When the Cassini spacecraft (launched October 15, 1997) flew within 175 km (109 miles) of Enceladus in July 2005, everything believed known about the Saturnian moon had to be discarded. With the unexpected discovery of a huge water geyser and the heavy presence of water vapor in its atmosphere, a satellite previously believed to be small and “dead” (meaning no geological activity) became the best hope to date for finding extraterrestrial life. Even though everything must be in place to support the existence of life – water, “an adequate heat source, proper chemical nutrients, and precise environmental conditions [that] result in the necessary chemical reactions…”[1], Dr. Robert Brown, a planetary scientist at the University of Arizona and a senior scientist working on the Cassini project “told a major conference in Vienna, Austria [that] Enceladus contains… the ingredients for life.”[2]
History and Description:
Enceladus, discovered on August 28, 1789 by German born British Astronomer Sir William Herschel (1738-1822), best known for his discovery of Uranus, is the sixth largest moon of Saturn, with a mean diameter of 504 km (313 miles), approximately seven times smaller than the Earth’s moon. It orbits Saturn “at a distance of 238,000 km (147,886 miles) from the planet’s center and 180,000 km (111,847 miles) from its surface, between the orbits of Mimas and Tethys (two other Saturnian moons), rotating “synchronously… keeping one face pointed towards Saturn” and completes each revolution in 32.9 hours.[3]
Based on findings from Cassini, Enceladus consists of a core with a higher percentage of irons (FE) and silicates (compound consisting of Silicon (Si) and Oxygen (O), one or more metals, and possibly hydrogen (H)) that may have undergone more heating from radioactive decay than the interiors of Saturn’s other mid-sized icy moons. Enceladus has a light gravitational field, yielding a density of 1.61 grams per cubic centimeter.
Though known to exist for nearly two centuries, Enceladus, “one of only three outer solar system bodies (along with Jupiter’s moon Io and Neptune’s moon Triton) where active eruptions have been observed” remained a mere speck until the Voyager program. When Voyager I flew by within 202,000 km (125,517 miles) of Enceladus on November 12, 1980, it revealed “a highly reflective surface devoid of impact craters, [indicative of] a youthful surface.” Voyager II, which flew by within 87,010 km (54,065 miles) of Enceladus on August 26, 1981 revealed a diverse surface – some of it older and heavily cratered (mid-to-high northern latitude region), others lightly cratered (near the equator) and the remainder generally smooth and youthful.[4]
The February 17, March 9, and July 14, 2005 flybys of Cassini revealed Enceladus’ surface in significantly greater detail. “Smooth plains resolved into relatively crater-free regions filled with numerous small ridges and scarps. …Numerous fractures (possibly caused by the build up of pressure from the temperature differential between the moon’s warm subsurface and frigid surface and exterior environment) were found within the older, cratered terrain… and several additional young terrain were discovered… such as… near the [moon’s] South Pole …[including] intriguing dark spots, 125 and 750 meters (410 and 2461 feet) wide, which appear to run parallel to narrow fractures [and are believed to be] collapse pits” filled with thick blue ice. Cassini also imaged the moon’s smooth plains (Sarandib Planitia and Diyar Planitia) leading to the estimate that this terrain is between 170 million to 3.7 billion years old, as well as the surface area facing Saturn, which was found to be “covered in numerous criss-crossing sets of troughs and ridges,” and the geologically active South Pole, which revealed the presence of an active geyser whose gushing water adds to Saturn’s e-ring, and a “distinctive tectonically-deformed region [estimated to be between 10 to 100 million years old that reaches] as far north as 60° south latitude… covered in tectonic fractures and ridges [including four such fractures bounded on each side by ridges called ‘tiger stripes’ covered in ice and even boulders ranging from 10 to 100 meters (33 to 328 feet) wide, which appear to be less than 1000 years old].”[5]
The discovery of the cryovolcanism (“eruption of water and/or other liquid or vapor-phase volatiles, together with gas-driven solid fragments onto the surface of a planet or moon due to internal heating”)[6] at Enceladus’ South Pole, in which a geyser gushes water and other volatiles instead of silicate rock, along with the presence of an inconstant atmosphere (thickest around the South Pole) that consists mainly of water vapor (H2O)(91{a3acb2be643effc4b7b4b3705df5cb2eae026059a1d075d54be8af4d92e7e619}) along with smaller amounts of molecular nitrogen (N) (4{a3acb2be643effc4b7b4b3705df5cb2eae026059a1d075d54be8af4d92e7e619}), carbon dioxide (CO2)(3.2{a3acb2be643effc4b7b4b3705df5cb2eae026059a1d075d54be8af4d92e7e619}) and methane (CH4) (1.7{a3acb2be643effc4b7b4b3705df5cb2eae026059a1d075d54be8af4d92e7e619}) provides the greatest hope for the existence of life somewhere on or beneath the moon’s surface despite a mean surface temperature of about -325° Fahrenheit.
