During Mars’ warm season, surface features called recurring slope linea appear in Palikir Crater, resembling water flowing. Liquid brines near the surface might explain this activity, but the exact mechanism and source of the water are not understood. Credit: NASA/JPL-Caltech/Univ. of Arizona
In elementary school, I learned that all life on Earth requires three essential ingredients: carbon and various raw materials, usable energy, and liquid water. I found that interesting but somewhat depressing. The problem was—I wanted to believe in the existence of extraterrestrial life, and these three requirements ruled out most of the known universe as potential sites for finding it, at least as we understood the universe at that time.
Since then, I’ve become much more hopeful. What changed? Our scientific understanding of the things life needs to originate, as well as the many places where we now know where we can find them . Those three essential ingredients that I had worried were so rare are, in fact, ubiquitous, even in what we would generally consider the least hospitable regions of our own solar system. This new information has forced us to expand our classical definition of the “habitable zone”—but what do we mean by habitable?
This artist’s concept depicts one possible appearance of the planet Kepler-452b, the first near-Earth-size world to be found in the habitable zone of star that is similar to our sun. Credit: NASA/Ames/JPL-Caltech
The Earth, as we know, is “habitable” by the classical definition because liquid water is stable on its surface. About 74% of Earth’s surface is covered by water, not all of it is in liquid form. Most of the Arctic Ocean is covered year-round by ice, and kilometers-thick ice glaciers cover the Antarctic Continent. If the Earth were just 10% closer to the Sun, all that ice would melt. Conversely, if we moved the Earth outward by about 40%, nearly all of the world’s oceans would freeze. The balance of liquid and solid water makes life on Earth possible by stabilizing our climate. The range of distances from the Sun needed to achieve this stability defines the so-called classical “habitable zone.”
Thanks to research done in some of Earth’s most extreme environments, we know that simple life can thrive in the kinds of conditions found well outside this classical habitable zone, even at distances from our Sun that we would never have imagined to be amenable to supporting life.
Let’s take a tour through our solar system, and beyond, to see where life might be hiding.
This illustration portrays possible ways that methane might be added to Mars’ atmosphere (sources) and removed from the atmosphere (sinks). Credit: NASA/JPL-Caltech/SAM-GSFC/Univ. of Michigan
When the Red Planet comes to mind, most people picture a dry and dusty landscape devoid of any life as we saw in the images of Mars returned from the Viking missions in 1976. But this view of Mars has changed thanks in part to data returned from NASA’s Curiosity Rover. We know that most of Mars’ water exists as underground permafrost, and we now know that liquid water exists below the surface of Mars and breaks out onto the surface occasionally creating gullies.
This subsurface region of Mars could be a potential breeding ground for a certain type of life called extremophiles, which “love” extreme environmental conditions such as high salt content, high pressure, or high temperatures. Scientists suspect that a class of these extremophiles called “methanogens,” which are bacteria that produce methane, may be thriving beneath the surface. Plumes of methane have been spotted in the Martian atmosphere by telescopes on Earth, and burps of methane were detected by Curiosity emanating from the Gale crater. Is all that methane the byproduct of a methanogen’s metabolism, or is it a signature of geochemical processes that have yet to be discovered? ESA’s ExoMars Mission hopes to find some of the answers in 2017.
Moving further outward from the Sun, we encounter the dwarf planet, Ceres, currently being studied by the Dawn spacecraft. Ceres is notable for the bright spots that we now believe are mounds of salts left behind when the subsurface “brine” pushes its way to the surface and evaporates. This would imply a subsurface briny ocean. But what forces are pushing that liquid to the surface? On other planets, convection processes powered by an internal heat source are capable of moving subsurface liquid up to the surface. Is an internal heat source at work on Ceres? If so, Ceres has all the essential ingredients for life as we know it—liquid water, energy to keep it warm, and raw materials.
JUPITER’S GALILEAN MOONS:
This “family portrait,” a composite of the Jovian system, includes the edge of Jupiter with its Great Red Spot, and Jupiter’s four largest moons, known as the Galilean satellites. From top to bottom, the moons shown are Io, Europa, Ganymede and Callisto. Credit: NASA/JPL/DLR
Further out from Ceres, we encounter Jupiter and its four large Galilean moons Io, Europa, Ganymede, and Callisto. Of these four, Europa is generally considered the most promising world for extraterrestrial life because it has a subsurface ocean of liquid water about 10-to 60 kilometers beneath its ice surface, that is heated by the pull-push of tidal interactions between Jupiter and Io on one side of its orbit, and Ganymede and Callisto on the other.
Because of the heat generated inside Europa due to these gravitational tides, the floor of the subsurface ocean may have volcanoes and hydrothermal vents, similar to the ones found in Earth’s deep oceans. In our oceans, hydrothermal vents release a rich reactive chemical soup that is utilized by nearby life forms to drive their metabolic engines. Many scientists think that life may have originated at the sites of hydrothermal vents since some of the bacteria found there contain genetic characteristics that point to an extremely ancient origin.
