Solar System Sites That Might Support Life
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3/25/2020 Alien life on Saturn? NASA 'to reveal MAJOR breakthrough with habitable ocean discovery' NASA could be about to announce a major alien Solar System Sites breakthrough on one of Saturn's moons. By Jess Bell / Published 12th April 2017 that might support life Saturn and its Moons Titan & Enceladus Dr. Alka Misra Department of Mathematics & Astronomy University of Lucknow, Lucknow Saturn Moons -82 [2020] • At opposition, when Saturn is at • Saturn was the outermost planet its brightest, it can lie within 8 known to ancient A.U. of the Earth. astronomers. Named after the father of Jupiter in Greek and Roman mythology. • However, its great distance from • Saturn orbits the Sun at almost twice the Sun still makes it the distance of Jupiter, with an orbital considerably fainter than either semi-major axis of 9.54 A.U. (1430 Jupiter or Mars. million km) and an orbital eccentricity of 0.06. • Saturn ranks behind Jupiter, the • The planet's sidereal orbital period of inner planets, and several of the 29.5 Earth years was the longest brightest stars in the sky in natural unit of time known to the terms of apparent brightness. ancient world. • Less than one-third the mass of • From Saturn's distance and angular size, Jupiter, Saturn is still an enormous the planet's radius--and hence the average planet, at least by terrestrial density--quickly follow. standards. • Saturn's equatorial radius is 60,000 km, or • As with Jupiter, Saturn's many moons 9.4 Earth radii. allowed an accurate determination of 3 the planet's mass long before the • The average density then is 700 kg/m (0.7 g/cm3)--less than the density of water arrival of (which is 1000 kg/m3). the Pioneer and Voyager missions. • Here we have a planet that would float in 26 • Saturn's mass is 5.6×10 kg, or 95 the ocean--if Earth had one big enough! times the mass of Earth. 1 3/25/2020 • Saturn's low average density indicates • that, like Jupiter, it is composed The rotation period at the equator is 10h14m, about 26 minutes shorter. primarily of hydrogen and helium. • Because of Saturn's lower density, this • Saturn's lower mass, however, results in rapid rotation makes Saturn even more lower interior pressure, so that these flattened than Jupiter. gases are less compressed than in Jupiter's case. • Saturn's polar radius is only 54,000 km, about 6000 km less than the equatorial • Saturn, like Jupiter, rotates very rapidly radius. and differentially. • Careful calculations show that this • The rotation period at high planetary degree of flattening is less than would be latitudes (determined by tracking weather expected for a planet composed of features observed in Saturn's hydrogen and helium alone. atmosphere) is 10h40m. • Astronomers believe that Saturn also has a • Saturn is much less colorful than Jupiter. rocky core, perhaps twice the mass of Jupiter's. • Saturn shows yellowish and tan cloud • Saturn's best-known feature is its belts that parallel the equator, but these spectacular ring system. regions display less atmospheric structure than do the belts on Jupiter. • Because the rings lie in the equatorial plane, • No obvious large "spots" or "ovals" adorn their appearance (as seen from Earth) changes Saturn's cloud decks. in a seasonal manner. • Bands and storms do exist, but the color changes that distinguish them on Jupiter are largely absent on Saturn • Astronomers finally made the first accurate determinations of the hydrogen and helium content in the late 1960s. • As on Jupiter, hydrogen and helium dominate--these most • These Earth-based measurements were abundant elements never escaped later confirmed with arrival of from Saturn's atmosphere the Pioneer and Voyager spacecraft in the 1970s. because of the planet's large mass and low temperature. • Saturn's atmosphere consists of molecular hydrogen (92.4 %), helium (7.4 %), methane (0.2 %), & ammonia (0.02 %). • However, the fraction of helium on Saturn is far less than is observed • In Saturn's cold upper atmosphere, most on Jupiter (where, as it is found, ammonia is in the solid or liquid form, with helium accounts for nearly 14% of relatively little of it present as a gas to absorb sunlight and create spectral lines. the atmosphere) or in the Sun. 2 3/25/2020 • In September 1990, Amateur • Infrared measurements indicate that Saturn's Astronomers detected a large white surface (i.e, cloud-top) temp. is 97 K, substantially spot in Saturn's southern higher than the temperature at which Saturn would hemisphere, just below the equator. reradiate all the energy it receives from the Sun. • In November of that year, when • In fact, Saturn radiates away almost three times the Hubble Space Telescope imaged the phenomenon in more detail, the more energy than it absorbs. spot