Centauri Dreams

Every time I mention a Brian Aldiss novel, I have to be careful to check the original title against the one published in the US. The terrific novel Non-Stop (1958) became Starship in the States, rather reducing the suspense of decoding its strange setting. Hothouse (1962) became The Long Afternoon of Earth when abridged in the US following serialization in The Magazine of Fantasy & Science Fiction. I much prefer the poetic US title with its air of brooding fin de siècle decline as Aldiss imagines our deep, deep future.

Imagine an Earth orbiting a Sun far hotter than it is today, a world where our planet is now tidally locked to that Sun, which Aldiss describes as “paralyzing half the heaven.” The planet is choked with vegetation so dense and rapidly evolving that humans are on the edge of extinction, living within a continent-spanning tree. The memory of reading all this always stays with me when I think about distant futures, which by most accounts involve an ever-hotter Sun and the eventual collapse of our biosphere.

Image: The dust jacket of the first edition of Brian Aldiss’ novel Hothouse.

Indeed, warming over the next billion years will inevitably affect the carbon-silicate cycle. Its regulation of atmospheric carbon dioxide is a process that takes CO2 all the way from rainfall through ocean sediments, their subduction into the mantle and the eventual return of CO2 to the atmosphere by means of volcanism. Scientists have thought that the warming Sun will cause CO2 to be drawn out of the atmosphere at rates sufficient to starve out land plants, spelling an end to habitability. That long afternoon of Earth, though, may be longer than we have hitherto assumed.

A new study now questions not only whether CO2 starvation is the greatest threat but also manages to extend the lifetime of a habitable Earth far beyond the generally cited one billion years. The scientists involved apply ‘global mean models,’ which help to analyze how vegetation affects the carbon cycle. Lead author Robert Graham (University of Chicago), working with colleagues at Israel’s Weizmann Institute of Science, is attempting to better understand the mechanisms of plant extinction. Their new constraints on silicate weathering push the conclusion that the terrestrial biosphere will eventually succumb to temperatures near runaway greenhouse conditions. The biosphere dies from simple overheating rather than CO2 starvation.

The implications are intriguing and offer fodder for a new generation of science fiction writers working far-future themes. For in the authors’ models, the lifespan of our biosphere may be almost twice as long as has been previously expected. Decreases in plant productivity act to slow and eventually (if only temporarily) reverse the future decrease in CO2 as the Sun continues to brighten.

Here’s the crux of the matter: Rocks undergo weathering as CO2 laden rainwater carrying carbonic acid reacts with silicate minerals, part of the complicated process of sequestering CO2 in the oceans. The authors’ models show that if this process of silicate weathering is only weakly dependent on temperature – so that even large temperature changes have comparatively little effect – or strongly CO2 dependent, then “…progressive decreases in plant productivity can slow, halt, and even temporarily reverse the expected future decrease in CO2 as insolation continues to increase.”

From the paper:

Although this compromises the ability of the silicate weathering feedback to slow the warming of the Earth induced by higher insolation, it can also delay or prevent CO2 starvation of land plants, allowing the continued existence of a complex land biosphere until the surface temperature becomes too hot. In this regime, contrary to previous results, expected future decreases in CO2 outgassing and increases in land area would result in longer lifespans for the biosphere by delaying the point when land plants overheat.

How much heat can plants take? The paper cites a grass called Dichanthelium lanuginosum that grows in geothermal settings (with the aid of a symbiotic relationship with a fungus) as holding the record for survival, at temperatures as high as 338 K. The authors take this as the upper temperature limit for plants, adding this:

Importantly, with a revised thermotolerance limit for vascular land plants of 338 K, these results imply that the biotic feedback on weathering may allow complex land life to persist up to the moist or runaway greenhouse transition on Earth (and potentially Earth-like exoplanets). (Italics mine)

The long afternoon of Earth indeed. The authors point out that the adaptation of land plants (Aldiss’ continent-spanning tree, for example) could push their extinction to even later dates, limited perhaps by the eventual loss of Earth’s oceans.

