* This is the sixth installment of “Problematica.” It is written by Max Dresow…
Crack a textbook in geology or paleontology and you are likely to encounter the expression, “the present is the key to the past.” You are less likely to encounter variations on this theme, like the claim that the past supplies a key to the past, or even that the past supplies a key to the future. But this is not for want of examples. To take a random one, in 1995, John Grotzinger and Andrew Knoll proposed that “the key” to deciphering the reef structures of the late Permian is found in the Precambrian. This is because environmental conditions in the late Permian favored the deposition of large volumes of precipitated microbialites and inorganic cements, much as they did prior to the rise of reef-building animals. To understand how microbialites and seafloor cements build reefs, the thing to do is to examine a period of time in which this phenomenon is especially well developed. This means the Precambrian. So when it comes to late Permian reefs, the past supplies the key to (another compartment of) the past.
There are also examples of earth scientists using the past to understand the future. The entire field of conservation paleobiology is geared to operate in this way. Likewise the project that studies the Paleocene-Eocene Thermal Maximum (PETM) to learn about our likely future under climate change. These projects are finally starting to garner philosophical attention, and several people have written about them on this very weblog. The present essay, however, concerns a different strategy. Here I am not concerned with efforts to use Earth’s past to understand either its past or its future. Instead, I am concerned with two attempts to use the present state of Mars to understand Earth’s future and past, respectively.
Mars, painted red by iron oxides (and a filter—the true color of Mars is a duller red-brown)
My account is split into two parts, which cover episodes separated by about a hundred years. In the first, I will consider an attempt to use Mars to understand Earth’s future, launched at a time when the surface features of Mars had only just become visible. Then, in the sequel, I will consider recent efforts to enlist Mars as an analog for the Earth before plate tectonics. The exercise is mostly exploratory: I am not driving at any big philosophical conclusion apart from the obvious one that “the key to the past [or future]” may sometimes reside on other planets. Still, I will offer a few remarks on the philosophical implications of these projects before I am finished.
“How the Earth will Follow Mars”
Regular readers of The San Francisco Call would have encountered this headline near the end of the Theodore Roosevelt administration:
WILL the NEW YEAR SOLVE the RIDDLE of MARS?
Positive Assertion by Professor Lowell, Based on his Latest Discoveries, Indicate that our Nearest Neighbor is Peopled by a Race Superior to Mankind
The year was 1907, and the “Professor” was Percival Lowell: a wealthy Boston eccentric who was not actually a college professor. In 1894, trailing a minor reputation as an orientalist, Lowell arrived in Flagstaff, Arizona. There, away from the city lights, at an altitude of 7,000 feet, he set up an observatory. The Lowell Observatory would make headlines around the world when a young astronomer named Clyde Tombaugh discovered the ninth planet in our solar system in 1930. (Lowell had commissioned the search for “Planet X” before his death in 1916. Today, the dwarf planet Pluto memorializes him in its planetary symbol, PL.) However, between 1894 and 1907, it was the fourth planet that commanded the bulk of Lowell’s attention.
Percival Lowell seated at his telescope at the Lowell Observatory
His interest had been stimulated by a book, La Planète Mars, written by the Frenchman Camille Flammarion. In this book, Flammarion argued that the surface of Mars is crisscrossed by linear structures, which the Italian astronomer Giovanni Schiaparelli* had earlier dubbed canali, or “channels.” Schiaparelli interpreted the canali as shallow depressions, perhaps two hundred kilometers across, which run for thousands of kilometers in a single direction. He suspected that they were formed by running water, and that they “[constitute] the primary mechanism… by which water (and with it organic life) may be diffused across the arid surface of the planet.” But he declined to interpret them as technological artifacts or “canals”—a point lost on his English translators. Flammarion, by contrast, took this imaginative leap. With a novelist’s vision, he proposed that the canali functioned as a life-support system for a dying planet. Probably they were “rectification[s] of old rivers aimed at the general distribution of water to the surface of the continents” (Flammarion 1892, 589). Anyway, they were strong evidence of a technologically advanced civilization capable of undertaking engineering projects on a truly planetary scale.
