The search for life outside of planet Earth has captivated humanity for centuries. While we have found evidence of water and organic compounds on some planetary bodies in our solar system, determining which planets definitively do not harbor life remains an open question.
Quick overview: Which planets might have life?
Of the eight planets in our solar system, the following are considered to have the highest potential to host life:
– Mars: Mars has liquid water in the form of briny flows on its surface, organic molecules, and a climate that was more Earth-like in the ancient past. While no definitive signs of life have been found, some researchers believe microbes could be living deep underground.
– Europa: Jupiter’s moon Europa has an icy surface but a global subsurface ocean in contact with a rocky core. This ocean has the ingredients needed for life, but the surface is bathed in radiation. Life would need to exist deep underwater, if at all.
– Enceladus: Saturn’s moon Enceladus also has a global ocean beneath its icy crust. Plumes of water erupt through the south pole, allowing a sample of the interior ocean to be studied. No life has been detected, but conditions are thought to be potentially habitable.
– Titan: Saturn’s largest moon Titan has lakes and seas of liquid hydrocarbons on its surface. It also has a thick atmosphere rich in organic compounds. While it lacks liquid water, some hypotheses suggest life could exist with a different biochemical foundation.
Quick overview: Which planets likely cannot support life?
On the other hand, the following planets and planetary bodies are considered unlikely to host life due to inhospitable conditions:
– Mercury: As the closest planet to the Sun, Mercury experiences extremely high daytime temperatures and lacks an atmosphere to insulate the surface. It does not seem to have the requisite conditions for life as we know it.
– Venus: While Venus is Earth’s twin in size, its extreme greenhouse effect makes it the hottest planet in the solar system. Its atmosphere consists mainly of carbon dioxide with sulfuric acid clouds. The intense heat and acidic conditions are overwhelmingly hostile for life.
– Mars’ moons: The two Martian moons, Phobos and Deimos, are small, airless bodies. With no atmosphere and no liquid water, they are not thought to be promising sites to search for life.
– Jupiter: Jupiter is a gas giant planet without a solid surface. While some of its many moons show promise, the planet itself lacks the conditions needed for Earth-like life.
– Saturn: As another gas giant, Saturn also lacks a habitable environment apart from some potentially promising moons like Enceladus and Titan. The planet itself, however, appears inhospitable to life as we know it.
Why is Mars considered the most promising place to search for life in our solar system?
Of all the planets and bodies in our solar system, Mars is considered the most promising place to search for microbial life forms for several reasons:
– Mars once had abundant liquid water on its surface in rivers, lakes, and possibly oceans. Liquid water is thought to be necessary for all life on Earth. Though the planet is dry and cold today, water may persist underground.
– The Martian surface has complex long-chain organic molecules that could be building blocks for living organisms. Methane in the atmosphere could also potentially derive from biological sources underground.
– Mars has a 24.5-hour day-night cycle similar to Earth and moderate temperatures at certain latitudes that get above freezing. This gentle warmth could support life.
– Mars has distinct seasons, polar ice caps, volcanoes, canyons, and many other reminders of Earth. Of all the planets, its environmental conditions and geologic features most closely mimic those of our home planet.
– NASA’s rovers and Mars orbiters have found evidence that the ancient Martian environment used to be much more Earth-like than it is today. Conditions may have been habitable earlier in its history and some microbes could have adapted to survive.
– Future crewed missions to Mars will be able to directly search for signs of life. Upcoming rovers with sample return capability will bring back rocks and soil to analyze in Earth labs for biosignatures.
What current evidence is there that life may have existed on Mars?
While no definitive proof of life has been found on Mars yet, multiple rover missions have collected evidence that habitable environments did exist in the ancient past and potentially in the present day as well:
Ancient habitable environments
– Alluvial fans, dried up lake beds, deltas, and ancient river valleys have been imaged from orbit. These require extended periods of liquid water long ago on the Martian surface.
– Minerals such as borates, clays, and sulfates detected by rovers also require water to form. They’ve been found in areas like the Gale Crater lake bed explored by Curiosity.
– Curiosity found a rocky area called Yellowknife Bay to have substantial evidence that its ancient lake environment billions of years ago was habitable for microbial life. The chemistry of the rocks showed neutral pH, key elements present, and energy sources.
Organic compounds
– NASA’s Curiosity rover detected background levels of methane in the atmosphere that changed seasonally. On Earth, methane is primarily a biological gas emitted by living organisms.
