最近寂静中两道高频题背景: life in Europa and Hydrothermal vent

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1. Europa (moon)
Europa Listeni/jʊˈroʊpə/[9] (Jupiter II), is the sixth-closest moon of the planet Jupiter, and the smallest of its four Galilean satellites, but still the sixth-largest moon in the Solar System. Europa was discovered in 1610 by Galileo Galilei[1] and possibly independently by Simon Marius around the same time. Progressively more in-depth observations of Europa have occurred over the centuries by Earth-bound telescopes, and by space probe flybys starting in the 1970s.
Slightly smaller than the Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of water ice and is one of the smoothest in the Solar System.[10] This surface is striated by cracks and streaks, whereas craters are relatively rare. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably serve as an abode for extraterrestrial life.[11] This hypothesis proposes that heat from tidal flexing causes the ocean to remain liquid and drives geological activity similar to plate tectonics.[12]
The Galileo mission, launched in 1989, provided the bulk of current data on Europa. Although only fly-by missions have visited Europa, its intriguing characteristics have led to several ambitious exploration proposals. The next mission to Europa is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022.[13]

Europa has emerged as one of the top locations in the Solar System in terms of potential habitability and the possibility of hosting extraterrestrial life.[63] Life could exist in its under-ice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean hydrothermal vents. Life in such an ocean could possibly be similar to microbial life on Earth in the deep ocean.[64][65] So far, there is no evidence that life exists on Europa, but the likely presence of liquid water has spurred calls to send a probe there.[66]
Until the 1970s, life, at least as the concept is generally understood, was believed to be entirely dependent on energy from the Sun. Plants on Earth's surface capture energy from sunlight to photosynthesize sugars from carbon dioxide and water, releasing oxygen in the process, and are then eaten by oxygen-respiring animals, passing their energy up the food chain. Even life in the deep ocean, far below the reach of sunlight, was believed to obtain its nourishment either from the organic detritus raining down from the surface, or by eating animals that in turn depend on that stream of nutrients.[67] An environment's ability to support life was thus thought to depend on its access to sunlight.


This giant tube worm colony dwells beside a Pacific Ocean vent. Although the worms require oxygen (hence their blood-red color), methanogens and some other microbes in the vent communities do not.
However, in 1977, during an exploratory dive to the Galapagos Rift in the deep-sea exploration submersible Alvin, scientists discovered colonies of giant tube worms, clams, crustaceans, mussels, and other assorted creatures clustered around undersea volcanic features known as black smokers.[67] These creatures thrive despite having no access to sunlight, and it was soon discovered that they comprise an entirely independent food chain. Instead of plants, the basis for this food chain was a form of bacterium that derived its energy from oxidization of reactive chemicals, such as hydrogen or hydrogen sulfide, that bubbled up from Earth's interior. This chemosynthesis revolutionized the study of biology by revealing that life need not be sun-dependent; it only requires water and an energy gradient in order to exist. It opened up a new avenue in astrobiology by massively expanding the number of possible extraterrestrial habitats.
Although the tube worms and other multicellular eukaryotic organisms around these hydrothermal vents respire oxygen and thus are indirectly dependent on photosynthesis, anaerobic chemosynthetic bacteria and archaea that inhabit these ecosystems provide a possible model for life in Europa's ocean.[61] The energy provided by tidal flexing drives active geological processes within Europa's interior, just as they do to a far more obvious degree on its sister moon Io. Although Europa, like the Earth, may possess an internal energy source from radioactive decay, the energy generated by tidal flexing would be several orders of magnitude greater than any radiological source.[68] However, such an energy source could never support an ecosystem as large and diverse as the photosynthesis-based ecosystem on Earth's surface.[69] Life on Europa could exist clustered around hydrothermal vents on the ocean floor, or below the ocean floor, where endoliths are known to inhabit on Earth. Alternatively, it could exist clinging to the lower surface of Europa's ice layer, much like algae and bacteria in Earth's polar regions, or float freely in Europa's ocean.[70] However, if Europa's ocean were too cold, biological processes similar to those known on Earth could not take place. Similarly, if it were too salty, only extreme halophiles could survive in its environment.[70] In September 2009, planetary scientist Richard Greenberg calculated that cosmic rays impacting on Europa's surface convert some water ice into free oxygen (O2), which could then be absorbed into the ocean below as water wells up to fill cracks. Via this process, Greenberg estimates that Europa's ocean could eventually achieve an oxygen concentration greater than that of Earth's oceans within just a few million years. This would enable Europa to support not merely anaerobic microbial life but potentially larger, aerobic organisms such as fish.[71]
In 2006, Robert T. Pappalardo, an assistant professor in the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder said,
We’ve spent quite a bit of time and effort trying to understand if Mars was once a habitable environment. Europa today, probably, is a habitable environment. We need to confirm this … but Europa, potentially, has all the ingredients for life … and not just four billion years ago … but today.[72]
In November 2011, a team of researchers presented evidence in the journal Nature suggesting the existence of vast lakes of liquid water entirely encased in Europa's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell.[40][41] If confirmed, the lakes could be yet another potential habitat for life.
A paper published in March 2013 suggests that hydrogen peroxide is abundant across much of the surface of Jupiter's moon Europa.[73] The authors argue that if the peroxide on the surface of Europa mixes into the ocean below, it could be an important energy supply for simple forms of life, if life were to exist there. The scientists think hydrogen peroxide is an important factor for the habitability of the global liquid water ocean under Europa's icy crust because hydrogen peroxide decays to oxygen when mixed into liquid water.