[* Schiaparelli was the first person to make detailed observations of the Martian surface. And in a delightful irony, he was colorblind. Concerning the complexion of the red planet, Schiaparelli observed that “[the] general appearance of the planet for me was almost that of a chiaroscuro made with Chinese ink upon a general bright background…”]
A version of Schiaparelli’s 1888 map of Mars, complete with maria (seas), terrae (continents), and canali (channels)
To say that Percival Lowell accepted this hypothesis is to undersell his enthusiasm for it. Although he feigned even-handedness at first, it is clear that he never doubted it for a moment, at least after his first public remarks on the matter in 1894. His major statement appeared in 1906, and provided the occasion for the write-up in The San Francisco Call. This was his book Mars and its Canals, which he followed two years later with a companion, Mars as the Abode of Life.
The case for intelligent life on Mars is set out as a string of inferences. Lowell begins with the notion that Mars is thickly vegetated, which is based on the observation that large parts of the Martian surface undergo seasonal changes in coloration from blue-green to ochre. From here, he infers that there must also exist life “of a higher and more immediately appealing kind” (i.e., animals), since the evolution of life on Earth has seen a parallel development of plants and animals with arrows of influence running in both directions (Lowell 1906, 348). He observes that Mars has undergone more secular cooling than Earth, and suggests that secular cooling is the main engine of evolutionary progress. This leads him to conclude that Martian life is highly advanced and that it probably includes beings exceeding Homo sapiens in their intelligence and sociability. Finally, he infers from the configuration of the canali that they are technological artifacts whose “end and aim” is the “husbanding” of water from the snowcaps to the more habitable (but also more parched) equatorial regions (373). “From the fact, therefore, that the reticulate canal system is an elaborate entity embracing the whole planet… we have not only proof of the world-wide sagacity of its builders, but a very suggestive side-light, to the fact that only a universal necessity such as water could well be its underlying cause” (378).
So far, Lowell’s argument is an attempt to use Earth to understand Mars, including those features of Mars that lack earthly analogs (e.g., canali). But then he turns things around. In Mars as the Abode of Life, Lowell writes that Mars “[occupies] earthwise… the post of prophet” (Lowell 1908, 111). By observing Mars, we are able “to foresee what eventually will overtake the earth in process of time; inasmuch as from a scrutiny of Mars coming events cast not their shadows, but their light, before” (Lowell 1908, 111). The present state of Mars prefigures our own planetary destiny.
But what future for Earth does the Red Planet prefigure? In a word, a dry one. As beautiful as Mars’s “opaline tints” appear through the telescope, “they represent a really terrible reality.”
That rose-ochre enchantment is but a mind mirage. A vast expanse of arid ground, world-wide in its extent, girding the planet completely in circumference, and stretching in places from pole to pole, is what those opaline glamours signify. Their bare rock gives them color… But this very color, unchanging in its hue, means the extinction of life. Pitilessly persistent, the opal here bears out its sinister intent. (Lowell 1908, 134)
This, you might think, is melodrama enough for a scientific treatise. But Lowell is only getting started. “To let one’s mind dwell on these Martian Saharas,” he writes, is to gain an insight into “the essence of Mars.” A barren land bereft of oceans and “level as a polished shield.” Rain that falls sporadically, but evaporates just as quickly. A water supply “tied up for the greater part of the year at one pole or the other.” Life hanging on through the efforts of its dominant species, whose network of canals directs seasonal meltwater to equatorial oases. In short, climate catastrophe—this is the essence of Mars, and its prophecy for its nearest celestial neighbor.
An image of Mars captured by NASA’s Perseverance rover, which would have done nothing to allay Lowell’s concerns
When will Earth go the way of Mars? Not soon, Lowell thinks. Still, signs of its inevitable transformation are manifest. For one thing, “[a]ttention shows that loss of water has been going on through the eons that have passed, and that the process is taking place under our very eyes to-day” (Lowell 1908, 118). Geological evidence confirms this picture: “Once laid down, the earth’s oceans have been slowly disappearing since.” Inland seas are likewise evaporating, so that eventually Earth will have no large water bodies to speak of. At the same time, desert-belts are widening. Lowell compares these belts to “life-extinguishing serpent’s coils”—a description, he says, which involves “but little personification” (124). As evidence he points to the histories of Egypt and Palestine: “The land which once flowed with milk and honey can hardly flow with bad water now” (129). Even the petrified forests near Flagstaff betoken a degraded climate. Where once the hills were clothed in greenery, there now exists only a ruin of trees scattered about like the bones of an ancient colossus.
