A recent study published a few days ago suggest that there is oxygen-rich water just below the surface of Mars. Oxygen is important for life on Earth, where chemicals are oxidized to produce energy, and so there is the exciting prospect that it’s possible to have oxygen-rich water on another planet. Of course, there are other ways organisms can get energy without oxygen, but it just makes things easier if oxygen is present.
When it comes to life on other planets or moons, there is a great deal of debate and interest among scientists and the public. A quick Google news search, of popular articles published only in this past week, found no fewer than a several hundred.
Since the question “does life exist on other planets in our solar system?” cannot be directly answered at the moment, we have to rephrase the question to be “can life exist on other planets in our solar system?” and then do the experiments on Earth to narrow down the field of areas to explore.
Narrowing down the number of target exploration sites is critical because the solar system is huge, and our current abilities to explore are limited. We still have a hard time exploring our own planet. Just think about, how many new species are found every year, and in how many new and crazy environments do we find life thriving in areas that we didn’t think life could possibly exist:
Harsh environments where people thought life could not exist:
- super hot hydrothermal vents at the bottom of the ocean
- deep earth and in the rock
- freezing cold temperatures of the Antarctic, and glacial brine pools
- inside crystals - yup, and that’s where we will go because this is a story of minerals.
So where to find life in crystals? One place is salt.
When a lake (or ocean) dries up, it leaves behind a super-salty brine that can take weeks, or months, to completely evaporate. Chances are that you have done this experiment before, by dissolving sugar in water, to form rock candy. The water evaporates to leave behind a rich concentration of sugar water. When the water (not the sugar) continues to evaporate, there isn’t enough water to keep the sugar dissolved, so it precipitates out to form sweet sugar crystals.
Let’s get back to the salt lake… in this very salty liquid, bacteria called halophiles (salt-loving creatures) thrive, and even requiring these extreme chemical environments to live. As the water evaporates away into the atmosphere, salt minerals grow (like halite) and a whole host of other interesting minerals (mirabilite, epsomite, and others). Eventually, the water completely dries, but not before a good number of the bacteria get trapped and entombed in the crystals. You would think that this is certain doom for the bacteria; but no, they continue to survive and can hibernate for thousands or millions of years (actually, nobody really knows how long they can survive).
Even though the survival of halophiles in crystals is not totally understood, what we do know is fascinating. For example, the halobacteria that live in the water and salt of Searles Lake produce a carotene (think beta-carotene that makes carrots orange) to protect themselves from harmful UV rays. Ultra-violet radiation will damage DNA, and will ultimately kill any unprotected bacteria. The carotenes absorb UV, thus making an effective sunscreen for protection. These carotenes turn the water and the crystals pink, red, and purple, depending on what and how many carotenes are present.
The shapes of the bacteria also change through time. The bacteria often start off as rod-like cells, but when nutrients deplete, they change shape (likely to conserve energy). Lots of things happening in salt water!
There are still many open questions about how they eat, what happens to their waste, how do they hibernate, and how long can they survive in stasis, before their DNA breaks down beyond the point of repair, and many other questions. There is a mountain of work that still needs to be done, but the subject is fascinating and can teach us about how life+minerals on our own planet interact, as well how they might interact in extraterrestrial ecosystems.
So what role do the crystals play? The crystals are the preserving medium and therefore are the crux of the whole problem. You would think there is a ton of research addressing that issue, however surprisingly very little has been done. The crystal likely acts a stasis chamber (or an aquarium), holding the bacteria alive enough, so when new water comes and dissolved the salt crystal, the bacteria are released from the chamber. Something like this could be happening on Mars within its seasonal brine seeps.
The mineral-microbe interface is a difficult experiment from which meaningful data can be obtained. Both the crystal and bacteria move constantly under the microscope, and the direct interaction between crystal and bacteria only lasts for milliseconds. So, new techniques need to be developed to better understand this mineral+life system.
Here is a thought experiments that might be testable, and would be great for a science fair project: as the water evaporates, the bacteria need to find their way to the safe haven of the fluid, that will eventually be included in the crystal — but which crystal? As said before, there are several types of salt crystals that form when the water dries up, so how do they determine in which crystals to take up residence? Luckily, there is a sequence of crystallization that occurs in brines, and this sequence is predetermined by the chemical composition of the brine itself. Just based on the brine’s starting composition, we can predict (with a great deal of accuracy) the sequence of crystallization. Here is a simple example of a brine with only two salts — sodium sulfate, and sodium chloride:
- first, sodium sulfate crystallizes in the form of the mirabilite.
- second, sodium chloride will crystallize in the form of halite (just your normal table salt). If there were some magnesium in the water, the sequence might be mirabilite, epsomite, then halite.
As the water evaporates, the concentration of the still dissolved salts increases. So if bacteria love to live in the saltiest of salt water, then they simply have to go where the greatest amount of salt forming. Maybe they can‘taste’the water somehow using an osmotic pressure system via its cell membrane, I have no idea right now. But if there is a correlation between the concentration of salt and the number of bacteria around the crystals, then that answers an important question of how they find their way to the fluid (but of course the devil is in the details). And that’s just one experiment of many that needs to be performed to better understand the mineral-microbe interfaces.
Aaron Celestian is the Mineralogy Curator at the Natural History Museum of Los Angeles. He researches how minerals interact with their environments and with living things, and how those minerals can be used to solve problems like climate change, pollution, and disease.
media credits: salt photos by Stan Celestian, brine photos by Susan Celestian, animation by Bailee DesRocher, microscope video by Aaron Celestian.