Around 3.7 billion years ago,  a little north of the equator,   a flash flood swept up a large  boulder into a fast-running river.

It was carried downstream into a delta where  the river met a lake that had formed inside a   huge crater 45 kilometers wide - the impact  site of an asteroid that had hit long ago.

Finally, the boulder splashed into the  crater lake, sinking to the bottom.

Floods like this were fairly common on  ancient Earth, but this was not Earth.

This was Mars, but at a time in the deep  past when it was much more Earth-like.

It had big bodies of liquid water on its surface,  it had a protective and insulating atmosphere,   it had a warmer climate, and  maybe - just maybe - it had life.

As far as we can tell though, it doesn’t  have any of those things anymore.

Here on Eons, we often talk about  the radical changes and countless   revolutions that our planet has  gone through over deep time.

But while Earth’s natural history has been  playing out over the last few billion years,   another epic planetary saga has  been unfolding right next door.

Even though we’ve been asking questions of  that red dot in the night sky for a long time,   we’ve only just recently begun  to figure out some answers,   thanks in large part to our  curiosity and perseverance.

So what happened on Mars?

How and  why did it go from that warm, wet,   potentially habitable planet to the cold,  dry, inhospitable one that we know today?

Well, the core reason seems to be that while we  had some surface-level similarities at one point,   internally, our two worlds were  on very different trajectories.

Mars has meant a lot of different things to  lots of different people over human history.

This strange, mobile point of light was counted   by the ancient Greeks as one of  the ‘planetes’ or ‘wanderers’.

And its foreboding reddish tinge  meant that several ancient cultures   interpreted it as a divine symbol  of war, violence, and conflict.

But it wasn't until Galileo first gazed  at the planet through a telescope in   the early 1600s that Mars started  to mean something very different.

As astrology gave way to astronomy,   our views of Mars transformed from a mystical  celestial body to an actual, physical place… …an entire other world, complete  with geologic features and vast   landscapes that we could observe,  map out, and dream of visiting.

And it opened the door to another  tantalizing possibility, too.

As we gazed at Mars, was anything gazing back?

It was a question that we could only speculate  about from a distance - and boy, did we,   but we didn’t have the technology  to actually get any good answers.

It would take until the second half of  the 20th century, when our relationship   with Mars transformed yet again, for us  to finally investigate this possibility.

Because Mars went from being a place that  we could only observe from far away to a   destination that we could actually travel to  - or at least send machines to on our behalf.

Orbiters, landers, and now rovers  have revealed to us a dry, rocky,   barren world with no obvious signs  of life, and they’ve also offered us   the long-sought-after opportunity to gain new  insight into the natural history of the planet.

Take the Perseverance rover, for example, our most   recent robotic representative  to arrive on the red planet.

Since 2021, Perseverance has been  working on a very important mission:   to explore and study the now-dried up  lakebed of that crater we mentioned earlier.

It’s looking for evidence of any ancient life   that might have called that crater  lake home, billions of years ago.

And the data that Perseverance  has collected has also allowed   us to trace ancient geological events  from that more dynamic period of Mars’s   history… Like the story of that  flood nearly 4 billion years ago.

Back then, the planet had a thicker atmosphere,   and was much warmer, wetter,  and suitable for life.

But to understand how and why Mars  changed into what it is today,   we need to go back to its formation.

See, Mars’ origin story was very similar to  Earth’s, with a few important differences.

Both   planets formed around 4.6 billion years ago from  clouds of dust and rocky debris orbiting our sun.

At their centers were partially molten cores of   iron that gave both planets  powerful magnetic fields.

This was a result of the dynamo effect  - the generation of a magnetosphere by   the rotating, convecting, and electrically  conducting liquid iron in the outer core.

Now, we’re lucky enough to still have  an active and powerful magnetosphere   here on Earth more than 4 billion years later.

It acts as an invisible shield,  protecting our atmosphere from   being eroded by solar wind – charged  particles belched out from the sun.

