Does Mars have rights? And what about Europa, Ganymede, and Titan for that matter? The first question was asked in a 1990 essay by NASA astrobiologist Christopher McKay. The other celestial bodies mentioned are moons of Jupiter and Saturn that some researchers believe could harbor extraterrestrial life.

The 1967 Outer Space Treaty requires spacefaring nations to conduct exploration of the Moon and other celestial bodies "so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter and where necessary, shall adopt appropriate measures for this purpose." Overseeing this requirement for planetary protection of not only the Earth, but also other planets, moons, asteroids, and comets, is the international Committee on Space Exploration (COSPAR) headquartered in Paris and NASA's Planetary Protection Office. The goal of the treaty is to prevent back contamination—that is the introduction of extraterrestrial life to Earth—and forward contamination—the introduction of Earth life to extraterrestrial environments.

Concerning back contamination, the 1969 Apollo 11 moon mission crew and their haul of moon rocks were immediately isolated after splashdown in a Mobile Quarantine Facility on board the aircraft carrier that picked them up and were then transported to the Lunar Receiving Lab in Houston where they were quarantined for two weeks. Ultimately, the Apollo moon missions brought back about 400 kilograms of moon rocks and years of testing found no indication of life in them.

With regard to forward contamination of other parts of the solar system with terrestrial life, the two Viking landers sent by the U.S. to Mars in the mid-1970s were extensively cleaned and sterilized. The Viking landers conducted experiments trying to determine if there was life on the Martian surface. The tests produced results that were initially interpreted as negative. One of the main goals of sterilization is to prevent the experiments from mistaking hitchhiking Earth life for Martian life. The other chief goal is to protect the Red Planet from being infected by Earth microbes. The Mars landers launched by the Soviets in the 1970s all essentially crashed. What effective planetary protection measures the Soviets took to prevent the introduction of Earth life are not known.

The only sample return missions other than to the moon were the Genesis mission which sampled the solar wind (and crashed landed in Utah), the Stardust mission which collected samples from the tail of Comet Wild 2 in 2006, and the Japanese probe Hayabusa which sampled the Itokawa asteroid in 2010. None of the missions found anything that could be called alive. The next sample return mission, the Russian Phobos-Grunt robotic lander, will launch tomorrow with the goal of landing on Mars' moon Phobos to scoop up and return a sample of its soil.

The reason for avoiding back contamination is pretty clear; we want to avoid an Andromeda Strain scenario in which an unleashed alien life form harms Earth life including people. But what is the rationale for preventing forward contamination? As already mentioned, one chief reason is to prevent inadvertent contamination by Earth microbes from being mistaken as evidence for the existence of extraterrestrial life. But do we have an ethical obligation to prevent harm that might be caused by Terran life to extraterrestrial life? Even more broadly, do we have the right to change the environments of other worlds even if they do not contain any living organisms?

While still in the early stages of space exploration, it makes sense to try to prevent the inadvertent introduction of terrestrial life to other worlds while researchers pursue their search for extraterrestrial life. But others argue that sometime later in this century, humanity should begin the process of terraforming other worlds, most probably beginning with Mars. British planetary scientist Martyn Fogg provides a good definition of terraforming as "a process of planetary engineering, specifically directed at enhancing the capacity of an extraterrestrial planetary environment to support life. The ultimate in terraforming would be to create an uncontained planetary biosphere emulating all the functions of the biosphere of the Earth—one that would be fully habitable for human beings."

Mars as it is is not a promising home for Earth life; its average temperature is -60°C, well below Earth's average of 15°C; the pressure of its carbon dioxide atmosphere is one-hundredth that of Earth's; and it lacks an ozone layer so its surface is blasted by DNA destroying UV rays from the sun. Can it be made more hospitable? Science fiction author Jack Williamson coined the word terraforming in a 1942 short story in Astounding Science Fiction. Arthur C. Clarke further developed the idea of terraforming in his 1951 novel The Sands of Mars. In 1973, young astronomer Carl Sagan devised a proposal for melting Mars' South Pole by darkening it. This would boost carbon dioxide in its atmosphere, creating a greenhouse effect, which would warm the planet and allow water to flow.

In a research review in 1998, Martyn Fogg evaluated various suggested technical means to begin terraforming Mars. A runaway greenhouse effect releasing carbon dioxide might be jumpstarted by pumping potent man-made greenhouse gases like perfluorocarbons into the atmosphere or by directing extra sunlight onto the South Pole using a space mirror 250 kilometers in diameter. Once started, Fogg estimates it would take 100 years to build up a thick atmosphere and warm the planet enough so that anaerobic Earth life could successfully colonize the planet. Candidates for pioneering terrestrial microbes include the dessication-resistant cyanobacterium Chroococcidiopsis, the lime-boring cyanobacterium Matteia, and the ionizing-radiation resistant heterotrophic bacterium Deinococcus radiodurans. In addition, the Planetary Society is flying 10 hardy organisms as part of its Living Interplanetary Flight Experiment (LIFE) aboard the Phobos-Grunt mission. They will be returned with soil from Phobos to see how they fare in long exposure to space.

