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.
Curious about the microcosmos?
Join hosts Hank Green and Deboki Chakravarti
on another show produced by our team, Journey to the Microcosmos.
If you haven't watched their videos yet, you are really missing out! Each episode
uses incredible footage to take you on a dive into the tiny, unseen world that surrounds us.
Check them out at youtube.com/microcosmos.
We know that Mercury, Venus, and Mars are all super low on water – so why do
we have so much of it? Find out in our episode, “Where Did Water Come From?”
And thanks to this month’s a-mars-ing Eontologists!
Raphael Haase, Jake Hart, Juan M, Annie & Eric Higgins, John Davison Ng,
Melanie Lam Carnevale, and Colton. Become an Eonite at patreon.com/eons
and you can get fun perks like submitting a joke for me to read! Here’s one from Sarah W.
What are the basal animals' favorite herb? Basil.
Ba-dum-chhhhhhh... And as always thanks
for joining me in the Adam Lowe studio. Subscribe at youtube.com/eons for more prehistoric stories.
Both planets formed around 4.6 billion years ago
from clouds of dust and rocky debris or d- [loses the words]
Wow...
See, Marge is -
Marge...