Enceladus’ Water Geysers:
When Cassini flew over Enceladus in November 2005, it confirmed the January 16, 2005 discovery of numerous geyser-like jets of water and ice particles (the composition was determined during the July 2005 flyby when Cassini flew directly through the plume), rising from multiple numbers of fractures or vents (“tiger stripes”) in the moon’s icy crust. One of the plumes rose as high as 500 km (311 miles), powered by pressurized sub-surface chambers, temperature differentials, the moon’s weak gravity – about 12½ times weaker than the Earth’s gravitational force, and to some degree the gravitational pull of Saturn.
Based on “the combined analysis of imaging, mass spectrometry, and magnetospheric data,” it is likely that Enceladus’ plumes of water and ice particles emanate from “pressurized sub-surface chambers [located less than 100 meters (328 feet) below the moon’s icy surface that consist of near pure water heated to about 26°-32°Fahrenheit prior to ejection], similar to geysers on Earth.”[7] Further confirmation that the water is liquid beneath the surface came from an analysis conducted by Cassini on the ice surrounding the “tiger stripe” fractures. “That ice was amorphous and virtually crater-free, indicating that it welled up relatively recently.”[8]
Furthermore, because of the absence of ammonia (NH3), which can serve as anti-freeze for water, it is also likely that the moon’s sub-surface water is heated by tidal (frictional forces arising from flexure or shifting caused by the gravitational pull of Saturn, 2:1 “mean motion orbital resonance with Dione,”[9] a nearby moon, meaning that Enceladus completes two orbits of Saturn for every one by Dione, and to a lesser degree the gravitational pull of Tethys, another nearby moon) or radiogenic (caused by radioactivity or a radioactive transformation) sources, since Enceladus’ South Pole temperature is about -177°Fahrenheit versus the frigid -298° to -325°Fahrenheit for much of the rest of the moon’s surface and because the water and ice-particles must “have a certain density… that implies surprisingly warm temperatures” to be carried aloft.[10] The difference is too great to be explained by solar heating since Enceladus’ icy surface reflects more 90{a3acb2be643effc4b7b4b3705df5cb2eae026059a1d075d54be8af4d92e7e619} of the sun’s weak energy back into space. Accordingly the moon “has the highest albedo (ratio of reflected to incident light) of any body in the solar system” with a measurement of >0.9.[11]
According to research presented at a European Geosciences Union (EGU) conference in April 2006, Enceladus’ core of molten rock may be as hot as 2060°Fahrenheit further bolstering the theory that the moon’s geological activity is fueled by tidal and radiogenic sources.
Likely Life Forms on Enceladus
If life is to be found on Enceladus, it is likely to be in the form of extremely simple microbes that can exist in harsh, seemingly uninhabitable environments as long as chemical nutrients, biomolecules such as amino acids, an energy source and liquid water are present, which appears to be the case in when it comes to the pressurized chambers that provide geothermal warming to the moon.
The Case for Possible Extraterrestrial Life:
Ingredients For Life:
Two important ingredients for life are water (H2O) and an energy source (though it has been found to be unnecessary for some chemosynthetic cryophiles) to fuel and sustain an organism’s metabolism. Both are present on Eceladus. Resevoirs of liquid water run beneath the moon’s surface while about 99.9{a3acb2be643effc4b7b4b3705df5cb2eae026059a1d075d54be8af4d92e7e619} of its topography is covered in water (H2O) ice that is constantly refreshed by the shooting geysers that rain down as ice particles and snow. At the same time, the hydrothermal jets that power Enceladus’ geysers provide an optimal habitat for microorganisms in the same way the deep-water and Yellowstone National Park’s hydrothermal vents do on Earth.
