Swapping Rocks - Exchange of Surface Material Among the Planets

Extract from the July 1994 issue of Planetary Report



The returning Apollo 11 astronauts' triumphal reception in July 1969 was somewhat delayed by a strict and lengthy biological quarantine. In those days, no one was certain that the Moon was entirely sterile. No one knew whether the lunar rocks might harbor deadly microorganisms. One wonders whether the level of concern would have been as high if scientists had known that dozens of lunar rocks had been lying in the Antarctic ice for thousands of years, or that about 10 small fragments of the Moon must fall onto Earth's surface every year. Unfortunately for the astronauts, the first lunar meteorite was not recognized until 1982. Before that time, no one seriously believed that nearly unaltered rocks could be blasted off the surface of one planet and later fall onto the surface of another.

Now, however, not only do we know that lunar rocks occasionally fall to Earth, but we are also reasonably certain that a group of nine meteorites, the so-called SNCs (for Shergony, Nakhla and Chassigny, named after the sites where they landed), originated on the planet Mars. Although all of the lunar meteorites were collected long after they fell, four of the SNCs were observed dropping from the sky. In 1911, a piece of Nakhla, which fell near Alexandria, Egypt, killed a dog, scoring the only known fatality (of a mammal) caused by a meteorite. [update: more recent work suggests otherwise]



The total flux of martian material falling onto Earth has been estimated at about half a ton per year. Under these circumstances, it may seem silly to worry about hypothetical martian organisms contaminating Earth, since martian material has evidently rained down on our planet throughout its history. Although a good case can be made for limiting modern biological contamination of Mars by terrestrial spacecraft, the discovery of Mars rocks on Earth brings up the immediate question of whether Earth rocks have been ejected into space, eventually to fall onto Mars, thus closing the circle of potential contamination.

Blasting Rocks off Planets

Older work on the maximum velocities achieved by impact ejecta focused on the relationship between the pressure in the shock wave generated by the impact and the velocity of material just behind the shock. Measured directly in laboratory experiments, the shock pressure needed to accelerate material to planetary escape velocities--2.4 kilometers per second (about 5,000 miles per hour) for the Moon, and 5.0 kilometers per second (about 11,000 miles per hour) for Mars, implying pressures of 0.44 and 1.5 megabars (a megabar equals 1 million times Earth's atmospheric pressure at sea level) for lunar and martian basalts, respectively-would have been high enough to melt or even vaporize the ejected rock. Yet study of the lunar meteorites indicates that their ejection was accompanied by no more than about 0.2 megabar of shock, and the most highly shocked martian meteorites (which contain pockets of once-melted glass) still indicate only about 0.4 megabar.

The problem with the pressure-velocity relationship is that it applies only to material completely engulfed by the shock wave. Very close to the target surface, however, the ambient pressure is zero. No matter how strong the impinging shock wave, the free surface can never be raised to a pressure higher than zero. This effectively shields surface rocks from strong compression. However, the pressure increases very rapidly with depth below the surface, which translates into a powerful acceleration that throws lightly shocked surface rocks our at speeds comparable to the original impactor's speed.

An experiment performed several years ago by Andy Gratz and colleagues at the Lawrence Livermore Laboratory has verified the general correctness of this model. An aluminum projectile about the size of a penny was fired at a granite block at about 4 kilometers per second (9,000 miles per hour). Material from the face of the block was ejected at about 1 kilometer per second (2,000 miles per hour). This material was caught in a foam cylinder and, upon analysis. proved to be composed of millimeter-size, lightly shocked fragments of granite.

Furthermore, blocks up to a meter in diameter from the uppermost limestone layer surrounding the 24-kilometer diameter (l5-mile) Ries impact crater in southern Germany have been found nearly 200 kilometers away in Switzerland. Although they were not actually ejected from Earth, these blocks again show a combination of low shock damage (less than 10 kilobars. 10,000 times Earth's atmospheric pressure at sea level) and high ejection velocity (1.4 kilometers per second or about 3,000 miles per hour). Thus, current theory, experiment and observation all agree in indicating that a small quantity of material near the surface surrounding the site of an impact is ejected at high speed while suffering little shock damage.