2）Hydrothermal vent
A hydrothermal vent is a fissure in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots. Hydrothermal vents exist because the earth is both geologically active and has large amounts of water on its surface and within its crust. Common land types include hot springs, fumaroles and geysers. Under the sea, hydrothermal vents may form features called black smokers. Relative to the majority of the deep sea, the areas around submarine hydrothermal vents are biologically more productive, often hosting complex communities fueled by the chemicals dissolved in the vent fluids. Chemosynthetic bacteria and archaea form the base of the food chain, supporting diverse organisms, including giant tube worms, clams, limpets and shrimp. Active hydrothermal vents are believed to exist on Jupiter's moon Europa, and ancient hydrothermal vents have been speculated to exist on Mars.[1]

Some hydrothermal vents form roughly cylindrical chimney structures. These form from minerals that are dissolved in the vent fluid. When the superheated water contacts the near-freezing sea water, the minerals precipitate out to form particles which add to the height of the stacks. Some of these chimney structures can reach heights of 60 m.[6] An example of such a towering vent was "Godzilla", a structure in the Pacific Ocean near Oregon that rose to 40 m before it fell over.
A black smoker or sea vent is a type of hydrothermal vent found on the seabed, typically in the abyssal and hadal zones. They appear as black, chimney-like structures that emit a cloud of black material. The black smokers typically emit particles with high levels of sulfur-bearing minerals, or sulfides. Black smokers are formed in fields hundreds of meters wide when superheated water from below Earth's crust comes through the ocean floor. This water is rich in dissolved minerals from the crust, most notably sulfides. When it comes in contact with cold ocean water, many minerals precipitate, forming a black, chimney-like structure around each vent. The deposited metal sulfides can become massive sulfide ore deposits in time.
Black smokers were first discovered in 1977 on the East Pacific Rise by scientists from Scripps Institution of Oceanography. They were observed using a deep submergence vehicle called ALVIN belonging to the Woods Hole Oceanographic Institution. Now, black smokers are known to exist in the Atlantic and Pacific Oceans, at an average depth of 2100 metres. The most northerly black smokers are a cluster of five named Loki's Castle,[7] discovered in 2008 by scientists from the University of Bergen at 73°N, on the Mid-Atlantic Ridge between Greenland and Norway. These black smokers are of interest as they are in a more stable area of the Earth's crust, where tectonic forces are less and consequently fields of hydrothermal vents are less common.[8] The world's deepest black smokers are located in the Cayman Trough, 5,000 m (3.1 miles) below the ocean's surface.[9]
White smoker vents emit lighter-hued minerals, such as those containing barium, calcium and silicon. These vents also tend to have lower temperature plumes. These alkaline hydrothermal vents also continuously generate acetyl thioesters, providing both the starting point for more complex organic molecules and the energy needed to produce them. Microscopic structures in such alkaline vents "show interconnected compartments that provide an ideal hatchery for the origin of life".[10]