Another later step that might be taken is to genetically engineer earth plants so that they could survive in a low-oxygen environment and begin to pump oxygen into the atmosphere. It might take 10,000 to 100,000 years for the terraformed Martian atmosphere to contain enough oxygen for people to breathe unassisted.

It may be technically possible to transform Mars so that it is more hospitable to terrestrial life, but some ethicists argue that it would be wrong to do so. Do we have a moral obligation to leave Mars and other worlds alone?

Yes, argues Australian philosopher, Robert Sparrow in a 1999 article, "The Ethics of Terraforming [PDF]," in Environmental Ethics. An effort to terraform Mars, asserts Sparrow, "demonstrates two serious defects of moral character: an aesthetic insensitivity and the sin of hubris. Trying to change whole planets to suit our ends is arrogant vandalism." Developing what he calls an agent-based virtue ethics, Sparrow argues that what makes actions right or wrong is the character of the moral agent. Terraforming Mars indicates an ethically significant aesthetic insensitivity in the same way that a hiker wantonly whacks a transient but beautiful set of icicles on a wintry day that only she will see. "What is significant is the blindness the hiker has displayed to beauty even though no one else may suffer from its loss," he writes. The blindness is a vice. Similarly greening up with genetically engineered redwoods the Valles Marineris, the largest canyon in the solar system, would indicate that we do not properly appreciate its present desolate beauty.

The second moral defect demonstrated by terraforming is hubris, which "occurs when humans willfully ignore their limits and seek to become like gods." We should stay in our proper place, asserts Sparrow. "A proper place is one which one can flourish without too much of a struggle," he argues. In his terms our proper place is Earth and before daring to terraform Mars "we must show that we are capable of looking after our current home before we could claim to have any place on another."

Sparrow acknowledges that he does not offer an objective account of beauty, so it still resides in the eye of the beholder, as does desolate ugliness. As awesome as the view down Valle Marineris might be now, it would arguably be even more so with vistas teeming with life. To use Sparrow's metaphor, instead of icicles, one could see and appreciate a waterfall that has replaced them. With regard to the hubris of terraforming, one initial response should be a hearty so what? On the other hand, efforts at terraforming could help humanity toward moral improvement by increasing our understanding of just how precious terrestrial life is and aid us in managing it toward greater integrity, stability, and beauty.

Mars may not be lifeless. Some researchers believe that Martian life may have retreated to warm underground refugia as the planet's oceans dried up and froze hundreds of millions of years ago. Do we have any moral obligations toward Martian microbes, should they exist?

"If life is present on another world, the introduction of terrestrial life forms could lead to an ecological holocaust, a moral and aesthetic tragedy, as well as an immense loss to science," argued University of Oregon sociologist Richard York in a 2005 article, "Toward a Martian Land Ethic [PDF]," in Human Ecology Review. York is extending environmentalist Aldo Leopold's notion of a land ethic developed in his seminal 1949 A Sand County Almanac. Leopold argues that aesthetic and moral concerns must also be considered along with economic ones when it comes to figuring out what is the right thing to do with any piece of land and its community of living things. "A thing is right when it tends to preserve the integrity, the stability, and beauty of the biotic community," wrote Leopold. "It is wrong when it tends otherwise." But does that terrestrial land ethic apply to alien topographies and biota?

Martian life might be a "second genesis," that is, it arose independently of Earth life, or it might be the result of transpermia in which organisms were spread via meteors between planets where they took up residence. Perhaps life originated on Mars and eventually reached Earth where it thrived. In the transpermia case, what we could learn from Martian life would likely be more limited since it would be similar to terrestrial life.

Astrobiologist Christopher McKay argues [PDF] that if Martian life is a second genesis, then "its enormous potential for practical benefit to humans in terms of knowledge should motivate us to preserve it and to enhance conditions for its growth." Its utilitarian value might "exceed the opportunity cost of not establishing human settlements on Mars." In fact, McKay suggests that we might even try to restore the Martian environment to an earlier state under which indigenous organisms evolved so that they could better flourish. However, finding a second genesis so close to Earth would also suggest that the emergence of life is a relatively common occurrence in the cosmos, reducing the moral force of arguments for preserving Martian microbes. Preserving samples of Martian life for later study and use would be prudent before embarking on terraformation.

Dead planets and moons are not intrinsically valuable. And as fascinating as they might be, Martian microbes are no more moral agents than are terrestrial microbes. They simply do not have an ethical point of view that we must consider. On that account, there is no good moral reason why humans should limit the expansion of terrestrial life, including themselves, throughout the solar system.

Ronald Bailey is Reason's science correspondent. His book Liberation Biology: The Scientific and Moral Case for the Biotech Revolution is available from Prometheus Books.