The Small Mars Problem

Figure 1. All planets and dwarf planets orbiting within 6 astronomical units (AU) of our Sun, shown at their relative diameters.

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From afar, our Solar System looks regular and well-organized. At its center is a large, massive sphere of incandescent gases (the Sun) surrounded by eight smaller and less massive spheres of heavier elements (the planets) distributed in concentric orbits out to a distance of about 4.5 billion km/2.8 billion miles.

The orbital distribution of the eight planets also seems regular, at least at first glance. Smaller, rocky worlds are confined to the inner system, while larger, gaseous worlds dominate the outer system. Planet sizes follow a curve, rising from the inner to the middle planets and then declining again from the middle to the outer planets.

In more specific terms, mass and radius increase along with distance among the three planets closest to the Sun (Mercury through Earth). Both parameters peak at the orbit of the fifth planet, Jupiter, which is almost a dozen times the radius and more than 300 times the mass of Earth. Then, from Jupiter through Uranus, the seventh planet, both mass and radius decline substantially along with distance from the Sun.

But this orderly progression of planet sizes has two notable interruptions: Mars and Neptune. If the distribution of planets were truly regular, Mars would be larger and more massive than Earth, and Neptune would be smaller and less massive than Uranus. Instead, the Red Planet has only 53% of Earth’s radius (0.53 Rea) and 11% of its mass (0.11 Mea), while the Azure Planet, at 17.2 Mea and 3.9 Rea, has about 98% of the radius of Uranus but 119% of its mass.

How did that happen?

In the present post I’m going to ignore the oddity of Neptune and concentrate on the Martian half of the question. My rationale is that Mars occupies our system’s classical habitable zone, and therefore – along with Earth and Venus – plays a critical role in theories of the habitability of extrasolar planets. If mass had been more uniformly distributed in the inner Solar System, Mars would be more massive than it is. If its mass were in the range of 1 to 2 Mea, Mars would likely be able to sustain a magnetic field, plate tectonics, surface water, and long-term habitability. Therefore, if we want to understand the potential system architectures that might support life-bearing planets, we need to understand why Mars is so small.

Figures 1 and 2 highlight the Small Mars Problem and the Great Martian Gap, which is the name I just invented for the general depletion of mass between Earth and Jupiter. The planet Mars and the dwarf planet Ceres orbit within this gap at 1.52 AU and 2.77 AU, respectively. With a little more than 1% of the mass of our Moon, Ceres accounts for fully one-third of all mass in the Asteroid Belt, which is concentrated between 2.2 and 3.3 AU (the latter boundary provided by the 2:1 resonance with Jupiter's orbit; Jewitt et al. 2009). The entire region between the orbits of Earth and Jupiter contains less than 0.12 Mea, with little Mars accounting for 99% of the total. By contrast, the region extending inward from Earth’s orbit to the Sun contains 1.87 Mea, yet Earth, the most massive object, accounts for only 53% of the total.

Figure 2. The Great Martian Gap

Blue numbers along the bottom refer to astronomical units (AU), where the Earth/Sun separation = 1. Planets are shown at their relative sizes and relative distances from the Sun, with separate scales for radius and distance. As astronomers have long noted, mass is severely depleted between the orbit of Jupiter at 5.2 AU and the orbit of Earth at 1 AU (see Weidenschilling 1977).

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zigzag migration

Recent studies by Konstantin Batygin & Greg Laughlin (2015) and by Sean Raymond & colleagues (2016) have presented conflicting scenarios to explain the Small Mars Problem and the Great Martian Gap. Both involve zigzag migratory paths for Jupiter during the primordial phase of system evolution.

Batygin & Laughlin based their approach on earlier models by Kevin Walsh & colleagues (2011) and Pierens & Raymond (2011), in which Jupiter formed in the outer Solar System (somewhere beyond 3 AU) and then migrated first inward and then outward again. The popular name for this scenario (blogged here and here) is the Grand Tack. According to Batygin & Laughlin, these maneuvers not only swept most solid mass out of the region exterior to Earth’s present orbit, but also created an instability that emptied the region interior to 0.7 AU.

