Centauri Dreams

‘Oumuamua continues to inspire questions and provoke media attention, not only because of its unusual characteristics, but because of the discussion that has emerged on whether it may be a derelict (or active) technology. Harvard’s Avi Loeb examined the interstellar object in these terms in a paper with Shmuel Bialy, one we talked about at length in these pages (see ‘Oumuamua, Thin Films and Lightsails). The paper would quickly go viral.

Those who have been following his work on ‘Oumuamua will want to know about two articles in the popular press in which Loeb answers questions. From the Israeli newspaper Ha’aretz comes an interview conducted by Oded Carmeli, while at Der Spiegel Johann Grolle asks the questions. From the latter, a snippet, in which Grolle asks Loeb what the moment would be like if and when humanity discovers an extraterrestrial intelligence. Loeb’s answer raises intriguing questions:

I can’t tell you what this moment will look like. But it will be shocking. Because we are biased by our own experiences. We imagine other beings to be similar to us. But maybe they are radically different. For example, it is quite possible that we won’t encounter the life forms themselves, but rather only their artifacts. In any case, we ourselves are not designed for interstellar journeys. The only reason astronauts survive in space is that they are under the protection of the Earth’s magnetic field. Even when traveling to Mars, cosmic rays will become a major problem.

Image: Avi Loeb (center) at the Daniel K. Inoue Solar Telescope (DKIST) in June of 2017. Credit: Avi Loeb.

Intriguing, given our conversations here about artificial intelligence and the emergence of non-biological civilizations. After all, we are in the nearby galactic company of numerous stars far older than our own. Would robotic beings supplant their biological cousins, or would the scenario be more like biological beings using artilects as their way of achieving interstellar travel? Either way, Loeb’s guess is that our first evidence will be an encounter with technological debris. The interview goes on to cover the ‘Oumuamua story’s outline thus far.

Meanwhile, two new papers from Loeb have appeared, the first written with John C. Forbes. “Turning Up the Heat on ‘Oumuamua” looks at the interstellar object, whatever it is, from another angle. If we were to discover more objects like this, how could we best analyze them? In earlier work with Manasvi Lingam, Loeb examined the population of interstellar objects that could be trapped within the Solar System, slung by Jupiter into parabolic orbits around the Sun.

The number could be as high as 6,000, a figure based on the deduced abundance of interstellar objects given the fact that we observed ‘Oumuamua as early as we did with instrumentation of the sensitivity of the Pan-STARRS telescopes. The paper references work on the overall abundance of these objects performed in 2017 by Greg Laughlin (UC-Santa Cruz) and Konstantin Batygin (Caltech), as well as a 2018 paper from Aaron Do (University of Hawai’i).

Learning more could involve a flyby mission, says Loeb, but there may be a better way:

In our new paper with John Forbes we proposed instead studying the vapor produced when such objects pass close to the Sun and get evaporated by the intense solar heat. We calculated the likelihood of that happening, keeping in mind that `Oumuamua did not show any signs of a cometary tail or carbon-based gas since it did not pass close enough to the Sun.

We used the known orbit of `Oumumua and assume a population of similar interstellar objects on random orbits in the vicinity of the Sun. This provided us with a likelihood of passages close to the Sun.

These objects would be expected to show a high orbital inclination, and assuming a population of this size, they should be readily detectable by future telescopes, such as the forthcoming Daniel K. Inoue Solar Telescope (DKIST). Another marker of interstellar origin, according to the paper, would be anomalous oxygen isotope ratios. If we can find interstellar objects that pass close to the Sun, we should be able to learn something about their composition. Loeb and Forbes use Monte Carlo methods to determine that such objects collide with the Sun once every 30 years, while about two should pass within the orbit of Mercury each year.

Usefully, spectroscopic study of cometary tails is a well-practiced science. As the paper notes:

Generally these studies are able to classify comets into different groups depending on the inferred production rates of H₂O, C₂, CN, and NH₂ as well as dynamical properties, which likely reflect formation in different parts of the protoplanetary disk (Levison 1996)… The promise of using close encounters with the sun to learn about extrasolar small bodies is that the sun has the ability to disrupt even large cometary nuclei via its intense radiation, sublimating not just surface volatiles but even silicates and iron. In principle this exposes the interiors of these objects to remote spectroscopy, which could place strong constraints on the composition of these objects.

