We have seen that Earth’s abundant life is due in large part to its surface oceans of liquid water. We’ve also seen that Earth is the only world in our solar system with such oceans, at least in part because of its rocky composition, its large size for a rocky world, and its distance from the Sun that places it within the habitable zone.

Nevertheless, there is a least some chance that other worlds in our solar system might have life, and many more possibilities exist for planets around other stars. Let’s take a brief look at what science tells us about possibilities for life beyond Earth.

“Follow the Water”

The first question scientists must ask when considering life beyond Earth is: Where should we look? After all, even our own solar system has a vast number of worlds when we include all the asteroids and comets, and we can’t possibly look everywhere. For this reason, scientists have in general used a strategy that is sometimes called “follow the water.” Here’s the idea:

You will recall from life science that all life on Earth requires liquid water to survive. This is true not only of plants and animals, but also of all the microscopic organisms found on our planet. Moreover, virtually everywhere that we find liquid water, we find life. That is, if you were to take a drop of water from any place on Earth — even deep underground — and look at it under a microscope, you’d almost certainly see some living organisms swimming around (Figure 4.62).

You might therefore wonder if this same idea would hold true elsewhere. That is, if we find liquid water on another world, should we expect to see life in it? No one knows, but it certainly seems like the answer should at least be “maybe.” For this reason, scientists looking for extraterrestrial life assume that a good first step is to look for worlds that have liquid water. In other words, the search for life should begin by “following the water” wherever we might find it.

Note that it does not matter whether this water is on the surface of a world or underground. It seems at least possible that life could exist in either case. Nevertheless, this guidance of “follow the water” allows us to significantly narrow the search for life down. Instead of wondering about all worlds, we need only to wonder about worlds that have liquid water either on their surfaces or underground. This allows us to rule out most of the many worlds in our solar system as places worth searching for life:

• We can rule out Mercury and the Moon, which certainly have no liquid water on their surfaces, and are unlikely to have any underground either.
• We can rule out virtually all small asteroids and comets, because small objects have no atmosphere to allow for liquid water on their surfaces and too little internal heat to keep water liquid underground.
• We can generally rule out Venus, because its surface is much too hot to have liquid water, and it is even hotter underground (just as on Earth, temperatures increase with depth in Venus’s interior).
• We can probably rule out the four large, hydrogen-rich planets (Jupiter, Saturn, Uranus, Neptune), because although their clouds almost certainly contain some droplets of liquid water, the strong winds on these planets make it unlikely that individual droplets ever last very long.

That narrows the possibilities for life in our solar system down to a much more manageable number of worlds: Mars, which apparently had surface liquid water in the distant past and might still have liquid water underground today, and a few of the largest ice-rich worlds, some of which show evidence of subsurface oceans. We’ll discuss these possibilities shortly, but first let’s consider how scientists learn about these distant worlds.

Spacecraft Exploration

If we are going to search for water (and life), we are going to need to get as “up close and personal” with other worlds as possible. Beyond our solar system, the great distances to other stars means we’ll have no choice but to rely on telescopes. But here in our own solar system, we’ve developed technology that allows us to send spacecraft to other worlds, where we can study them in much greater depth than we can with telescopes on or near Earth. It’s therefore worth talking briefly about the different types of spacecraft that we use.

First, remember that so far, the Moon is the only world besides Earth that humans have ever visited in-person. The Moon visits occurred between 1969 and 1972 as part of NASA’s Apollo program. Aside from those Apollo missions to the Moon, all other spacecraft that we have sent to other worlds have been robots , meaning machines that can carry out complex tasks. In other words, we have conducted a fair amount of robotic exploration of other worlds, but human exploration of these worlds remains in the future.

Slide Show 4.63 presents a gallery of a few famous robotic missions. Note that, generally speaking, we can divide these missions into three major types:

• Orbiters : An orbiter is a spacecraft that conducts its studies by looking down from above as it orbits some world. This gives an orbiter a “global” vantage point that is very useful; we can often obtain data from orbiters that we could not get from the surface, which is why we use many orbiting spacecraft to study our own planet Earth. We’ve sent orbiters to all of the planets except for Uranus and Neptune (as well as to the Moon and to a few asteroids and comets). As long as an orbiter is high enough up to avoid any air resistance from a planet’s atmosphere, it can remain in orbit indefinitely. Therefore, orbiter missions usually continue for many years, until the spacecraft either stops working or scientists deliberately end the mission.
• Landers : The most “up close and personal” study of other worlds comes from spacecraft that actually land on their surfaces. Some landers simply stay in the place where they land, others are rovers that can drive around. The Perseverance rover, which should land on Mars in 2021, even carries a small, experimental helicopter (called Ingenuity) that it will test out on Mars.

