Has there ever been life on Mars?

No, probably not-- or maybe?

I don't know.

[THEME MUSIC] There is no greater hero in our search for life on Mars than a little robot named "Opportunity."

15 years after its mission began, 57 times longer than it signed up for, "Opportunity" has gone silent.

As we wait and hope while NASA races to re-establish contact, we thought we'd reflect on the greater quest that "Opportunity" has contributed so much to-- the search for life on Mars.

Mars, the Red Planet, fourth rock from the sun, it's currently just past its closest approach to Earth, making it an angry red eye in the early night sky.

This is a planet that, as far as we know, is inhabited entirely by robots.

But at the end of July, something remarkable was announced that may prove the possibility of life on Mars.

There's a giant underground lake of liquid water, right now, under the south pole.

And in 2020, the race to find life on Mars will intensify with both ESA and NASA launching missions.

They'll shift the focus from the search for water-- because we now know there is water-- to the search for life, past or present.

NASA's also planning a mission to take a sample of Martian soil and rocket it back to Earth.

Applying our best earthbound technologies to an uncontaminated sample may provide the proof of Martian life we've always been hoping for.

But as good scientists, we have to suggest some caution because people have been searching for life on Mars and believing they found it for over 300 years.

So today, we're going to look back at the long history of life on Mars or, at least, of our hunt for it.

We begin our story at the end of the 17th century when Dutch polymath, Christiaan Huygens, made amazing advances on Galileo's astronomical telescope.

This allowed him to observe a dark patch on the Martian surface, and so measure Mars's 24-hour rotation rate, as well as a white spot on its south pole.

He guessed that both of these resulted from surface water, clouds, and ice.

Huygens saw this as evidence that Mars was habitable and so, perhaps, inhabited.

In fact, he was the first to postulate that liquid water is necessary for life.

A few centuries of extra science has only convinced us further of this, as we've discussed before.

Huygens was a visionary in many ways but only half right in his interpretation of his observations of Mars.

The white spot was indeed a polar ice cap.

But the dark spot was probably the volcanic rock of the Syrtis Major Planum.

Nonetheless, the prospect of a watery Mars kicked off centuries of optimism for an inhabited planetary neighborhood.

Astronomer Giacomo Maraldi quickly followed Huygens and in the early-18th century discovered ice caps on both poles, correctly inferring their nature.

Later that century, William Herschel watched the ice caps grow and shrink annually and thought he saw what he calls a comfortable but modest atmosphere.

Herschel would generously decreed of Mars that its inhabitants probably enjoy a situation in many respects similar to ours.

Now, Herschel was a fantastic astronomer, but his wrongness about Mars's atmosphere is a nice example that even the smartest of us see what we want to see.

As a scientist, it's so important to stay aware of that very human bias because it's led to the erroneous discovery of life on Mars on multiple occasions.

Fast forward to 1877.

Giovanni Schiaparelli was studying Mars during its perihelion opposition.

That's the configuration that also just passed recently.

Mars was at its closest approach to the sun and the Earth at its closest to Mars.

He observed streaks on the surface of the planet that he named "canali."

That's Italian for "channels," presumably implying naturally formed flows of water.

Perhaps, in part, with the newly made Suez Canal still fresh on people's minds, canali was mistranslated in English to "canals," implying artificially made structures.

People see what they want to see, and they hear what they want to hear.

The prospect of a technological civilization on Mars inspired businessman-turned-astronomer Percival Lowell.

He built himself an observatory and dedicated his career to studying Mars.

The Lowell Observatory is famous for the discovery of Pluto, but that was later.

Lowell was obsessed with the Martian canals.

He spent 15 years making sophisticated drawings of its surface.

Then he went full-on "Ancient Aliens," Martian edition.

He believed that the canals were a relic of an ancient advanced civilization that built them to move water from the polar ice caps as a last-ditch source of water on a drying planet.

Lowell wrote three books on the subject, the last presenting his full theory on the subject of life on Mars.

This popularized the belief in the general public of life on Mars, ultimately inspiring works of fiction like HG Wells, "The War of the Worlds," or Ray Bradbury's "The Martian Chronicles."

It turns out that Lowell's canals were just as fictional, probably a combination of artifacts from imperfect telescope optics plus actual erosion features.

We had to actually go to Mars to really confirm this.

In the 1960s and early '70s the Mariner missions conducted the first Mars flybys, giving us our very first close-ups of the planet.

Mariner didn't find canals, but it did find evidence of ancient riverbeds carved into the landscape.

This evidence for ancient flowing water wasn't definitive at the time.

A closer look was needed.

NASA landed "Viking 1 and 2" on the surface of Mars in 1976.

They conducted four experiments looking for the biosignatures of past or present microbial life.

Three of the tests were negative, but the fourth was positive.

It was the so-called labeled release experiments.

In it a soil sample is fed nutrients laced with radioactive carbon-14.

Any microbes should metabolize the nutrients and expel radioactive CO2.

Right away, both probes detected these radioactive gases.

A general rule in science is that a result isn't real until it's repeatable.

And this experiment failed that test a week later.