had developed into a band of clouds completely encircling the • Thus Saturn, like Jupiter, has an internal planet's equator. energy source. • But the explanation behind Jupiter's excess energy--that the planet has a large reservoir(basin/tank) of heat left over from its formation--doesn't work for Saturn. • • In Saturn, where the internal Saturn is smaller than Jupiter and so temperature is lower, the helium doesn't must have cooled more rapidly--rapidly dissolve so easily and tends to form enough that its original supply of energy droplets instead. has long ago been used up. • The phenomenon is familiar to cooks, • What then is happening inside Saturn to who know that it is generally much produce this extra heat? easier to dissolve ingredients in hot liquids than in cold ones. • The explanation for this strange state of affairs also explains the mystery of • Saturn probably started out with a fairly Saturn's apparent helium deficit. uniform solution of helium dissolved in hydrogen, • At the temperatures and high pressures • But the helium tended to condense out found in Jupiter's interior, liquid of the surrounding hydrogen, much as helium dissolves in liquid hydrogen. water vapor condenses out of Earth's atmosphere to form a mist (fog/smog). • The amount of helium condensation was greatest in the planet's cool outer layers, • The answer is simple: As the helium where the mist turned to rain about 2 billion years ago. sinks toward the center, the planet's gravitational field compresses it and • A light shower of liquid helium has been heats it up. falling through Saturn's interior ever since. • The gravitational energy thus released • This helium precipitation is responsible for is the source of Saturn's internal heat. depleting the outer layers of their helium content. • In the distant future, the helium rain • So we can account for the unusually low will stop, and Saturn will cool until its abundance of helium in Saturn's atmosphere- outermost layers radiate only as much -much of it has rained down to lower levels. energy as they receive from the Sun. • But what about the excess heating? 3 3/25/2020 • Careful measurements of Saturn's energy budget (balance of energy absorbed versus energy radiated) • When that happens, the show that the planet radiates 1.5–2.5 times more temperature at Saturn's cloud energy into space than it receives from the Sun. tops will be 74 K. • This radiated energy indicates that the planet must have an internal heat source. Scientists accept that Saturn draws its extra energy from two sources: • As Jupiter cools, it too may someday experience helium (1) heat left over from the planet's formation approximately 4.5 billion years ago, still radiating precipitation in its interior, out into space, and causing its surface temperature (2) the "raining out" of atmospheric helium. to rise once again. • Just as water condenses in terrestrial clouds to produce rain, droplets of liquid helium form in Saturn's atmosphere. • As these droplets fall through Saturn's • Support for the helium-condensation model atmosphere they acquire kinetic energy. was obtained during the Voyager encounters, when it was found that the abundance of • This energy is absorbed into deeper layers helium in Saturn's atmosphere was much where the droplets meet resistance and slow lower than that observed in Jupiter's. their fall, and the temperature in those regions increases. • Upper-atmospheric depletion of helium has • This thermal energy is eventually circulated not yet occurred on Jupiter because its by convection back up through the higher layers atmosphere has only recently become cool of the atmosphere and radiated into space. enough to permit helium condensation; • The helium raining out of Saturn's upper layers • On Saturn, in contrast, helium has been is left over from the planet's formation; in about raining out for about two billion years, settling two billion more years all of Saturn's helium will half the available helium toward the core have sunk deep into the planet, at which time heating by helium condensation will cease. • At the resolution of his small telescope, the Ring Systems of Saturn rings looked like bumps on the planet, or • The most obvious aspect of Saturn's perhaps components of a triple system of appearance is, of course, its planetary some sort. ring system. • In 1659, the Dutch astronomer Christian • Astronomers now know that all the jovian Huygens realized what the "bump" was--a planets have rings, but Saturn's are by far thin, flat ring, completely encircling the the brightest, the most extensive, and the planet. most beautiful. • In 1675, the French-Italian Astronomer • Galileo saw them first in 1610, but he did Giovanni Domenico Cassini discovered the not recognize what he saw as a planet first ring feature, a dark band about two- with a ring.