“The earth, then, is going the way of Mars,” and the best we can do is cultivate those virtues that allow Martians to cooperate on the planet-wide projects needed to stave off pitiless extinction.
REDUX 2022
Needless to say there are no canali. The appearance of linear structures on Mars is an illusion, although in fairness to Lowell this was not immediately apparent, and one can find supporters of the canal theory well into the twentieth century (as well as advocates of Martian vegetation). The Mariner 4 spacecraft decisively settled the issue in 1965, when a successful flyby returned the first detailed pictures of the Martian surface. However, other planks of Lowell’s argument were shown to be rotten well before this. Alfred Russel Wallace penned a particularly incisive criticism in 1907, even while accepting “unreservedly” the accuracy of Lowell’s observations. As Wallace observed, Lowell’s interpretation ignores “the totally inadequate water-supply” for an irrigation system spanning the entire Martian surface (Wallace 1907, 103). It also fails to consider “the extreme irrationality of constructing so vast a canal-system the waste from which… would use up ten times the probable supply.” Such an undertaking “would be the work of a body of madmen rather than intelligent beings.” Wallace also criticized Lowell’s suggestion that average surface conditions on Mars probably resemble “the south of England.” According to Wallace, the average temperature was unlikely to exceed –35° F, much too cold to support any civilization resembling our own, or even, for that matter, running water. (Current estimates place Mars’s average surface temperature at about –80° F.)
All this is quite damning. And yet, like many good stories, this one may have a twist ending. Last year, researchers at the University of Arizona produced a series of models indicating that climate change may have done in life on Mars (Sauterey et al. 2022). One was a geophysical model of a rocky planet whose crust is soaked in water and incompletely encased in ice. They used this to constrain the climate and atmospheric composition of early Mars (ca. 4.1 to 3.7 billion years ago), as well as the thermal profile of the crust and likely crust-atmosphere gas exchanges. The model was then coupled to a model of a microbial ecosystem to assess the habitability of the subsurface to a population of hydrogen-munching, methane-producing microbes. What they found was that conditions on early Mars would have enabled such an ecosystem to thrive—but only for a while. As the microbes gradually altered the composition of the atmosphere, a major episode of global cooling would have taken place. (Yes, even with the increased methane concentration. The reason has to do with the depletion of atmospheric hydrogen, which is a powerful indirect greenhouse gas.) In a relatively short time, temperatures would have plummeted, driving the microbes deeper into the lithosphere. With their major food source (H2) depleted, the continued existence of the population would have hinged on its ability to locate an alternative source of energy. The result, almost certainly, would have been ecosystem collapse, rendering the planet barren of life.
There is an analogy here with anthropogenic climate change, albeit an imperfect one. Still, it would not be unlike a prophet to speak somewhat obscurely.
Mars as A Key to Early Earth
Lowell’s prophecies turned on the notion that Mars “models” or otherwise represents the future state of Earth. But that is not the only way to leverage Mars to better understand our planet. A false prophet, Mars might nonetheless have a promising career as a “time machine,” or as I would prefer to put it, a time capsule (Lapôtre et al. 2022). Let me explain.
The mills of plate tectonics grind slowly, but they grind exceedingly fine. What this means is that traces of Earth’s past are progressively lost as continents jostle and collide and ocean crust sinks back into the mantle at subduction zones. No rocks survive from the first 10% of Earth’s history, and when it comes to the Eoarchean Era (4.0–3.6 Bya) things are not much better. Only about a dozen exposures are known to contain rocks of Eoarchean age, and these have been extensively altered during their long sojourn on the planet. Indeed, as a general rule, the older the rocks you’re interested in, the less of them there are, and the more likely they are to be cooked and deformed.
