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Humans have pondered the question of life beyond our planet for millennia. Only in the past few decades, however, has musing given way to observation.<\/p>\n
Given its promising physical features and proximity, Mars was the obvious target for humanity\u2019s first efforts in \u201cboots on the ground\u201d astrobiological exploration, but it is not our solar system\u2019s only body of interest. Venus is something of an anti-\u00adMars, its mean surface temperature a scorching 464\u00b0C (867\u00b0F), maintained by a runaway greenhouse atmosphere. Some, however, propose that earlier in its history, Venus was more temperate, perhaps a potential abode for life. It has even been suggested that the clouds that envelop the present-\u00adday Venusian surface are sufficiently cool to support life. Indeed, phosphine (PH3), a gas produced by organisms on Earth, has been reported from Venusian clouds, although both the measurement and its interpretation have been debated vigorously. Neither do all planetary scientists agree that the surface of Venus was once habitable.<\/p>\n
For now, then, Earth remains the inner solar system\u2019s Goldilocks planet for life: Mars is too cold, and Venus too hot. Earth? Just right.<\/p>\n
The concept of a habitable zone\u2014\u00ad the range of orbits around a star within which liquid water can be stable on revolving planets\u2014\u00ad permeates modern discussions of astrobiology. It turns out, there is more than one way to sustain liquid water in our solar system.<\/p>\n
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Cover of “Earth & Life” by Andrew H. Knoll Courtesy Princeton University Press<\/p>\n
Europa is a moon of Jupiter, far too distant from the Sun to be heated by its rays. Despite this, Europa has an ocean hidden beneath a veneer of water ice. Beginning in the 1970s, analysis of light absorbed or reflected by Europa identified H2O on its surface. Later, satellite images confirmed the moon\u2019s icy face, and ensuing measurements of Europa\u2019s gravity demonstrated that this surface is only skin deep, extending downward 80 to 170 km (50 to 106 miles) to a rocky interior. Finally, studies of magnetism indicated that the lower part of Europa\u2019s watery mantle is liquid. Together, then, light, gravity, and magnetism revealed a subsurface ocean deep within the solar system.<\/p>\n
How can liquid water be maintained so far from the Sun? The answer is \u201ctides.\u201d Most readers, whether they live in Atlantic City or Saskatoon, are familiar with tides on Earth. Our planet and moon are locked in a gravitational dance, and as Earth rotates beneath the moon, seawater is alternately drawn toward the moon or, on the other side of our planet, away from it, generating the oscillating tides observed along coastlines. (The Sun also influences tides on Earth, much as the moon does, but less strongly.) Tides affect the solid Earth as well, but because both Earth and the moon are minor players in the gravitational relationships found throughout our solar system, and because the moon\u2019s orbit is nearly circular, tidal influence on the solid Earth is small. Not so for the moons revolving around our solar system\u2019s giant planets, Jupiter and Saturn. Together, these planets have some 369 documented moons, mostly small bodies with highly eccentric orbits. (No fewer than 128 of Saturn\u2019s are sufficiently tiny that they were discovered only in early 2025.) Jupiter\u2019s strong gravitational pull induces tides in the moons, and as their rocky interiors are pushed and pulled, the resulting friction generates heat.<\/p>\n
Io, the closest moon to Jupiter, gets so hot that its rocky interior melts; because of this, Io is our solar system\u2019s most active volcanic body. The next three closest moons, Europa, Ganymede, and Callisto, don\u2019t generate volcanoes, but they heat up enough to melt the lower part of their icy surfaces.<\/p>\n
Currently, much astrobiological interest focuses on Europa. Light cannot penetrate beneath the moon\u2019s icy surface, but geochemical models suggest that chemical reactions between Europa\u2019s ocean and its rocky interior could provide energy for at least a limited biosphere. Indeed, magnetic data indicate that Europa\u2019s subsurface ocean is salty, telling us that that water does interact chemically with underlying rocks. Moreover, the strong tides induced by Jupiter crack Europa\u2019s icy shell, allowing subsurface ocean water to spread onto the surface, depositing sodium chloride (NaCl, or table salt) and perhaps other materials on top of the ice. There is also evidence for carbon dioxide ice (the \u201cdry ice\u201d of high school science demonstrations), documenting the presence of carbon at and near Europa\u2019s surface. Thus, observations of Europa check some of the boxes of interest to astrobiologists. Liquid water? Check. Source of energy? Check. Carbon? Check. But is there nitrogen? Phosphorus? We don\u2019t yet know.<\/p>\n
