The Possibility of Extraterrestrial Life in Our Solar System and on Exoplanets Stephen R. Cantor Teaching Lecturer Plymouth State University Plymouth, NH Sunday Morning University Congregation Bnai Israel St. Petersburg, FL 33710 February 22, 2015 3 Adar 5775
Life History of a Star Similar to Our Sun Life history of a star whose mass is near that of the sun. The sun is now halfway through its estimated lifetime of 10 billion years. For the sun, we can expect a billion more years of warming as the sun s temperature gradually increases. Life on earth will ultimately become impossible, not as one thought which was because the sun would cool off, but rather because it will grow too hot. In 5 or 6 billion years, the sun will expand to reach its maximum size as a giant larger across than the earth s current orbit. Later its fuel gone. The sun will collapse into a white dwarf as its outer layers stream off as a planetary nebula.
Mars Over billions of years, solar winds stripped away the Martian upper atmosphere. This atmospheric erosion transformed Mars from warm and wet, and presumably more hospitable to life, to cold and dry. The Opportunity Robotic Rover discovered evidence for the most ancient Martian environment where life could have existed, roughly 4 billion years ago. Dry lake beds and river channels carved into the Martian landscape suggest that the planet once had a thick encasing atmosphere. Mars should have been cold and dry in the past, as it is now, and it has been a mystery how warm and wet periods occurred in its history. One possibility is that water vapor and carbon dioxide injected into the Martian atmosphere by volcanoes were responsible.
Mars (Continued) Another possibility follows from the discovery that the Martian river valleys and craters made by giant asteroid impacts are about the same age. The kinetic energy of an asteroid of rock and ice 100 to 200 kilometers across (62 to 124 miles across) that smashed into Mars would have heated the planet s surface, and the asteroid s ice content would have become a scalding rain that fell for many years, caused flash floods, and fed the streams that carved the valleys seen today.
Mars (Concluded) Frozen ground water would also have melted and come to the surface. After some thousands of years, Mars would have cooled down and the water turned to ice, although underground water could have remained liquid for much longer. There seem to have been 25 such impacts 3.5 billion years ago, spread 10 to 20 million years apart.
Is There Life On Mars? Mars rotates on its axis in a little over 24 hours. Its revolution about the sun requires nearly 2 Earth years, and its axis is inclined to the plane of its orbit at nearly the same angle as the Earth s. Thus the Martian day and night have about the same lengths as ours. The Martian seasons are 6 months long, and at least as pronounced as ours. Although the conditions on Mars were once as hospitable to life as those on Earth, if life does exist today on Mars, it would have adapted to an environment that would soon destroy most Earthly organisms.
Is There Life On Mars? (Continued 1) Martian climates are severe by our standards. Over 50% further from the sun than the Earth, Mars receives much less solar energy per square foot than we do. The Martian atmosphere, which is largely carbon dioxide (only 0.13 percent is oxygen) is extremely thin, which is equivalent to the Earth s atmosphere at an altitude of nearly 40 km (24.8 miles), so little of the heat from the sun remains after nightfall. Daytime temperatures in the Martian summer at its equator rise to over 0 C (32 F), but at night drop to a chilly -23 C (-9.4 F). The average surface temperature of the entire planet is about -55 C (- 67 F). The scanty Martian atmosphere is unable to screen out harmful solar ultraviolet radiation, a function carried out in the Earth s atmosphere by the ozone present at high altitudes. At times, giant dust storms blow for months across the entire planet.
Is There Life On Mars? (Continued 2) NASA s Curiosity Rover recently recorded (reported in December 2014) a burst of methane gas that lasted at least 2 months. One possibility of the explanation for the methane is that it is the waste product of certain living microbes. The presence of methane is significant because that gas can not last for long. Calculations indicate that sunlight and chemical reactions in the Martian atmosphere would break up the molecules within a few hundred years, so any methane there now must have been created recently. On Earth, most of the methane in the air is produced by plants and bacteria.
Is There Life On Mars? (Continued 3) Scientists have also confirmed the presence of carbon-based organic molecules in a rock sample. The so-called organics are not direct signs of life, but they lend weight to the possibility that Mars had the ingredients required for life, and may still have them. Extreme Life The fact that most terrestrial life requires a regular supply of liquid water and oxygen plus protection from solar ultraviolet radiation does not mean that life of some kind could not exist without them.
Is There Life On Mars? (Continued 4) The life processes of certain bacteria on Earth do not require oxygen, so an oxygen-containing atmosphere is not indispensible, at least for primitive forms of life. Sulfur is abundant on Mars, and organisms that obtain the energy they need from chemical reactions based on sulfur are, in principal, possible. Conceivably there could have evolved on Mars life forms that can thrive on traces of water gleaned from minerals in surface rocks. Also, shells of some sort might protect Martian life from ultraviolet radiation. Or, life on Mars could exist underground where perhaps water ice or liquid water is present, the energy source being heat from the interior rather than sunlight, as in some places on Earth.