A map of the oldest rocks and minerals on the planet (top) and a picture of some of these rocks belonging to the Nuvvuagittuq Greenstone Belt (credit for both images: Jonathan O’Neil). Note that the age of the Nuvvuagittuq Greenstone Belt is disputed, with one study placing it at 4.4 billion years and another at 3.8 billion years. If the earlier date is confirmed, it will be necessary to modify my statement that no rocks survive from the first 10% of Earth’s history. Also note that the Jack Hills zircons are only mineral grains—the host rock has apparently been destroyed
This creates an obvious problem for geologists interested in the structure and dynamics of early Earth. They want to answer questions like, what did Earth look like prior to the onset of plate tectonics? What mechanisms controlled the deformation of the crust? And were surface environments conducive to the origin and evolution of life? Yet in the absence of empirical constraints, these questions are difficult to answer. The relevant geological record simply does not exist. It is a casualty of the same restless dynamism that enabled our planet to evolve creatures desirous of peering so deep into the abyss of time.
Enter Mars. In stark contrast to Earth, over 80% of Martian surface rocks are thought to have formed during Earth’s Archean period (a figure that swells to 90% when we include the Proterozoic). These include some very ancient rocks indeed, which have existed for most of the history of the planet without much in the way of crunching or cooking. Released from the engine of plate tectonics, rocks can enjoy lengthy periods of tectonic repose, and so it has evidently gone with Mars. This means that the Red Planet preserves “a near pristine record of surface environments in a world without plate tectonics or complex life”—a world, in other words, that bears some striking similarities to early Earth (Lapôtre et al. 2022, 1).
Of course, because Mars is its own planet, Martian rocks will be of little help in answering questions like, when did plate tectonics begin on Earth, and what sequence of tectonic regimes preceded the present one? Yet assuming Mars’s geodynamic past really does resemble Earth’s, its panorama of ancient rocks may hold the key to resolving some truly thorny problems. An example concerns sediment fluxes. On Earth, life profoundly affects the operation of surface processes over a range of spatial and temporal scales. River function, for example, is closely bound up with vegetation, and presumably has been since the greening of the land in the Silurian Period. Now, because most interpretations of the pre-vegetation rock record are based on observations of the present world, our general picture of river function may be seriously biased. But how can we tell whether this is the case, and—more importantly—how can we arrive at a more accurate picture? Earth’s ancient fluvial record is relatively poor with a few noteworthy exceptions. However, Mars hosts many meandering river deposits dating to the planet’s wet (but plant-free) past, affording new windows on the morphodynamics of rivers in the absence of abundant vegetation. These suggest that rivers meander much more rapidly in the absence of vegetation, leading to extensive reworking of floodplain deposits. No less important, they “have direct implications for [attempts to constrain] the duration of sediment storage within fluvial floodplains[,]… the pace of geochemical cycling and Earth’s palaeoclimate” (Lapôtre et al. 2020, 174). It is beginning to seem that in some cases at least, the “key to [Earth’s] past” might be found on the Red Planet.
An inverted fluvial channel near the Martian equator showing “scroll-bars” typical of river meanders. Inverted channels form when the sediments that fill them become more resistant to erosion than the surrounding material. When the surrounding material erodes, the channel is left standing as a ridge
But that’s not all. Mars hosts a variety of tectonic features including faults, folds, valleys, and so-called “wrinkle ridges” (low sinuous landforms resembling crinkles in wet paper). Most striking, no doubt, is the Valles Marineris: a long system of canyons that stretches like a laceration across the belly of the planet. All of these were produced in the absence of plate movement, or at least that is the general opinion among earthly geologists. But this is exactly what makes them useful for understanding how the crust may have behaved on early Earth, as well as what the surface may have looked like before the wheels of plate tectonics started to turn.