…an important implication of our work is that the factors controlling Earth’s transitions into exotic hot climate states could be a primary control on the lifespan of the complex biosphere, motivating further study of the moist and runaway greenhouse transitions with 3D models. Generalizing to exoplanets, this suggests that the inner edge of the “complex life habitable zone” may be coterminous with the inner edge of the classical circumstellar habitable zone, with relevance for where exoplanet astronomers might expect to find plant biosignatures like the “vegetation red edge” (Seager et al. 2005).

The paper is Graham, Halevy & Abbot, “Substantial extension of the lifetime of the terrestrial biosphere,” accepted at Planetary Science Journal (preprint).

I want to return to Mars this morning because an emerging idea on how to terraform it is in the news. The idea is to block infrared radiation from escaping into space by releasing engineered dust particles about half as long as the wavelength of this radiation, which is centered around wavelengths of 22 and 10 μm, into the atmosphere. Block those escape routes and the possibility of warming Mars in a far more efficient way than has previously been suggested emerges. The paper on this work even suggests a SETI implication (!), but more about that in a moment.

Grad student Samaneh Ansari (Northwestern University) is lead author of the paper, working with among others Ramses Ramirez (University of Central Florida), whose investigations into planetary habitability and the nature of the habitable zone have appeared frequently in these pages (see, for example, Revising the Classical ‘Habitable Zone’). The engineered ‘nanorods’ at the heart of the concept could raise the surface temperature enough to allow survivability of microbial life, which would at least be a beginning to the long process of making the Red Planet habitable.

As opposed to using artificial greenhouse gases, a method that would involve vast amounts of fluorine scarce on the Martian surface, the nanorod approach takes advantage of the properties of the planet’s dust, which is lofted to high altitudes as an aerosol. The authors calculate, using the Mars Weather Research and Forecasting global climate model, that releasing 9-μm-long conductive nanorods made of aluminum “not much smaller than commercially available glitter” would provide the needed infrared blocking that natural dust cannot, and once at high altitude settle more slowly to the surface.

What stands out in the authors’ modeling is that their method is over 5,000 times more efficient than other methods of terraforming, and relies on materials already available on Mars. Natural dust particles, you would think, should warm the planet if released in greater quantities, but the result of doing so is actually to cool the surface even more. Let me quote the paper on this counter-intuitive (to me at least) result:

Because of its small size (1.5-μm effective radius), Mars dust is lofted to high altitude (altitude of peak dust mass mixing ratio, 15 to 25 km), is always visible in the Mars sky, and is present up to >60 km altitude (14–15). Natural Mars dust aerosol lowers daytime surface temperature [e.g., (16)], but this is due to compositional and geometric specifics that can be modified in the case of engineered dust. For example, a nanorod about half as long as the wavelength of upwelling thermal infrared radiation should interact strongly with that radiation (17).

Edwin Kite (University of Chicago) is a co-author on the work:

“You’d still need millions of tons to warm the planet, but that’s five thousand times less than you would need with previous proposals to globally warm Mars. This significantly increases the feasibility of the project… This suggests that the barrier to warming Mars to allow liquid water is not as high as previously thought.”

Image: This is Figure 3 from the paper. Caption: The proposed nanoparticle warming method. Figure credit: Aaron M. Geller, Northwestern, Center for Interdisciplinary Exploration and Research in Astrophysics + IT-RCDS.

Strikingly, the effects begin to emerge quite quickly. Within months of the beginning of the process, atmospheric pressure rises by 20 percent as CO2 ice sublimes, creating a positive warming feedback. Note this from the paper:

On a warmed Mars, atmospheric pressure will further increase by a factor of 2 to 20 as adsorbed CO2 desorbs (35), and polar CO2 ice (36) is volatilized on a timescale that could be as long as centuries. This will further increase the area that is suitable for liquid water (6).

That said, we’re still not in range for creating a surface habitable by humans. We have to deal with barriers to oxygenic photosynthesis, including the makeup of the Martian sands, which are laden with potentially toxic levels of nitrates, and an atmosphere with little oxygen. Toxic perchlorates in the soil would require ‘bioremediation’ involving perchlorate-reducing bacteria, which yield molecular oxygen as a byproduct. We’re a long way from creating an atmosphere humans can breathe, but we’re in range of the intermediate goal of warming the surface, possibly enough to sustain food crops.