Raymond & colleagues took a very different approach based on the “inside out” model of planet formation presented by Chatterjee & Tan (2014). Contrary to his own earlier work, Raymond’s group proposed that Jupiter formed in the inner Solar System near the Sun, and then migrated first outward, then inward, and finally outward again, depleting the region inward of Venus and wreaking havoc beyond Earth.

sweeping secular resonances

While the Grand Tack has been more widely discussed and endorsed than the inside-out scenario, both explanations have been faulted. Now Benjamin Bromley & Scott Kenyon (2017) present an alternative approach in which “sweeping secular resonances” with Jupiter’s orbital motion, rather than any migratory scenario, become the mechanism for clearing the Great Martian Gap. Their model implies a less dramatic but equally consequential role for Jupiter, and I suspect that it can be extended to explain similar gaps observed in the architecture of multiplanet systems around other stars.

To develop their model, Bromley & Kenyon (hereafter BK17) conducted extensive numerical simulations based on earlier work by and with their collaborators Makiko Nagasawa and Edward Thommes (Nagasawa et al. 2007, Thommes et al. 2008). They also note recent work on the same problem by Xiaochen Zheng & colleagues (2017).

BK17 begin with the familiar theoretical construct of the Minimum Mass Solar Nebula (blogged here). They assume that a dusty gas nebula (generally known as a protoplanetary disk) is present at the outset of their simulations. Jupiter is fully formed at its current semimajor axis of 5.2 AU (Figure 2), having cleared a gap in the disk for 1 AU on either side of its orbital path. A swarm of planetesimals orbits inward of this gap, while Saturn orbits well beyond it. Both gas giants exert gravitational effects on their surroundings, and the disk itself has gravity. In addition, the orbit of Jupiter is slightly eccentric, but probably less so than its present value of 0.05. BK17 assume an eccentricity of 0.03 in their simulations.

The key factor in their approach is the n5 resonance (“nu-5,” Greek letter nu with superscript 5), a “secular” or “very long-term” resonance between the motion of the protoplanetary disk and Jupiter’s orbital period. BK17 define the nu-5 resonance as the location “where the local apsidal precession rate matches Jupiter’s rate of precession,” and note that a planetesimal or protoplanet at this location will be perturbed by Jupiter’s gravitational influence onto a highly eccentric orbit. The likely result will then be either collision with another planet or protoplanet, engulfment by the Sun, or ejection from the Solar System.

In the early Solar System, when the gas disk was still present, the nu-5 resonance was located in the vicinity of the present Asteroid Belt (Zheng et al. 2012). As the gas dissipated, the resonance moved inward, destabilizing (“shaking up”) the orbits of protoplanets and planetesimals and effectively clearing out a substantial mass in solids. After the gas was completely depleted, the nu-5 resonance reached its present position inside the orbit of Venus. This sweeping shake-up created the Great Martian Gap while leaving behind enough mass to build Earth and Venus, as well as their two by-blows, Mercury and Mars.

BK17 discovered that several different factors were critical to reproducing the mass of Mars and the present-day Asteroid Belt within the time constraints provided by the known age of Mars. These include the mass of the perturbing planet, its distance from the system habitable zone, and the timing and speed of the sweeping secular resonance generated by its orbital motion.

Regarding mass, BK17 find that only a “Jupiter-mass planet” can produce the magnitude of perturbation required to induce a shake-up in the protoplanetary disk of a Sun-like star. Unfortunately, they don’t provide a precise value for the necessary mass – for example, would an object of Saturn’s mass (95 Mea) be sufficient? They also note that a “super-Earth” would be massive enough to produce sweeping secular resonances in an M dwarf system, likely referring to an object in the range of 1-10 Mea (see, e.g., Kenyon & Bromley 2009).

Regarding orbital location, they find that the masses of Earth and Mars depend sensitively on the semimajor axis of Jupiter at the time of the sweeping resonance. If Jupiter had been substantially farther from the Sun, the resonance would never have reached the orbit of Mars, and Mars would have grown much bigger than it actually did – presumably massive enough to support a habitable environment. But if Jupiter had been substantially closer to the Sun, and thus closer to the system habitable zone, the resonance would have inhibited the formation of Earth in the same way that it stunted the growth of Mars when Jupiter was at 5.2 AU. Instead of one living planet, our system would have none at all.