A third ingredient is Nitrogen, which is present on Enceladus and in its atmosphere. “Nitrogen is an essential ingredient of amino acids (“organic compounds containing an amino group (NH2), a carboxylic acid group (COOH), and any of various side groups, especially any of the 20 compounds that have the basic formula NH2CHRCOOH, and that link together by peptide bonds to form proteins or that function as chemical messengers and as intermediates in metabolism”), commonly referred to as the building blocks of life) and nucleic acids (“complex compounds found in all living cells and viruses, composed of purines, pyrimidines, carbohydrates, and phosphoric acid. Nucleic acids in the form of DNA and RNA control cellular function and heredity”).[12]
A fourth ingredient is the presence of organic compounds, substances consisting of carbon (C), the signature element of life. Based on Cassini’s flybys, the existence of simple organic compounds have been detected along the “tiger stripe” fractures and in the plumes of water and ice crystals gushing from the moon’s geysers. Small amounts of methane (CH4), carbon dioxide (CO2) and propane (C3H8) have been detected in the plumes while small amounts of carbon dioxide (CO2) and methane (CH4) have been found in Enceladus’ atmosphere. In addition, Cassini also detected the presence of ethane (C2H6) and ethylene (C2H4) two additional organic compounds.
A fifth ingredient is the existence of some kind of “basis for complex chemistry.”[13] When viewing scientific theories about the beginning of life on Earth, such a chemistry is not unattainable for bodies elsewhere in the space. In both the “primordial soup” (in which biological compounds such as amino acids emerged from an “organic broth”) and “metabolism first” (in which life emerged from small molecules, even tinier than RNA, recently bolstered by the discovery of Methanosarcina acetivorans, an undersea microbe, “which eats carbon monoxide (CO) and expels methane (CH4) and acetate (CH3COO)”[14] using only “two very simple proteins” for chemosynthesis to sustain its metabolism) theories, the prerequisites for the beginning of life on Earth were not insurmountably great, especially since this life originally arose under harsh conditions when the Earth’s atmosphere consisted primarily of toxic substances such as ammonia (NH3), carbon dioxide (CO2), methane (CH4), and steam.
The prospects for life may also be enhanced because Enceladus does not have an intense radiation field and because of the reduced potency of the sun’s harmful ultraviolet (UV) rays due to time (longer to reach), distance (1.427 billion km or 886 million miles from the sun) and shielding (parts of the moon’s surface are shielded by Saturn because of its synchronous rotation) factors.
Cryophiles
Living and fossilized cryophilic (cold-loving) microbes have been found in frigid Arctic environments where temperatures can drop as low as -90°Fahrenheit (Greenland and northern Siberia) to lower than -125°Fahrenheit (Antarctica). They have even been found to exist at Sverrefjell Volcano located on Svalbard, an island group north of Norway where “no living organisms would have been expected [to exist, having] adapted to extremely cold conditions.”[15]
Examples include a moss “that remained alive yet dormant while frozen for 40,000 years in the permafrost of the Kolyma Lowlands of northeastern Siberia,”[16] orange mosses covered with black films of cyanobacteria” have been found growing “on the ice of the Matanuska Glaciar in Alaska” as well as actinomycetes (“filamentous or rod-shaped…microorganisms [that] resemble bacteria and fungi”) and other metabolically diverse microbes found in ancient ice samples retrieved from Beacon Valley, Antarctica and other arctic locales.[17]
The recent discovery of “a new species of polychaete worm (also known as pink “ice worms” that are about 1 to 2 inches in length) found living on the exposed surface of methane (CH4) gas hydrate mound[s]” in frigid waters deep beneath the ocean surface are another positive sign.[18] However, the greatest encouragement comes from the discovery of chemosynthetic cryophiles that require no energy source for metabolism. In lieu of such a source, these organisms obtain energy merely from “chemical reactions between rock and water (H2O).”[19]
Thermophiles
Living and fossilized microbes have been found in geothermal or geologically active environments. One example is the existence of chemosynthetic, thermophilic (heat-loving) microbes that exist in Yellowstone’s Norris Geyser Basin where temperatures consistently exceed 158°Fahrenheit and photosynthesis cannot occur. Accordingly they use hydrogen (H2) to fuel their metabolism. This is especially encouraging since hydrogen (H) is a major component of water (H2O) found in Enceladus’ geysers and because the sunlight reaching Enceladus’ surface is likely insufficient for photosynthesis.
In addition, chemosynthetic, thermophilic or hyperthermophilic (extreme-heat-loving) microbes utilizing hydrogen sulfide (H2S) for metabolic functions (e.g. bacterium Aquifex aeolicus) and prokaryotic bacteria and cyanobacteria, along with larger organisms such as giant tube worms (Rifita pachyptila), huge clams (Caliptogena), and mussels), have also been found by the Earth’s deep water geothermal vents where temperatures can reach 716ºFahrenheit and sunlight cannot penetrate.