Addendum: I made a mistake above, soon caught by Alex Tolley. Let me insert his comment here to straighten out my mistake:

“… which are laden with potentially toxic levels of nitrates,”

I think you misinterpreted the sentence from the paper:

“…is not sufficient to make the planet’s surface habitable for oxygenic photosynthetic life: barriers remain (7). For example, Mars’ sands have ~300 ppmw nitrates (37), and Mars’ air contains very little O2, as did Earth’s air prior to the arrival of cyanobacteria. Remediating perchlorate-rich soil…”

300 ppm nitrates is very low and will not support much plant or bacterial life. [You want ~ 10,000 ppm ] That is why N and P are added to simulated Mars regolith when testing plant growth for farming or terraforming. IIRC, there have been suggestions of importing nitrogen from Titan to meet its needs on Mars.

Thanks for catching this, Alex!

Although nanoparticles could warm Mars… both the benefits and potential costs of this course of action are now uncertain. For example, in the unlikely event that Mars’ soil contains irremediable compounds toxic to all Earth-derived life (this can be tested with Mars Sample Return), then the benefit of warming Mars is nil. On the other hand, if a photosynthetic biosphere can be established on the surface of Mars, perhaps with the aid of synthetic biology, then that might increase the Solar System’s capacity for human flourishing. On the cost side, if Mars has extant life, then study of that life could have great benefits that warrant robust protections for its habitat. More immediately, further research into nanoparticle design and manufacture coupled with modeling of their interaction with the climate could reduce the expense of this method.

That’s a robust way forward, one the authors suggest could involve wind tunnel experiments at Mars pressure to analyze how both dust and nanomaterials are released from modeled Mars surfaces, from dusty flat terrain to the ice of the poles. Large eddy simulations (LES) are ways to model larger flows such as winds and weather patterns. Deploying these should be useful in learning how the proposed nanorods will disperse in the atmosphere, while local warming methods also demand consideration.

A question I had never thought to ask about terraforming was how long the effects can be expected to last, and indeed the authors point out how little is known about long-term sustainability. A 2018 paper on current loss rates in the Martian atmosphere suggests that it would take at least 300 million years to fully deplete the atmosphere. The big unknown here is the Martian ice, and what may lie beneath it:

…if the ground ice observed at meters to tens of meters depth is underlain by empty pore space, then excessive warming over centuries could allow water to drain away, requiring careful management of long-term warming. Subsurface exploration by electromagnetic methods could address this uncertainty regarding how much water remains on Mars deep underground.

Image: Will we ever get to this? The ‘nanorod’ approach oculd be the beginning. Credit: Daein Ballard, Wikimedia Commons CC BY-SA 3.0.

The SETI implication? Nanoparticle warming is efficient, so much so that we might expect other civilizations to use the technique. A potential technosignature emerges in the polarization of light, because a terrestrial world with a magnetic field will show the interaction of polarized light with the planet’s atmosphere, the latter conceivably laden with the nanoparticles at work in terraforming. Polarization will occur when light interacts with nanoparticles, aerosols, or dust in the atmosphere or the magnetic field. This would be an elusive signature to spot, but not outside the range of possibility.

In the absence of an active geodynamo to drive a magnetic field, Mars would not be a candidate for this kind of remote observation. But an exoplanet of terrestrial class with a magnetic field should, by these calculations, be a candidate for this kind of study.

The paper is Ansari et al., “Feasibility of keeping Mars warm with nanoparticles,” Science Advances Vol. 10, No. 32 (7 August 2024). Abstract / Preprint. Thanks to Centauri Dreams reader Ivan Vuletich for the pointer to this paper.

Mars was a lively destination in early science fiction because of its proximity. When H. G. Wells needed a danger from outer space, The War of the Worlds naturally looked toward Mars, as a place close to Earth and one with the ability to provoke curiosity. Closely studied at opposition in 1877, Mars provoked in Giovanni Schiaparelli the prospect of a network of canals, surely feeding a civilization that might still be alive. No wonder new technologies turned toward the Red Planet as they became available to move beyond visible light and even attempt to make contact with its inhabitants.