Regarding the timing of the sweeping secular resonance, BK17 note that its schedule is determined by the lifetime of the protoplanetary disk. As we saw in an earlier post, the system age when gas dissipation commences can fall anywhere between 1 and 10 million years. At the early end of that range, according to BK17, dissipation accompanied by shake-up would have extremely negative consequences for rocky planet formation, as it would destroy planetesimals before they had time to accrete into protoplanets. At the latter end, however, the effects would be modest, since accretion would already be well advanced, potentially permitting the growth of Earth-size planets out to a distance of 3 AU. In the case of our Solar System, we can assume that the shake-up happened before a system age of about 4 million years, given radiometric evidence that Mars was fully formed by then.

The rate of disk dispersal also matters. Although many studies have found that gas dissipation happens rapidly, requiring less than half a million years from start to finish (Williams & Cieza 2011), variation is inevitable: some disks take longer than others to disperse. BK17 find that the relative speed of dissipation strongly affects the outcomes of secular resonance sweeping. If the gas dissipates quickly, the resonance sweeps inward at the same rate, resulting in minimal disruption of the planetesimal population. If the gas dissipates more slowly, the resonance becomes increasingly more destructive, clearing larger and larger quantities of solid mass from the system.

Figure 3. Calaveras street sweepers on the Day of the Dead

From a print by José Guadalupe Posada (1852-1913)

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extrasolar asteroids and orbital gaps

Although BK17 are interested primarily in the evolution of our Solar System, they attempt to generalize some of their results to extrasolar locales. Their chief concern is the occurrence of extrasolar analogs of the Asteroid Belt. They argue that most systems with a gas giant in Jupiter’s approximate location (i.e., just outside the system ice line, where accretion is maximized) will experience a sweeping secular resonance whose outcome will be a ring of rocky debris in the inner system. While they concede that few such structures have been discovered to date (HD 69830 is most familiar), they attribute these limited findings to the difficulty of discerning modest aggregations of warm debris even around nearby stars. In the future, they predict, more sensitive searches will be more successful.

I suggest that the implications of their model are much broader than their relevance to extrasolar asteroid belts, and far more dispiriting. If sweeping secular resonances are common in systems with cool gas giants, then the outlook for habitable planets is even less promising than I thought. Here’s why.

An important focus of this blog is the possibility of Solar System analogs – that is, exoplanetary systems containing cool giants whose orbital parameters would permit the survival of Earth-mass planets (0.5-2 Mea) in the local habitable zone. (For recent posts on this topic, see here and here.) My January search of the Extrasolar Planets Encyclopaedia identified 17 such systems located within 60 parsecs/196 light years. All center on Sun-like stars in the range of 0.85-1.15 Solar masses, so their habitable zones have boundaries similar to those proposed for our own system (0.99-1.70 AU; Kopparapu et al. 2013).

Among the Jupiter analogs in these systems, semimajor axes range from 3 AU to 5.2 AU, and more than half orbit inside 4 AU. According to the findings of BK17, virtually all these systems will have experienced a sweeping secular resonance very similar to the one they propose for the Solar System. Because all but one of the 17 confirmed Jupiter analogs orbits closer to the local habitable zone than does our own Jupiter, the depletion of mass in this favored region is likely to be even more extreme than it was at home. Therefore, habitable planets appear to be less likely in the existing sample of Solar System analogs than they are in the Solar System.

To put it another way: BK17 have just shown that gas giant planets are even more unfriendly to the formation and survival of habitable planets than we already suspected. It’s not enough for the giant to reside outside the system ice line in a configuration that permits an Earth-like planet to maintain an Earth-like orbit. The giant must also be distant enough from the central star that the sweeping secular resonance generated by its orbit was insufficient to clear solid mass from the local habitable zone. Even Jupiter managed to evacuate mass from more than half of the radial extent of our own habitable zone, drastically reducing our system’s potential to produce life-bearing planets. Now it looks like extrasolar Jupiters might be still more likely to foreclose the possibility of life around other stars.

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REFERENCES

Batygin K, Laughlin G. (2015) Jupiter’s decisive role in the inner Solar System’s early evolution. Proceedings of the National Academy of Sciences 112, 4214-4217. Abstract: 2015PNAS..112.4214B

Bromley BC, Kenyon SJ. (2017) Terrestrial planet formation: Dynamical shake-up and the low mass of Mars. Astronomical Journal 153, 216. Abstract: 2017AJ....153..216B

Chatterjee S, Tan JC. (2014) Inside-out planet formation. Astrophysical Journal 780, 53.

