One of the biggest questions that humanity has asked is, “Are we alone in the universe?”
The possibility that there are other civilizations that we could one day communicate with has tantalized people for years.
However, before we can answer that question, there are other simpler questions that we need to answer, such as, is life even possible outside of Earth?
The place where we might be able to answer that question is in our own backyard.
Learn more about if there is extraterrestrial life in our solar system on this episode of Everything Everywhere Daily.
Before I go any further, let me say that this episode is not about alien intelligence, alien civilizations, UFOs, or alien encounters.
Those might be worthy of discussion at some point in the future, but this episode is focused on something much simpler.
This episode is focused on the search for life. Not necessarily intelligent or complex life, just very simple life. Not even multicellular life, like plants or animals, just very simple life on a par with single-cell organisms.
The discovery of any sort of life that exists outside of Earth would be a groundbreaking discovery and one of the most important in the history of humanity.
The search for alien life and single single-celled organisms would indeed be alien life is part of the nascent scientific field called astrobiology.
Astrobiology is a multidisciplinary field of scientific inquiry that focuses on the study of life in the universe.
It combines principles and methods from various scientific fields, including biology, chemistry, physics, astronomy, and geology, to explore the origins, evolution, distribution, and future of life, both on Earth and beyond.
The main goal of astrobiology is to understand the conditions and processes that give rise to life and to determine if life exists elsewhere in the universe.
At this point, astrobiology is primarily a theoretical discipline because we haven’t found life anywhere else.
We are limited by our current technology on where we can search. Even if we should find exoplanets that would be suitable for life around other stars, there is no way we could possibly reach them.
This means that, like it or not, our search for life has to take place in our own backyard, our solar system.
So what exactly are we looking for?
We aren’t necessarily looking for life as it would exist on Earth. Life on Earth is based on DNA and, save for viruses, is cellular.
That could be what exists outside of Earth, but we don’t know that. We are looking for something that meets the basic definitions of life.
The definition of life, and there is no universal agreement on this, include the following attributes:
Organization – Something living would need to be organized and show some sort of complexity to differentiate it from non-living things.
Metabolism – This would involve chemical reactions which would allow the lifeform to acquire and utilize energy.
Growth and development – this would imply the ability to grow and change over time as it moves into different phases of its life cycle.
Reproduction – the ability to create offspring with similar characteristics to itself.
There is a great deal of debate as to what other criteria define life, but for the most part, I think most biologists would agree on these features as a minimum.
So, where exactly do we start looking to try and find life?
Well, if a life form is to show metabolism, it means the transportation of energy. In a solid, nothing would really move, so if atoms and molecules are to move to support metabolism in the form of chemical energy, that would imply some sort of fluid.
There is one fluid that we know for sure works, and it also happens to be rather abundant in the solar system….water.
Water has been called the universal solvent. It plays a part in every cellular form of life on Earth. Moreover, there is evidence of water, at least in the form of ice, all over the solar system.
So, the search for liquid water would be a pretty good place to start.
The next question is, where can we find liquid water in the solar system outside of Earth?
Liquid water can only exist within a certain range of pressure and temperature.
The first place to look, and the place we have been looking, sort of, is on Mars.
Mars is much colder than Earth. It has a much thinner atmosphere, and it can’t trap heat as well. That means that liquid water can’t easily exist on the surface like it can on Earth.
But we know that there is water on Mars. Both the north and south poles of Mars have ice caps which consist mostly of water ice but also some CO2 ice, aka dry ice.
Moreover, ample satellite observations of the Martian surface as well as surface rovers have found evidence that, at least at some point in the past, liquid water was flowing on the surface. There are dry river beds and erosional features.
The big question is if liquid water can exist on the surface. Right now, the evidence seems to indicate that it can, perhaps at least temporarily. There have been observations of what are known as Recurring Slope Lineae.
These are erosional features on slopes that change every season. In particular, they tend to change when temperatures are warmest on Mars. These erosional features look just like what you would find on Earth.
While temperatures on Mars are overall much colder than Earth, near the equator it is possible for temperatures to reach as high as 21°C or 71°F during the middle of the day.
This might allow for a liquid brine to seep out of the soil and flow for short periods of time. The brine part is important because salty water has a lower freezing point than regular water, which is why you use salt to melt ice in the winter.
If liquid water was more abundant in the past, and that seems to be the case, it might have been possible for some sort of microbial life to have arisen on Mars.
Some descendants of those ancient microbes might still be able to eke out an existence in these places where water can briefly melt.
We know that there are extremophiles on Earth that can exist under such conditions, so it isn’t out of the question.
Any sort of life that could possibly exist there would have an extremely slow metabolism, similar to the microbes that live deep under the surface of the Earth. They have lifespans that reach into the centuries.
Despite all of the probes, landers, and rovers that have been sent to Mars, only two have actually carried instruments to check for signs of life directly…the Viking Landers.
They ran several tests on Martian soil, and some of the results were….inconclusive. I’ve previously done an entire episode on this topic if you wish to revisit it.
