Martian Soil Is Deadly. And That's Why It Might Support Life.

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0:03 The tardigrade, the water bear,

0:05 the moss piglet, capable of surviving extreme drying,

0:08 freezing heat, radiation, the vacuum of space, maybe even asteroid impact.

0:13 And who knows, maybe some tardigrades have hitchhiked on impact ejector to Mars.

0:18 And according to a new study,

0:21 the tardigrade can survive most of the ravages of the Martian surface.

0:26 Most.

0:27 When these researchers tried adding a chemical

0:30 that is both abundant on Mars and horrifically toxic,

0:35 even the tardigrade wounds down.

0:38 So if Earth's most intrepid extreophile can't handle

0:42 the hostility of the red planet, can anything?

0:46 Well, maybe yes.

0:48 Maybe even today.

0:50 Let's see where Martian life and tardigrade astronauts may still be hiding.

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1:51 Now, on to the episode.

1:53 There are a few discoveries that will

1:55 change everything if and when we make them.

1:58 But one of the biggest is extraterrestrial life.

2:00 That's why Mars has been such a focus

2:03 for NASA's exploration of the solar system.

2:06 It was once so much like the Earth,

2:09 a real atmosphere, liquid water on the surface.

2:12 For a while, we thought we might find

2:15 primitive life cling to existence on that surface.

2:18 This was boyed by Viking Landers labeled release

2:22 experiment which tentatively detected metabolic byproducts in Martian soil.

2:26 But NASA ultimately concluded that this was due to abiotic soil chemistry.

2:30 And as the inhospitability of the Martian surface became clearer,

2:35 we downgraded our hopes.

2:37 For some time now, we've mostly been focused

2:41 on our searches for signs of past life.

2:44 And just last year, the Perseverance rover found

2:48 the most tantalizing such sign in the Jezero crater.

2:53 Mineral composition and patterning that closely resembles

2:56 the metabolic byproducts of certain Earth microbes.

2:59 And yes, we did cover that.

3:02 This potential bio signature is not conclusive, yet it's extremely exciting.

3:08 Although maybe exciting is now a relative term relative

3:12 to our diminishing hope of finding actual living life on Mars.

3:17 Now, don't get me wrong, signs of past life would be huge.

3:21 But what if we're not ready to give up?

3:24 Maybe the Jezero crater discovery should raise our hopes that somewhere

3:28 the descendants of whatever left those marks are still there,

3:33 somehow eluding our best efforts.

3:35 If so, where might they be?

3:38 There are at least a few regions on Mars that could host some sort of ecosystem.

3:43 So, let's find them.

3:45 First up, let's review why Mars used to be

3:48 an oasis and why it's now a hell hole.

3:51 A wealth of evidence from minerals that only form in water to actual dried

3:56 river and lake beds showed that liquid

3:58 water was once abundant on the Martian surface.

4:01 We also know that Mars once had a much thicker atmosphere.

4:05 Back in the day, Mars also probably had a magnetic field powered, as with Earth,

4:10 by a natural dynamo, a solid iron core spinning within a molten envelope.

4:14 But Mars is a lot smaller than Earth,

4:17 and it shed its smaller reservoir of heat more quickly.

4:21 The core solidified, the magnetic field shut off,

4:23 and the Martian atmosphere was exposed to the ablative effect of the solar wind.

4:28 As that atmosphere was sheared away, temperatures dropped,

4:31 more atmospheric gases were frozen into the surface.

4:35 As was the water.

4:36 We see that water now in Martian ice caps

4:40 and it seems occasionally leaking up from a warmer interior.

4:43 But the surface of Mars is now bone dry with an esphixxiatingly thin

4:49 atmosphere that leaves it exposed to intense

4:52 solar ultraviolet radiation and cosmic rays.

4:55 But it's even worse than that.

4:57 The exposed surface is almost all regalith.

5:01 Dust, sand, and rock fragments built up from billions

5:05 of years of meteorite impacts amplified by weathering and erosion.

5:10 And with no tectonic activity to recycle this stuff, baking under hard UV,

5:17 this regolith is photochemically altered

5:20 into various salts of chlorine and sulfur.