Jewitt D, Moro-Martín A, Lacerda P. (2009) The Kuiper Belt and Other Debris Disks. In Astrophysics in the Next Decade, edited by Harley A. Thronson, Massimo Stiavelli, Alexander Tielens. Springer. Abstract: 2009ASSP...10...53J

Kopparapu R, Ramirez RM, Kasting JF, Eymet V, Robinson TD, Mahadevan S, Terrien RC, Domagal-Goldman S, Meadows V, Deshpande R. (2013) Habitable zones around main-sequence stars: New estimates. Astrophysical Journal 65, 131.

Kenyon SJ, Bromley BC. (2009) Rapid formation of icy super-Earths and the cores of gas giant planets. Astrophysical Journal 690, L140-L143.

Nagasawa M, Thommes EW, Kenyon SJ, Bromley BC, Lin DNC. (2007) The diverse origins of terrestrial-planet systems. In Protostars and Planets V, edited by B. Reipurth, D. Jewitt, K. Keil. University of Arizona Press, pages 639-654. Abstract: 2007prpl.conf..639N

Pierens A, Raymond SN. (2011) Two phase, inward-then-outward migration of Jupiter and Saturn in the gaseous solar nebula. Astronomy & Astrophysics 533, A131. Abstract: 2011A&A...533A.131P

Raymond SN, Izidoro A, Bitsch B, Jacobson SA. (2016) Did Jupiter’s core form in the innermost parts of the Sun’s protoplanetary disk? Monthly Notices of the Royal Astronomical Society 458, 2962-2972. Abstract: 2016MNRAS.458.2962R

Thommes E, Nagasawa M, Lin DNC. (2008) Dynamical shake-up of planetary systems. II. N-body simulations of Solar System terrestrial planet formation induced by secular resonance sweeping. Astrophysical Journal 676, 728-739. Abstract: 2008ApJ...676..728T

Weidenschilling JS. (1977) The distribution of mass in the planetary system and solar nebula. Astrophysics and Space Science 51, 153-158.

Williams JP, Cieza LC. (2011) Protoplanetary disks and their evolution. Annual Review of Astronomy and Astrophysics 49, 67-117. Abstract: 2011ARA&A..49...67W

Zheng X, Lin DNC, Kouwenhoven MBN. (2017) Planetesimal clearing and size-dependent asteroid retention by secular resonance sweeping during the depletion of the Solar Nebula. Astrophysical Journal 836, 207.

Mighty Throxeus

Imaginary view of ancient Mars, ca. 4.4 billion years ago, with clouds and an ocean.
Composite of NASA and NOAA images.

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Just catching up with the big news about that ocean on Mars! As many of you probably know already, successive robotic missions have returned plenty of evidence that water once flowed on the Red Planet. The evidence includes dry riverbeds, deltas, and gullies photographed by orbiters; sedimentary rocks photographed by surface rovers; and hydrated minerals detected through direct analysis by rovers. In fact, it’s widely believed that Gale Crater, the landing site of the Curiosity rover, was once a freshwater lake.

Nevertheless, most of these surface features could be explained by intermittent episodes when surface water flowed briefly and then sublimated or froze. Such activity might be related to asteroid impacts or volcanic eruptions. Given this possibility, the evidence so far available has not offered unambiguous proof that Mars ever supported open bodies of water for biologically significant stretches of time.

Channels of ancient riverbeds on Mars. (Source unknown)

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Now an international team led by Geronimo Villanueva argues that ancient Mars had an ocean that persisted for hundreds of millions of years. They base their findings on an analysis of the relative abundance of water isotopes in the Martian atmosphere, as measured through spectrographic observations by telescopes on Earth. Compared to water down here, water on Mars is enriched in deuterium. Villanueva and colleagues interpret this enrichment as evidence that most of the original reservoir of water on Mars has dissipated into space. They propose that, not long after Mars formed out of the Solar nebula, it supported an ocean covering about 20% of the surface, and as deep as the Mediterranean Sea. They estimate that the ocean was near its full extent about 4.5 billion years ago, but had lost a substantial amount of water by 4.1 billion years ago. On the basis of the current topography of Mars, they hypothesize that water covered much of the planet’s northern hemisphere, concentrated on a region called the Vastitas Borealis.

