While Mars is known as a frozen, red planet today, it has all the evidence we could ask for of a ... [+] watery past, lasting for approximately the first 1.5 billion years of the Solar System. Could it have been Earth-like, even to the point of having had life on it, for the first third of our Solar System's history?

For as long as humanity has been watching the skies, weve been fascinated with the possibility that other worlds much like Earth might contain living organisms. While our visits to the Moon taught us that its completely barren and uninhabited, other worlds within our Solar System remain full of potential. Venus might have life in its cloud-tops. Europa and Enceladus might have life teeming in a sub-surface ocean of liquid water. Even Titans liquid hydrocarbon lakes provide a fascinating place to search for exotic living organisms.

But by far, the most fascinating possibility is the red planet: Mars. 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 the plausibility of life on Mars, not only in the ancient past, but possibly still living, and perhaps occasionally active, even today. There are five possibilities for life on Mars. Heres what we know so far.

Oxbow bends only occur in the final stages of a slowly flowing river's life, and this one is found ... [+] on Mars. While many of Mars's channel-like features originate from a glacial past, there is ample evidence of a history of liquid water on the surface, such as this dried-up riverbed.

With the information weve obtained from various orbiters, landers, and rovers, weve made a slew of fascinating discoveries on Mars. We see dried-up riverbeds and evidence of ancient glacial events on the Martian surface. We find tiny hematite spheres on Mars as well as copious evidence for sedimentary rock, both of which only form on Earth in aqueous environments. And weve observed solid sub-surface ice, snows, and even frozen surface water on Mars in real-time.

Weve even observed whats likely to be briny surface water actively flowing down the walls of various craters, although that result is still controversial. All the raw ingredients that are required for life on Earth were abundant on early Mars as well, including a thick atmosphere and liquid water on its surface. Although Mars no longer appears as though its teeming with life today, there are three pieces of evidence that past or even present life might be a possibility.

The hematite spheres (or 'Martian blueberries') as imaged by the Mars Exploration Rover. These are ... [+] almost certainly evidence of past liquid water on Mars, and possibly of past life. NASA scientists must be certain that this site -- and this planet -- are not contaminated by the very act of our observing. As of yet, there is no surefire evidence for either past or present Martian life.

The first compelling piece of evidence came from the instruments on board NASAs Mars Viking landers in 1976. There were three biology experiments performed: a gas exchange experiment, a labeled release experiment, and a pyrolytic release experiment, followed-up by a gas chromatograph mass spectrometer experiment. The labeled release experiment yielded a positive result when performed on both Viking landers, but only the first time the test occurred. All other experiments came back negative.

The second piece of evidence came when a fragment of a Martian meteorite Allan Hills 84001 was recovered on December 27, 1984. As it turns out, approximately 3% of all meteorites that fall to Earth originate from Mars, but this one was particularly large: nearly 2 kilograms (over 4 pounds) heavy. It originally formed on Mars some 4 billion years ago, and landed on Earth only some 13,000 years ago. When we looked inside of it in 1996, it appears to contain material that could be the remnants of fossilized organic life forms, although they could have arisen from inorganic processes as well.

Most recently, the Mars Curiosity rover detected Methane vents on Mars, which could have been ... [+] produced either organically or inorganically. If it's organics, the author will lose a bet with physicist Robert Garisto!

And finally, the third piece of evidence came out with NASAs latest Mars rover: Curiosity. As the seasons changed on Mars, Curiosity detected burps of methane emitted from specific underground locations, but only at the end of Martian winter and with the onset of spring. This is, again, an ambiguous signal at best, as inorganic, geochemical processes could be seasonal and result in the release of methane, but organic, biological processes could cause this as well.

When we look at the full suite of evidence at everything weve learned about Mars there are five possibilities for the history of life on the Red Planet. It could be an eternally barren world; it could be a world where life thrived for a time but then hit a dead-end; it could have extant life on it today; it could have been seeded by Earth life early on; or it could only have Earth-based organisms that made their way there since the dawn of the space age.

Heres what each possibility would mean.

Mars, along with its thin atmosphere, as photographed from the Viking orbiter. From afar as well as ... [+] up close, there are no obvious, compelling signs of past or present life on the planet, although there are some ambiguous points that could either favor or disfavor life.

1.) Mars never had life on it. Despite having the same raw ingredients as early Earth and similar, watery conditions, the necessary circumstances that enable life to form simply never occurred on Mars. All the geological and chemical processes that occur inorganically still happened, but nothing organic. Then, a little more than three billion years ago, Marss atmosphere was stripped away by the Sun, drying up any liquid surface water and leading to Marss current appearance.

This is the most conservative stance, and would require that all three of the purported positive tests have either an inorganic or contamination-based resolution. This is eminently possible, and remains in the mind of many the default assumption. Until some very compelling evidence comes along that robustly points to either past or present life on Mars, this will likely remain the leading hypothesis.

Seasonal frozen lakes appear throughout Mars, showing evidence of (not liquid) water on the surface. ... [+] These are just a few of the many lines of evidence that point to a watery past on Mars. Whether water indicates life or not has not yet been determined.

