The Rarest Ingredient for Life

Europa — one of Jupiter’s moons

Where should we look for life in the solar system? Which places have the right ingredients for life? What are the right ingredients for life?

We don’t know what the full set of necessary ingredients for life is, but by looking at life on Earth, we can come up with several that have to be part of the answer: water, an energy source and the main elemental ingredients of life here on Earth (mainly CHNOPS).

As we’ve explored the solar system, we’ve found compelling evidence for these conditions in the past and even the present on Mars and on the icy moons of Jupiter and Saturn. We’ve found evidence of past lakes on Mars and most of the chemicals that life needs. Even better, these chemicals were present in forms that could power certain lifeforms here on Earth.

As for Europa and Enceladus (moons of Jupiter and Saturn respectively), we have evidence that they currently have vast oceans under their icy surfaces. The gravitational kneading caused by tidal forces as these moons orbit closer and farther from their large planets keeps their interiors warm. And the radiation that their surfaces encounter presents an extra source of energy that can transform chemicals that cycle up to the icy surface (making the brown material seen in the cracks of Europa) and then are resubmerged as more energized species. And both have plumes coming out of their south poles that we can fly spacecraft through to directly investigate the oceans beneath the ice! (We’ve already done this for Saturn’s moon Enceladus and found evidence for Earth-like pH levels in the ocean and hydrothermal activity!!)

This looks great! Maybe there’s life currently there now! There’s at least some reason to believe that it could be there. We’ve checked off those important ingredients of water, energy and chemical substrate. But if it turns out that there isn’t life there, what’s missing? What’s the most important ingredient that we haven’t seen definite evidence of?

Artist’s depiction of Enceladus’s small ocean with hydrothermal vents and geyser plumes

So what is the rarest ingredient?

I asked Eric Smith (who’s book I’ve previously reviewed) this summer that exact question: what’s the rarest ingredient for the emergence of life in the solar system?

His answer: a long lived redox disequilibrium.

Now, that’s quite a mouthful, but together we can parse that statement and see how fundamental redox disequilibrium is to the emergence of life and how Earth has some very special conditions that have maintained that disequilibrium while it ground to a halt on Mars and it’s unknown how long it could have lasted or if it existed on Europa or Enceladus.

Starting at the end of the sentence, a disequilibrium represents a source of unused energy in the environment. In essence, the environment hasn’t balanced everything out yet and settled into an unchanging equilibrium. The best arguments for how and where life formed are that it formed in hydrothermal vents. The core chemistry of life happens mostly spontaneously in those conditions and many other indirect lines of evidence also point to that as the birthplace of life. Why hydrothermal vents? Well, they’re where hell and heaven meet, more specifically the reduced mantle and oxidized atmosphere.

Hydrothermal vent chemistry (from Dr. Schieber’s site)

The heat and pressure of the mantle cause the iron and magnesium in the mantle to be in a chemical state that wants to give away some electrons (a reduced state). The atmosphere due to the sun’s UV splitting water and subsequent hydrogen escape is in a state that wants to suck up electrons (an oxidized state). Separate these two conditions with a barrier (the crust) and focus where they meet to specific points (vents) and you get in essence a battery that runs on the reduction-oxidation difference or redox potential. The amount of energy that’s doled out in this process is the perfect amount and type to do the organic chemistry that’s central to life. Other sources of energy are either too diffuse (the Earth’s heat) or too energetic (light) to be good for life’s chemistry. (Photosynthesis now harnesses light, but it’s a complex process that involves breaking the huge amount of photon energy into smaller manageable chunks of chemical energy.)

Great, we have a decent idea of the conditions that were fundamental to life arising on Earth: a redox disequilibrium. Now, we’ve seen evidence that this once happened on Mars and perhaps it’s also happened on Europa or Enceladus. But there’s one more part to that phrase I haven’t touched on yet, “long-lived”. Mars’s tectonic activity has come to a halt and its atmosphere has mostly been blown away. There is no longer a sustained accessible redox disequilibrium on Mars. The question is how long did life on Mars have to become able to use other energy sources like light before the original energy source shut off? Was there enough time? Was there a large enough difference between mantle and surface to start up life in the first place?

Why Earth’s redox disequilibrium was long lived

Earth actually has fairly specialized conditions for its redox potential to persist. It’s gravity is strong enough to hold on to most of the elements of its atmosphere except the hydrogen, amplifying the oxidizing nature of the atmosphere. Earth is also bigger than Mars, meaning it stays hot for longer, lengthening the time the mantle can convect and keeping our magnetic field intact which also preserves our atmosphere. The long-lived atmosphere helped maintain Earth’s global oceans which feed back into the cracks of the crust (creating vents!) actually make it much easier for the mantle to flow allowing for more dynamic plate tectonics and mantle convection. This may be another reason Earth’s tectonics have lasted longer than on Mars.

It’s an even bigger question if a sufficient redox potential can exist on the icy moons and, if so, for how long? Does the warm mantle of the moons mix with the surface too quickly shutting off the redox difference much faster than life can start? Is there even a mantle that’s reduced? Is the ocean or the icy surface sufficiently oxidized?

I think these are some of the most interesting questions for planetary science and for studying the origin of life. What were the redox conditions on other planets and moons throughout history? How strong were they and how long did they last? Knowing those answers would be a great help to knowing whether life could have started elsewhere in the solar system. And it doesn’t look that easy for a long-lived redox potential to have happened elsewhere. Maybe there was enough time on Mars. It’s certainly the place we know the most about. Maybe there are the right conditions on Europa or Enceladus although we know much less. We’ve found liquid water, the chemicals of life, even energized forms that could fuel life, but without a long term sustained source of energy in the right form, I’m skeptical life could form or persist. So, we’ve got to learn more!

Proposed Europa Clipper mission

Fortunately, there is another rover headed to Mars to search for biosignatures and previous habitability. And there’s a proposed mission to explore Europa in the coming decade and even fly through its plumes like Cassini did for Enceladus. There is so much yet to explore!

Shoutout to Konrad for our late night discussion about Europa’s plumes.

(Originally posted on my Wordpress blog)