Thank you to The Economist for supporting PBS. The tardigrade, the water bear, the moss piglet,
capable of surviving extreme drying, freezing heat, radiation, the vacuum of space,
maybe even asteroid impact. And who knows, maybe some tardigrades have hitchhiked on
impact ejector to Mars. And according to a new study, the tardigrade can survive most of the
ravages of the Martian surface. Most. When these researchers tried adding a chemical that is both
abundant on Mars and horrifically toxic, even the tardigrade wounds down. So if Earth's most
intrepid extreophile can't handle the hostility of the red planet, can anything? Well, maybe yes.
Maybe even today. Let's see where Martian life and tardigrade astronauts may still be hiding.
dark energy mugs are all still available. Links in the description. Now, on to the episode. There are
a few discoveries that will change everything if and when we make them. But one of the biggest is
extraterrestrial life. That's why Mars has been such a focus for NASA's exploration of the solar
system. It was once so much like the Earth, a real atmosphere, liquid water on the surface.
For a while, we thought we might find primitive life cling to existence on that surface. This was
boyed by Viking Landers labeled release experiment which tentatively detected metabolic byproducts in
Martian soil. But NASA ultimately concluded that this was due to abiotic soil chemistry. And as
the inhospitability of the Martian surface became clearer, we downgraded our hopes. For some time
now, we've mostly been focused on our searches for signs of past life. And just last year, the
Perseverance rover found the most tantalizing such sign in the Jezero crater. Mineral composition and
patterning that closely resembles the metabolic byproducts of certain Earth microbes. And yes,
we did cover that. This potential bio signature is not conclusive, yet it's extremely exciting.
Although maybe exciting is now a relative term relative to our diminishing hope of finding actual
living life on Mars. Now, don't get me wrong, signs of past life would be huge. But what if
we're not ready to give up? Maybe the Jezero crater discovery should raise our hopes that
somewhere the descendants of whatever left those marks are still there, somehow eluding our best
efforts. If so, where might they be? There are at least a few regions on Mars that could host
some sort of ecosystem. So, let's find them. First up, let's review why Mars used to be an oasis and
why it's now a hell hole. A wealth of evidence from minerals that only form in water to actual
dried river and lake beds showed that liquid water was once abundant on the Martian surface.
We also know that Mars once had a much thicker atmosphere. Back in the day, Mars also probably
had a magnetic field powered, as with Earth, by a natural dynamo, a solid iron core spinning within
a molten envelope. But Mars is a lot smaller than Earth, and it shed its smaller reservoir of heat
more quickly. The core solidified, the magnetic field shut off, and the Martian atmosphere was
exposed to the ablative effect of the solar wind. As that atmosphere was sheared away,
temperatures dropped, more atmospheric gases were frozen into the surface. As was the water. We see
that water now in Martian ice caps and it seems occasionally leaking up from a warmer interior.
But the surface of Mars is now bone dry with an esphixxiatingly thin atmosphere that leaves it
exposed to intense solar ultraviolet radiation and cosmic rays. But it's even worse than that.
The exposed surface is almost all regalith. Dust, sand, and rock fragments built up from billions of
years of meteorite impacts amplified by weathering and erosion. And with no tectonic activity to
recycle this stuff, baking under hard UV, this regolith is photochemically altered into various
salts of chlorine and sulfur. But without that tectonic recycling or dissolution by water, these
just build up and up and up. Now, many of them, like the perch chlorates, are extremely oxidizing
and generally bad news for delicate creatures made from organic molecules. Long story short,
the Martian surface sucks. If we're searching for extent life on Mars, we need places that are the
bullseye of a habitability ven diagram. Now, Earth boasts some species that are tolerant of some
pretty extreme circumstances. So, we don't need to be too prescriptive here, but there are some basic
conditions that are going to be necessary for any life. First, we need liquid water. Now, persistent
large bodies of water may not exist, so we'll need to get creative there. Second, we need protection
from all that super bad stuff that Mars tries to inflict on life, like the extreme UV and the
cosmic rays, the toxic chemicals at the surface, and the extreme temperature swings. Third,
we need some source of energy. Now, for most life on Earth, that's ultimately the sun. But on Mars,
direct exposure to the sun means exposure to all of the bad things. So, we need to get creative
here, too. So, we want to come up with a mission to search for where these non-negotiables actually
overlap. Water, protection, energy. The easiest way to deal with the protection issue is to get
below the surface. And essentially, all of our possible habitats will be subterranean or sub
Martian-ian. The less we have to dig,
the sooner we'll be able to engineer our mission. So, let's start
as close to the surface as possible. In fact, we only need to go down a few centimeters into the
Martian surface to dramatically reduce ultraviolet radiation and a few tens of centime to give some
buffer from temperature swings. This doesn't fix the other major problems of the surface at
these depths. It's always still below the freezing point of water and we do need liquid water and the
hazardous Martian salts remain extremely abundant. But strangely, it turns out that those same salts
may solve the freezing problem. Salts like perchlorates are highly oxidizing, but they're
