Suitability criteria

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Mass

Low-mass planets are poor candidates for life for two reasons. First, their lesser gravity makes atmosphere retention difficult. Constituent molecules are more likely to reach escape velocity and be lost to space when buffeted by solar wind or stirred by collision. Planets without a thick atmosphere lack the matter necessary for primal biochemistry, have little insulation and poor heat transfer across their surfaces (for example, Mars, with its thin atmosphere, is colder than the Earth would be if it were at a similar distance from the Sun), and provide less protection against meteoroids and high-frequency radiation. Further, where an atmosphere is less dense than 0.006 Earth atmospheres, water cannot exist in liquid form as the required atmospheric pressure, 4.56 mm Hg (608 Pa) (0.18 inch Hg), does not occur. The temperature range at which water is liquid is smaller at low pressures generally.

Secondly, smaller planets have smaller diameters and thus higher surface-to-volume ratios than their larger cousins. Such bodies tend to lose the energy left over from their formation quickly and end up geologically dead, lacking the volcanoes, earthquakes and tectonic activity which supply the surface with life-sustaining material and the atmosphere with temperature moderators like carbon dioxide. Plate tectonics appear particularly crucial, at least on Earth: not only does the process recycle important chemicals and minerals, it also fosters bio-diversity through continent creation and increased environmental complexity and helps create the convective cells necessary to generate Earth's magnetic field.<ref>Ward, pp. 191–220</ref>

"Low mass" is partly a relative label: the Earth is low mass when compared to the Solar System's gas giants, but it is the largest, by diameter and mass, and the densest of all terrestrial bodies.<ref group=lower-alpha>There is a "mass-gap" in the Solar System between Earth and the two smallest gas giants, Uranus and Neptune, which are 13 and 17 Earth masses. This is probably just chance, as there is no geophysical barrier to the formation of intermediate bodies (see for instance OGLE-2005-BLG-390Lb and Super-Earth) and we should expect to find planets throughout the galaxy between two and twelve Earth masses. If the star system is otherwise favorable, such planets would be good candidates for life as they would be large enough to remain internally dynamic and to retain an atmosphere for billions of years but not so large as to accrete a gaseous shell which limits the possibility of life formation.</ref> It is large enough to retain an atmosphere through gravity alone and large enough that its molten core remains a heat engine, driving the diverse geology of the surface (the decay of radioactive elements within a planet's core is the other significant component of planetary heating). Mars, by contrast, is nearly (or perhaps totally) geologically dead and has lost much of its atmosphere.<ref>Template:Cite web</ref> Thus it would be fair to infer that the lower mass limit for habitability lies somewhere between that of Mars and that of Earth or Venus: 0.3 Earth masses has been offered as a rough dividing line for habitable planets.<ref>Template:Cite journal</ref> However, a 2008 study by the Harvard-Smithsonian Center for Astrophysics suggests that the dividing line may be higher. Earth may in fact lie on the lower boundary of habitability: if it were any smaller, plate tectonics would be impossible. Venus, which has 85% of Earth's mass, shows no signs of tectonic activity. Conversely, "super-Earths", terrestrial planets with higher masses than Earth, would have higher levels of plate tectonics and thus be firmly placed in the habitable range.<ref>Template:Cite web</ref>

Exceptional circumstances do offer exceptional cases: Jupiter's moon Io (which is smaller than any of the terrestrial planets) is volcanically dynamic because of the gravitational stresses induced by its orbit, and its neighbor Europa may have a liquid ocean or icy slush underneath a frozen shell also due to power generated from orbiting a gas giant.

Saturn's Titan, meanwhile, has an outside chance of harbouring life, as it has retained a thick atmosphere and has liquid methane seas on its surface. Organic-chemical reactions that only require minimum energy are possible in these seas, but whether any living system can be based on such minimal reactions is unclear, and would seem unlikely. These satellites are exceptions, but they prove that mass, as a criterion for habitability, cannot necessarily be considered definitive at this stage of our understanding.<ref>Template:Cite news</ref>

A larger planet is likely to have a more massive atmosphere. A combination of higher escape velocity to retain lighter atoms, and extensive outgassing from enhanced plate tectonics may greatly increase the atmospheric pressure and temperature at the surface compared to Earth. The enhanced greenhouse effect of such a heavy atmosphere would tend to suggest that the habitable zone should be further out from the central star for such massive planets.