2.) Mars had life early on, but it died out. This scenario, in many ways, is just as compelling as the prior one. Its very easy to imagine that a world with:

could lead to life. To many, its virtually impossible to imagine that these conditions after more than a billion years wouldnt lead to life, considering that life arose on Earth no more than a few hundred million years after its formation.

However, the loss of the Martian atmosphere had a profound effect on the planet, and could have resulted in the extinction of all life on Mars. Drilling down into the sedimentary rock of Mars and searching for fossilized life forms, or even metamorphosed carbon-rich inclusions, could potentially reveal the evidence necessary to validate this scenario.

Recurring slope lineae, like this one on the south-facing slope of a crater on the floor of Melas ... [+] Chasma, have not only been shown to grow over time and then fade away as the martian landscape fills them in with dust, but are known to be caused by the flowing of briny, liquid water. Perhaps, in those flows, life processes are occurring.

3.) Mars had early life, and it still persists in a mostly-dormant form beneath the surface. This is the most optimistic, but still scientifically viable, view of life on Mars. Perhaps life took hold early on, and when Mars lost its atmosphere, a few extremophiles remained in a sort of frozen, suspended-animation state. When the right conditions emerged perhaps underground, where liquid water can occasionally flow that life wakes up and begins performing its critical biological functions.

If this is the case, then there are still organisms to be found beneath the Martian surface, perhaps in the shallow sands just a few feet or even mere inches below our spacecraft. Were likely only talking about single-celled life, perhaps not even reaching the complexity of a eukaryotic cell, but life on any world other than Earth would still be a revolution for science. NASAs Perseverance rover, which launched successfully on July 30, 2020, will collect critical soil samples to attempt to test this hypothetical scenario.

A planetoid colliding with Earth, larger than even the asteroid strike that wiped out the dinosaurs, ... [+] could easily kick up sufficient amounts of material that some of it would make it to Mars, possibly contaminating the ancient Red Planet with Earth-like material, as well as Earth-based biological organisms.

4.) Mars didnt have life until Earth seeded it, naturally. 65 million years ago, a very large, fast-moving body impacted Earth, creating Chixulub crater and kicking up enough material to blanket the Earth in a cloud of debris, leading to the fifth great mass extinction in Earths history. And, like many massive impacts, this one likely kicked up small pieces of Earth all the way into space, the same way that impactors on the Moon or Mars send meteors throughout the Solar System, where some of them eventually land on Earth.

Well, a few impacts likely go the other way as well: sending Earth-borne material to other worlds, including Mars. It seems unreasonable that the material in Earths crust, rich in organic life, wouldnt make it to Mars at all. Instead, its eminently plausible that Earth-based organisms made it to Mars and began reproducing there, whether they thrived or not. Perhaps someday, well be able to know the full history of life on Mars, and determine whether any of it has the same common ancestor that all extant Earth life is descended from. Its a fascinating possibility that isnt easy to dismiss.

The first truly successful landers, Viking 1 and 2, returned data and images for years, including ... [+] providing a controversial signal that may have indicated life's presence on the red planet.

5.) Our modern space program spread Earth-based life to Mars. And, finally, perhaps Mars truly was a barren, lifeless planet at least for billions of years until the dawn of the space age. Perhaps spaceborne materials that werent 100% decontaminated or sterilized landed on the Martian surface, bringing modern Earth organisms with them as stowaways.

Its the ultimate nightmare of astrobiologists: that theres a fascinating history of life to uncover on another world, but well contaminate it with our own organisms before we ever learn the true history of life on that world. In the worst case scenario, it could be the case that was surviving simple life on Mars of Martian origin, but that Earth life arrived and out-competed it, driving it to a rapid extinction. This very real, healthy fear is why were frequently so conservative, from a biological perspective, when we explore other planets and foreign worlds.

An Atlas V rocket with NASA's Perseverance Mars rover launches from pad 41 at Cape Canaveral Air ... [+] Force Station. The Mars 2020 mission plans to land the Perseverance rover on the Red Planet in February 2021, where it will seek signs of ancient life and collect rock and soil samples for possible return to Earth. (Paul Hennessy/SOPA Images/LightRocket via Getty Images)

There is a tremendous hope that current and future generations of Mars rovers and orbiters will help us finally puzzle out whether Mars either now or at any point in its past has ever harbored life. If the answer to that question is affirmative, then it leads to an important follow-up question: is that life related to or independent of life on Earth? It is possible that life originated on Earth and seeded Mars with life; its possible that life originated on Mars and then seeded Earth; its even possible that life predated both Earth and Mars, and early forms of it took hold on both planets.

But at this point in time, we have no overwhelming evidence that life ever existed on Mars at all. We have a few hints that could be indicators of past or present life there, but entirely inorganic processes could explain each and every one of those observed results.

As always, the only way well find out the truth is by conducting more and better science with superior instruments and techniques. As NASAs Perseverance rover moves ahead to collect a variety of soil samples, the next step will be returning them to Earth for laboratory analysis. If we succeed at that, we could know for certain, within the next decade, which of these five possibilities is most consistent with the truth about Mars.

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The 5 Possibilities For Life On Mars - Forbes