Impacts such as the one that created the 180-kilometer diameter (110-mile) Chicxulub crater in Yucatan 65 million years ago (and incidentally caused a profound extinction that wiped out the dinosaurs, among others) may have launched millions of rock fragments 10 meters (30 feet) or more in diameter into interplanetary space. Of these fragments, a small fraction, perhaps 1 in 500, would have been so lightly shocked that internal temperatures remained below 100 degrees Celsius (212 degrees Fahrenheit). Higher temperatures would presumably kill any microorganisms present in the rock, but a few thousand of the ejected rocks, those originating nearest the free surface, could have carried viable organisms into interplanetary space. Although such impacts are fortunately rare at the present time (the only comparable craters known are the 1.85-billion-year-old Sudbury crater in Ontario and the 1.97-billion-year-old Vredefon crater in South Africa), the much higher cratering rate early in solar system history during the period of late heavy bombardment that lasted up to about 3.8 billion years ago would have made ejection of microorganisms a much more common occurrence at that time.

The most lightly shocked rocks ejected at high speed are necessarily those closest to the free surface. The surface is also the place where biological activity is highest, so that a large impact on Earth, or on an earlier life-harboring Mars, would be very likely to throw rocks that might contain microorganisms into interplanetary space. Larger organisms, even if present, would be unlikely to survive the 10,000 g accelerations accompanying the launch process.

Current cratering calculations indicate that large lmpacts even on Venus, despite its dense atmosphere, could eject surface rocks into interplanetary space. Meteorites from Venus have not yet been discovered, but there appears to be no reason why they might not someday be found on Earth. Large impacts on all of the terrestrial planets are thus capable of ejecting lightly shocked surface rocks into interplanetary space. If there should be microorganisms on the surfaces of these planets, then they too have a chance of journeying to another planet.

Between the Planets

After a long series of such encounters. a few fragments' orbits get "pumped up" sufficiently to cross Earth's orbit. Then the more massive Earth takes over this cosmic volleyball game. changing the orbit still more, until the fragment may become Venus crossing. Sometimes the fragment is deflected all the way out to Jupiter or Saturn, which themselves may eject it from the solar system entirely. At any stage of this random walk through the solar system, the fragment may actually hit one of the planets, ending its journey.

Natural orbital perturbations thus supply the means for rocks ejected from one planer to spread throughout the solar system and eventually fall onto another planet (or leave the solar system entirely). This is presumably how the SNC meteorites reached Earth. Any microorganism contained in these rocks would thus have an opportunity to colonize the new planet, if it was able to survive both the journey and the fall to its destination.

Surviving the journey

Many microorganisms stand up surprisingly well to the space environment. Subjected to high vacuum, some bacteria quickly dehydrate and enter a state of suspended animation from which they are readily revived by contact with water and nutrients. Medical laboratories routinely use high vacuums for preservation of bacteria. Viable microorganisms were recovered from pans of the Surveyor 3 camera system after three years exposure to the lunar environment. However, these instances of preservation have only been tested over times approaching decades, not over the tens to hundreds of millions of years necessary for interplanetary travel.

Nature, however, has been kind enough to give us several instances of really long-term preservation of viable microorganisms. Chris McKay of NASA Ames Research Center has extracted microorganisms preserved for perhaps as long as 3 million years from deep cores in the Siberian permafrost. Even more impressive is the discovery of bacteria that were preserved for some 255 million years in salt beds of Permian age discovered at a site in New Mexico. Dehydrated by contact with salt and protected from radiation by the salt's low content of radioactive elements, these ancient bacteria demonstrated their viability by causing the decay of fish that had been packed with the salt.

Living bacteria can tolerate extremely high radiation doses, far higher than any multicellular organism can withstand. They can resist the effects of radiation largely because of active DNA repair systems. It is less clear that a dormant bacterium could tolerate large amounts of radiation. However, if the microorganisms happened to be living in cracks or pores of rocks that were ejected as large blocks, the rock itself might provide adequate shielding against both cosmic rays and ultraviolet light. Since it requires about 3 meters (about 10 feet) of rock to shield against high-energy galactic cosmic rays, if the impact event were to throw out rock fragments of about 10 meters (30 feet) diameter or larger, a significant interior volume would be protected against this radiation. Ultraviolet light can be screened by only a few microns of silicate dust, so the interiors of large ejecta blocks might be excellent havens for spacefaring bacteria.