– Organic carbon compounds have been found on the surface. While these could have non-biological origins, it shows the raw ingredients for life are present on Mars.
Potential subsurface habitat
– Under certain surface conditions, thin films of water containing dissolved salts could allow hardy microbes to persist in the present day. Features like Recurring Slope Lineae are hypothesized to be caused by the flow of briny water.
– Deep underground radiation-protected caves theoretically could support microbial communities. As technology improves, drilling on Mars could directly sample these environments.
What characteristics of Mars make it uninhabitable for complex life forms?
Although Mars may harbor simple microbial life deep beneath its surface, the dry, cold, irradiated surface makes the planet uninhabitable for more complex life forms for the following reasons:
Lack of a thick atmosphere
– The thin, cold Martian atmosphere provides little insulation. Surface temperatures average -55°C and swing wildly from as high as 20°C at noon to -153°C at night.
– Atmospheric pressure on Mars is less than 1% that of Earth at only about 6 millibars. Liquid water would rapidly boil away. Ice sublimates into vapor due to lack of pressure.
– No ozone layer exists to shield the surface from ultraviolet radiation and cosmic radiation. This is lethal to complex life.
Freezing cold surface temperatures
– The average surface temperature is around -55° C. Maximum temperatures only reach 20°C for brief periods during summer noontimes in some equatorial regions.
– Most water on the surface is thought to exist as ice, with ice caps extending from the poles down to around 60° latitude.
– Overnight lows can reach as cold as –153° C, far below the freezing point of water. This extreme cold is inhospitable for life as we know it.
Water scarcity
– With low temperatures and pressures, liquid water is not stable for prolonged periods on the surface. Any water would rapidly evaporate or freeze.
– Subsurface water sources like recurrent slope lineae involve only thin flowing films of salty brine, not large stable bodies of liquid water.
– The location and amount of any deep groundwater reservoirs is unknown but thought to be small. The largest estimates are around those of Lake Superior in volume.
– Water vapor levels in the atmosphere are less than 1% relative humidity due to the cold.
Intense radiation levels
– Mars lacks a protective global magnetic field to deflect solar and cosmic radiation.
– UV radiation reaches the surface at levels high enough to kill even dormant microbes. This UV flux would be hazardous to surface-exposed life.
– Cosmic rays produce intensely damaging secondary radiation when they hit the topsoil, creating hostile surface conditions.
How do conditions found on Mars compare to Earth’s most extreme environments?
The most uninhabitable regions of Earth, while extreme, still provide more relatively more habitable conditions than the surface of Mars:
Driest deserts
The Atacama Desert in Chile is one of the driest places on Earth with average rainfall less than 15 mm per year. However, it still has major humidity and precipitation advantages over Mars:
– Average daytime relative humidity still reaches 75% due to coastal fog.
– Air contains dan enough water for hygroscopic salts to absorb it. No liquid brines are stable on Mars’ surface.
– Lichens and other microbes are found throughout the Atacama, showing it remains habitable.
Coldest temperatures
East Antarctica reaches average winter temperatures around -60°C, comparable to Mars’ -55°C average. However, summers still get well above freezing even here on Earth:
– The lowest air temperature record on Earth is -89.2°C, warmer than Mars extremes.
– Max summer temperatures average -12°C, allowing meltwater. Mars barely exceeds -10°C even at its warmest.
– The stable water cycle allows liquid water to persist under the East Antarctic ice sheets, supporting life.
Peak radiation
The Atacama desert and Antarctica receive high UV exposure from a depleted ozone layer and lack of cloud cover. But unlike Mars:
– Earth’s protective magnetic field deflects the most damaging cosmic radiation and energetic solar protons.
– The thin Martian atmosphere allows nearly all UV radiation through.
– Antarctic soils receive less than one-thousandth the cosmic radiation dose of the Martian surface.
Conclusion
Based on these comparisons to Earth environments and analysis of the surface conditions on Mars, it is clear that no known complex life forms could survive on the exposed Martian surface. Subsurface environments do offer some potential refuges for microbial life to have persisted into the present day, giving hope that a basic Martian biosphere may yet be discovered. But Earth remains the most hospitable habitat in the solar system for rich diversity of life as we know it. The search continues for life that has adapted to the extremes of Mars, but hope is low for creatures more complex than microscopic in scale.
