Seeking life on other worlds necessarily makes us examine our assumptions about the detectability of living things in extreme environments. We’re learning that our own planet supports life in regions we once would have ruled out for survival, and as we examine such extremophiles, it makes sense to wonder how similar organisms might have emerged elsewhere. Pondering these questions in today’s essay, Centauri Dreams regular Alex Tolley asks whether we are failing to consider possibly rich biospheres that could thrive without the need for surface water.
By Alex Tolley
Image: An endolithic lifeform showing as a green layer a few millimeters inside a clear rock. The rock has been split open. Antarctica. Credit: https://en.wikipedia.org/wiki/Endolith#/media/File:Cryptoendolith.jpg, Creative Commons).
A policeman sees a drunk man searching for something under a streetlight and asks what the drunk has lost. He says he lost his keys and they both look under the streetlight together. After a few minutes the policeman asks if he is sure he lost them here, and the drunk replies, no, and that he lost them in the park. The policeman asks why he is searching here, and the drunk replies, “this is where the light is” – The Streetlight Effect
I’m going to make a bold claim that we are searching for life where the starlight can reach, and not where it is most common, in the lithosphere.
One of the outstanding big questions is whether life is common or rare in the universe. With the rapid discovery of thousands of exoplanets, the race is now on to determine if any of those planets have life. This means using spectroscopic techniques to find proxies, such as atmospheric composition, chlorophyll “red edge”, and other signatures that indicate life as we know it. There is the exciting prospect that new telescopes and instruments will give us the answer to whether life exists elsewhere within a decade or two.
The search for life on exoplanets starts with locating rocky planets in the habitable zone (HZ). The HZ is defined as potentially having liquid surface water, which requires an atmosphere dense enough to ensure that water is retained. While complex, multicellular life that visibly populates our planet is the vision most people have of life, as I have argued previously , it is most likely that we will detect the signatures of bacterial life, particularly archaean methanogens, as prokaryotes were the only form of life on Earth for over 85% of its existence. Most worlds in the HZ will probably look more like Venus or Mars, either too dry and/or with an insufficient atmosphere to allow surface water. Such worlds will be bypassed for more attractive Earth analogs.
This is particularly important for the most common star type, the M-dwarfs. These stars are often downgraded as having habitable planets due to the flaring of their stars which can strip atmospheres and irradiate the surface. This reduces the likelihood for life at the surface, and for many, is a showstopper.
However, if life established well below the surface, these factors affecting the surface become relatively unimportant. All stars, including M-dwarfs, may well have a retinue of living worlds, but with their life undetectable by current means.
Despite mid-20th-century hopes for multicellular life to be found on Mars or Venus, it is now clear that the surfaces of these planets are devoid of any sort of multicellular based ecosystems. Venus’ surface is too hot for any carbon-based life to survive. The various Martian orbiters and landers have found no multicellular life, and so far no unambiguous evidence of microbial life on or near the surface. The Moon is the only world where surface rock samples have been returned to Earth, and these samples suggest, unsurprisingly, that the lunar surface is sterile [10,12].
NASA’s mantra for the search for life, echoing the HZ requirement, is “Follow the water!” On its face, this makes the lunar surface unlikely as a habitat, similarly Mars, although Mars’ does have an abundance of frozen water below the surface. This leaves the subsurface icy moons as the current favorite for the discovery of life in our solar system, particularly around any hypothetical “hot vents” that mimic Earth’s.
However, when following the trail of liquid water, we now know that the Earth has a huge inventory of water in the mantle, providing a new source of water for the crustal rocks. This water is most likely primordial, sourced from the chondritic material during formation.[6,9] If the Earth has primordial water in the mantle, so might the Moon, as it was formed from the same material as the Earth. A recent analysis of lunar rocks indicates that the bulk of the water in the Moon is also primordial, with concentrations only an order of magnitude less than the water in the Earth’s mantle . While we know Mars has water just below the surface, the same argument about primordial water deep within Mars also follows.
The question then becomes whether this water is in a form suitable for life. Is there a zone in these worlds where water is both liquid and at a temperature below the maximum we know terrestrial thermophiles can survive?
Table 1 below shows some estimates for Earth, Mars and the Moon where a suitable liquid water temperature range exists. The estimated thermal gradients are used to suggest the depths where life might start to be found as temperatures and pressures result in liquid water, and the maximum depth life might survive.
On Earth, the reference planet, the high thermal gradient, and warm surface suggest life can be found at any depth, up to about 5 – 6 km. The Moon, due to a low thermal gradient might only have a habitable zone starting at 15 km below the surface but reaching down to nearly 120 km. Mars is intermediate, with a habitable zone 6-29 km in extent.
Table 1. Estimates of thermal gradients and range of depths where water is liquid, but below 120C as a current approximate maximum for thermophiles
at 120C (with
|Depth (km) at|
|Depth (km) at
|Moon||-18 *||1.17 ***||103||15||118|
* Assumes the Moon surface temperature would be the same as the Earth without an atmosphere
So we have 2 possible rocky worlds in our solar system that may have water reservoirs in their mantles due to primordial asteroids and therefore liquid water in their lithospheres deep below the surface, protected from radiation and with fairly constant temperatures within the range of terrestrial organisms. So our necessary condition of liquid water may exist in these worlds, rather than at the surface.