All this comes to mind this morning because of an intriguing story sent along by my friend Al Jackson, whose work on interstellar propulsion is well known in these pages, as is his deep involvement with the Apollo program. Al had never heard of the incident described in the story. It occurred in 1924, when at another Martian opposition (an orbital alignment bringing Earth and Mars as close as they’ll get during its 26-month orbit), the U. S. Navy imposed radio silence nationwide for five minutes once an hour from August 21 to 24. The plan: Allow observatories worldwide to listen for Martians.

Image: The cover of the Edgar Rice Burroughs novel that would have been on Mars enthusiasts’ shelves when the 1924 opposition occurred. Burroughs’ depiction of Mars was hugely popular in its day.

This was serious SETI for its day. A dirigible was launched from the U. S. Naval Observatory carrying radio equipment for these observations, with the capability of relaying its signals back to a laboratory on the ground. A military cryptographer was brought in to monitor the situation, as attested by a provocative New York Times headline from August 23 of that year: “Code Expert Ready for Message.; RADIO HEARS THINGS AS MARS NEARS US.”

All this was news to me too, and thus I was entranced by the new article, a Times essay from August 20 of this year, written by Becky Ferreira. Because something indeed happened and was reported in August 28 of 1924, again in the Times: “SEEKS SIGN FROM MARS IN 38-FOOT RADIO FILM; Dr. Todd Will Study Photograph of Mysterious Dots and Dashes Recently Recorded.”

As Ferreira explains::

A series of dots and dashes, captured by an airborne antenna, produced a photographic record of “a crudely drawn face,” according to news reports. The tantalizing results and subsequent media frenzy inflamed the public’s imagination. It seemed as if Mars was speaking, but what was it trying to say?

“The film shows a repetition, at intervals of about a half hour, of what appears to be a man’s face,” one of the experiment’s leaders said days later.

You may recall that when Frank Drake began Project Ozma at Green Bank in 1960, he homed in on nearby stars Tau Ceti and Epsilon Eridani. And relatively soon he got a strong signal, causing him to ponder whether detecting other civilizations might be easy if you just pointed your antenna and began to listen. But the signal turned out to be from an aircraft in the skies of West Virginia, an early SETI frustration, for radio frequency interference (RFI) is a source of constant concern, as witness the stir caused briefly in 2019 by what appeared to be a signal from Proxima Centauri, but was not.

I don’t think the 1960 RFI experience got much media play, if any, though Project Ozma itself received a certain degree of coverage. But the ‘face’ found in the Mars radio reception of 1924 would have caused newspaper readers in that year to recall Guglielmo Marconi’s 1920 claim that he had detected signals “sent by the inhabitants of other planets to the inhabitants of Earth.” This was an era bristling with the new exploration of radio wavelengths, which if they could offer communications across a continent or ocean, could surely make possible a signal from one planet to another.

The interest was international, as another Times headline makes clear, this one from August 23, 1924: “RADIO HEARS THINGS AS MARS NEARS US; A 24-Tube Set in England Picks Up Strong Signals Made in Harsh Dots. VANCOUVER ALSO FAVORED At Washington the Translator of McLean Telegrams Stands by to Decode Any Message.”

Back to the ‘face’ found in the research effort on the American side of the Atlantic, dug out of data relayed from the dirigible. It was an astronomer named David Peck Todd who went to work with inventor Charles Francis Jenkins, using a radio from the National Electrical Supply Company designed to support troops in combat. Jenkins would use it to pick up any signals from Mars as detected by the airship. He had for his part built a ‘radio camera’ that would convert the radio data into optical flashes that would be imprinted on photographic paper, and it was within the result that what seemed to be a face emerged. But it was one that not everyone saw.

Jenkins himself was unimpressed, as I learned from a story titled “Freakish Radio writings on Mars Machine” that ran in the Daily News on August 27. Let me quote the small piece in its entirety:

C. Francis Jenkins, Washington inventor, is investigating to ascertain cause of a series of freakish writings received on his special machine designed to record any possible radio signals from Mars.

The film record shows an arrangement of dots and dashes and pictures resembling a human face.

“I do not think the results have anything to do with Mars,” Jenkins said.