And indeed, two comets — 96P/Machholz 1 and Yanaka (1998r) — have been found to have depleted levels of CN and C₂ relative to water. Sun-grazing comets of interstellar origin, assuming we can identify them early through instrumentation like the LSST (Large Synoptic Survey Telescope) should be available for such examination, a way to probe their composition without the need for sending fast flyby missions, although the latter would obviously be useful.

In a second paper, just accepted at Research Notes of the American Astronomical Society. Loeb and Harvard colleague Amir Siraj note that ‘Oumuamua’s shape may be more extreme than we have thought. Noting that the axis ratio for the object has been pegged at between 6:1 and 10:1, the paper delves into the lightcurve, with a startling result, as Loeb explained in an email this morning:

The lightcurve of the interstellar object Oumuamua showed a net brightening by one magnitude between October and November 2017, after corrections for the changing distances to the Sun and Earth and solar phase angle, assuming isotropic uniform albedo and the canonical phase function slope value for cometary and D-class objects of -0.04 magnitude per degree. We used the change in the orientation of `Oumuamua between October and November 2017 to show that this brightening implies a more extreme shape for the object. We inferred a ratio between its brightest and dimmest phases of at least 50:1 for a cigar shape and 20:1 for a pancake-like geometry. The revised values can be avoided if the phase function slope is 3 times larger than the canonical value, implying in turn another unusual property of `Oumuamua.

Variations in albedo could be in play, although here we would be looking at sharp variations for a minor change in viewing angle of ~ 11°, which Loeb and Forbes consider a possibility, though one without precedent in previous studies of asteroids and comets.

The papers are Forbes and Loeb, “Turning Up the Heat on ‘Oumuamua,” submitted to The Astrophysical Journal Letters (preprint); and Siraj and Loeb, “‘Oumuamua’s Geometry Could be More Extreme than Previously Inferred,” accepted at Research Notes of the American Astronomical Society (full text).

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 [13], 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 [1]. 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

* Assumes the Moon surface temperature would be the same as the Earth without an atmosphere
** [7]
*** [8]

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. [11]. 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.
Source: [11]

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 [5]. 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.

References

1. Barnes, J. J., Tartèse, R., Anand, M., Mccubbin, F. M., Franchi, I. A., Starkey, N. A., & Russell, S. S. (2014). The origin of water in the primitive Moon as revealed by the lunar highlands samples. Earth and Planetary Science Letters, 390, 244-252. doi:10.1016/j.epsl.2014.01.015

2. Davies, P. C., Benner, S. A., Cleland, C. E., Lineweaver, C. H., Mckay, C. P., & Wolfe-Simon, F. (2009). Signatures of a Shadow Biosphere. Astrobiology, 9(2), 241-249. doi:10.1089/ast.2008.0251

3. Davies, P. C. (2011). ? The eerie silence: Renewing our search for alien intelligence. ? Boston: Mariner Books, Houghton Mifflin Harcourt.

4. Gold, T. (1992). The deep, hot biosphere. Proceedings of the National Academy of Sciences, 89(13), 6045-6049. doi:10.1073/pnas.89.13.6045

5. Gold, T. (2010). ? The deep hot biosphere: The myth of fossil fuels. New York, NY: Copernicus Books.

6. Hallis, L. J., Huss, G. R., Nagashima, K., Taylor, G. J., Halldórsson, S. A., Hilton, D. R., . . . Meech, K. J. (2015). Evidence for primordial water in Earth’s deep mantle. Science, 350(6262), 795-797. doi:10.1126/science.aac4834

7. Hoffman N.(2001) Modern geothermal gradients on Mars and implications for subsurface liquids. Conference on the Geophysical Detection of Subsurface Water on Mars (2001)

8. Kuskov O (2018) Geochemical Constraints on the Cold and Hot Models of the Moon’s Interior: 1–Bulk Composition. Solar System Research, 2018, Vol. 52, No. 6, pp. 467–479.

