Terrestrial analogue sites

An important notion in the evaluation of analogue sites is that of "fidelity", which describes the resemblance of the analogue to its extraterrestrial correspondent. Fidelity is used in comparative planetary science to express the analogy of a terrestrial site to a target extraterrestrial surface. This classification is possible based on various criteria such as geomorphology, geochemistry, exobiology or exploration conditions.

Geomorphology is the scientific study of landforms and the processes that shape them. In terms of analogue sites, scientists search for locations on Earth that exhibit similar landforms such as can be found on exploration targets like the Moon, Mars or even asteroids and comets. The idea is to confront astronauts, robots or scientific equipment with sites that resemble in their geologic appearance those extraterrestrial surfaces. Examples are volcanic sites which resemble lunar terrain (regolith), polar locations and glaciers that can be compared to the poles of Mars or of Jupiter moon Europa, or terrestrial lava tubes which can also be found on the Moon or Mars.

Geochemistry is the science that uses the principles of chemistry to explain the mechanisms behind major geological systems. The aspect of geochemistry is of importance for analogue sites when locations offer the possibility to test analysis instruments for future space missions (manned or robotic). Geochemical fidelity is also of importance for the development and test of equipment used for in-situ resource utilization. Examples for such analogue sites are terrestrial volcanoes that offer rocks similar to those found on the Moon or hematite concretions which can be found in Earth deserts and also on Mars (so-called "Blueberries").

Exobiology or astrobiology is the study of the origin and evolution of extraterrestrial life. In terrestrial analogues efforts are put on the identification of so-called extremophile organisms, which are life forms that live and survive in extreme conditions such as can be found on other planets or moons. The objective of this research is to understand how such organisms survive and how they can be identified (or their remnants).

Examples of exobiology analogue sites are the Rio Tinto in Spain, which hosts bacteria that can survive high temperatures and harsh chemical conditions, or black smokers in the deep sea that host colonies of life forms in high-pressure and high-temperature conditions. The cold dry hyperarid core of the Atacama desert is one of our closest analogues for Martian surface conditions and is often used for testing rovers and life detection equipment that one day may be sent to Mars.[3][4][5][6][7] Scientists can test at such analogue sites sampling equipment designed to search and identify lifeforms.

Another criterion to search for analogue sites are locations where the exploration conditions of future astronauts can be simulated. Future explorers of the Moon or Mars will have to handle various conditions, such as reduced gravity, radiation, work in pressurized space suits and extreme temperatures. Preparing astronauts for these conditions calls for training on sites that exhibit some of those conditions. The operations that can be simulated reach from living in isolation, to extra-vehicular activity (EVA) in reduced gravity to the construction of habitats. Examples for analogue sites that offer such exploration conditions are research stations at the poles or underwater EVA training as it is done at NEEMO by NASA, or at the Marseilles subsea analogue by COMEX.[8] Underwater analogue sites allow for the training of astronauts in neutral buoyancy conditions (such as is done in test pools at NASA, ESA or Star City in Russia) while operating on a natural terrain. Potential targets for such training are missions to the Moon and Mars, to test sampling, drilling and field explorations in 1/6th or 1/3rd of Earth's gravity, or asteroids, and to test anchoring systems in microgravity.

Space analogues can help to train personnel in using technologies and instruments, and in knowing how to behave with a spacesuit. Thus two types of analogue sites exist: underwater sites and surface sites.

• Underwater sites simulate a reduced-gravity environment by compensating weight by the Archimedes' principle, thus simulating zero gravity or reduced gravity (lunar gravity, for example).
• Surface sites serve to train astronauts to walk and move within a spacesuit, and to test the Mars Exploration Rover (for example). Expeditions to surface sites also help teach geology to astronauts, who mostly trained as pilots.

Space analogues may have potential similarities to environments for exobiology. In some places on Earth the conditions allow only certain types of organisms - extremophile organisms - to live.