EARTH'S MOON is a luminous silver-white charm--a gleaming, beckoning object, as well as an ancient symbol for that which is feminine. As the largest and brightest object in the star-splattered night sky, it long has inspired wonder and curiosity. It also is associated with love and frequently serves as a sign of elusive beauty. Yet, it is the only natural body beyond our Earth that we have set foot upon.
A moon can be defined as a natural body that has attained orbit around a planet It is kept in its orbit by the force of the host planet's gravity and the gravity of the moon itself. Some planets host a moon and some do not. There are a few theories about where Earth's moon came from and how it managed to form. The most credible is termed the giant impact theory, which sometimes is termed the Big Whack or Big Splash theory by astronomers when they are in a playful frame of mind. These impish nicknames arise from the fundamental basis of the theory: a Mars-sized protoplanet, named Theia by astronomers, crashed into the primordial Earth billions of years ago. The collision caused part of the Earth's crust to be blasted off into space. Some of this debris was captured into Earth-or-bit, where it eventually was pulled together by the force of gravity to become our moon.
Most of the Big Whack theory first was suggested back in 1975 by William K. Hartmann and Donald R. Davis of the Planetary Science Institute in Tucson, Ariz. Their theory is based on geological evidence gathered by Apollo astronauts when they made their historic journey to the moon in July 1969. Oxygen isotopes embedded in moon rocks proved to be almost identical to those on Earth. Also, other pieces of evidence indicate that the moon is made up of, at least in part, the same material as Earth's mantle.
Our closest--and very bewitching--companion, however, hardly is unique in the cosmic scheme of things. More than 100 moons circle planets in our solar system. Most of them are frozen bodies, composed of ices and rocky material. Yet, a few may not be lifeless after all. In particular, Europa of Jupiter may harbor a subsurface ocean beneath its cracked icy crust, warmed by tidal flexing into a life-loving, liquid water state. Primitive forms of aquatic life may be swimming around in Europa's still-hypothetical global, subsurface ocean. In addition, the second largest moon in our solar system, Titan of Saturn, possesses an environment hauntingly akin to that of our Earth before life developed here (prebiotic). Large raindrops of liquid hydrocarbons fall to the surface of this cold, tortured moon, creating seas and lakes composed of liquid methane and ethane that play the same role as water on our Earth. It is possible that life, as we do not know it, can develop using liquids other than water.
Meanwhile, the largest moon in our solar system, Ganymede of Jupiter, is bigger than the planet Mercury. Like its sister moon Europa, Ganymede may harbor a global ocean of liquid water beneath its crust of icy rock. Likewise, a tiny frozen moon, Enceladus of Saturn, sprays out geysers of ammonia-laced water from its so-called "tiger stripes." Hence, Enceladus may bear water beneath its devastatingly frigid crust of ice.
We have known since 1995 that our solar system is not the only game in town. There are hundreds of extrasolar planets circling stars other than our own sun. Astronomers believe that our Milky Way could be bursting with billions of planets--and an even greater number of moons. Some of these moons could possess the precious, mysterious recipe that allows them to become bubbling cauldrons of life. "Moons form so commonly in our solar system that it would be ludicrous to think that Ibis is unique," indicates Peter Ward, coauthor of Rare Earth.
As far back as ancient Greece, humanity has suspected that there are other solar systems in the universe in addition to our own. This speculation has not always been met with open arms by the powers that be. For instance, in 1584, when the Catholic monk Giordano Bruno asserted that there were "countless suns and countless earths all rotating around their suns," he was accused of heresy, and burned at the stake. Our Earth was booted out of its exalted status as the most important entity in the universe early in the 16th century, when Nicolaus Copernicus calculated that our planet orbits the sun, instead of the other way around. His revolutionary insight, although reluctantly accepted, shattered the traditional Judeo-Christian dogma that we and our planet hold a special, central place in the cosmos.
Astronomers are discovering extrasolar planets at a frenetic pace. In the 1990s, they uncovered about two planets every year. For most of the past decade, however, the rate has accelerated to a pair per month. Today, extrasolar planets are being discovered on an almost daily basis. Astronomers are on the verge of finding an Earthanalog--a small rocky planet that is just the right distance from its star for an abundance of liquid water to exist--and where there is water, there is the possibility of life.
In our own solar system, moons are becoming primary targets of future space missions. Yet, when we look beyond our own extended solar family, it only is the extrasolar planets that astronomers have been able to detect so far. However, contends David Kipping, a predoctoral fellow at the Harvard-Smithsonian Center for Astrophysics, "I think exomoons are just as interesting as exoplanets."
Our solar system has eight planets (sorry, Pluto) and 170 moons (at last count). Most of these moons am frigid, barren rocks. However, as we have noted, a few of these icy baubles may be sanctuaries for some sort of living material. Other stellar systems very well may have similar numbers of planets and moons, and a subgroup of these bodies could dwell at just the fight "Goldilocks" distance from their star for liquid water to exist. "There may be just as many habitable moons as habitable planets in our galaxy," Kipping maintains.
