Humanity has always tried to understand the origin of the building blocks of life, how the biogenic compounds combine to create life, how life affects and gets affected by the environment from which it arose, and finally, whether and how life expands beyond its planet of origin. But before moving to other possibly habitable worlds in the solar system, let’s find out why life exists on Earth!
Why does life exist on Earth?
Earth, the third kiddo from the Sun, is indeed an amazing planet. It has a gaseous atmosphere which plays an important role in making the existence of life possible on it. Our atmosphere contains water vapor which helps to moderate our daily temperatures. It further contains 21% oxygen, which is necessary for us to breathe, 78% nitrogen, and 0.9% argon. The other 0.1% consists of water vapor, carbon dioxide, neon, methane, krypton, helium, xenon, hydrogen, nitrous oxide, carbon monoxide, nitrogen dioxide, sulfur dioxide, and ozone.
Although present in meager quantities, the latter elements are important as they help absorb harmful solar radiations before they reach the Earth’s surface. However, most of them would act as a poison to humans if present in larger amounts. So we can say that there is a perfect balance of elements in the Earth’s atmosphere, which is a prerequisite for a planet to support life. The atmosphere further protects us from meteors as well. Due to the friction generated between a meteor and the atmospheric gases, most meteors burn up before hitting Earth’s surface as a meteorite.
Seasons and magnetic field
Earth rotates on an imaginary axis, tilted at a 23.5-degree angle. The rotation causes the transition from day to night and vice versa, while the tilt determines the change in seasons. If the Earth were not tilted, we would have experienced the same season all year long. Earth has a core of molten iron-nickel. The rapid spin of the Earth and the liquid, hot metallic core causes a magnetic field to surround the Earth. This magnetic field traps the charged particles hurled at the Earth by the Sun during solar wind activity.
When these charged particles react with the gases present in our atmosphere, the gases begin to glow. This gives rise to a phenomenon known as aurorae, or glowing gases, visible in the Arctic Circle and the Antarctic Circle. As is the case with all inner planets, Earth’s surface has also been affected by volcanism, tectonic activity, and to a lesser degree, meteorite impacts. Earth has one naturally occurring satellite, the Moon, which governs the water bodies’ tidal activity.
Distance from the Sun
For living things, the correct temperature is critical. Most of the life on Earth lives in warm to moderate temperatures. There aren’t nearly as many life forms that live in freezing temperatures, such as at the poles or in scorching environments such as inside volcanoes. One factor that affects the temperature of a planet’s atmosphere is its distance from the star (sun in our case) it orbits. The planets closest to the sun are very hot, while planets that are far away are much colder, making life almost impossible on either of the planets.
Life is the solar system
Although it seems kind of impossible for life to exist on other planets in our solar system, still, time and again, humankind has come across several shreds of evidence that makes us doubt our assumption of being all alone in our solar system. While we are moving our curiosity towards “exoplanets” that are potentially habitable worlds orbiting other stars, our close neighbors in our own solar system are also providing a lot to explore. This has given a hope that there might be life on other planets, or even the moons of other planets near us, although quite different from what it is on Earth! So let’s have a look at some of the suitable candidates one by one!
Mars: The Red Planet
Among our discoveries about Mars, one stands out above all others: the possible presence of liquid water on Mars, either in its ancient past or preserved in the subsurface today. Everywhere we find water on Earth, we find life. So, if Mars once had liquid water, or still does today, it’s compelling to ask whether any microscopic life forms could have developed on its surface. About 3.8-3.5 billion years ago, Mars and Earth were much more similar. Evidence from Mars missions suggests that the red planet may have been much warmer and wetter than we observe it today. Scientists find the first evidence of microbial life on Earth in this ancient timeframe.
This smaller, colder, more distant cousin of Earth most certainly had a wet past, where liquid water clearly flowed on the surface for more than a billion years. Circumstantial evidence has pointed to life’s plausibility on Mars, not only in the ancient past but possibly still living, and perhaps occasionally active even today.
While robotic spacecraft have given us wonderful views, no human has ever tried to visit Mars, and no such missions will be attempted for many years. In fact, whoever will turn out to be the first people on Mars may be your age today, and when you are an adult, perhaps you will watch, or even participate, as people make the first voyage to the red planet.