5:23 But without that tectonic recycling or dissolution by water,

5:28 these just build up and up and up.

5:31 Now, many of them, like the perch chlorates,

5:35 are extremely oxidizing and generally bad news

5:38 for delicate creatures made from organic molecules.

5:40 Long story short, the Martian surface sucks.

5:43 If we're searching for extent life on Mars,

5:46 we need places that are the bullseye of a habitability ven diagram.

5:51 Now, Earth boasts some species that are

5:54 tolerant of some pretty extreme circumstances.

5:56 So, we don't need to be too prescriptive here,

6:00 but there are some basic conditions that are going to be necessary for any life.

6:06 First, we need liquid water.

6:08 Now, persistent large bodies of water may not exist,

6:12 so we'll need to get creative there.

6:14 Second, we need protection from all that super

6:17 bad stuff that Mars tries to inflict on life,

6:19 like the extreme UV and the cosmic rays,

6:22 the toxic chemicals at the surface, and the extreme temperature swings.

6:26 Third, we need some source of energy.

6:29 Now, for most life on Earth, that's ultimately the sun.

6:32 But on Mars, direct exposure to the sun means exposure to all of the bad things.

6:37 So, we need to get creative here, too.

6:40 So, we want to come up with a mission

6:45 to search for where these non-negotiables actually overlap.

6:49 Water, protection, energy.

6:50 The easiest way to deal with the protection issue is to get below the surface.

6:57 And essentially, all of our possible

6:59 habitats will be subterranean or sub Martian-ian.

7:02 The less we have to dig, the sooner we'll be able to engineer our mission.

7:07 So, let's start as close to the surface as possible.

7:10 In fact, we only need to go down

7:12 a few centimeters into the Martian surface to dramatically

7:15 reduce ultraviolet radiation and a few tens

7:18 of centime to give some buffer from temperature swings.

7:22 This doesn't fix the other major problems of the surface at these depths.

7:26 It's always still below the freezing point of water and we

7:30 do need liquid water and the hazardous Martian salts remain extremely abundant.

7:34 But strangely, it turns out that those same

7:38 salts may solve the freezing problem.

7:41 Salts like perchlorates are highly oxidizing, but they're also hygroscopic,

7:46 meaning they can absorb water vapor directly from the atmosphere,

7:51 like those little packets of silica.

7:53 They're also powerful antifreeze agents,

7:56 allowing the accumulated water to remain a liquid

8:00 at temperatures as low as -70 C.

8:03 Under the right temperature and humidity conditions, these salts can deliquesce.

8:08 They form thin films of brine around

8:11 particles and within gaps of the Martian regolith.

8:15 These would likely form in the Martian night and then evaporate in the day.

8:21 So you could imagine primitive life forms that go into stasis,

8:24 perhaps even dehydrating to avoid the most

8:27 dangerous oxidation when the UV is high.

8:31 But at night they'd activate metabolizing

8:34 the chemical gradients in the Martian soil.

8:38 Are such organisms even plausible?

8:40 Well, actually, yeah.

8:41 We have things on Earth that do some of the required things.

8:47 There are various halophilic or salt loving microbes,

8:50 some of which even use perchlorates as a key part of their metabolic cycle.

8:55 For microbes in the Martian regolith,

8:58 the sun would be the ultimate source of energy.

9:01 For example, a perchlorate metabolizing critter will eat those molecules during

9:06 the night while the sun's UV will replenish them during the day.

9:10 And this points to a question that we can address without even going to Mars.

9:16 How would the hardiest Earth microbes fare in simulated Martian conditions?

9:21 It's not that hard to fake the Martian surface.

9:25 ultraviolet radiation, low pressure CO2 atmosphere,

9:28 cold and a regolith-like soil containing a variety of these salts.

9:33 Now, I already mentioned the recent tardigrade study.

9:37 These things do pretty well until perchlorates are introduced.

9:41 Then they really suffer.