But Mars has lost its magnetosphere, and  with it, protection from those solar winds.

Over time, they stripped away its thick  atmosphere, its warmth, its protection   from radiation, its liquid water, and, it  seems, any life it might have once had, too.

Because Mars’ dynamo switched off  sometime after 3.7 billion years ago.

We can tell because those are the youngest  Martian rocks that are still magnetized,   which happens when lava cools in  the presence of a magnetic field.

We’re still figuring out  exactly why it switched off,   but a big part of it may have  simply come down to size.

See, Mars is only around  half the diameter of Earth,   so its core cooled down much  faster after the planets formed.

This shut off the dynamo-effect and exposed the  atmosphere to the full force of the solar winds.

The planet’s rivers, lakes, and perhaps even  an ancient ocean all vanished as the water   either froze, got trapped in rocks, or was lost  to space without a thick atmosphere to retain it… …Though they left behind the valleys, lake  beds, and shorelines that they carved into   the planet’s surface – prime targets in our  search for past or present Martian life.

'Cause the question is: if life had emerged  during that long-gone age of habitability,   could some especially resilient lineages have  stuck around – maybe by moving underground,   or to the polar ice caps, where some  amount of liquid water still exists?

After all, if studying life on Earth has taught  us anything, it’s that life, hm, finds a way.

And   species - especially microbes - can often adapt to  thrive in even the most extreme of environments.

Or, if not, could any evidence of past life  have survived after all those billions of years?

While we haven't found any definitive evidence  of Martian life yet, we have found some clues… In 2018 for example, Perseverance’s older  sibling, Curiosity, made an intriguing discovery.

It collected data showing that the concentration  of methane in Mars's thin atmosphere - a gas which   on Earth is primarily produced by living  organisms - seemed to cycle seasonally.

This suggested that the gas might be  seeping out from underground reservoirs… …which could potentially be a by-product  of microbial life below the surface where   there might be liquid water and where  there's better protection from radiation.

Plus, both Curiosity and Perseverance have  also found a variety of organic molecules   in Martian rocks on the surface  that are over 3 billion years old.

All life that we know of is carbon-based,  so organic molecules are the necessary   raw materials for and potential  trace of ancient carbon-based life.

Now to be clear, neither of these discoveries   means that life either definitely used  to exist on Mars or still does today.

Both could also just be a result of  chemical and geological processes   rather than biological ones, we  still don't know that for sure.

But there are basically three possible  options: Mars never had life, Mars had   life at some point but it died out, or  Mars had life and still has it today.

And figuring it out might even require another   transformation in our relationship with the  red planet.

We may need to bring Mars to us.

By returning some of the most  promising samples to Earth,   we could study them in much more detail than  our rovers alone can on the planet itself.

So, in late 2022, Perseverance carried out one  of its most important orders: it collected a rock   sample from the crater, packaged it in a titanium  tube, and left it at an assigned spot nearby.

This was the first of many  samples of rock, sediment,   and atmosphere that Perseverance has  been tasked with carefully collecting.

And in the early 2030s, a  robotic mission will arrive,   retrieve some of the samples, and  return them to Earth for analysis.

Scientists hope that studying actual samples  of Mars in our labs from that ancient crater   lake and other promising sites could be the key to  figuring out the question of life on the planet.

Whatever the answer is, it will radically change  how we understand the nature of life itself.

Is biology common or rare in the universe?

Is life on Mars related to life on Earth?

Is that life based on the same underlying  biochemistry and genetics or a totally   different system?

In other words, are  there other ways of being ‘alive’?

And is life infinitely resilient  - capable of adapting to even the   most difficult times and persisting in some form?

Or can a planet lose its entire  biosphere - right down to the   last microbe - if conditions get bad enough?

And you can see how answering these  questions - especially those last   two - with the data from Mars’ past might be  important for the future of life on Earth.