In the meantime, NASA is working hard now to discover whether there is life on Mars or not. The United States and other countries have been sending spacecraft to orbit or land there since the 1960s, and each mission teaches us more about this fascinating planet. We have learned that even though Mars is more similar to Earth than anywhere else in the solar system, and therefore is a good place to look for life, it is still different from Earth in many ways.
In 2021, three spacecraft will arrive at Mars: the United State’s Mars 2020 spacecraft carrying the Perseverance rover and mars helicopter Ingenuity, China’s Tianwen-1 carrying a rover, a lander, and an orbiter; and UAE’s Emirates Mars mission, carrying an orbiter called Hope. Perseverance and Tianwen-1 will search for microbial life on Mars. Hope will study the planet’s weather patterns.
A compass points to the north pole on Earth because our whole planet acts like a giant magnet, but Mars does not act this way. Besides turning a compass needle, Earth’s magnetic field also turns away dangerous space radiation particles. The absence of a magnetic field and a thin atmosphere make the red planet’s surface vulnerable to harmful space radiation. Although several measurements show there’s water on Mars, it is scarce compared to Earth. Besides, Mars is so chilly that most of the water is probably not liquid but rather ice. Overall, Mars would be a pretty uncomfortable place to live!
It would be exciting to know whether the planet hosted life in the past. So, in addition to looking for living bacteria, NASA will be searching for tiny fossils that might indicate that life got a start early in Mars’ history but, unlike on our home planet, it did not survive and evolve into larger life forms.
Many of the studies of Mars will involve robots, like the ones that have gone there before but getting more advanced with each flight. Someday a spacecraft like Perseverance may pick up samples from Mars and bring them back to Earth, where they can be studied in our best laboratories. Eventually, humans may make a daring journey, but many important problems have to be solved before trying such an expensive, difficult, and exciting voyage.
Europa – Jupiter’s Glow-In-The-Dark Moon
Europa was in the news two months ago when NASA scientists found that the Jovian moon might be glowing in the dark. For the past several years, everyone believed that Mars was the most likely body in our solar system to support life beyond Earth. But after years of telescopic observations, decades of spacecraft exploration, and several robotic voyages exploring its surface, the promise of discovering life on Mars remains elusive.
Scientific attention is being focused on Europa, the fourth-largest of Jupiter’s 79 confirmed moons. It may be an even better candidate for finding life than Mars. For life to be present, the three basic requirements are:
- Liquid water
- Chemical building blocks
- A source of energy.
And Europa, seems to have all three.
Europa’s Liquid Water
The surface of Europa is covered with ice. This ice forms a “crust” on the Jovian moon that is thought to be several kilometers thick. Beneath the crust, a subsurface ocean of liquid water up to 100 kilometers deep is thought to exist. Investigators believe that the ocean is rich in dissolved ions, particularly magnesium, sodium, potassium, and chlorine. Organisms on Earth live in ion-rich solutions, so there is a good chance that they live in them on Europa.
Europa’s Building Blocks of Life
The ice and other materials on Europa’s surface are bombarded with radiation from Jupiter that could alter them into some of the chemical building blocks of life. These include free oxygen (O2), hydrogen peroxide (H2O2), carbon dioxide (CO2), and sulfur dioxide (SO2). If these compounds reach the subsurface ocean, they can act as valuable nutrients to start and sustain life. The ocean water can react with the rocks and minerals of the subsurface ocean’s floor to liberate other nutrients to support life.
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Europa’s Energy Source
Europa’s position in space is within the powerful gravitational field of Jupiter. This strong gravitational “pull” has the moon locked into an orbit with one hemisphere constantly facing Jupiter. The elliptical orbit carries Europa closer to and farther away from the planet. This alternating increase and decrease of gravitational force on Europa results in the moon elongating and relaxing with each trip around the planet. This internal movement, combined with gravitational forces exerted by neighboring moons, produces internal friction and heat within Europa, which can further prove to be a boon for life to exist.
Life on Europa might be easier to detect. The presence of magnesium compounds on Europa’s surface suggests that water from the subsurface ocean reaches the surface through springs or vents. If this occurs, these eruptions will deliver up ions and microbes from the ocean below. So, if there is life in Europa’s subsurface ocean, it could be scattered about the planet’s surface where landers or rovers might find it. A mission to the Europa’s surface might easily find evidence of life or even some of the microbes by sampling surface materials. This makes Europa an exciting target in the search for extraterrestrial life.
