Imagine being roused by an attendant floating in zero gravity from a years-long sleep just before landing on a distant inhabited moon, Pandora. What you would soon discover is an exotic DNA carbon-based alien world.
In the new blockbuster Avatar, director James Cameron places his bet squarely on the “life as carbon-based DNA” camp. With NASA’s Kepler mission showing the potential to detect Earth-sized objects, habitable moons may soon become science fact. If we find them nearby, a new paper by Smithsonian astronomer Lisa Kaltenegger shows that the James Webb Space Telescope (JWST) will be able to study their atmospheres and detect key gases like carbon dioxide, oxygen, and water vapor.
“If Pandora existed, we potentially could detect it and study its atmosphere in the next decade,” said Lisa Kaltenegger of the Harvard-Smithsonian Center for Astrophysics (CfA).
So far, planet searches have spotted hundreds of Jupiter-sized objects in a range of orbits. Gas giants, while easier to detect, could not serve as homes for life as we know it. However, scientists have speculated whether a rocky moon orbiting a gas giant could be life-friendly, if that planet
orbited within the star’s habitable zone (the region warm enough for liquid
water to exist).
“All of the gas giant planets in our solar system have rocky and icy moons,” said Kaltenegger. “That raises the possibility that alien Jupiters will also have moons. Some of those may be Earth-sized and able to hold onto an atmosphere.”
Kepler looks for planets that cross in front of their host stars, which creates a mini-eclipse and dims the star by a small but detectable amount. Such a transit lasts only hours and requires exact alignment of star and planet along our line of sight. Kepler will examine thousands of stars to
find a few with transiting worlds.
Once they have found an alien Jupiter, astronomers can look for orbiting moons, or exomoons. A moon’s gravity would tug on the planet and either speed or slow its transit, depending on whether the moon leads or trails the planet. The resulting transit duration variations would indicate the moon’s existence.
Once a moon is found, the next obvious question would be: Does it have an atmosphere? If it does, those gases will absorb a fraction of the star’s light during the transit, leaving a tiny, telltale fingerprint to the atmosphere’s composition.
The signal is strongest for large worlds with hot, puffy atmospheres, but an Earth-sized moon could be studied if conditions are just right. For example, the separation of moon and planet needs to be large enough that we could catch just the moon in transit, while its planet is off to one side of the star.
Kaltenegger calculated what conditions are best for examining the atmospheres of alien moons. She found that alpha Centauri A, the system featured in Avatar, would be an excellent target.
“Alpha Centauri A is a bright, nearby star very similar to our Sun, so it gives us a strong signal” Kaltenegger explained. “You would only need a handful of transits to find water, oxygen, carbon dioxide, and methane on an Earth-like moon such as Pandora.”
“If the Avatar movie is right in its vision, we could characterize that moon with the James Webb Space Telescope in the near future,” she added.
While Alpha Centauri A offers tantalizing possibilities, small, dim, red dwarf stars are better targets in the hunt for habitable planets or moons. The habitable zone for a red dwarf is closer to the star, which increases the probability of a transit.
Astronomers have debated whether tidal locking could be a problem for red dwarfs. A planet close enough to be in the habitable zone would also be close enough for the star’s gravity to slow it until one side always faces the star. (The same process keeps one side of the Moon always facing Earth.) One side of the planet then would be baked in constant sunlight, while the
other side would freeze in constant darkness.
An exomoon in the habitable zone wouldn’t face this dilemma. The moon would be tidally locked to its planet, not to the star, and therefore would have regular day-night cycles just like Earth. Its atmosphere would moderate temperatures, and plant life would have a source of energy moon-wide.
“Alien moons orbiting gas giant planets may be more likely to be habitable than tidally locked Earth-sized planets or super-Earths,” said Kaltenegger. “We should certainly keep them in mind as we work toward the ultimate goal of finding alien life.”
So what about Cameron’s Na’vi? Will our first discovery of life beyond Earth be DNA and carbon-based? At once humanoid and tantalizingly exotic, the 10-foot, blue-skinned Na'vi come with supermodel dimensions; long articulated digits, the better to grip with; and the slanted eyes and ears of a cat.
In his famous lecture, “Life in the Universe,” Stephen Hawking observed that what we normally think of as ‘life’ is based on chains of carbon atoms, with a few other atoms, such as nitrogen or phosphorous. We can imagine that one might have life with some other chemical basis, such as silicon, “but carbon seems the most favorable case, because it has the richest chemistry.”