Finally, a larger planet is likely to have a large iron core. This allows for a magnetic field to protect the planet from stellar wind and cosmic radiation, which otherwise would tend to strip away planetary atmosphere and to bombard living things with ionized particles. Mass is not the only criterion for producing a magnetic field—as the planet must also rotate fast enough to produce a dynamo effect within its core<ref>Template:Cite web</ref>—but it is a significant component of the process.

Orbit and rotation

As with other criteria, stability is the critical consideration in evaluating the effect of orbital and rotational characteristics on planetary habitability. Orbital eccentricity is the difference between a planet's farthest and closest approach to its parent star divided by the sum of said distances. It is a ratio describing the shape of the elliptical orbit. The greater the eccentricity the greater the temperature fluctuation on a planet's surface. Although they are adaptive, living organisms can stand only so much variation, particularly if the fluctuations overlap both the freezing point and boiling point of the planet's main biotic solvent (e.g., water on Earth). If, for example, Earth's oceans were alternately boiling and freezing solid, it is difficult to imagine life as we know it having evolved. The more complex the organism, the greater the temperature sensitivity.<ref>Ward, pp. 122–123.</ref> The Earth's orbit is almost wholly circular, with an eccentricity of less than 0.02; other planets in the Solar System (with the exception of Mercury) have eccentricities that are similarly benign.

Habitability is also influenced by the architecture of the planetary system around a star. The evolution and stability of these systems are determined by gravitational dynamics, which drive the orbital evolution of terrestrial planets. Data collected on the orbital eccentricities of extrasolar planets has surprised most researchers: 90% have an orbital eccentricity greater than that found within the Solar System, and the average is fully 0.25.<ref>Template:Cite web</ref> This means that the vast majority of planets have highly eccentric orbits and of these, even if their average distance from their star is deemed to be within the HZ, they nonetheless would be spending only a small portion of their time within the zone.

A planet's movement around its rotational axis must also meet certain criteria if life is to have the opportunity to evolve. A first assumption is that the planet should have moderate seasons. If there is little or no axial tilt (or obliquity) relative to the perpendicular of the ecliptic, seasons will not occur and a main stimulant to biospheric dynamism will disappear. The planet would also be colder than it would be with a significant tilt: when the greatest intensity of radiation is always within a few degrees of the equator, warm weather cannot move poleward and a planet's climate becomes dominated by colder polar weather systems.

If a planet is radically tilted, seasons will be extreme and make it more difficult for a biosphere to achieve homeostasis. The axial tilt of the Earth is higher now (in the Quaternary) than it has been in the past, coinciding with reduced polar ice, warmer temperatures and less seasonal variation. Scientists do not know whether this trend will continue indefinitely with further increases in axial tilt (see Snowball Earth).

The exact effects of these changes can only be computer modelled at present, and studies have shown that even extreme tilts of up to 85 degrees do not absolutely preclude life "provided it does not occupy continental surfaces plagued seasonally by the highest temperature."<ref>Template:Cite press release</ref> Not only the mean axial tilt, but also its variation over time must be considered. The Earth's tilt varies between 21.5 and 24.5 degrees over 41,000 years. A more drastic variation, or a much shorter periodicity, would induce climatic effects such as variations in seasonal severity.

Other orbital considerations include:

  • The planet should rotate relatively quickly so that the day-night cycle is not overlong. If a day takes years, the temperature differential between the day and night side will be pronounced, and problems similar to those noted with extreme orbital eccentricity will come to the fore.
  • The planet also should rotate quickly enough so that a magnetic dynamo may be started in its iron core to produce a magnetic field.
  • Change in the direction of the axis rotation (precession) should not be pronounced. In itself, precession need not affect habitability as it changes the direction of the tilt, not its degree. However, precession tends to accentuate variations caused by other orbital deviations; see Milankovitch cycles. Precession on Earth occurs over a 26,000-year cycle.