Enceladus: The Icy Beauty
According to a new study from Southwest Research Institute (SwRI), the interior of Saturn’s moon Enceladus is geochemically complex, making its subsurface ocean quite habitable.
Enceladus’ geysers, which are the huge plumes of water vapor erupting through cracks at its south pole, contain water vapor, ice, salts, methane, and simple and complex organic molecules, the kind of molecules normally found in living systems. Scientists think that there’s an ocean below Enceladus’ icy crust. A new study has found that the interior of Enceladus is more geochemically complex than once thought, boosting the prospects for life.
The researchers from SwRI have developed a new geochemical model of the moon. It’s based on an analysis of material in the moon’s water vapor plumes. The analysis suggests that carbon dioxide is controlled by chemical reactions on the seafloor of Enceladus’ interior ocean. The findings have opened up intriguing new possibilities for life in Enceladus’ subsurface watery abyss. Enceladus erupts a plume that contains gases and frozen sea spray into space. By understanding the composition of the plume, we can learn about what the ocean is like, how it got to be this way, and whether it provides the environments where life, as we know, could survive.
Titan: The Methane World
Saturn’s largest moon Titan is an extraordinary and exceptional world. Among our solar system’s more than 150 known moons, Titan is the only known one having a substantial atmosphere. Of all the places in the solar system, Titan is the only place besides Earth known to have liquids in the form of rivers, lakes, and seas on its surface.
Titan is larger than Mercury, and it’s the largest moon in our solar system after Jupiter’s Ganymede. Titan’s atmosphere primarily comprises nitrogen, like Earth’s, but with a surface pressure 50 percent higher than Earth’s. Titan has clouds, rain, rivers, lakes, and seas of liquid hydrocarbons like methane and ethane, the largest of them being hundreds of feet deep and hundreds of miles wide.
Beneath Titan’s thick crust of water ice probably exists an ocean primarily of water rather than methane. Titan’s subsurface water could be a place to harbor life. In contrast, its surface lakes and seas of liquid hydrocarbons could conceivably harbor life that uses different chemistry than we’re used to. And in another scenario, Titan could also be a lifeless world and shattering all our expectations.
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Venus: An unexpected candidate
While all this research was going on, Venus: The hottest planet in our solar system, made an unexpected entry in the race for supporting life. Venus is hot and poisonous and was long considered too inhospitable for anything to survive.
But now, digging through archival NASA data, scientists have found a hint of phosphine picked up by Pioneer 13 — a probe that reached Venus in December 1978. Eventually, on September 14, 2020, a team of scientists announced that its members had detected phosphine gas in the caustic, hot atmosphere of Venus. Phosphine is a toxic gas made up of one phosphorus and three hydrogen atoms (PH₃).
On giants like Jupiter and Saturn, immense heat and pressures can jam the phosphorus and hydrogen atoms together to form the molecule. But on smaller, rocky planets like Earth and Venus, there isn’t enough energy to produce copious amounts of phosphine in the same way. And the only possible way to produce it is with the help of anaerobic life or microbial organisms that don’t require or use oxygen.
Here on Earth, phosphine is found in our intestines, in the feces of badgers and penguins, and some deep-sea worms. Phosphine is extremely toxic. Terrorists use it as a chemical agent in warfare. Not to forget, on the TV show “Breaking Bad,” Walter White produced phosphine in his meth lab to kill two rivals.
But, who is making it in Venusian clouds? As far as the chemical and thermodynamic conditions on Venus are known, there doesn’t seem any artificial process that can produce phosphine in the amounts it has been observed. So, scientists are left with no other option to think of anything that might have produced it other than microbes in the upper part of the planet’s thick atmosphere. And this is how phosphine is indicating signs of life on Venus.
As Albert Einstein once said, “the universe is stranger than we can imagine.” None the less, armed with this caveat, Astrobiologists should never stop trying to imagine how the universe works – nor shy away from attempting to find life in the most unexpected worlds because we never know what our neighbors are hiding from us!
This article on ET life in is a guest article by Krishna Bhutada, an Engineering student from Pune, India.
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