Astronomers have considered how these enticing extrasolar moons might be detected. These modes of detection usually demand a chance, fortuitous alignment of parent star, extrasolar planet, and moon--although Kipping and his colleagues are proposing an alternative to this chancy mode of detection that should increase the odds of astronomers finally bagging an exomoon. Their new method depends on the careful observation of a transiting planet. Just as a witch flies across the luminous face of our moon on her broomstick, a transiting extrasolar planet crosses in front of the face of its star.
"If a transiting planet has a moon, it will cause a wobble in the planet's orbit," Kipping explains. The much sought-after wobble shows itself as an alteration in the time between two transits, which is how long it takes for the extrasolar planet to complete a single orbit around its star. Astronomers have, in the past, searched for transit timing variations (TTVs), but so far no sign of a remote exomoon has been spotted.
One of the primary difficulties is that several things can cause the TTVs, although Kipping's group has devised a method to avoid this conundrum. They have demonstrated that, by measuring alterations in the speed at which an extrasolar planet floats in front of its starve transit timing duration (TTD)--a moon hunter unambiguously can detect the desired exomoon. Furthermore, by measuring the two effects--TTVs and TTDs--and combining the variations, an astronomer can calculate the moon's orbital period and mass.
Lisa Kaltenegger, also of the Harvard-Smithsonian Center for Astrophysics, has written a paper discussing whether any definitive statements about the habitability of such moons can be made. Her contention is yes--provided astronomers are using the right technologies, such as the 6.5-meter instrument on the upcoming James Webb Space Telescope, a large, infrared optimized instrument scheduled for launch in 2014. In addition, the exomoon in question should be close to maximum separation so that astronomers are able to make the necessary measurements to characterize it.
Life on moons
Caleb Scharf, an astrobiologist at Columbia University, suggests that exomoons circling giant hot Jupiter-like planets could be life-supporting oases in the otherwise hellish orbits of such masting planets. "They might be the most likely places to find life in the galaxy." Scharf believes that life could thrive much farther away from stars than scientists ever supposed. This is, of course, relevant news for astrobiologists, who study what life might be like beyond our own planet.
Gravitational interactions among moons can be strong. The gravitational interactions among Europa and its sister moons--orbiting very close to Jupiter and each other--is what provides the energy and warmth necessary to sustain a subsurface global ocean of liquid water beneath a cracked icy crust.
This "tidal heating" makes Jupiter's four Galilean moons much balmier than astronomers would expect from the relatively meager quantity of sunlight bathing them, according to Scharf: "Exactly the same thing will happen in moon systems around extrasolar gas giants." His calculations have revealed that extrasolar moons as large as Earth could undergo at least 100 times as much healing as Io, the innermost Galilean moon. He thinks this combination of tidal heating and warmth from the parent star could keep such fortunate exomoons snug enough for water to remain a life-friendly liquid. "I believe it could double the size of the habitable zone around a star."
Kaltenegger adds, "Habitable-zone exomoons may be detected in the near future with missions like Kepler and could be orbiting their planet at a distance that allows for spatially separate transit events."
NASA's planet-hunting Kepler space telescope--that astronomers hope will spot Earthanalogs circling other stars--also might be capable of discovering those very elusive and possibly life-friendly exomoons that dwell in other solar systems. Kepler's main mission is to monitor thousands of stars, searching for telltale dips in brightness as orbiting planets float in front of them in transit events. The orbiting observatory was launched in March 2009 aboard a Delta-2 rocket.
It is possible for Earth-bound observatories, and some space-borne telescopes, such as Spitzer and Hubble, to bag Jupiter-sized extrasolar planets. Kepler, however, is the first telescope designed especially to detect alien worlds close to Earth in size. Astronomers have studied a large number of hypothetical planetary systems, and discovered that a bouncy Saturn-like planet, which would be low in mass for its size, provides the best of all possible chances for detecting a moon. A Saturn-like planet is preferable to a heavier Jupiter-like world. This is because planets such as Saturn are very large, and are capable of blotting out a great amount of light as they float in front of their parent star--however, because Saturn-like planets are very light, they will wobble much more than a more massive planet such as Jupiter.
"For the first time, we have demonstrated that potentially habitable moons up to hundreds of light-years away may be detected with current instrumentation," Kipping asserted in Space News. His team found that habitable exomoons down to 0.2 times the mass of Earth are spied easily by Kepler. "As we ran the simulations, even we were surprised that moons as small as one-fifth of the Earth's mass could be spotted.
Kepler will look for Earth-mass habitable exomoons around 25,000 stars up to 500 light-years away from our sun. "It seems probable that many thousands, possible millions, of habitable exomoons exist in the galaxy and now we can start to look for them," Kipping concludes.
Judith Braffman-Miller is a freelance journalist.
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|Title Annotation:||Science & Technology|
|Publication:||USA Today (Magazine)|
|Date:||Mar 1, 2011|
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