There have been other positive signs which could indicate life on Mars. This includes the presence of methane in the atmosphere and the discovery of organic chemicals in the soil.
Future planned Mars missions will search more directly for life. The ExoMars mission by the European Space Agency will carry the Rosalind Franklin rover, which will contain a laboratory for searching for biosignatures as well as a drill that can take samples from two meters deep.
The mission that researchers are really waiting for is the Mars sample return mission. This would literally bring samples of soil back to Earth, where they can be studied more extensively.
While Mars is a good candidate for finding life, it is not the only one.
Another target that has tantalized researchers is Jupiter’s moon Europa.
Europa is basically a ball of ice. On the surface, it would be too cold for anything to survive.
However, beneath the surface, it is believed that there could be an entire ocean of water. The tidal forces created from its orbit around Jupiter provide enough internal friction to melt the water under the surface. Moreover, the tidal forces could create thermal vents, which would serve as a transportation vehicle for nutrients into the water.
Surface features seem to indicate the presence of water underneath the icy crust. Cracks and ridges on the surface seem to indicate liquid water filling the cracks when they open up and freezing.
Europa arguably has far greater potential for life than Mars does because of the enormous reservoir of liquid water.
The problem is that Europa is much farther away than Mars and is much more difficult to reach, and if there is life, it would be buried under 15 to 25 kilometers or 10 to 15 miles of ice.
Almost everything we know about Europa has come from fly-by missions to Jupiter. There are future missions to Europa that are planned, but as of right now, they, too, are all fly-by missions.
There is a Europa lander mission that has been discussed, but as of today, nothing has been approved, and nothing has been confirmed.
While Europa has one of the best potentials for life in the solar system, it is not the only ice moon in the solar system.
Saturn’s moon of Enceladus is an ice moon, and geysers of water vapor have been observed on its south pole, indicating a liquid sea below the surface.
Moreover, the Cassini mission detected complex organic molecules in the ice plumes, which are necessary for life.
The problem is that Saturn is even farther away than Jupiter. It has all the problems of exploring Europa, but even further away.
Two other moons of Jupiter, Ganymede, and Callisto, are also potential places where life could exist.
Ganymede is the largest moon in the solar system and the only one with its own magnetic field. It consists of rock and water, and it also might have a subsurface ocean.
Callisto is very similar. It is composed of approximately equal amounts of rock and ice, with plenty of organic compounds that have been identified, and it, too, may have a subsurface ocean. It is actually the third-largest moon in the solar system and is larger than the planet Mercury.
There is one other moon that some think could harbor life, but it is different from the other.
It is a moon that has an atmosphere almost as thick as Earth, and it is the only other body in the solar system that has liquid on the surface. It even is believed to have rain, rivers, and lakes.
It is Saturn’s moon, Titan.
The catch is that the liquid on the surface isn’t water, it’s hydrocarbons such as methane and ethane.
The surface of Titan is cold enough for these hydrocarbons to exist in a liquid form, but its far too cold for liquid water. The hydrocarbons show that there is plenty of the basic molecular building blocks of life available. Here I’ll reference my previous episode on the Miller-Urey Experiment.
What is totally unknown is if life could exist without water. Is it possible to use some other liquid like methane or ethane? If life did evolve on Titan, it would be radically different than life that arose on Earth or even potentially life found on Mars or the icy moons of the solar system.
These interplanetary bodies that I’ve listed are the best and most obvious places to look for life in the solar system, but they certainly are not the only places where some sort of simple life could exist.
One possible location would be in the atmosphere of Venus. The pressure on the surface of Venus is 95 times greater than that of Earth, and the average surface temperature is 900° F or 480° C.
However, higher in the atmosphere, conditions are quite different. Temperatures are moderate, and there exists a zone where life can live. The big question is whether life could exist without a solid surface or a liquid medium. Could life use gas as a fluid?
There are far more questions than answers when it comes to life outside of Earth. So far, we haven’t found anything. But, in the big scheme of things, we also haven’t looked very hard. We’ve landed a few robots on Mars, and we’ve taken distant images of Moons. That’s about it.
The process of searching for life might take decades or even centuries, but at least have an idea of where to look.
If we can find signs of life, any life, no matter how simple, elsewhere in our solar system, it will perhaps be the greatest scientific discovery in history.
The Executive Producer of Everything Everywhere Daily is Charles Daniel.
The associate producers are Thor Thomsen and Peter Bennett.
Today’s review comes from listener Sofia over on Apple Podcasts in the United States: She writes:
Very interesting and informational
I love this podcast! I listen to it a lot I’m not in the completionist’s club because I am on the younger side and do not like scary stuff, but I am very grateful that you make this podcast because it makes boring days a lot more interesting. Thank you so much!
Thanks, Sophia! I’m really glad to know that you are enjoying the show. I hope the show keeps you entertain and you remain a listener for years to come.
Remember, if you leave a review or send me a boostagram, you too can have it read on the show.