Prospects for Life On Mars

We are now ready to undertake a brief tour of places where scientists hope to search for life on other worlds. Using the “follow the water” strategy, our first stop is Mars.

Slide Show 4.64 – Mars today does not have enough atmospheric pressure to have surface liquid water, but these slides show some of the evidence indicating that it had surface liquid water in the past.

In addition to the evidence that you see visually in the slide show, the rovers we’ve sent to Mars have also conducted chemical analysis of soil and rocks on Mars. These studies further confirm that Mars once had abundant liquid water on its surface.

Did Mars Have Life Long Ago?

Could Mars Still Have Life Today?

Mars no longer has any surface liquid water, but scientists suspect that there could still be small amounts of liquid water underground. Two key facts point to this possibility.

The first fact is that Mars still has a substantial amount of water ice (Figure 4.66). Some of this water ice is found in Mars’s polar caps, and more appears to be mixed in with soil in other areas of the planet. The second is the fact that Mars has volcanoes that have been active in “geologically recent” times. As we’ve discussed, this fact implies that Mars still has a moderate amount of internal heat (just as we expect for its intermediate size). In principle, this heat ought to be sufficient to melt some of the underground ice into liquid water. In fact, scientists have found evidence for at least one underground lake on Mars.

Figure 4.66 – Mars no longer has liquid water on its surface, but it still has a significant amount of water ice.

These facts suggest the intriguing possibility that if Mars once had life on its surface, some of that life might have evolved to survive in the underground liquid water. If such life exists, scientists suspect it would consist only of microscopic organisms (in part because of the limited energy available to underground life). Still, finding living microbes on Mars would revolutionize biology, because for the first time we’d be able to study life from another world.

What happened to Mars’s Water?

The evidence showing that Mars once had lakes, rivers, and rainfall is extremely strong, and yet Mars no longer has this water today. So it’s natural to wonder what happened to all that water.

You might at first guess that this water might now be frozen at the polar caps and elsewhere. However, careful studies have found that there is far too little ice on Mars today to account for all the water it must have had in the past. Similar studies tell us that Mars has also lost most of the atmospheric gas that once gave the surface warmer temperatures and a much higher atmospheric pressure. Where did all this water and atmospheric gas go?

Interestingly, scientists trace the answer to an idea you can understand by thinking about a beautiful natural light show that occurs on Earth: the aurora , which can often be seen on clear nights at high latitudes. Figure/Video 4.67 shows what the aurora looks like.

The aurora is a phenomenon that occurs because Earth has a global magnetic field . You are familiar with this magnetic field because it is what makes a compass needle point north. But it has other important effects as well. In particular, the magnetic field creates what is, in essence, an invisible protective “bubble” around our planet (more formally called a magnetosphere ).

Figure 4.68 shows the idea. In addition to emitting sunlight, the Sun also sends a stream of high-energy, charged particles outward into space (called the solar wind ). If Earth lacked a magnetic field, these particles would all slam into atoms and molecules in Earth’s upper atmosphere. Instead, the magnetic “bubble” deflects most of these particles around our planet, while channeling a few of them toward the poles, where they cause the aurora.

Scientists trace Mars’s loss of atmospheric gas and water to the fact that Mars does not have an aurora or a strong magnetic field, and therefore does not have a protective magnetic bubble like Earth. As a result, the particles from the Sun do slam into the atoms and molecules of Mars’s upper atmosphere, sometimes hitting them in a way that knocks them out of the atmosphere and into space, never to return.

Over billions of years, this process has apparently stripped away most of the atmospheric gas that Mars once had. This, in turn, allowed water to evaporate, and once water vapor is in the atmosphere, the molecules could be split apart and the hydrogen could escape to space. In other words, scientists ultimately trace Mars’s loss of water to its lack of a protective magnetic field.