also hygroscopic, meaning they can absorb water vapor directly from the atmosphere, like those
little packets of silica. They're also powerful antifreeze agents, allowing the accumulated water
to remain a liquid at temperatures as low as -70 C. Under the right temperature and humidity
conditions, these salts can deliquesce. They form thin films of brine around particles and within
gaps of the Martian regolith. These would likely form in the Martian night and then evaporate in
the day. So you could imagine primitive life forms that go into stasis, perhaps even dehydrating to
avoid the most dangerous oxidation when the UV is high. But at night they'd activate metabolizing
the chemical gradients in the Martian soil. Are such organisms even plausible? Well, actually,
yeah. We have things on Earth that do some of the required things. There are various halophilic or
salt loving microbes, some of which even use perchlorates as a key part of their metabolic
cycle. For microbes in the Martian regolith, the sun would be the ultimate source of energy.
For example, a perchlorate metabolizing critter will eat those molecules during the night while
the sun's UV will replenish them during the day. And this points to a question that we can
address without even going to Mars. How would the hardiest Earth microbes fare in simulated
Martian conditions? It's not that hard to fake the Martian surface. ultraviolet radiation, low
pressure CO2 atmosphere, cold and a regolith-like soil containing a variety of these salts. Now,
I already mentioned the recent tardigrade study. These things do pretty well until perchlorates are
introduced. Then they really suffer. And in a 2024 study, halaphilic bacteria and fungi were
found to survive pretty well in Mars-like regolith conditions,
especially at several centimeters depth.
But again, this was only until the perchlorates were introduced. Then everything died. So on the
one hand, perchlorates may be the best of the Martian salts at forming brines. They suck in
more water and more critically they suppress the freezing point below the actual Martian
surface temperature. But on the other hand, they're lethal. But don't underestimate the
ingenuity of life. Even here on Earth, we have microbes that can use those perchlorates as a key
part of their metabolism. And on Mars, that could keep the abundance of this stuff in check. And a
study just last year found that when you expose E.coli to increasing perchlorate concentrations,
they activate pathways for DNA repair and stress mitigation, showing that there are biological
solutions to the perchlorate toxicity. On Mars, these perchlorates built up relatively slowly over
billions of years. And so it's plausible that micro species there have evolved to withstand
and even utilize this stuff. So life could exist at shallow depths in the Martian regolith even
if it's a marginal and tortured existence. But the upside is that this stuff is the easiest to
search for. And because of that our putitive life finding missions are already underway. The Rosalyn
Franklin rover. now targeted for a 2028 launch is designed with a 2 m drill that'll get to depths
where cosmic ray protection is good enough even for humans. The mission is focused on looking
for signs of past life, but it's possible it'll detect biomarkers in the samples that it draws up.
It could even be that the Perseverance rover has already dug up lifebearing material from shallower
depths and packaged it for return to Earth. Now, that sample return mission is now in question due
to budgetary chaos. But if we can get it to a proper lab on Earth and take a look for all of
the bio signatures, even the subtlest or sparsest ones that our Mars robots haven't been able to
catch could be found. Okay, let's see if we can find slightly more hospitable habitats. Mars has
widespread near surface ground ice, especially at high latitudes, and orbital data suggests
buried ice deposits at mid latitudes, too. This ice would solve our protection issues. Water in
solid or liquid form is a fantastic UV and cosmic ray shield. A meter of it is a common shielding
plan for speculative human space missions. And that same UV blocking power and separation from
the atmosphere slows the chemistry that leads to the formation of perchlorates, which is
probably a good thing. In either phase, water has wonderful temperature stability, which means the
layer below this surface ice is protected from the wild temperature fluctuations on the surface. Now,
solid water is not liquid water, which is what we really want. But a 2024 NASA study found that melt
water might occasionally form beneath the ground ice. Now, these would be thin layers of water that
are warmed from below by weak geothermal energy and from above by the UV depleted sunlight that
filters through the ice. That liquid phase is also helped by the increased pressure from
the ice layer and perhaps some lowering of the freezing point from a hopefully not too toxic
salt concentration. Ice also supports another intriguing possibility, radiolysis. Ionizing
radiation interacting with ice can split water molecules producing hydrogen and oxidants. On
Earth, some deep subsurface microbes do live off hydrogen produced by water rock interactions and
radiolysis. The Martian radiation environment while hostile at the surface could generate
chemical energy in protected icerich regions. Now finding this sub ice life would also require
a shallow drilling mission and the Rosalyn Franklin mission may actually find subsurface
ice to investigate. But there's also the Mars Life Explorer proposal. If green lit, this would work
towards a mid- latatitude lander that'll aim for a location with surface ice. It would have a 2 m
drill just like Rosal and Franklin and onboard labs to look for bio signatures and signs of
metabolism both in the bore hole and perhaps seeping up from layers of Mars deeper down.