Given that liquid water may be found deep below the surface, is there any evidence that life exists there too?
In 1999, the iconoclast astrophysicist and astronomer Thomas Gold published a popular account of his theory that fossil fuels were not derived from biological sources, but rather from primordial methane that was contaminated by organisms living deep within the Earth’s crust.[4,5]. While his theory remains controversial, his suggestion that organisms live in the lithosphere has been proven correct. . Bores have shown that microorganisms have been found living at least 4 km below the surface. It has been suggested that the biomass of these organisms may exceed that of humanity on Earth, so life in the lithosphere is not trivial compared to that on the surface of our planet.
Figure 1. Illustration of the search for life in the lithosphere. At this time, life has been found at depths of nearly 4 km, but absent at 9 km where the temperatures were too high.
1. Deep-sea, manned submersibles and remotely operated vehicles collect fluid samples that exit natural points of access to the oceanic crust, such as underwater volcanoes or hydrothermal vents. These samples contain microbes living in the crust beneath.
2. Drilling holes into the Earth’s crust allows retrieval of rock and sediment cores reaching kilometers below the surface. The holes can then be filled with monitoring equipment to make long-term measurements of the deep biosphere.
3. Deep mines provide access points for researchers to journey into the Earth’s continental crust, from where they can drill even deeper into the ground or search for microbes living in water seeping directly out of the rock.
From the article:
To date, studies of crustal sites all over the world—both oceanic and continental—have documented all sorts of organisms getting by in environments that, until recently, were deemed inhospitable, with some theoretical estimates now suggesting life might survive at least 10 kilometers into the crust. And the deep biosphere doesn’t just comprise bacteria and archaea, as once thought; researchers now know that the subsurface contains various fungal species, and even the occasional animal. Following the 2011 discovery of nematode worms in a South African gold mine, an intensive two-year survey turned up members of four invertebrate phyla—flatworms, rotifers, segmented worms, and arthropods—living 1.4 kilometers below the Earth’s surface.
With our existence proof of a deep, hot biosphere in Earth, is it possible that similar life could exist in the lithospheres of other rocky worlds in our solar system, including our Moon?
Mars is particularly attractive, as there is evidence Mars was both warmer and wetter in the past. There was geologic activity as clearly evident by the Tharsis bulge and the shield volcanoes like Olympus Mons. We know there is frozen water below the surface on Mars. What we are not certain of is whether Mars’ core is still molten and hot, and what the areothermal gradient is. One of the scientific goals of the Insight lander, currently on Mars, is to determine heat flow in Mars. This will help provide the data necessary to determine the range of the habitable zone in the lithosphere.
In contrast, we do have samples of Moon rock. An analysis of the Apollo 11 samples showed that organic material was present, but there was no sign of life except for terrestrial contamination [10, 12]. Since then, very little effort has been applied to look for life in the lunar rocks. The theory that the Moon is desiccated, hostile to life, and sterile, seems to have deterred further work. The early analyses indicated that methane (CH4) is present in the Apollo 11 samples. This may be primordial or delivered subsequently by impacts from asteroids or comets. If we ever discovered pockets of natural gas, even petroleum, on the Moon, this would be a staggering confirmation of Gold’s theory.
So where should we look?
Although the Moon is in our proverbial backyard, the expected depth of liquid water starts well below the bottom of the deepest craters.. This suggests that either deep boring would be necessary, or we must hope for impact ejecta to be recoverable from the needed depths. The prospects for either seem rather remote, although scientific and commercial activities on the Moon might make this possible in this century.
Despite its remoteness, Mars may be more attractive. Sampling at the bottom of crater walls and the sides of the Valles Marineris may give us relatively easy access to samples at the needed depths. Should the transient dark marks on the sides of crater walls prove to be liquid water, we would have samples within easy reach. The recent discovery of a possible subsurface water deposit just 1.5 km beneath the surface of Mars might be another possible target to reach.
The requirement that water is a necessary, but insufficient, condition for life has focused efforts on looking for life where liquid surface water exists. Because of the available techniques, exoplanet targets will be those that satisfy the HZ requirements. While these may prove the first confirmation of extraterrestrial life, they cannot answer some of the fundamental questions that we would like to know, for example, is abiogenesis common, or rare, and is panspermia the means to spread life. For that, we will need samples of such life. For the foreseeable future, that means sampling the solar system. We have 2 nearby worlds, and Gold suggested that there might be 10 suitable Moon-sized and above worlds that might have deep biospheres . That might be ample.
To date, our search for life beyond Earth has been little more than looking for fish in the waves lapping the shore. We need to search more comprehensively. I am arguing that this search needs to focus on the habitable regions of lithospheres of any suitable rocky world. We might start with signs of bacterial fossils in exposed rock strata and ejecta, and then core samples taken from boreholes to look for living organisms. Finding life, especially that from a different genesis would indicate that life is indeed ubiquitous in the universe.
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