The Earth's Moon appears to play a crucial role in moderating the Earth's climate by stabilising the axial tilt. It has been suggested that a chaotic tilt may be a "deal-breaker" in terms of habitability—i.e. a satellite the size of the Moon is not only helpful but required to produce stability.<ref>Template:Cite journal</ref> This position remains controversial.<ref group=lower-alpha>According to prevailing theory, the formation of the Moon commenced when a Mars-sized body struck the Earth in a glancing collision late in its formation, and the ejected material coalesced and fell into orbit (see giant impact hypothesis). In Rare Earth Ward and Brownlee emphasize that such impacts ought to be rare, reducing the probability of other Earth-Moon type systems and hence the probability of other habitable planets. Other moon formation processes are possible, however, and the proposition that a planet may be habitable in the absence of a moon has not been disproven.</ref>

In the case of the Earth, the sole Moon is sufficiently massive and orbits so as to significantly contribute to ocean tides, which in turn aids the dynamic churning of Earth's large liquid water oceans. These lunar forces not only help ensure that the oceans do not stagnate, but also play a critical role in Earth's dynamic climate.<ref>Template:Cite web</ref><ref>File:Tidalwaves1.gif</ref>

Orbital stability

For a stable orbit the ratio between the moon's orbital period Ps around its primary and that of the primary around its star Pp must be < Template:Frac, e.g. if a planet takes 90 days to orbit its star, the maximum stable orbit for a moon of that planet is less than 10 days.<ref name=Kipping_2009a>Template:Cite journal</ref><ref name="Heller2012">Template:Cite journal</ref> Simulations suggest that a moon with an orbital period less than about 45 to 60 days will remain safely bound to a massive giant planet or brown dwarf that orbits 1 AU from a Sun-like star.<ref name="skyandtelescope">Template:Cite web</ref>

Tidal effects

While the effects of tidal acceleration are relatively modest on planets, it can be a significant source of energy for natural satellites and an alternative energy source for sustaining life.

Moons orbiting gas giants or brown dwarfs are likely to be tidally locked to their primary: that is, their days are as long as their orbits. While tidal locking may adversely affect planets within habitable zones by interfering with the distribution of stellar radiation, it may work in favour of satellite habitability by allowing tidal heating. Scientists at the NASA Ames Research Center modelled the temperature on tide-locked exoplanets in the habitability zone of red dwarf stars. They found that an atmosphere with a carbon dioxide (Template:Chem) pressure of only Template:Convert not only allows habitable temperatures, but allows liquid water on the dark side of the satellite. The temperature range of a moon that is tidally locked to a gas giant could be less extreme than with a planet locked to a star. Even though no studies have been done on the subject, modest amounts of Template:Chem are speculated to make the temperature habitable.<ref name="skyandtelescope"/>

Tidal effects could also allow a moon to sustain plate tectonics, which would cause volcanic activity to regulate the moon's temperature<ref name=volcanoes-climate>Template:Cite web</ref><ref name=nasa-io>Template:Cite web</ref> and create a geodynamo effect which would give the satellite a strong magnetic field.<ref name=hyperphysics-geodynamo>Template:Cite web</ref>

Axial tilt and climate

Provided gravitational interaction of a moon with other satellites can be neglected, moons tend to be tidally locked with their planets. In addition to the rotational locking mentioned above, there will also be a process termed 'tilt erosion', which has originally been coined for the tidal erosion of planetary obliquity against a planet's orbit around its host star.<ref name=Heller_2011>Template:Cite journal</ref> The final spin state of a moon then consists of a rotational period equal to its orbital period around the planet and a rotational axis that is perpendicular to the orbital plane.

If the moon's mass is not too low compared to the planet, it may in turn stabilize the planet's axial tilt, i.e. its obliquity against the orbit around the star. On Earth, the Moon has played an important role in stabilizing the axial tilt of the Earth, thereby reducing the impact of gravitational perturbations from the other planets and ensuring only moderate climate variations throughout the planet.<ref name=earthmoonaxialtilt>Template:Cite web</ref> On Mars, however, a planet without significant tidal effects from its relatively low-mass moons Phobos and Deimos, axial tilt can undergo extreme changes from 13° to 40° on timescales of Template:Nowrap to Template:Nowrap.<ref name=mars101axialtilt>Template:Cite web</ref><ref name=Armstrong>Template:Cite journal</ref>

Being tidally locked to a giant planet or sub-brown dwarf would allow for more moderate climates on a moon than there would be if the moon were a similar-sized planet orbiting in locked rotation in the habitable zone of the star.<ref name=space-dot-com-exomoons>Template:Cite news</ref> This is especially true of red dwarf systems, where comparatively high gravitational forces and low luminosities leave the habitable zone in an area where tidal locking would occur. If tidally locked, one rotation about the axis may take a long time relative to a planet (for example, ignoring the slight axial tilt of Earth's moon and topographical shadowing, any given point on it has two weeks – in Earth time – of sunshine and two weeks of night in its lunar day) but these long periods of light and darkness are not as challenging for habitability as the eternal days and eternal nights on a planet tidally locked to its star.