Another potential habitat is the Martian lava tubes. Orbital imagery has revealed skylights,
collapsed ceilings exposing underground voids that likely connect to lava tubes that would have
formed billions of years ago when Mars was still volcanically active. These natural caverns could
provide excellent radiation shielding and thermal stability. The great unknown here is whether
there's water. There could be ice deposits or brines, perhaps even occasional water welling up
from below, or they could be bone dry. In the case of lava tubes, we're going to need to send digging
or drilling equipment much heavier than any lander or rover can handle. Maybe we just send humans to
explore the labyrinthine Martian underworld. What could go wrong? Anywhere close to the Martian
surface, water is probably transient and energy is limited. That means we're unlikely to find dense,
diverse, or expansive ecosystems. More like sparse, scattered communities that may be
dormant much of the time and metabolize and grow at glacial speeds. Just like the Earth microbes
that live in sedimentary layers deep beneath the seafloors with their cell division rates
of hundreds of thousands of years. In the case of Mars, going deeper should improve habitability.
Just meters down, there's full protection from the worst ravages of the surface. Deeper still,
temperature starts to increase. Although the Martian interior is solid to the core,
it's still heated by the decay of radioactive material. Same as on Earth. Temperature rises
by about 10 Kelvin per kilometer. By the time you reach about 6 km or so, it should be warm enough
for water to be liquid, if there's any water there. But intriguingly, recent work published
in 2024 used seismic data from NASA's Insight lander to postulate that parts of the Martian
midcrust may contain gigantic aquifers. Extensive regions of basically groundwater at depths of
roughly 10 to 20 km. The interpretation of the seismic data is still debated, but if confirmed,
this is potentially the largest region on Mars that could support an ecosystem. The key here is
that the seismic data is consistent with aquifers. So, water saturated fractured rock as opposed
to say water carrying clays or solid rock. Water needs to be able to flow in order to not stagnate.
Any aquous ecosystem will reach chemical and energetic equilibrium unless there's a way
to maintain chemical gradients for energy and to replenish nutrients and to export waste products
for a deep aquifer. That means the fractures need to connect regions over a sufficiently
large scale. Now we absolutely do have analog life fors on Earth, diverse bacteria and archa that
thrive in deep aquifers persisting on geochemical energy pathways. If Mars has the right geology,
there's no reason that such a deep biosphere should not also exist there. The main issue
with this last prospect is that it's going to be enormously difficult to test the idea. It's
going to be a long, long time before we can put together a mission to drill several kilometers
beneath the Martian surface. Given the scale and complexity of that effort, this might be
something that also has to wait for an actual human presence. Look, we honestly have no idea
whether there's currently life on Mars or ever was. But the implications of extent life would be
so huge that we got to look. Maybe Earth life was actually spawned by hitchhikers on impact ejecta
from Mars. Or maybe the other way around. Only the discovery of living life on Mars with its intact
DNA would tell us that. And if Martian life formed independently of Earth's, well, that tells us that
life really does form easily and often, improving the odds that we'll keep finding weird and
wonderful life forms as we expand our reach across interstellar spacetime. Thank you to The Economist
video series where senior editors debate the forces shaping geopolitics, defense, economics,
and technology. As a viewer of PBS Spacetime, you can get a special 35% off to enjoy The Economist.
video series where senior editors debate the forces shaping geopolitics, defense, economics,
and technology. As a viewer of PBS Spacetime, you can get a special 35% off to enjoy The Economist.