Entering a New World

The fate of a meteorite entering a planetary atmosphere depends largely upon its initial size and speed. Small meteorites, smaller than a few centimeters, burn up in Earth's atmosphere. Very large ones, a kilometer or more in diameter, traverse it without slowing and make craters. Meteorites of intermediate sizes, a few meters to tens of meters, however, are significantly slowed by the atmosphere. Buffeted by kilobars of aerodynamic pressure, they break up in the atmosphere (as did the famous Peekskill meteorite that disintegrated over the eastern United States on October 9, 1992) and may eventually fall to the ground in a shower of small fragments. Even on the modem Mars, with its tenuous atmosphere, meter-size meteorites are greatly slowed before striking the surface.

This scenario of slowing and breakup of intermediate-size meteorites is nearly ideal for the dispersion of microorganisms onto the new planet. Whether or not these organisms can survive and multiply depends, of course, on conditions at their new home. It seems unlikely that terrestrial organisms arriving on the modern Mars or Venus would survive. However, in the past conditions may have been much more hospitable on Mars at least, and perhaps at that time microorganisms from Earth found a home on Mars, or vice versa.

The current impact-exchange rates among the terrestrial planets are relatively low. However, during the era of heavy bombardment, when most of the visible craters on the Moon and Mars formed, cratering rates were thousands of times higher than current rates. Blue-green algae were apparently present on Earth as early as 3.5 billion years ago. and life may have been present even earlier, overlapping the period of heavy bombardment. Given the possibility of exchange of life among the planets by large impacts, we may have to regard the terrestrial planets not as biologically isolated, but rather as a single ecological system with components, like islands in the sea, that occasionally communicate with one another.

Although this scenario is highly speculative, it may be testable: If sample returns from former lake deposits on Mars should contain evidence of the existence of a microbiota, it may be possible to extract organic molecules from the samples. If familiar terrestrial molecules such as DNA, RNA and proteins are discovered, and especially if a genetic code similar to that of terrestrial organisms is found, then it would provide very strong verification of the idea that Earth and Mars have exchanged microorganisms in the past. Naturally, any such test requires that we be very careful not to contaminate the samples beforehand with terrestrial organic molecules.

H. Jay Melosh is a professor of planetary science at the Lunar and Planetary Laboratory at the University of Arizona. His latest book, Impact Cratering: A Geologic Process, has been published by Oxford University Press.

In an article titled "Some mysteries of Planetary Science" in the May/June 1988 issue of Planetary Report, Carl Sagan wrote:

A very recent and interesting analysis by H.J. Melosh at the University of Arizona shows that in the course of generating a l00-kilometer impact crater, debris can be transferred from Mars to Earth or vlce versa; the chunks being transported are big enough that anything inside them does not suffer significant radiation damage from the solar wind or solar ultraviolet light or cosmic rays during the roughly million-year time scale it takes to be ejected from the one world and swept up by the other. This has a serious biological implication. It suggests that on time scales of tens to hundreds of millions of years microorganisms from Earth arrive at Mars.

Is this true? If they survive the journey, would they survive the martian environment? And if we go to Mars looking for microbes and find one, is it indigenous, or is it some contaminant from Earth? How do we find out? And then there's even the remote possibility that a long time ago terrestial microbes got transported to Mars, sat around and maybe proliferated a while, and then another big impact on Mars ejected their remote descendants back to Earth. How much did they change in the meantime?

Jay Melosh is also participating in a gigantic collaboration to do a careful analysis of the transfer of viable microorganisms from one planet to another that will be published soon in Icarus:

Mileikowsky, C., Cucinotta, F., Wilson, J. W., Horneck, G., Lindgrin, L., Melosh, H. J., Rickman, H. and Valtonen, M. (1998) Natural transfer of viable microbes in space, Part 1: From Mars to Earth and Earth to Mars, submitted to Icarus. (not published as at May 1999) - Paul Davies: "It turns out to be even easier than we thought to cross-contaminate the planets."