Geochemistry

Template:Main It is generally assumed that any extraterrestrial life that might exist will be based on the same fundamental biochemistry as found on Earth, as the four elements most vital for life, carbon, hydrogen, oxygen, and nitrogen, are also the most common chemically reactive elements in the universe. Indeed, simple biogenic compounds, such as very simple amino acids such as glycine, have been found in meteorites and in the interstellar medium.<ref>Template:Cite web</ref> These four elements together comprise over 96% of Earth's collective biomass. Carbon has an unparalleled ability to bond with itself and to form a massive array of intricate and varied structures, making it an ideal material for the complex mechanisms that form living cells. Hydrogen and oxygen, in the form of water, compose the solvent in which biological processes take place and in which the first reactions occurred that led to life's emergence. The energy released in the formation of powerful covalent bonds between carbon and oxygen, available by oxidizing organic compounds, is the fuel of all complex life-forms. These four elements together make up amino acids, which in turn are the building blocks of proteins, the substance of living tissue. In addition, neither sulfur, required for the building of proteins, nor phosphorus, needed for the formation of DNA, RNA, and the adenosine phosphates essential to metabolism, is rare.

Relative abundance in space does not always mirror differentiated abundance within planets; of the four life elements, for instance, only oxygen is present in any abundance in the Earth's crust.<ref>Template:Cite web</ref> This can be partly explained by the fact that many of these elements, such as hydrogen and nitrogen, along with their simplest and most common compounds, such as carbon dioxide, carbon monoxide, methane, ammonia, and water, are gaseous at warm temperatures. In the hot region close to the Sun, these volatile compounds could not have played a significant role in the planets' geological formation. Instead, they were trapped as gases underneath the newly formed crusts, which were largely made of rocky, involatile compounds such as silica (a compound of silicon and oxygen, accounting for oxygen's relative abundance). Outgassing of volatile compounds through the first volcanoes would have contributed to the formation of the planets' atmospheres. The Miller–Urey experiment showed that, with the application of energy, simple inorganic compounds exposed to a primordial atmosphere can react to synthesize amino acids.<ref>Template:Cite web</ref>

Even so, volcanic outgassing could not have accounted for the amount of water in Earth's oceans.<ref>Template:Cite web</ref> The vast majority of the water —and arguably carbon— necessary for life must have come from the outer Solar System, away from the Sun's heat, where it could remain solid. Comets impacting with the Earth in the Solar System's early years would have deposited vast amounts of water, along with the other volatile compounds life requires onto the early Earth, providing a kick-start to the origin of life.

Thus, while there is reason to suspect that the four "life elements" ought to be readily available elsewhere, a habitable system probably also requires a supply of long-term orbiting bodies to seed inner planets. Without comets there is a possibility that life as we know it would not exist on Earth.

Microenvironments and extremophiles

File:Atacama.png
The Atacama Desert in South America provides an analog to Mars and an ideal environment to study the boundary between sterility and habitability.

One important qualification to habitability criteria is that only a tiny portion of a planet is required to support life. Astrobiologists often concern themselves with "micro-environments", noting that "we lack a fundamental understanding of how evolutionary forces, such as mutation, selection, and genetic drift, operate in micro-organisms that act on and respond to changing micro-environments."<ref>Template:Cite web</ref> Extremophiles are Earth organisms that live in niche environments under severe conditions generally considered inimical to life. Usually (although not always) unicellular, extremophiles include acutely alkaliphilic and acidophilic organisms and others that can survive water temperatures above 100 °C in hydrothermal vents.