Filling out this picture is important for a number of reasons. I have already mentioned that Earth’s surface processes are profoundly affected by the presence of complex life, but just as important is the influence of plate tectonics. Hydrological, sedimentary, and geochemical cycling are all significantly influenced by plate movements, making it difficult to know how these processes operated before the current regime came online. Here again, Mars and other planets may offer a clue. The most tantalizing prospect is that the knowledge gained will illuminate the conditions under which Earth’s earliest life forms emerged and diversified. On Mars and early Earth, meteor impacts seem to have “exerted a dominant control on topography, acting as sources (crater rims) and sinks (crater basins) for sediments and bio-essential elements” (Lapôtre et al. 2022, 8). Could these impacts have also generated the energy, chemical gradients, and habitats needed for life to emerge? The traditional view has been that impacts are more likely to have frustrated abiogenesis than facilitated it, but if “impact-generated hydrothermal springs and surficial lakes created environments on Mars that are analogous to those found around black and white smokers,” then perhaps this view will have to be revised. If that happens, then Mars—for all we know, a lifeless planet—will have contributed a major insight to the solution of biology’s greatest mystery.
ACTUALISM Ad Astra
Adrian Currie has argued that historical scientists are “methodological omnivores.” Just as omnivorous animals are able to use both animal and plant matter for sustenance, historical scientists are able to metabolize all kinds of evidence to support substantive claims. They do this by constructing “purpose-built epistemic tools tailored to generate evidence about highly specific targets” (Currie 2015, 187). Basically, historical scientists are comfortable borrowing or adapting a range of external resources to achieve their epistemic goals. In the same vein, Robert Chapman and Alison Wylie speak of archeology’s “expansive and sometimes wildly eclectic… opportunism,” requiring “the cultivation of a working knowledge of resources… developed in a rich array of external fields” (Chapman and Wylie 2016, 11).
I can think of no better illustration of this “expansive… opportunism” than the proposal that we might “[probe] space to understand Earth.” The sheer extravagance of the project is breathtaking, but it is no less opportunistic for this. Consider the motivation. The present Earth is a vast repository of resources for deciphering the records of the past. But it is not an all-sufficient resource. If we picture the history of Earth as a flip book, then we have direct access to only the last few pages. These are almost indecently helpful for understanding the rest, for the simple reason that we have access to them and can scrutinize them as carefully as we want to. Still, they are only a few pages, and there is no guarantee that what we can learn from them will enable us to reconstruct the remainder. The “book,” after all, contains many worlds: hot worlds, cool worlds, lifeless worlds, and worlds before plate tectonics began its world-altering work. Some of the most conspicuous features of these worlds lack analogs on the modern Earth. Others left no surviving records. Curiosity demands that they be reconstructed anyway. What is an unlucky geoscientist to do but look to the heavens?
Self-portrait of the Mars rover Curiosity
Happily, the heavens have a number of uses for those interested in the history of Earth. As Lapôtre et al. put it, the “rich diversity of planetary environments and composition” can function as analogs, experiments, and archives for better understanding our planet. Begin with archives. As noted, our planet lacks many rocks older than about 3.5 billion years. This leaves a gap of about a billion years in the geological record, during which time a lot of pretty momentous stuff happened. Scientists naturally want to learn about this stuff, but in the absence of detailed records the challenge of reconstructing it lacks important empirical constraints. That is, unless pieces of the ancient Earth survive off-planet or records of important processes can be found on other worlds.
Planets that do not contain actual chunks of Earth may nonetheless contain valuable archives, but only insofar as an analogy exists between the planetary environment/s represented in the archive and the associated environment/s on Earth. So in the case of Mars, the proposed analogy is between the surface environments of early Mars and those of early Earth. This has the look of a good comparison despite the paucity of ground-truthed geological data on Mars and the lack of information about conditions on early Earth. However, Percival Lowell’s example stands as a reminder that both analogies and disanalogies need to be taken into account when engaging in this kind of reasoning. Lowell’s prophesies for Earth were based on the notion that Earth and Mars are following the same climate trajectory, with Mars slightly ahead in its evolution. But this is not the case, so the whole activity was an exercise in self-deception.
