A tour of our solar system reveals a stunning diversity of worlds, from charbroiled Mercury and Venus to the frozen outer reaches of the Oort Cloud.
In between are a few tantalizing prospects for life beyond Earth – subterranean Mars, maybe, or the moons of giant planets with their hidden oceans – but so far, it’s just us.
“There’s nothing else in the solar system with lots of life on it,” said Mary Voytek, senior scientist for astrobiology at NASA Headquarters in Washington, D.C. “Otherwise, we would have likely detected it.”
Still, NASA continues searching the solar system for signs of life, past or present, and decades of investigation have begun to narrow down the possibilities. The broiling inner solar system seems unlikely (though the high-altitude clouds of Venus remain a possibility).
The same goes for the cloud-covered gas giants, with their crushing atmospheric pressures and seemingly bottomless depths – perhaps no solid surface at all, or if there is one, it’s no place for any living being.
The farthest provinces, with their dwarf planets and would-be comets locked in deep freeze, also seem a poor bet, though they can’t be ruled out. Same for dwarf planet Ceres in the asteroid belt, considered a possible “water world” either now or earlier in its history.
That brings us back to those tantalizing prospects. There’s Mars, now a cold, nearly airless desert, but once temperate and flowing with water.
And much hope remains out among the gas giants – not the big planets themselves, but their long list of moons. Jupiter’s Europa and Saturn’s Enceladus, despite their frozen, forbidding surfaces, are hiding vast oceans beneath the ice – among several moons with subsurface oceans.
Let’s begin the tour with our hottest planet.
Venus, a tantalizing target
Often called our “sister planet,” Venus, of similar size and structure to Earth, has critical differences: a surface hot enough to melt lead, a crushingly heavy atmosphere and an extremely volcanic geology. Venus began its existence much as Earth did, perhaps even with globe-spanning oceans. But the two planets took very different paths. A runaway greenhouse effect likely boiled off Venus’s oceans and turned the planet into a perpetual inferno – the hottest world in the solar system.
Yet Venus also exerts an irresistible pull for astrobiologists – scientists who study how life begins, its necessary ingredients and the planetary environments that it might require. Venus is a kind of negative to Earth’s positive; by studying what went so very wrong, we might learn what it takes to get life right.
“Venus gives us an example of an alternative evolution for planets,” said Vikki Meadows, an astrobiologist who heads the Virtual Planetary Laboratory in NASA’s Nexus for Exoplanet System Science.
The planet’s divergent path includes “loss of habitability, loss of water on the surface, sulfuric acid clouds, and a dense carbon-dioxide atmosphere,” Meadows said. “It’s also a warning – how terrestrial planets die.”
Venus has deep implications as well for the study of exoplanets – planets that orbit other stars. Many close to their stars are probably Venus-like worlds; Venus is a nearby laboratory showing how such planets might evolve.
Persistent, dark streaks in Venus’s clouds, where temperatures and pressure are more congenial, also prompt intriguing speculation: Could they be wind-whipped bands of microbial lifeforms? A recent study even suggested the presence of one potential life sign, a gas called phosphine, in the Venusian atmosphere. Bacteria on Earth produce it. For now, this possibility remains in the “unlikely but possible” column, scientists say; only further investigation will offer a definite answer.
Earth as an analog in search for life
As we cruise past our sole example of a life-bearing world, we might take a page from an earlier era of planetary exploration, courtesy of Carl Sagan. The astronomer and prize-winning author also was a key member of science teams for a variety of NASA’s solar system exploration missions, including Galileo.
In 1990, as the space probe zipped past Earth for a gravitational kick that would hurtle it toward the outer solar system, it turned its instruments on the home planet. Sagan’s question: Could Galileo detect signs of life on Earth?
And it did. Oxygen. Methane. A spike in the infrared part of the light spectrum, called a “red edge,” the telltale sign of reflective vegetation on the surface. Galileo even detected what today might be called a “technosignature” – a sign of intelligent life. In this case, powerful radio waves that were unlikely to come from natural sources.
“It’s vital to think about what our own planet would look like to an alien,” said Giada Arney, an astronomer and astrobiologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “It’s important to think about what signs of life they could actually see from space.”
Arney, who says much of her work involves “thinking about Earth as an exoplanet,” focuses on haze-shrouded worlds. As we search for signs of life around other stars, she reminds us that our own planet would have looked very different at various epochs in the deep past.
The Earth of billions of years ago, in the Archean era, might not even have been Sagan’s “pale blue dot.” Before the atmosphere became oxygen rich, Earth might occasionally have been a “pale orange dot,” Arney says, its orange haze created by complex atmospheric chemistry involving methane generated by microbes. A similar haze is found today in the atmosphere of Saturn’s moon, Titan, though in this case, not generated by life.
To find an analog of our own planet out among the stars, we must consider “not just modern Earth, but Earth through time,” she said. “The kinds of planets that could be (considered) Earth-like may be very different from modern Earth.”
Mars: Potentially habitable at some point
In a sense, the Red Planet tells a tale echoing that of Venus, but from the other side of the temperature scale. Investigations by orbiters, and rovers on the surface, confirm that Mars was once wet, with rivers, lakes and perhaps even oceans, and like Earth potentially habitable.