Is There Life On Mars? (Concluded) In fact, extra heat may not be needed: bacteria have been found deep in the ice sheets of Greenland and Antarctica that seem to have survived for hundreds or thousands of years at temperatures as low as -40 C (-40 F). One species remained alive at -196 C (-321 F) when immersed in liquid nitrogen. Since conditions long ago on Mars may have been comparable for a long period to those on the Earth, life of some kind could have come into being on Mars. The loss of most of the carbon dioxide of its atmosphere, vital for the greenhouse effect, and the disappearance of its surface water, some ending up underground, were gradual, and it is not at all absurd to speculate that living things could have adapted to the progressively harsher environment and survived in some form to the present.
Europa The Jovian satellite Europa is nearly the size of our moon, but unlike our moon, it is covered with a layer of water ice several kilometers thick. The ice is scarred by a network of curved, intersecting fracture lines, like a cracked eggshell. A surface of this kind is expected if the ice is floating on on a vast ocean of liquid water. Currents in the ocean regularly break up the ice, and water wells up into the cracks and freezes there to give the patterns we see.
Europa (Continued) Where does the heat come from that keeps Europa s subsurface water liquid and powers its currents? Some may be the result of radioactivity in Europa s rocky core, but most arises from the continued squeezing and stretching due to tidal forces caused by the gravitational pulls of Jupiter and its other large satellites, as in the case of Io. Given liquid water and heat, could life have developed on Europa?
Europa (Concluded) After all, organisms have been discovered on Earth that thrive in perpetual darkness with no need for sunlight, and living things have been discovered beneath Antarctica ice. Comet impacts on Europa may well have dumped on its surface billions of tons of such elements needed for life as carbon, nitrogen, and sulfur. Many scientists think that Europa, not Mars, is the most likely candidate fror extraterrestrial life in our solar system today. NASA hopes to launch a spacecraft to visit Europa, Callisto, and Ganymede. If liquid water is indeed under Europa s ice coat, a lander may follow that will inspect the cracks, and burrow through the ice to look for life.
Life Thrives Under Antarctica An unseen ecosystem flourishes in the darkness, entombed beneath 800 meters (1/2 mile) of ice in Antarctica. In January 2013, a U.S. team reported finding live cells in water from Lake Whillans, 800 meters below the West Antarctica ice sheet. In August 2014, that team announced that it had found not only life but a thriving ecosystem there. The team identified genetic traces of 3,931 microbial species or groups of species in water from the lake.
Life Thrives Under Antarctica (Concluded) The finding bolsters the idea that life could exist elsewhere in the solar system, such as under Mars s polar ice caps or in a subsurface ocean on Jupiter frozen moon Europa.
Titan Titan, Saturn s largest satellite, is believed to have a rocky core surrounded by a thick layer of water ice. It is the only satellite in the solar system with a dense atmosphere, which seems to consist largely of nitrogen with some methane and small amounts of other organic compounds. Titan s atmosphere is more like ours than of any other body in the solar system. Reddish clouds of organic compounds float in the atmosphere and send showers of methane rain and perhaps snow from time to time to feed the liquid methane rivers, lakes, and seas on Titan s surface.
Titan (Continued) Methane apparently plays the same geological role on Titan that water plays on Earth. Titan and the Earth are the only bodies in the solar system on which rain or snow of some kind falls to the ground. Could life have developed on Titan? The answer seems to be that it might have if Titan had been warmer. Titan s atmosphere is similar to the Earth s before life emerged. However, chemical processes are slower at lower temperatures.
Titan (Concluded) If life took a half billon years to come into being on the Earth, which is a reasonable estimate, then the solar system is not old enough for for this process to have occurred on the frigid (-179 C [- 290 F]) surface of Titan. However, conceivably heat from Titan s interior or from comet impacts produced pools of liquid water that could persist under the crusts of ice on its surface for long periods, which makes a search for life more reasonable.
Enceladus There appears to be a saltwater ocean, and a potentially habitable environment, beneath the icy crust of Saturn s moon Enceladus. Salty ice geysers erupt through cracks in Enceladus south polar ice sheet. Heat wells up through the fissures, presumably from a deep, warm reservoir. In April, 2014, data from the spacecraft Cassini was used to map an ocean hiding underneath Enceladus south pole. The sea holds about as much water as Lake Superior.
Enceladus (Continued) In July 2014, evidence was found connecting the fountains to an underground water supply. Using images from the Cassini spacecraft and maps, it was found that warm spots at the base of each geyser are too small to power the jets, so that spray could not originate at the surface. Most likely, Saturn s gravity repeatedly opens and closes the fissures allowing water and heat to escape from the interior and vent into space.