Several eminent scientists think otherwise, that life in the universe could have a myriad of possible biochemical foundations ranging from life in ammonia to life in hydrocarbons and silicon. Silicates have a rich chemistry with a propensity for forming chains, rings, and sheets. One of the founders on modern genetics, Cairn-Smith, argued that layers of crystalline silicates functioned as a primitive form of life on early Earth, before they evolved into carbon-based life forms.
The Earth (and we’re assuming, Pandora) was formed largely out of the heavier elements, including carbon and oxygen. Somehow, Hawking observes, “some of these atoms came to be arranged in the form of molecules of DNA. One possibility is that the formation of something like DNA, which could reproduce itself, is extremely unlikely. However, in a universe with a very large, or infinite, number of stars, one would expect it to occur in a few stellar systems, but they would be very widely separated.”
Other prominent scientists have warned that we humans may be blinded by our familiarity with carbon and Earth-like conditions. In other words, what we're looking for may not even lie in our version of a "sweet spot" in the Galactic Habitable Zone. After all, even here on Earth, one species "sweet spot" is another species worst nightmare. In any case, it is not beyond the realm of feasibility that our first encounter with extraterrestrial life will not be a solely carbon-based fete.
Alternative biochemists speculate that there are several atoms and solvents that could potentially spawn life. Because carbon has worked for the conditions on Earth, we speculate that the same must be true throughout the universe. In reality, there are many elements that could potentially do the trick. Even counter-intuitive elements such as arsenic may be capable of supporting life under the right conditions. Even on Earth some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines.
Several other small life forms use arsenic to generate energy and facilitate growth. Chlorine and sulfur are also possible elemental replacements for carbon. Sulfur is capably of forming long-chain molecules like carbon. Some terrestrial bacteria have already been discovered to survive on sulfur rather than oxygen, by reducing sulfur to hydrogen sulfide.
Nitrogen and phosphorus could also potentially form biochemical molecules. Phosphorus is similar to carbon in that it can form long chain molecules on its own, which would conceivably allow for formation of complex macromolecules. When combined with nitrogen, it can create quite a wide range of molecules, including rings.
So what about water? Isn't at least water essential to life?
Not necessarily. Ammonia, for example, as we mentioned above has many of the same properties as water. An ammonia or ammonia-water mixture stays liquid at much colder temperatures than plain water. Such biochemistries may exist outside the conventional water-based “habitability zone”. One example of such a location would be right here in our own solar system on Saturn’s largest moon Titan.
Hydrogen fluoride methanol, hydrogen sulfide, hydrogen chloride, and formamide have all been suggested as suitable solvents that could theoretically support alternative biochemistry. All of these "water replacements" have pros and cons when considered in our terrestrial environment. What needs to be considered is that with a radically different environment, comes radically different reactions. Water and carbon might be the very last things capable of supporting life in some extreme planetary conditions.
While some of these scenarios may seem the stuff of science finction, it’s important to keep in mind that the foundations of life on Earth, the association of a protein with a nucleic acid when view abstractly, does little to convey the endgame wonders such as blue whales and Mozart’s operas.
A billion years from now our descendants may have discovered other systems of physical life such as plasma within stars which would be based on the reciprocal influence of patterns of magnetic force and the ordered motion of charged particles. In fact, such life may well exists within our Sun.
Another form would be based on radiation emitted by isolated atoms and molecules in a dense interstellar cloud similar to the one physicist Fred Hoyle described in his scifi thriller, The Black Cloud. Such clouds can have a long lifetime lasting millions of years before they collapse.
Our personal favorite at The Daily Galaxy is the possibility of life in Neutron stars which would be based on the properties of polymeric atoms which which could form chains that could store and transmit information in a way that bears an uncanny similarity to the functions of nucleic acids -the molecules that carry genetic information or form structures within cells.
In the meantime, you can be sure I’ll be dreaming of the exotic and very sexy carbon-based Neytiri, the slinky Na’vi heroine -tail and all.
Casey Kazan - sourced from materials provided by the Harvard-Smithsonian Center for
Astrophysics (CfA) and New York Times article by Manhola Dargis
0 comentários:
Enviar um comentário