Contemporary scientists are not as careless as Lowell in assessing analogies and disanalogies. To the contrary, they are possessed of sophisticated methods for evaluating them, ranging from spectroscopic observations to modeling and statistical studies to exploratory missions. (And then there are meteorites. Many chunks of Mars have made their way to Earth, toting with them a diagnostic cocktail of atmospheric gasses. When analyzed isotopically, these meteorites suggest that Mars’s mantle is relatively inactive, “possibly pointing to a stagnant- or sluggish-lid tectonic regime that may offer similarities to Earth’s Hadean and Archean tectonics” (Lapôtre et al. 2022, 4).) The hope is that a better understanding of similarities and differences between planets will enable scientists to develop “quantitative models of geological and atmospheric processes that pertain to our understanding of the Earth” (Lapôtre et al. 2020, 170). This is why Lapôtre et al. call comparative planetology “a grand, full-scale experiment” capable of illuminating various features of Earth’s dynamic history.
A graphic from Lapôtre et al. (2020) comparing Venus, Earth, the Moon, and Mars through time. R = radius, g = surface gravity, T = surface temperature, and p = atmospheric pressure at the surface
Let me conclude with a playful thought. The physiologist August Krogh (1874–1949) is best remembered for his eponymous principle, which states that "for such a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied." Allow me one small modification. Instead of a “most convenient” animal, let us say there is a “most informative” one under the circumstances. Now ask yourself: might a similar principle apply to comparative planetology? Perhaps Mars is the best “animal” for the study of geochemical cycling before plate tectonics—better even than Earth itself. But for studies of the initiation of plate tectonics, maybe Venus is a better option. And if we want to study something like the influence of magma oceans on the atmosphere, maybe the best animal lies outside our solar system. Ease of access will tend to favor Earth as the most informative study animal, but it is not inconceivable that other benefits might sometimes outweigh convenience. Scientists should not emulate the drunkard searching for his keys beneath the street lamp; and anyway, it is never perfectly dark outside the halo described by the lamp.
We have come a long way since Percival Lowell filled his notebooks with visions of Martian canals. Still, I find it gratifying that we can celebrate with him
the allurement we feel toward what is least like us. For the wider the separation from the familiar, the greater the parallax the new affords for cosmic comprehension… Some day, our own geology, meteorology, and the rest will stand indebted to the study of the planet Mars… and what that other world shall have taught us will redound to a better knowledge of our own, and of that cosmos of which the two form part. (Lowell 1906, 382–383)
References
Chapman, R. and Wylie, A. 2016. Evidential Reasoning in Archaeology. London: Bloomsbury.
Currie, A.M. 2015. Marsupial lions and methodological omnivory: function, success and reconstruction in paleobiology. Biology & Philosophy 30:187–209.
Flammarion, C. 1892. La planète Mars et ses conditions d’habitabilité [volume 1]. Paris: Gauthier-Villars et Fils.
Grotzinger, J.P. and Knoll, A.H. 1995. Anomalous carbonate precipitates: is the Precambrian the key to the Permian? PALAIOS 10:578–596.
Lapôtre, M.G.A., O’Rourke, J.G., et al. 2020. Probing space to understand Earth. Nature Reivews Earth and Environment 1:170–181. https://doi.org/10.1038/s43017-020-0029-y.
Lapôtre, M.G.A., Bishop, J.L., et al. 2022. Mars as a time machine to Precambrian Earth. Journal of the Geological Society 179. https://doi.org/10.1144/jgs2022-047.
Lowell, P. 1906. Mars and its Canals. New York: Macmillan.
Lowell, P. 1908. Mars as the Abode of Life. New York: Macmillan.
Sautery, B., Charnay, B., et al. 2022. Early Mars habitability and global cooling by H2-based methanogens. Nature Astronomy 6:1263–1271. https://doi.org/10.1038/s41550-022-01786-w.
Wallace, A.R. 1907. Is Mars Inhabited? New York: Macmillan.
For more on Mars and its Canals, see:
Sheehan, W. 1997. The Planet Mars: A History of Observation and Discovery. Tuscon: The University of Arizona Press. [This is a very readable overview of the history of Martian observation, interpretation, and exploration. For the canal business, see Chapters 5–9, and see especially Chapter 7 on Lowell.]
Gould, S.J. 1998. War of the worldviews. In Leonardo’s Mountain of Clams and the Diet of Worms, 339–354.
This essay from The Public Domain Review.
This article on Giovanni Schiaparelli.
And here is a 1994 documentary on the Lowell Observatory narrated by Captain Jean-Luc Picard himself.