Ganymede and Callisto also have subsurface oceans, but unlike Europa, theirs are much deeper—perhaps hundreds of kilometers below the surface. Ganymede is largely ice-covered, but Callisto has a larger fraction of rock. Searching for life in Ganymede’s or Callisto’s oceans would require technologies yet to be developed, but doing so would be worth the effort if we seek to find extraterrestrial life. As far as we can tell, the essential ingredients for life as we know it are in these surface oceans too.
This five-frame sequence of images from the New Horizons spacecraft captures the giant plume from Io’s Tvashtar volcano. Credits: NASA/JHU Applied Physics Laboratory/Southwest Research Institute
Jupiter and its moons are about five times farther from the Sun than Earth, which means that they receive about 25 times less energy from the sun than we do. If habitable regions do exist there, then our solar system’s habitable zone would have to include planetary bodies at least as far as Jupiter’s distance from the Sun. At this distance, the Sun’s energy is largely irrelevant for life, and other processes come to the forefront. We can see these at work as we move away from Jupiter towards Saturn.
NASA’s Cassini spacecraft pinged the surface of Titan with microwaves, finding that some channels are deep, steep-sided canyons filled with liquid hydrocarbons. One such feature is Vid Flumina, the branching network of narrow lines in the upper-left quadrant of the image. Credits: NASA/JPL-Caltech/ASI
Saturn is about ten times as far from the Sun as the Earth, yet even at this great distance, we find intriguing worlds rife with biological potential. Saturn’s largest moon, Titan, has an atmosphere that is like a natural organic chemistry laboratory. Similar to Earth’s atmosphere, the dominant gas on Titan is Nitrogen, but with about 5% methane. The methane is continually being chemically processed by solar ultraviolet radiation that drives a diverse realm of organic chemistry. These processes ultimately produce a wide range of hydrocarbons and nitrogen compounds that rain onto Titan’s surface creating lakes and icy dunes.
The subsurface of Titan may be even more complex than that of the large ice moons around Jupiter. Titan may have several different liquid, or semi-liquid, layers underneath its surface consisting of methane, ammonia, and liquid water. There may be layers or mixtures of each in Titan’s warm interior. We believe that water, carbon, and energy are all found inside Titan, making this moon one of the best candidates for finding life.
Narrow jets of gas and icy particles erupt from the south polar region of Enceladus, contributing to the moon’s giant plume. Credits: NASA/JPL/Space Science Institute
Not to be outdone, Saturn’s small moon, Enceladus also contains the three essential ingredients for life. Its surface is covered by water ice “snow,” and brine-spewing geysers have been spotted across its icy landscape. This brine contains a mixture of organic as well as other chemical compounds that some scientists hypothesize to have originated in hydrothermal systems similar to those on Earth. Thus, Enceladus is another one of our solar system’s best candidates for finding life, and all on a tiny moon only 300 miles in diameter.
The nitrogen ice glaciers on Pluto appear to carry an intriguing cargo: numerous, isolated hills that may be fragments of water ice from Pluto’s surrounding uplands. The hills, which are in the vast ice plain informally named Sputnik Planum within Pluto’s ‘heart,’ are likely miniature versions of the larger, jumbled mountains on Sputnik Planum’s western border. They are yet another example of Pluto’s fascinating and abundant geological activity. Credit: NASA/JHUAPL/SwRI
With an orbit way out in the Kuiper Belt at distances ranging from 30 to near 50 times the Sun-Earth distance, Pluto seems like an unlikely candidate for life. But even this ice world has potential habitability—at least on the inside. Scientists suspect that beneath Pluto’s diverse geologically active surface, an ocean of “warm” liquid water churns in its mantle, possibly providing the energy needed to drive biological processes. Pluto also has a rich deposit of hydrocarbons called tholins on its surface. But it will be some time before we can answer the question: is Pluto within the habitable zone?
NASA Hubble Space Telescope image of Fomalhaut b, a possibly rogue planet with an unusual elliptical orbit around nearby star, Fomalhaut. Credit: NASA/ESA/P. Kalas
Rogue worlds describe planets that aren’t gravitationally bound to stars, and so they almost certainly contain a large proportion of ice worlds. At first, one would think that the absence of sunlight would preclude a planet from being biologically relevant. But cues taken from planets in our own solar system suggest otherwise. For instance, if Jupiter and its moons were ejected from our solar system, not much would change for them. Jupiter emits more radiation than it receives from the Sun, and so it would continue to glow in the infrared for billions of years. Tidal heating between the Galilean moons and Jupiter would continue, keeping these ice worlds warm and habitable inside, for billions of years as well.
As our tour concludes…
…we are left with the problem of defining the habitable zone. The three ingredients for life are found well beyond where we had ever expected to find them, beyond the far edges of our solar system. We’re also left with more questions than answers, such as: What if life as we know it on Earth is actually an anomaly in the universe? What if the kinds of life hypothetically thriving in these ice worlds are the norm? If the past is any indication, our knowledge of life’s ideal conditions will continue to develop with our understanding of the many wonders of ice worlds.