Other Prospects for Life in the Solar System

Beyond Mars, it is too cold for water to stay liquid on the surfaces of any other worlds in our solar system. However, several ice-rich worlds show strong evidence of having subsurface oceans of liquid water. Again, the “follow the water” idea suggests there is at least a possibility that life might exist on some of these worlds. Let’s take a quick tour.

Jupiter’s Moons

Recall that Jupiter has four large moons that were first discovered by Galileo (see Section 3.2.2). Figure 4.69 shows global views of these four moons. The innermost of them, Io, is pockmarked by active volcanoes. In fact, Io is the most volcanically active world in our solar system. If Io ever had much water, it has probably expelled most of it through its volcanoes, making it highly unlikely that Io could have any subsurface liquid water.

The other three of these moons — Europa, Ganymede, and Callisto—are quite different from Io. All of them have surfaces made mostly of water ice, and all show at least some evidence of having a subsurface ocean. The strongest evidence comes from Europa.

Several different lines of evidence make scientists extremely confident that Europa has a subsurface ocean. The first piece of evidence comes from the fact that Europa has very few impact craters. You may already have explored this evidence in an earlier activity, but you should review it to be sure you understand it.

To review how the lack of impact craters points to a subsurface ocean, discuss the following questions with a classmate, then click to open the answer to see if they agree with what you came up with.

• Thinking back to what you learned about volcanoes and impact craters on rocky worlds, what does the lack of impact craters on Europa tell us about the age of its surface?
  
  Open

  Like all worlds in the solar system with solid surfaces, Europa should have had many impact craters when it was young. The lack of many impact craters today means that these ancient impact craters have all been erased. Therefore, its surface must be fairly young.

• Based on what you know about Europa’s surface, what substance seems most likely to have covered up ancient craters?
  
  Open

  Given that Europa’s surface is made largely of water ice, the most obvious explanation for the erasure of its craters is that they were covered up by a watery “lava” that erupted upward from underground.

Many other features of Europa’s surface also look like they were shaped by “eruptions” of liquid water from underground. Moreover, scientists know that the same heat source that makes Io so volcanically active (see this earlier box) also heats Europa’s interior, and this provides enough heat to melt some of Europa’s subsurface ice into liquid form. Further evidence comes from careful study of Europa’s magnetic field, which changes with time in a way that only makes sense if Europa has a salt-water ocean under an icy crust.

Slide show 4.70 summarizes some of the evidence for an ocean on Europa, and shows a model of what that ocean might look like beneath the surface.

Slide Show 4.70 – Evidence for an underground ocean on Europa.

The evidence for subsurface oceans on Ganymede and Callisto is less strong, but it still seems more likely than not that they have them. Together with Europa, this raises the intriguing possibility that there are three “ocean worlds” — offering three possible homes for life — orbiting the single planet Jupiter.

Note that, if any of these worlds do have life in their underground oceans, it cannot be getting energy from the Sun, because no sunlight would penetrate so deep underground. However, it is possible that the same internal heat that keeps the underground water from freezing might also power undersea volcanoes. If so, these volcanoes might provide energy for life in the same way that undersea volcanic vents on Earth provide energy for numerous lifeforms around them. Video 4.71 summarizes some of these ideas as they apply to Europa.

Saturn’s Moons

Saturn has two moons on which scientists imagine at least some possibility of life. The first is Titan, the only moon in our solar system with a substantial atmosphere and a world that has lakes of liquid methane and ethane on its surface. We’ve already discussed Titan in this Wow Factor box, which you may wish to review now (or read now if you haven’t read it already).

Although Titan is much too cold to have liquid water on its surface, some scientists wonder if it might have some type of life that uses liquid methane or ethane in place of liquid water. No one knows for sure whether this is even possible. In addition, some evidence points to a possible underground ocean of liquid water on Titan. If this ocean really exists, perhaps Titan could host both water-based and methane-based life.

The second of Saturn’s moons that offers a possibility of life is Enceladus, which startled scientists when the Cassini spacecraft first observed it shooting fountains of ice into space (Figure 4.72a). The Cassini mission flew past Enceladus dozens of times as it orbited Saturn. Based on these studies, scientists have concluded that Enceladus must also have a subsurface ocean containing liquid water (Figure 4.72b).