9:43 And in a 2024 study, halaphilic bacteria and fungi were found

9:49 to survive pretty well in Mars-like regolith conditions,

9:53 especially at several centimeters depth.

9:56 But again, this was only until the perchlorates were introduced.

10:01 Then everything died.

10:02 So on the one hand, perchlorates may be

10:05 the best of the Martian salts at forming brines.

10:08 They suck in more water and more critically they

10:12 suppress the freezing point below the actual Martian surface temperature.

10:17 But on the other hand, they're lethal.

10:20 But don't underestimate the ingenuity of life.

10:23 Even here on Earth, we have microbes that can

10:25 use those perchlorates as a key part of their metabolism.

10:29 And on Mars, that could keep the abundance of this stuff in check.

10:34 And a study just last year found

10:37 that when you expose E.coli to increasing perchlorate concentrations,

10:41 they activate pathways for DNA repair and stress mitigation,

10:45 showing that there are biological solutions to the perchlorate toxicity.

10:49 On Mars, these perchlorates built up relatively slowly over billions of years.

10:55 And so it's plausible that micro species there

10:59 have evolved to withstand and even utilize this stuff.

11:04 So life could exist at shallow depths in the Martian

11:08 regolith even if it's a marginal and tortured existence.

11:12 But the upside is that this stuff is the easiest to search for.

11:17 And because of that our putitive life finding missions are already underway.

11:23 The Rosalyn Franklin rover.

11:24 now targeted for a 2028 launch is designed with a 2 m drill

11:29 that'll get to depths where cosmic ray

11:32 protection is good enough even for humans.

11:34 The mission is focused on looking for signs of past life,

11:38 but it's possible it'll detect biomarkers in the samples that it draws up.

11:43 It could even be that the Perseverance rover has already dug up

11:48 lifebearing material from shallower depths and packaged it for return to Earth.

11:53 Now, that sample return mission is now in question due to budgetary chaos.

11:58 But if we can get it to a proper lab

12:01 on Earth and take a look for all of the bio signatures,

12:05 even the subtlest or sparsest ones that our Mars

12:08 robots haven't been able to catch could be found.

12:12 Okay, let's see if we can find slightly more hospitable habitats.

12:17 Mars has widespread near surface ground ice, especially at high latitudes,

12:23 and orbital data suggests buried ice deposits at mid latitudes, too.

12:28 This ice would solve our protection issues.

12:32 Water in solid or liquid form is a fantastic UV and cosmic ray shield.

12:37 A meter of it is a common shielding plan for speculative human space missions.

12:42 And that same UV blocking power and separation from the atmosphere

12:47 slows the chemistry that leads to the formation of perchlorates,

12:52 which is probably a good thing.

12:55 In either phase, water has wonderful temperature stability,

12:58 which means the layer below this surface ice is

13:02 protected from the wild temperature fluctuations on the surface.

13:06 Now, solid water is not liquid water, which is what we really want.

13:11 But a 2024 NASA study found that melt

13:15 water might occasionally form beneath the ground ice.

13:18 Now, these would be thin layers of water that are warmed from below by weak

13:24 geothermal energy and from above by the UV

13:27 depleted sunlight that filters through the ice.

13:29 That liquid phase is also helped by the increased pressure from the ice layer

13:34 and perhaps some lowering of the freezing point

13:37 from a hopefully not too toxic salt concentration.

13:40 Ice also supports another intriguing possibility, radiolysis.

13:45 Ionizing radiation interacting with ice can

13:48 split water molecules producing hydrogen and oxidants.

13:51 On Earth, some deep subsurface microbes do live

13:54 off hydrogen produced by water rock interactions and radiolysis.

13:58 The Martian radiation environment while hostile at the surface

14:02 could generate chemical energy in protected icerich regions.

14:06 Now finding this sub ice life would also require a shallow drilling

14:12 mission and the Rosalyn Franklin mission

14:15 may actually find subsurface ice to investigate.

14:18 But there's also the Mars Life Explorer proposal.

14:21 If green lit, this would work towards a mid-

14:25 latatitude lander that'll aim for a location with surface ice.

