to Mars?

For the direction from Earth to Mars, then it is the reverse problem. Almost all the meteorites would contain at least some microbes, but they have to find a habitat on Mars. These habitats may be rare, so what is the chance that a meteorite with Earth life in it would hit them? Also would the life on the meteorite be suitable for Mars?

Any microbe that depends on oxygen (aerobe) is unlikely to find a a suitable habitat on Mars, at least as far as we know. Also most microbes depend on conditions created by other microbes, which may not exist on Mars.

They also need to be able to survive the thin atmosphere, the UV and cosmic radiation, the extreme cold and the huge changes of temperature (in the equatorial region the top few cms can swing from far colder than Antarctica to as warm as the tropics in a single day, and further down is permanently tens of degrees below zero degrees centigrade).

The C-Pg impact event was an impact into a sulfur rich shallow tropical ocean, perhaps not the most ideal habitat for life capable of living on Mars.

An ideal microbial Martian colonist

The ideal microbial colonist of Mars would probably be an anaerobic primary producer (so only needs basic elements like rock, water, carbon dioxide etc), resistant to UV and cosmic radiation, and ideally able to survive in a single species ecosystem too.

So for instance, the cyanobacteria Chroococcidiopsis would almost certainly reproduce on Mars if it survived the journey and landed in a suitable habitat when it got there. It is a primary producer, would probably be able to survive on Mars, and occurs in single species ecosystems below the surface of rocks in the Atacama desert. Other microbes that live in salt deposits on Earth, e.g. dried up soda lakes, or a couple of meters below the surface in the Atacama desert would probably be suitable too.

So this could happen, but how likely is it that they would make the journey on a meteorite to Mars, and also end up in the right habitat when they get there?

Finally the bottom line is that all this is theoretical and to date no-one has found unambiguous evidence of transfer of life from one planet to another by meteorite. It is believed that it probably can happen, but we have no "ground truth" to test our theories and predictions about how it might happen.

If life did transfer between the planets, it had plenty of time to evolve in different directions

Even if life on Mars originated on Earth, say 40 million years ago, that is long enough for a tarsier to evolve into a human being. Also long enough for mammals and marusupials to evolve as distinct genera.

Especially with the different environment of Mars, with cold, vacuum, UV, cosmic radiation etc, you could expect microbes that have evolved on Mars for that long to be significantly different.

Is it really possible that Mars has life? We have been looking for a long time and not found any yet.

You could be forgiven for thinking that Mars is unlikely to have interestingly different forms of life since we have searched for several decades and not found anything. But there are also reasons why the search has been so hard on Mars.

The surface conditions are so very harsh there. The most habitable regions on Mars are similar to the cold dry deserts of the high Atacama, and the McMurdo dry valleys in Antarctica - but with almost no atmosphere, no oxygen to speak of, even greater extremes of cold, and less light. In these regions on the Earth life is found, but it is hard to spot. We haven't yet sent any life detection instruments to the most likely habitats. The last life detection instruments sent to Mars were in the 1970s on Viking I and II when we knew much less about Mars than we do now.

Instruments have been built that might have a good chance of detecting life on Mars if sent there. But they haven't yet been flown. And though our orbiters can study the entire surface of the planet, our rovers are currently limited in scope only able to travel at most a bit over 100 meters in a day. Also they can't land close to most of the most interesting terrain on Mars because they need a large flat "landing ellipse". Curiosity's landing list only just fitted into the gap between Mount Sharp and the wall of Gale crater.

Then, life on Mars may be hard to detect, if it has a different chemical basis from Earth life, and is also slowly metabolising and sparse in population. Especially if it is some primitive early form of life smaller than the archaea and similar in size to nanobes - one estimate for the size of a primitive pre-archaean cell is 50 nanometers, well below the nanometers diffraction limit for optical microscopy.

This is a nanobe, a few tens of nanometers across, too small to see in an optical microscope. Some think they may be alternative forms of life. Whether they are or not, certainly there must have been a stage in the past with cells smaller even than the ultramicrobacteria, one estimate is that primitive pre-archaean cells were about 50 nanometers in diameter, so similar in size to a nanobe

So how isolated are the planets from each other?

I think a good analogy to make is with continents for higher animals. Some migratory birds, bats, and insects fly from one continent to another. Sometimes a small mammal may be carried between islands on debris e.g. in a tsunami. But most of the higher animals will never make the transition.

So in the same way some rare microbes may be able to make the transition from Mars to Earth or vice versa, but most would never be able to take the trip, or highly unlikely to do it, or to survive when they get to their destination.

Arctic tern in flight. This small sea bird flies from the Arctic to Antarctica and back again every year, to take advantage of the polar summers By Mylen Tharssen

Arctic Tern - this small bird makes the migration from the Arctic summer to the Antarctic summer and back again every year (never sees winter). They usually travel far off shore while on migration, but may occasionally pass over land.

Some birds, especially sea birds, will be the same in Australia as anywhere else (some arctic terns for instance migrate through Australia on their annual migration from the Arctic to Antarctica and back again ). However, many of its mammals are unique to that island. Nowadays no scientist would recommend deliberate introduction rabbits, rats and other mammals to Australia or other isolated islands, except in extraordinary circumstances, and there are laws to prevent this from happening.

In the same way it's possible that Mars and Earth have some microbes with a relatively recent shared evolution. Who knows, perhaps Chroococcidiopsis might perhaps turn out to be an example of a microbe on both planets, this microbe might be the "arctic tern" of interplanetary microbes? Some of the shared life could have branched from each other as recently as a few tens of millions of years ago.

It is however more isolated than Australia. Even microbes that made the transfer tens of millions of yeas ago have had a great deal of time to evolve differently from their Earth counterparts. Any Martian Chrooccopidiosis probably wouldn't be exactly the same as its Earth counterpart.

Then, as for the higher animals and Australia, probably most microbes never made the transition to Mars.

If this is true, it has a positive and a negative side to it.

On the positive side, this is potentially great news for scientists. Martian life probably evolved independently at least for millions and probably for billions of years. In the best case, life on Mars may be is as interestingly different as life on planets light years away.

On the negative side, we may need to be cautious about sending humans to Mars in case they introduce Earth life. We also need to be cautious about returning Martian life to Earth.

However, Mars may not be the best place to colonize. And we can explore it "in situ" using real time telerobotics without sending humans to the surface itself. For more about all of this, see "Ten reasons not to live on Mars, great place to explore".

Zubrin has another argument, that even if life on Mars is different from Earth life, it is not going to be dangerous to colonists. I look at that in the next article: If Mars Is Interestingly Different For Life, Which Is Best, Freedom To Colonize, Or Freedom To Explore Mars In Its Pristine State?

See also

Video talks

I have started a series of short talks on contamination issues in my youtube channel on Mars and space colonization. Recently did some talks about whether we should return a sample from mars. The second one is about this topic of transfer of microbes on meteorites.

I've also done some video presentations with slides, on Reasons not to live on Mars, great to explore

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