For the moment, we can\u2019t tell whether Europa\u2019s subsurface ocean is habitable, perhaps even inhabited. If all goes well, however, we\u2019ll soon know much more. In October 2024, NASA launched its Europa Clipper, a mission that promises to enhance dramatically our understanding of the Europan ocean. Importantly, a suite of instruments will interrogate the chemistry of Europa\u2019s surface, providing a deeper understanding of the chemical makeup of subsurface ocean waters that episodically reach the surface. Those<\/p>\n
fissures in Europa\u2019s ice shell may also transport surface materials, including organic matter from micrometeorites, downward into the ocean. Direct observations of microorganisms are unlikely, but the mission team will be alert for possible indications of a conversation between moon and life in the details of Europa\u2019s surface chemistry. The Clipper should arrive at Europa in 2030.<\/p>\n
Jupiter is not alone in harboring moons of astrobiological interest. Indeed, two of the most fascinating moons in the solar system revolve around Saturn: Enceladus and Titan.<\/p>\n
Enceladus is a small moon, less than 600 km (375 miles) in diameter, with an icy surface that sits atop a rocky interior. So far, this sounds a bit like Europa, but there are intriguing differences. Notably, Enceladus\u2019s north polar region is pocked by numerous impact craters, evidence that this surface has existed for a long time, while the southern hemisphere is nearly smooth, revealing that it has been resurfaced relatively recently. A series of long, broadly parallel fissures colloquially known as tiger stripes marks the south polar region. Observations by NASA\u2019s Cassini spacecraft have revealed that the temperature of these stripes is high relative to the rest of the moon\u2019s surface. And Cassini discovered something else: The stripes put on one of the solar system\u2019s greatest shows. Geyser-\u00adlike fountains regularly spew subsurface liquids hundreds of kilometers into space. Like Europa, Enceladus has a subsurface ocean, and it is the spectacular emissions from this ocean that continually resurface its southern hemisphere.<\/p>\n
As was true for Europa, we will not soon drill through Enceladus\u2019s ice cover to sample its ocean directly. But Enceladus offers an additional option not available for Europa. We can sample the jets of salty water as they shoot into space.<\/p>\n
So far, we know that Enceladus\u2019s subsurface ocean is salty, indicating, once again, that subsurface waters react chemically with the moon\u2019s rocky interior. We also know that there is hydrogen (H2), nitrogen gas, traces of ammonia (NH3), and even amines\u2014\u00ad carbon-\u00adbearing molecules bound to hydrogen-and nitrogen-\u00ad rich structures, found, among other places, in the amino acids that form proteins. Other carbon-\u00adbearing compounds have also been detected, including carbon dioxide and trace amounts of simple organic molecules, such as methane, propane, acetylene, formaldehyde, and benzene. Moreover, there is reason to believe that the interior of Enceladus may contain phosphorus, a key element in terrestrial biomolecules.<\/p>\n
None of these molecules requires the presence of life, but a number of them\u2014\u00ad formaldehyde, for example, and those amines\u2014\u00ad are thought to have played a role in the origin of life on Earth.<\/p>\n
We don\u2019t know what future missions will find. Enceladus may or may not harbor a limited microbial biota. At the very least, however, Enceladus will be confirmed as another solar-\u00adsystem body where biologically relevant carbon chemistry takes place.<\/p>\n
This leaves Titan, perhaps the most interesting body in the solar system, save for Earth itself. Titan is the largest moon of Saturn. It has a thick, cloudy atmosphere and a surface patterned by lakes and rivers. This sounds a bit like Earth, and the first images of Titan\u2019s surface, beamed home by the European Space Agency\u2019s Huygens probe that dropped onto the moon in 2005, look eerily familiar.<\/p>\n
That familiarity, however, is misleading. Titan\u2019s atmosphere consists mostly of nitrogen gas, but its clouds are hydrocarbons; it rains methane, and its rivers and lakes consist of liquid methane as well. The surface of Titan is far too cold for liquid water; the mean surface temperature is \u2212179\u00b0C (\u2212290\u00b0F).\u00a0There is H2O at the surface of Titan, however; it makes up the moon\u2019s solid surface. There may also be liquid water at depth, heated, once again, by tidal friction. I doubt that we\u2019ll ever detect life on Titan, but its remarkable chemistry can tell us much about the possibilities of environments and carbon chemistry far from Earth.<\/p>\n
In sum, we may or may not be the only life to have taken root in our solar system. Mars and, perhaps, Venus might have harbored organisms in the past, and, just possibly, microbes may exist today in the subsurface oceans of moons that orbit Jupiter and Saturn. I\u2019m not convinced that any of these possibilities is likely, but their probability is not zero, and continuing exploration is warranted. What we can say with more certainty is that within our solar system, only on Earth has the conversation between life and its physical home transformed both through time. And only on Earth has persistent life evolved intelligent beings capable of asking questions about the universe we inhabit.<\/p>\n