Enceladus (Concluded) The findings suggest that water jetting out of Enceladus probably comes from a warm, briny subsurface sea atop a rocky core. The vapor plumes emanating from Enceladus show traces of organic compounds. Even though distant from the sun, Enceladus is warm enough for subsurface liquid water to be present, perhaps heated by the same tidal kneading that heats Jupiter s moon Europa. Given warmth, water, and organic compounds, did life of some kind appear? Since many of the ingredients of life seem in place, right now there is nothing to rule it out.
Charon Pluto, regarded as a dwarf planet, has 5 satellites, the largest of which is Charon. Charon s diameter is about 1200 km, a little over half that of Pluto s. Charon probably consists largely of ice with some rock as well. There are signs that Charon, like Jupiter s moon Europa, has an ocean of liquid water under a covering of ice.
Charon (Concluded) It is possible that Charon, like Europa, is heated by tidal forces that kneed its interior. Could there be underground life on Charon that draws its energy from this heat rather than from the feeble sunlight? What little we know about Charon does not seem to rule it out.
Exoplanets (Planets Orbiting Other Stars) Prior to 1992, no planets outside our solar system had been discovered To-date, over 1700 confirmed exoplanets have been discovered In 2014, 715 new worlds were discovered by the Kepler Space Telescope In April 2014, the Kepler team reported the smallest possible habitable exoplanet, about 490 light-years away. It is just 10 percent wider than the Earth, and orbits its dim, red star at a distance where liquid water could exist. However, astronomers do not yet know if that planet can support life. As of January 6, 2015, the number of confirmed exoplanet worlds that may be in their stars habitable zone with the potential conditions for life is now 13.
Exoplanets (Planets Orbiting Other Stars) (Concluded) As of January 6, 2015, the total number of exoplanet candidates discovered by the Kepler Space Telescope is 4,145, including 800 with diameters roughly the size of Earth s. Among those far-off locales are a few worlds that might not be too different from Earth.
Extraterrestrial Life The recipe for creating life begins with a warm rocky planet, whose gravity is able to hold an atmosphere and whose distance from its parent star is such that it receives enough but not too much radiant energy. Then, provide an atmosphere of certain quite common gases, allow the planet to cool down a bit so that water condenses out, and wait perhaps a billion years. This recipe was followed successfully on Earth.
Goldilocks Zone For Planets From the point of view of life in the universe, just how many billions of planets there are is not really important. What is important is that a certain proportion of these planets apparently meet the requirements of a suitable distance from its star, near-circular orbit, proper mass, a favorable atmosphere, and reasonable rotational period, all of which are necessary for life as we know it on Earth. Some of the rocky exoplanets that have been discovered are in the Goldilocks Zone, that is not so near their parent star to be too hot, nor too far away to be too cold for liquid water to exist on them.
Goldilocks Zone For Planets (Concluded) But, not any location in our galaxy will not do. Elements heavier than hydrogen and helium, which are needed for life and are created in supernova explosions are more abundant toward the center of the galaxy. However, supernova explosions, which would destroy life on a planet anywhere near it, are also more common toward the center of the galaxy. A region between 22,000 and 30,000 light-years from the center of the galaxy, about a tenth of the galactic disk, seems about right. Our sun is in the middle of this zone.
Would Extraterrestrial Life Resemble Life on Earth? Given a suitable parent star, size, composition, and environment in space, life could appear. Would that life necessarily resemble life on earth? Terrestrial life has many interesting examples of what is called convergent evolution. Organisms that came from different genetic lines are found to have developed similar forms and functions as they responded to similar environmental pressures. Thus eyes like ours independently evolved in six different types of animals.
Would Extra-Terrestrial Life Resemble Life on Earth? (Concluded) Bats and dolphins independently developed echolocation. Birds, bats, and pterodactyls independently developed flapping wings. Regarding intelligence, despite very different evolutionary histories and brain structures, crows and people both exhibit such aspects of intelligence as tool use, deception, and intricate social behavior. Extra-terrestrial life is not likely to mirror life on Earth exactly, but if a planet elsewhere is much like Earth, then the organisms that arise there may well be recognizable variations of organisms that now or in the past flourished here on Earth.
How Much Advanced Life? The Earth came into being about 4.6 billion years ago. Less than a billion years later, life of some primitive sort appeared. On a cosmic scale, that is pretty fast. This strongly suggests that life could come into being on planets around other stars. But, what kind of life? The next step from one-celled micro-organisms to multicellular organisms took over 3 billion more years on Earth. And not until now, 4.6 billion years since the Earth s origin, have living things on Earth progressed to the point where interstellar communications can be taken seriously.
How Much Advanced Life? (Concluded) The development of an advanced civilization, as distinct from merely complex life forms, may require too many chance events to be inevitable by now, wherever life occurs. The dinosaurs were successful enough to be the dominant animals on earth for 140 million years, but they never developed an advanced civilization. If it were not for the asteroid that drove them into extinction, we the human species may not be here today. Although life may probably be common in the universe, advanced life may not be.