While some thought that the initial Viking results were a positive act of biosignature on Mars, most agreed the results were inconclusive, likely a result of some abiotic, or nonliving, reaction in the soil and, sadly, not microscopic martians.

And then there's the famous Martian fossil meteorite.

This rock was discovered in Antarctica in 1984.

Based on its chemical composition, it must have come from the surface of Mars, probably ejected from Mars's gravitational field after another space rock smashed into the planet.

It was dated to around 4 billion years old from a time when we now believe Mars had liquid water on the surface.

This rock held a tiny secret for all those billions of years.

And it would hold that secret for another decade still until 1996 with the advent of high-resolution scanning electron microscopy and laser mass spectrometry.

That analysis revealed chain structures that look an awful lot like Earth's microfossils.

This direct detection of extraterrestrial life even prompted an announcement from President Clinton.

But once again, it was probably a false alarm.

Just like the Viking test results, these features could be explained by nonbiological processes.

It's never aliens until there's no other possible explanation.

All these near misses did serve a powerful good.

They inspired our continued obsession with the Red Planet, and so many more missions have followed.

A series of orbiters, landers, and rovers found increasing evidence of past and present activity of water on Mars.

The Mars "Odyssey" orbiter detected atmospheric hydrogen that told us of enormous quantities of water ice frozen into the Martian soil close to the poles.

And that brings us to "Opportunity," otherwise affectionately known as Mars Exploration Rover-B.

It landed in 2004 with its twin, MER-A, better known as "Spirit."

These six-wheeled, golf cart-sized robots were Swiss Army knives of geological lab instruments.

"Opportunity's" most spectacular discovery were these cute little, so-called, berries.

They turned out to be concretions of hematite, which means they were grown in wet environments with dissolved iron.

Wet means water, FYI.

Both rovers found a variety of other geological signs of past water activity, really solidifying the picture of Mars's much more liquid past.

Both rovers were supposed to shut down after around 90 days as Martian dust accumulated on their solar panels, cutting off their power supply.

But unexpected cleaning events from the Martian wind cleared those accumulations, allowing the massively extended lifespans of both rovers.

"Spirit" did great, but eventually got bogged in a sand trap before shutting down.

"Opportunity" just chugged on and on and on.

The vast Martian dust storm that has enveloped the planet since early June caused "Opportunity" to enter hibernation mode.

As of the filming of this video, the storm is only starting to settle down.

NASA and all of us remain hopeful the communication will be reestablished.

But even if not, you've earned your rest, brave "Opportunity."

The "Phoenix" lander was the first to taste Martian water, sampling the icy soil in the northern polar region.

It found an abundance of perchlorate, a type of salt.

And that's actually bad news for life.

Perchlorates act as antifreeze, lowering the freezing point of water.

And this will be important later.

Our latest Martian explorer, the "Curiosity" rover, has sampled soil at lower latitudes and confirmed the presence of H2O in the soil planet-wide.

This brings us just about to the present.

2018 has been a great year for the possibility of life on Mars.

Two papers came out on the same June day.

One reported the detection of methane in the atmosphere.

So have we seen Martian microbe farts?

Maybe, but not necessarily.

Again, there are nonbiotic processes that generate methane.

The second paper was about organic molecules found in mudstones on the Martian surface.

Now, remember, organic in chemistry just means a compound containing carbon.

We find small organic molecules in lots of places that life has never touched.

But the scientists think that these Martian molecules originated from large organic molecules called kerogens, which are often produced in the decay of once living matter.

And of course, there are other nonbiotic ways to produce them.

But all of these teasers have gotten scientists very excited.

But perhaps the coolest recent discovery is this giant underground lake I mentioned at the start of the episode.

At the end of July, the European Space Agency announced that this lake was discovered using radar reflections from the MARSIS instrument on the "Mars Express" orbiter.

This is the same way we map the ocean, by analyzing radio waves reflected from layers below the surface.

And the MARSIS radar revealed a boundary between the surface ice cap and liquid water 1.5 kilometers below the surface over a 20-kilometer-wide area.

It's odd to find liquid water in a region that is surely several 10s of degrees below freezing, but this may be where those perchlorates come back.

With enough of this stuff, the freezing point of water could be as low as negative 75 Celsius.

So it's an intensely cold, intensely salty lake that even Earth's most hardy extremophile organisms would have trouble with.

But it's a lake, nonetheless.

It's a simple matter of drilling 1.5 kilometers into the Martian ice cap to investigate.

Hey, here's an idea.

We should totally train a team of maverick deep-sea oil drillers to be astronauts and-- no, that's stupid.

So, do I think there's life on Mars?

Well, there's no real evidence for it yet.

But the planet was certainly wet and hospitable for long enough.

It didn't take life that long to get started on Earth.

It's very plausible and worth the hunt.

After all, the payoff would be enormous.

Finding even one incident of life beginning on another planet would tell us worlds about the likelihood of life in the universe-- kind of a big deal if we want to understand our own place in what so far seems an eerily empty spacetime.

Hey, guys, I'm going to be traveling for a little bit.

So we're going to have a little bit of a backlog in comment responses.

I wanted to let you know that we will be getting to them all, and we'll be releasing episodes on the normal schedule.

So, see you next week.