Interplanetary contamination
Interplanetary contamination refers to biological contamination of a planetary body by a space probe or spacecraft, either deliberate or unintentional.
There are two types of interplanetary contamination:
- Forward contamination is the transfer of life and other forms of contamination from Earth to another celestial body.
- Back contamination is the introduction of extraterrestrial organisms and other forms of contamination into Earth's biosphere. It also covers infection of humans and human habitats in space and on other celestial bodies by extraterrestrial organisms, if such organisms exist.
Current space missions are governed by the Outer Space Treaty and the COSPAR guidelines for planetary protection. Forward contamination is prevented primarily by sterilizing the spacecraft. In the case of sample-return missions, the aim of the mission is to return extraterrestrial samples to Earth, and sterilization of the samples would make them of much less interest. So, back contamination would be prevented mainly by containment, and breaking the chain of contact between the planet of origin and Earth. It would also require quarantine procedures for the materials and for anyone who comes into contact with them.
Overview
Most of the Solar System appears hostile to life as we know it. No extraterrestrial life has ever been discovered. But if extraterrestrial life exists, it may be vulnerable to interplanetary contamination by foreign microorganisms. Some extremophiles may be able to survive space travel to another planet, and foreign life could possibly be introduced by spacecraft from Earth. If possible, some believe this poses scientific and ethical concerns.Locations within the Solar System where life might exist today include the oceans of liquid water beneath the icy surface of Europa, Enceladus,
and Titan.
There are multiple consequences for both forward- and back-contamination. If a planet becomes contaminated with Earth life, it might then be difficult to tell whether any lifeforms discovered originated there or came from Earth. Furthermore, the organic chemicals produced by the introduced life would confuse sensitive searches for biosignatures of living or ancient native life. The same applies to other more complex biosignatures. Life on other planets could have a common origin with Earth life, since in the early Solar System there was much exchange of material between the planets which could have transferred life as well. If so, it might be based on nucleic acids too.
The majority of the species isolated are not well understood or characterized and cannot be cultured in labs, and are known only from DNA fragments obtained with swabs. On a contaminated planet, it might be difficult to distinguish the DNA of extraterrestrial life from the DNA of life brought to the planet by the exploring. Most species of microorganisms on Earth are not yet well understood or DNA sequenced. This particularly applies to the unculturable archaea, and so are difficult to study. This can be either because they depend on the presence of other microorganisms, are slow growing, or depend on other conditions not yet understood. In typical habitats, 99% of microorganisms are not culturable. Introduced Earth life could contaminate resources of value for future human missions, such as water.
Invasive species could outcompete native life or consume it, if there is life on the planet. However, the experience on earth shows that species moved from one continent to another may be able to out compete the native life adapted to that continent. Additionally, evolutionary processes on Earth might have developed biological pathways different from extraterrestrial organisms, and so may be able to outcompete it. The same is also possible the other way around for contamination introduced to Earth's biosphere.
In addition to science research concerns, there are also attempts to raise ethical and moral concerns regarding intentional or unintentional interplanetary transport of life.
Evidence for possible habitats outside Earth
and Europa show the best evidence for current habitats, mainly due to the possibility of their hosting liquid water and organic compounds.Mars
There is ample evidence to suggest that Mars once offered habitable conditions for microbial life. It is therefore possible that microbial life may have existed on Mars, although no evidence has been found.It is thought that many bacterial spores from Earth were transported on Mars spacecraft. Some may be protected within Martian rovers and landers on the shallow surface of the planet. In that sense, Mars may have already been contaminated.
Certain lichens from the arctic permafrost are able to photosynthesize and grow in the absence of any liquid water, simply by using the humidity from the atmosphere. They are also highly tolerant of UV radiation, using melanin and other more specialized chemicals to protect their cells.
Although numerous studies point to resistance to some of Mars conditions, they do so separately, and none have considered the full range of Martian surface conditions, including temperature, pressure, atmospheric composition, radiation, humidity, oxidizing regolith, and others, all at the same time and in combination. Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly.
Other studies have suggested the potential for life to survive using deliquescing salts. These, similarly to the lichens, use the humidity of the atmosphere. If the mixture of salts is right, the organisms may obtain liquid water at times of high atmospheric humidity, with salts capturing enough to be capable of supporting life.
Research published in July 2017 shows that when irradiated with a simulated Martian UV flux, perchlorates become even more lethal to bacteria. Even dormant spores lost viability within minutes. In addition, two other compounds of the Martian surface, iron oxides and hydrogen peroxide, act in synergy with irradiated perchlorates to cause a 10.8-fold increase in cell death when compared to cells exposed to UV radiation after 60 seconds of exposure. It was also found that abraded silicates lead to the formation of toxic reactive oxygen species. The researchers concluded that "the surface of Mars is lethal to vegetative cells and renders much of the surface and near-surface regions uninhabitable." This research demonstrates that the present-day surface is more uninhabitable than previously thought, and reinforces the notion to inspect at least a few meters into the ground to ensure the levels of radiation would be relatively low.
Enceladus
The Cassini spacecraft directly sampled the plumes escaping from Enceladus. Measured data indicates that these geysers are made primarily of salt rich particles with an 'ocean-like' composition, which is thought to originate from a subsurface ocean of liquid saltwater, rather than from the moon's icy surface. Data from the geyser flythroughs also indicate the presence of organic chemicals in the plumes. Heat scans of Enceladus's surface also indicate higher temperatures around the fissures where the geysers originate, with temperatures reaching −93 °C, which is 115 °C warmer than the surrounding surface regions.Europa
Europa has much indirect evidence for its sub-surface ocean. Models of how Europa is affected by tidal heating require a subsurface layer of liquid water in order to accurately reproduce the linear fracturing of the surface. Indeed, observations by the Galileo spacecraft of how Europa's magnetic field interacts with Jupiter's field strengthens the case for a liquid, rather than solid, layer; an electrically conductive fluid deep within Europa would explain these results. Observations from the Hubble Space Telescope in December 2012 appear to show an ice plume spouting from Europa's surface, which would immensely strengthen the case for a liquid subsurface ocean. As was the case for Enceladus, vapour geysers would allow for easy sampling of the liquid layer. Unfortunately, there appears to be little evidence that geysering is a frequent event on Europa due to the lack of water in the space near Europa.Planetary protection
Forward contamination is prevented by sterilizing space probes sent to sensitive areas of the Solar System. Missions are classified depending on whether their destinations are of interest for the search for life, and whether there is any chance that Earth life could reproduce there.NASA made these policies official with the issuing of Management Manual NMI-4-4-1, NASA Unmanned Spacecraft Decontamination Policy on September 9, 1963. Prior to NMI-4-4-1 the same sterilization requirements were required on all outgoing spacecraft regardless of their target. Difficulties in the sterilization of Ranger probes sent to the Moon are the primary reasons for NASA's change to a target-by-target basis in assessing the likelihood forward contamination.
Some destinations such as Mercury need no precautions at all. Others such as the Moon require documentation but nothing more, while destinations such as Mars require sterilization of the rovers sent there.
Back contamination would be prevented by containment or quarantine. However, there have been no sample-returns thought to have any possibility of a back contamination risk since the Apollo missions. The Apollo regulations have been rescinded and new regulations have yet to be developed. See [|suggested precautions for sample-returns].