The discovery of life in extreme conditions has complicated definitions of habitability, but also generated much excitement amongst researchers in greatly broadening the known range of conditions under which life can persist. For example, a planet that might otherwise be unable to support an atmosphere given the solar conditions in its vicinity, might be able to do so within a deep shadowed rift or volcanic cave.<ref>Template:Cite web</ref> Similarly, craterous terrain might offer a refuge for primitive life. The Lawn Hill crater has been studied as an astrobiological analog, with researchers suggesting rapid sediment infill created a protected microenvironment for microbial organisms; similar conditions may have occurred over the geological history of Mars.<ref>Template:Cite journal</ref>

Earth environments that cannot support life are still instructive to astrobiologists in defining the limits of what organisms can endure. The heart of the Atacama desert, generally considered the driest place on Earth, appears unable to support life, and it has been subject to study by NASA and ESA for that reason: it provides a Mars analog and the moisture gradients along its edges are ideal for studying the boundary between sterility and habitability.<ref>Template:Cite web</ref> The Atacama was the subject of study in 2003 that partly replicated experiments from the Viking landings on Mars in the 1970s; no DNA could be recovered from two soil samples, and incubation experiments were also negative for biosignatures.<ref>Template:Cite journal</ref>

Ecological factors

The two current ecological approaches for predicting the potential habitability use 19 or 20 environmental factors, with emphasis on water availability, temperature, presence of nutrients, an energy source, and protection from solar ultraviolet and galactic cosmic radiation.<ref name='D.C.Golden'>Template:Cite journal</ref><ref name=Beaty>Template:Citation</ref>

Some habitability factors<ref name=Beaty/>
Water Template:· Activity of liquid water
Template:· Past or future liquid (ice) inventories
Template:· Salinity, pH, and Eh of available water
Chemical environment Nutrients:
Template:· C, H, N, O, P, S, essential metals, essential micronutrients
Template:·Fixed nitrogen
Template:·Availability/mineralogy
Toxin abundances and lethality:
Template:· Heavy metals (e.g. Zn, Ni, Cu, Cr, As, Cd, etc.; some are essential, but toxic at high levels)
Template:· Globally distributed oxidizing soils
Energy for metabolism Solar (surface and near-surface only)
Geochemical (subsurface)
Template:· Oxidants
Template:· Reductants
Template:· Redox gradients
Conducive
physical conditions
Template:·Temperature
Template:·Extreme diurnal temperature fluctuations
Template:·Low pressure (is there a low-pressure threshold for terrestrial anaerobes?)
Template:·Strong ultraviolet germicidal irradiation
Template:·Galactic cosmic radiation and solar particle events (long-term accumulated effects)
Template:· Solar UV-induced volatile oxidants, e.g. O 2, O, H2O2, O3
Template:·Climate and its variability (geography, seasons, diurnal, and eventually, obliquity variations)
Template:·Substrate (soil processes, rock microenvironments, dust composition, shielding)
Template:·High CO2 concentrations in the global atmosphere
Template:·Transport (aeolian, ground water flow, surface water, glacial)

Liquid water

Template:Main Liquid water is thought by most astrobiologists to be an essential prerequisite for extraterrestrial life. There is growing evidence of subsurface liquid water on several moons in the Solar System orbiting the gas giants Jupiter, Saturn, Uranus, and Neptune. However, none of these subsurface bodies of water has been confirmed to date.


Atmosphere

An atmosphere is considered by astrobiologists to be important in developing prebiotic chemistry, sustaining life and for surface water to exist. Most natural satellites in the Solar System lack significant atmospheres, the sole exception being Saturn's moon Titan.

Sputtering, a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles, presents a significant problem for natural satellites. All the gas giants in the Solar System, and likely those orbiting other stars, have magnetospheres with radiation belts potent enough to completely erode an atmosphere of an Earth-like moon in just a few hundred million years. Strong stellar winds can also strip gas atoms from the top of an atmosphere causing them to be lost to space.

To support an Earth-like atmosphere for about 4.6 billion years (Earth's current age), a moon with a Mars-like density is estimated to need at least 7% of Earth's mass.<ref>Template:Cite web</ref> One way to decrease loss from sputtering is for the moon to have a strong magnetic field of its own that can deflect stellar wind and radiation belts. NASA's Galileo's measurements suggest that large moons can have magnetic fields; it found Ganymede has its own magnetosphere, even though its mass is only 2.5% of Earth's.<ref name="skyandtelescope"/> Alternatively, the moon's atmosphere may be constantly replenished by gases from subsurface sources, as thought by some scientists to be the case with Titan.Template:Citation needed