“The most exciting thing about Mars is that, at some point in time, 3.5 billion years ago, it’s clear the climate on Mars was more similar to Earth’s and had liquid water on its surface,” Voytek said.
Then solar wind and radiation stripped most of its atmosphere away. Its minimally active core ceased to generate a protective magnetic field. Its surface became forbiddingly cold and dry even as it was bombarded with radiation.
Is anything alive on Mars, perhaps beneath the surface, or in the frozen polar caps? Or might Earth’s future robotic explorers – one day maybe human explorers – stumble upon evidence of extinct forms from early Mars?
Two strikes against Mars, Voytek said, are its lack of available water and the absence of plate tectonics – the process on Earth that moves continents over eons and recycles buried nutrients back up to the surface.
“A lot of people think the planet may be dead – no life now because it doesn’t have that recycling going on,” she said.
Strikes in its favor might include detection of methane in the Martian atmosphere. On Earth, methane, otherwise short-lived in the atmosphere, is replenished by the metabolic action of life forms. Methane also can be produced through reactions of water and rock, but microbial life beneath the surface is another possibility.
“While surface conditions are not suitable, we may find evidence of past life, or perhaps some life that’s still hanging on,” said Morgan Cable, a researcher with the Astrobiology and Ocean Worlds Group at NASA’s Jet Propulsion Laboratory.
A newly launched Mars rover, Perseverance, is designed to collect samples of Martian soil – called regolith – that would be returned to Earth later for analysis. And the European Space Agency’s Rosalind Franklin lander, expected to launch in 2022, will drill beneath the Mars surface to search for signs of life.
Ocean worlds: The moons of gas giants
Our solar system’s majestic giants – Jupiter, Saturn, Uranus, Neptune – and their trains of moons might almost be considered solar systems in their own right. Some of these moons could well be habitable worlds; one of them, Titan, has a thick atmosphere, rain, rivers and lakes, though composed of methane and ethane instead of water.
We first glide toward Europa, a moon of Jupiter with an icy shell. Beneath the frozen surface, however, space probes have detected evidence of a vast ocean of liquid water. Two other Jovian moons, Ganymede and Callisto, also are likely to host subsurface oceans, though these might be sandwiched between layers of ice. That makes life less likely, Cable says.
“Europa, we think, has a nice contact between the liquid water ocean and the rocky interior,” she said. “That’s important because the energy you can generate through chemistry can be utilized by life.”
A potentially more accessible example can be found among the moons of Saturn, the next planet out. Enceladus, though tiny, also hides a liquid water ocean beneath an icy shell. But in this case, scientists know the little moon is doing something extraordinary.
“Luckily, it happens to be sending free samples from its ocean into space,” Cable says. “Enceladus is the only place in the solar system with guaranteed access to a subsurface ocean without the need to dig or drill.”
NASA’s Cassini spacecraft detected convincing evidence of hydrothermal vents on its sea floor, and jets of ocean water shoot through cracks in the moon’s surface, known as tiger stripes (Europa might have similar plumes). The material from Enceladus’s jets, in fact, forms one of Saturn’s rings.
Cassini flew through the plume, and although its instruments were not designed to analyze ocean-water samples – when it was built, the nature of these distant ocean worlds was unknown – it did pick up important clues.
These include complex organic molecules, salts similar to those in Earth’s oceans, and silicate “nanograins” and other evidence indicating the presence of hydrothermal activity.
Gases detected in the plume, hydrogen and methane, suggest enough energy is present to provide fuel for life.
“If there’s that much energy, why isn’t there life eating it?” Cable asks. So far, no one knows the answer.
“Hopefully a future mission will journey back to Enceladus and bring today’s modern sensitive instruments to this test,” she said.
Then there’s Titan.
Though smaller and with lighter gravity than Earth, Titan reminds us of our own world, if perhaps reflected through a fun-house mirror. Nitrogen dominates this moon’s atmosphere, as it does Earth’s. And Titan is the only other body in the solar system with rain, lakes and rivers – a whole hydrologic cycle in fact. Its flowing lakes and rivers are made of the hydrocarbons, methane and ethane.
Flowing water is not an option; Titan is nightmarishly cold, and water is essentially rock on its surface.
Titan also possesses a subsurface ocean of water, though deep down, and it’s unknown whether the ocean makes contact with anything from the surface. If it does, mixing with complex chemistry on the surface could provide fuel for life.
If it doesn’t, there’s another possibility. The chemical brew on the surface could power life as we don’t know it: exotic forms based on completely different components and chemical reactions.
“Titan allows us to test a completely separate hypothesis of life,” Cable said. “It has a completely different liquid on its surface.”
The extreme cold on Titan’s surface, of course, means chemistry happens very slowly if at all. That could make “weird life” far less likely.
NASA is planning a mission called “Dragonfly,” a rotary flier that will hop from place to place on the surface – and maybe solve some of Titan’s mysteries.
"The more we study our own cosmic backyard, the more surprises we find," Cable said. "And I'm excited. We'll be surprised more and more as we continue to extend our senses to the outer solar system and beyond."