When it comes to bacterium Aquifex aeolicus, its requirements are very simple. These heat-loving microorganisms “need little more than hydrogen (H), oxygen (O), carbon dioxide (CO2) and mineral salts to grow” [20] improving the odds that similar or like-kind chemosynthetic organisms may exist on Enceladus, especially in its geothermal pressure chambers below the surface.
Other Extremophiles
Along with thermophilic and cryophilic extremophiles (organisms that thrive in harsh “un-lifelike” environments), a third form also exists – anaerobic life that thrives in non-oxygen environments beneath the Earth’s crust. Their existence further improves the chances that extraterrestrial life may exist on Enceladus, especially since the most likely habitat for such life may be below the Saturnian moon’s surface.
“Lunar” Microbes
Perhaps the most fascinating example that points to the possibility of life on Enceladus is the proven fact that exposed microbes can survive in the harshness of space. When the Surveyor 3 probe landed on the Moon (Earth’s moon) on April 20, 1967, it had been inadvertently populated by between 50 to 100 microbes (Streptococcus mitis) that “survived launch, space vacuum, 3 years of radiation exposure, deep-freeze at an average temperature of only 20º above absolute zero (-453ºFahrenheit), and no nutrient, water or energy source.”[21] These organisms were still alive when the contaminated camera of Surveyor 3 was retrieved and returned to Earth by the Apollo 12 crew on November 12, 1969.
Conclusion:
The prospects of finding extraterrestria life on Enceladus are exceptionally encouraging. “Enceladus… has the fundamental chemical recipe for life. Add a pinch of phosphorous and you have all you need to make DNA – or perhaps some other DNA-like molecule,” Robert Brown, declared.”[22]
Based on the existence of critical ingredients of life, the diverse adaptability and growing numbers of discovered extremophiles thriving in places that appear uninhabitable and hostile to life, along with the amazing survival of the Surveyor 3 microbes, Enceladus appears to be the best hope yet of finding extraterrestrial life. However additional exploration, analyses and research need to be conducted to confirm the existence of this life, the age-old quest of planetology, since in the words of Carl Sagan (1934-1996), a legendary scientist, astronomer and author, “extraordinary claims require extraordinary evidence.”[23]
While Cassini will conduct flybys of Enceladus in 2006 and 2007, the greatest prospect for confirming the existence of extraterrestrial life or moving closer in this direction lies with the March 12, 2008 rendezvous. On this day, Cassini will fly through the moon’s plume of water and ice particles, passing within 23 km (15 miles) of Enceladus’ surface (if mission control deems it safe enough). However, the likely two-year extension of Cassini’s mission scheduled to end in 2008 and additional missions including landing probes (especially at Enceladus’ south pole) will be needed, since in the words of Carolyn Porco, a Cassini imaging team leader at the Space Science Institute, “It’s not clear that Cassini has the means to determine if the ice crystals themselves contain microbes. It may require a device with much greater compositional precision than we have, so that may have to be left for a future mission [since] we will want to do a better job of determining the composition of the ices comprising the fractures, which contain the simplest organic compounds not found anywhere else.”[24]
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[1] Jeffrey Zweerink, Ph.D. Enceladus: Liquid Water But No Life.
[2] Richard Black. Saturn’s moon ‘best bet for life.’
[3] Enceladus (moon). Wikipedia.com.
[4] Enceladus (moon). Wikipedia.com.
[5] Enceladus (moon). Wikipedia.com.
[6] David Darling. cryovolcanism.
[7] Enceladus (moon). Wikipedia.com.
[8] Alan Boyle. Liquid water on Saturn moon could support life.
[9] Enceladus (moon). Wikipedia.com.
[10] Enceladus Erupting. Astrobiology Magazine.
[11] Bill Arnett. Enceladus. Saturn II.
[12] Hydrothermal Environments on the Ocean Floor. and Dictionary.com.
[13] Peter N. Spotts. Life beyond Earth? Potential habitats in the solar system keep popping up.
[14] Michael Schirber. How Life Began: New Research Suggests Simple Approach.
[15] David L. Chandler. Mars-bound instruments detect Arctic microbes.
[16] Search for life on Mars will start in Siberia.
[17] Astrobiologists To Hunt Small Game in Siberia. and Dictionary.com.
[18] Cold Methane Environments on the Ocean Floor.
[19] Life in Extreme Environments.
[20] Hydrothermal Environments on the Ocean Floor.
[21] On Earth: Extreme Environments.
[22] Peter N. Spotts. Life beyond Earth? Potential habitats in the solar system keep popping up.
[23] Alan Boyle. Liquid water on Saturn moon could support life.
[24] Leonard David. Saturnian moon inspires a gusher of questions.
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