14:28 It would have a 2 m drill just like

14:32 Rosal and Franklin and onboard labs to look for bio

14:35 signatures and signs of metabolism both in the bore hole

14:39 and perhaps seeping up from layers of Mars deeper down.

14:43 Another potential habitat is the Martian lava tubes.

14:48 Orbital imagery has revealed skylights,

14:51 collapsed ceilings exposing underground voids that likely

14:55 connect to lava tubes that would

14:58 have formed billions of years ago when Mars was still volcanically active.

15:03 These natural caverns could provide

15:05 excellent radiation shielding and thermal stability.

15:07 The great unknown here is whether there's water.

15:10 There could be ice deposits or brines,

15:13 perhaps even occasional water welling up from below, or they could be bone dry.

15:17 In the case of lava tubes, we're going to need to send digging or drilling

15:22 equipment much heavier than any lander or rover can handle.

15:26 Maybe we just send humans to explore the labyrinthine Martian underworld.

15:30 What could go wrong?

15:31 Anywhere close to the Martian surface,

15:34 water is probably transient and energy is limited.

15:37 That means we're unlikely to find dense, diverse, or expansive ecosystems.

15:43 More like sparse, scattered communities that may be dormant much

15:48 of the time and metabolize and grow at glacial speeds.

15:52 Just like the Earth microbes that live in sedimentary layers deep beneath

15:57 the seafloors with their cell division rates of hundreds of thousands of years.

16:02 In the case of Mars, going deeper should improve habitability.

16:06 Just meters down, there's full protection from the worst ravages of the surface.

16:12 Deeper still, temperature starts to increase.

16:15 Although the Martian interior is solid to the core,

16:18 it's still heated by the decay of radioactive material.

16:22 Same as on Earth.

16:24 Temperature rises by about 10 Kelvin per kilometer.

16:27 By the time you reach about 6 km or so,

16:30 it should be warm enough for water to be liquid, if there's any water there.

16:36 But intriguingly, recent work published in 2024

16:40 used seismic data from NASA's Insight lander to postulate that parts

16:45 of the Martian midcrust may contain gigantic aquifers.

16:50 Extensive regions of basically groundwater at depths of roughly 10 to 20 km.

16:57 The interpretation of the seismic data is still debated, but if confirmed,

17:02 this is potentially the largest region on Mars that could support an ecosystem.

17:08 The key here is that the seismic data is consistent with aquifers.

17:13 So, water saturated fractured rock as opposed

17:16 to say water carrying clays or solid rock.

17:19 Water needs to be able to flow in order to not stagnate.

17:23 Any aquous ecosystem will reach chemical and energetic

17:27 equilibrium unless there's a way to maintain chemical

17:30 gradients for energy and to replenish nutrients

17:33 and to export waste products for a deep aquifer.

17:37 That means the fractures need to connect

17:39 regions over a sufficiently large scale.

17:41 Now we absolutely do have analog life fors on Earth,

17:45 diverse bacteria and archa that thrive

17:48 in deep aquifers persisting on geochemical energy pathways.

17:52 If Mars has the right geology,

17:55 there's no reason that such a deep biosphere should not also exist there.

18:00 The main issue with this last prospect is that it's

18:03 going to be enormously difficult to test the idea.

18:06 It's going to be a long, long time before we can put together a mission

18:09 to drill several kilometers beneath the Martian surface.

18:11 Given the scale and complexity of that effort,

18:14 this might be something that also has to wait for an actual human presence.

18:18 Look, we honestly have no idea whether

18:20 there's currently life on Mars or ever was.

18:24 But the implications of extent life would be so huge that we got to look.

18:30 Maybe Earth life was actually spawned by hitchhikers on impact ejecta from Mars.

18:36 Or maybe the other way around.

18:38 Only the discovery of living life on Mars

18:41 with its intact DNA would tell us that.

18:44 And if Martian life formed independently of Earth's, well,

18:48 that tells us that life really does form easily and often,

18:53 improving the odds that we'll keep finding weird and wonderful

18:56 life forms as we expand our reach across interstellar spacetime.

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