9. Mccubbin, F. M., Steele, A., Hauri, E. H., Nekvasil, H., Yamashita, S., & Hemley, R. J. (2010). Nominally hydrous magmatism on the Moon. Proceedings of the National Academy of Sciences, 107(25), 11223-11228. doi:10.1073/pnas.1006677107

10. Nagy, B., Drew, C. M., Hamilton, P. B., Modzeleski, V. E., Murphy, S. M., Scott, W. M., . . . Young, M. (1970). Organic Compounds in Lunar Samples: Pyrolysis Products, Hydrocarbons, Amino Acids. Science, 167(3918), 770-773. doi:10.1126/science.167.3918.770

11. Offord, C. (2018) Life Thrives Within the Earth’s Crust. The Scientist, October 1, 2018.

12. Oyama, V. I., Merek, E. L., & Silverman, M. P. (1970). A Search for Viable Organisms in a Lunar Sample. Science,167(3918), 773-775. doi:10.1126/science.167.3918.773

13. Tolley, A Detecting Early Life on Exoplanets. Centauri Dreams, February 2018

14. Way, M. J., Genio, A. D., Kiang, N. Y., Sohl, L. E., Grinspoon, D. H., Aleinov, I., . . . Clune, T. (2016). Was Venus the first habitable world of our solar system? Geophysical Research Letters, 43(16), 8376-8383. doi:10.1002/2016gl069790

15. Woo, M. The Hunt for Earth’s Deep Hidden Oceans. Quanta Magazine, July 11, 2018

Jill Tarter, an all but iconic figure in SETI, has just launched Technosearch, an Internet tool that includes all published SETI searches from 1960 to the present. A co-founder of the SETI Institute well known for her own research as well as her advocacy on behalf of the field, Tarter presents scientists with a way to track and update all SETI searches that have been conducted, allowing users to submit their own searches and keep the database current. The tool grows out of needs she identified in her own early research, as Tarter acknowledges:

“I started keeping this search archive when I was a graduate student. Some of the original papers were presented at conferences, or appear in obscure journals that are difficult for newcomers to the SETI field to access. I’m delighted that we now have a tool that can be used by the entire community and a methodology for keeping it current.”

Image: Screenshot of the Radio List on https://technosearch.seti.org/.

Among the materials included in Technosearch are:

• Title of the search paper
• Name(s) of observers
• Search date
• Objects observed
• Facility where the search was conducted
• Size and sensitivity of the telescope used
• Resolving power of the instrument
• Time spent observing each object
• A link to the original published research paper
• Comments that explain the search strategy
• Observer notes

Technosearch currently contains 102 radio searches and 38 optical searches. The tool was presented yesterday at the 2019 winter meeting of the American Astronomical Society in Seattle and will be maintained by the SETI Institute. The AAS meeting always produces interesting developments, including exoplanet investigations that I intend to discuss next week.

On Technosearch, a personal thought: No one who has not attempted a deep dive into the scholarship on SETI can know how frustrating it is to chase down lesser known investigations or details of major ones. The issue of ready availability extends to the broad field of interstellar flight research, as I learned when compiling materials for my Centauri Dreams book. The trail from conference presentation to published paper can be obscure, while materials relating to specific researchers can be scattered through library collections or spread over a range of journals, some of them with firewalls, or available only in expensive books..

I’ve long advocated for interstellar studies a return to what Robert Forward began with Eugene Mallove, a detailed bibliography whose last appearance was in the Journal of the British Interplanetary Society in 1980. Putting such a resource online opens it worldwide and strengthens a field whose online databases are in many cases incomplete and often do not include older materials. All fields of scholarship will be following this essential path even as we continue to wrestle with academic publishers over questions of access to complete texts.

Technosearch is a step forward for SETI that helps scientists work with consolidated information while building a useful archive of contemporary work going forward. Tarter developed the tool in collaboration with graduate students working with Jason Wright (Penn State), a well-known figure in Dysonian SETI, which culls astronomical data looking for the possible physical artifacts of advanced civilizations. Also in the mix is Research Experience for Undergraduates, a program supporting students in areas of research funded by the National Science Foundation.

Image: Jill Tarter and Andrew Garcia presenting the Technosearch Tool.

SETI Institute REU student Andrew Garcia worked with Tarter in the summer of 2018:

“I started helping Dr. Tarter with this project as a research opportunity during the summer. I’ve become convinced that Technosearch will become an important instrument for astronomers and amateurs interested in exploring the cosmos for indications of other technological civilizations. We can’t know where to look for evidence tomorrow if we don’t know where we have already looked. Technosearch will help us chronicle where and how we’ve looked at the sky. I would like to thank the NSF REU program and the CAMPARE program for their encouragement and support throughout this project.”