A little more digging in the newspaper archives revealed that Jenkins told Associated Press reporters, as recorded in the Buffalo Evening Times that same day (“Radio Signals Shown on Films, Puzzle Savants at Capital”) that he thought the results came from radio frequency interference, saying what appears to be a face is “a freak which we can’t explain.” The image was indeed part of a repeating pattern recorded on Jenkins’ machine, but people were reading into it what they wanted to see.

Image: What remains of the 1924 ‘face on Mars’ detection, as captured through photography of the original paper roll produced by Jenkins in his lab. Credit: Yale University Library.

So where is the 38-foot long roll of photographic paper that caused the ‘detection’ of a face from Mars? The original, according to Ferreira’s research, seems to have been lost, but Yale University Library lists three images from its collection of materials on David Peck Todd under the title “Martian signals recorded by Jenkins.” So we have at least three photographs of Jenkins’ work, but to me at least, no face seems apparent.

Also in Buffalo, the Buffalo American ran a much longer piece titled “Astronomers Scan Mars To Discover Human Life” for its August 28, 1924 issue, which looks at the whole issue of studying Mars, though without mention of Jenkins’ work. It includes this interesting paragraph:

…perhaps Mars does see what is happening on the earth. If you were on Mars and looked at the earth you would see a star twice as large as Mars appears to Buffalo, as the earth is double Mars’ size. In the far distance on the same side of the sky would be the sun but it would only be two-thirds as large as it appears here. On the other wise would be discerned a huge mass of vapor 1,300 times as large as the earth. That would be Jupiter, which has not solidified yet. You would also see a couple of moons. They light Mars at night and are responsible for the tides on its oceans.

The Buffalo American article takes us right into the Barsoom of Edgar Rice Burroughs’ imagination, which at that time had reached, in its 10-book series, The Chessmen of Mars (1922). If you’re a hard-core Burroughs fan, you may remember the chess game (known on Mars as Jetan) in which humans play the role of the chess pieces and fight to the death (Burroughs loved chess). Despite the Buffalo American’s mention of oceans, even in John Carter’s day Barsoom was depicted as a place where water resources were rare and tightly controlled.

And just why study Mars in the first place? The newspaper article explains:

They want to know if the earth is the only celestial globe on which the Creator put human beings and if the planets and stars beyond were designed merely for the people on Earth to admire.

The Mars of the day was an extraordinary place. In researching this piece, I came across this from an article on the 1924 opposition by Rowland Thomas that ran in the St. Louis Post-Dispatch:

For some time astronomers all over the world have been preparing to get close-ups of Mars with their telescopes. The observers at Lowell Observatory, Flagstaff, Ariz., where the late Percival Lowell carried on his lifelong study of the planet which confirmed his belief that intelligent life exists on it, reported that on the southern hemisphere of Mars, where the polar ice cap is now melting under the rays of what is there a spring-tide sun, vast areas of what may be continents, marshland, prairies and the beds of dired-up oceans are constantly changing in appearance.

Image: Mars as conceived by astronomer Percival Lowell (1855-1916) and discussed by him in three books: Mars (1895), Mars and Its Canals (1906), and Mars As the Abode of Life (1908). The canals are here shown filled, with the vegetation in vigorous growth. Painting by H. Seppings Wright (1850-1937).

We’re still fourteen years from Orson Welles’ “War of the Worlds” broadcast in October of 1938, which gave plenty of time for early SETI interest to grow along with magazine science fiction, which in the US began in the pages of Hugo Gernsback’s radio magazines starting with The Electrical Experimenter and moving on to Science and Invention, but would soon claim its own dedicated title in Amazing Stories, whose first issue appeared in April of 1926. A nod as well to a sprinkling of earlier SF stories in Street & Smith’s pulp The Thrill Book.

Ferreira’s article is terrific, and I’m glad to hear that she is working on a book on SETI. It took Mariner 4’s flyby in 1965 to finally demonstrate what the surface of Mars was really like, and by then the interstellar SETI effort was just beginning to get attention. I wonder how the Mars enthusiasts of 1924 would have reacted to the news that despite the SETI efforts of the ensuing 100 years, we still have no proof of intelligence or indeed life of any kind on another world?