Figure 4.72 – Saturn’s moon Enceladus sprays fountains of ice into space, telling us that it must have a subsurface ocean.

And More …

So far we have identified six worlds (besides Earth) in our solar system that seem to offer at least some possibility of having life: the planet Mars, three moons of Jupiter (Europa, Ganymede, Callisto), and two moons of Saturn (Titan, Enceladus). There could be even a few more.

It is even possible that dwarf planets like Ceres and Pluto might have some subsurface liquid water (or perhaps some other liquids), in which case they, too, might be added to the list of places to look for life.

Video 4.74 summarizes some of the prospects for life on moons or dwarf planets that could potentially have subsurface oceans.

Video 4.74 – This NASA video discusses possibilities for finding life on moons or dwarf planets in our solar system.

Life Around Other Stars

Scientists have by now discovered thousands of exoplanets, and these presumably offer far more possibilities of life than the relatively small number of possibilities in our own solar system. Unfortunately, the technology to actually search for life on any of these worlds remains well beyond us. They are too far away for us to send spacecraft to explore them, and our telescopes are not yet large enough to show them in any detail.

This situation is likely to change in coming decades, however, as scientists from around the world collaborate to build larger and more powerful telescopes. These telescopes are likely to teach us about possibilities for life around other stars in two general ways:

• First, as telescopes gradually become more powerful, they should enable us to begin to do spectroscopy of exoplanets. In fact, some limited spectroscopy has already been achieved, but so far only of planets that are unlikely to have life (either because they are very large, hydrogen-rich worlds or because they are too close to their stars). This type of study can tell us what exoplanet atmospheres are made of. In principle, we might find atmospheres containing gases (such as oxygen) that would allow us to infer the existence of life.
• With even more powerful telescopes, we might someday obtain images of other worlds that are detailed enough for us to see whether they have surface oceans, or to notice seasonal changes that might be indicative of life. Perhaps, someday, looking at the night side of a distant world, we might even see the lights of a civilization (Figure 4.75).

While there are no guarantees that we will ever find evidence of life, it is a near certainly that we will learn much more about planets around other stars during your lifetime.

Activity

Exoplanet Art Project

The painting below envisions the view from the surface of an imaginary planet that has Earth-like oceans and orbits a star located just outside our Milky Way Galaxy. Think about other possible worlds that might also exist around other stars, and make your own artwork to show your vision. You may choose any art form, including drawing, painting, making a model, doing computer art, or even writing a poem or story. You can feel free to be as imaginative as you wish, but try to make sure your world is scientifically possible.

Show Teacher Notes

This activity is a bit different for a science class, but will give your students an opportunity to showcase their art or writing skills, while also encouraging them to think about other worlds in a scientifically realistic way. It can be done either individually or pairs or small groups.

The Search for Extraterrestrial Intelligence

Most of the prospects of life we have discussed so far have concerned microscopic or other fairly simple forms of life. But what if we want to find intelligent life, meaning lifeforms that might build a civilization and that we could in principle talk to?

We can be quite certain that there is no intelligent life of this type on any other world in our own solar system. We know this because, by now, our spacecraft have photographed all of the major worlds on which such beings might reside, so if anyone was there, we would have already seen them.

For that reason, our only hope of finding other civilizations requires looking to the stars. Unfortunately, as we’ve discussed, our current telescopes are not capable of seeing civilizations on planets around other stars, if any exist. Scientists have therefore come up with an alternative way of trying to find out whether intelligent aliens might be out there: the search for extraterrestrial intelligence, or SETI .

SETI projects are in essence a search for signals that an alien civilization might be broadcasting in our direction, either deliberately or by accident. A deliberate signal would be one in which they are hoping someone will detect it and answer back. An accidental signal might be something like our television broadcasts , which travel out into space at the speed of light and therefore could in principle be picked up by alien beings.

Most SETI projects to date have “listened” for such signals using large radio telescopes (Figure 4.76). While a few strange signals have been detected, no one has yet identified a signal that clearly comes from another civilization. Still, the search is in its infancy, and it is possible that we will someday receive a signal that will prove we are not alone in the universe.