Layered buttes on the slopes of Mount Sharp on Mars. Credit: NASA/JPL-Caltech

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The study appeared in Science magazine and was accompanied by lots of media activity. A prime example is this nifty NASA video on YouTube, in which Villanueva and his colleague Michael Mumma speak optimistically about the habitability of ancient Mars. Below are examples of the many related image captures that flooded the Internet:

Two views of ancient Mars in the heyday of the northern ocean, based on the findings of
Villanueva et al. 2015. Credit: NASA/JPL-Caltech

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Two prerequisites for surface water on any planet are appropriate temperatures to maintain the liquid state and sufficient atmospheric pressure to prevent sublimation. Currently, the atmospheric pressure on Mars is less than 1% of Earth’s, so liquid water is impossible even at optimal temperatures. How ancient Mars might have harbored an atmosphere thick enough to produce the pressure necessary to sustain an ocean is an open question.

Another key question is the timing of events either known or hypothesized for the early history of Mars. Several lines of evidence indicate that all four terrestrial planets formed in a ring of planetesimals located between 0.7 and 1.0 astronomical units (AU) from the Sun (Hansen 2009). Exactly how this “birth ring” acquired its unique geometry is uncertain, but one hypothesis is that the formation and migration of Jupiter and Saturn during the first few million years of their existence depleted the inner regions of the Solar nebula and sculpted this narrow annulus (Brasser 2013). After the dissipation of the nebular gas – a process that occurred around 4 to 6 million years after the Sun’s ignition – protoplanets forming in the birth ring engaged in violent interactions that led to orbital perturbations and collisions. The small mass and eccentric orbit of Mars indicate that the planet was ejected from the ring relatively soon after the nebular gas dispersed, preventing further accretion of planetesimals. Accordingly, Ramon Brasser describes Mars as “essentially a planetary embryo that never grew to a full-fledged planet.” The planet acquired most of its mass during the first 4 million years of Solar System history, while the nebular gas was still present, and reached its final mass at a system age of about 10 million years. Thus Mars is older than the Earth, which evidently did not achieve its final mass until the Moon-forming event at a system age of about 30 to 50 million years.

By studying the effect of Jupiter and Saturn’s orbital excursions on the Solar birth ring, Brasser concludes that abundant water was available in the ring’s constituent planetesimals. Many of these would have been scattered inward from regions of the Solar nebula where ices were abundant. Given its wider orbit, Mars likely formed with an even larger bulk fraction of water than Earth, about 0.1% to 0.2% of its overall mass. Since Mars is only 11% as massive as Earth, it would have cooled much faster, enabling the condensation of a global ocean as soon as atmospheric conditions permitted. Nevertheless, Brasser notes that Mars’ gravity was too weak to retain all that water over billion-year time scales, although he does not address the process of mass loss.

As Villanueva and colleagues argue, the global Martian ocean was largely confined to the northern hemisphere, where the crust is 30 kilometers thinner than in the south and the average elevation is much lower (Marinova et al. 2008). This long-recognized north/south asymmetry is typically known as the Martian Hemispheric Dichotomy (or some variation thereof). Steep scarps and concentric massifs mark the elliptical boundary between the southern highlands and the northern basin, which is centered on a point 67 degrees N, 208 degrees E (Andrews-Hanna et al. 2008).

It is now widely accepted that this dichotomy was created by a “mega-impact” early in Martian history. Marinova and colleagues (2008) propose that the impactor struck at an angle of about 45 degrees and had a diameter of 1600-2700 km; it was notably smaller than our Moon (diameter 3474 km/2154 mi) and intermediate in size between Titania (1578 km), a moon of Uranus, and Triton (2706 km), Neptune’s largest moon. As for the timing of this catastrophe, Andrews-Hanna and colleagues describe the dichotomy as “one of the most ancient features on the planet.” Nimmo and colleagues (2008) infer that the collision occurred within the first 100 million years of Mars’ existence, “within the same time window as the Moon-forming impact on Earth.” They further argue that the southern highlands of Mars solidified out of the planet’s primordial magma ocean, while the northern lowlands arose from shock melting in the mantle. If the Martian mantle was rich in water, as Brasser concludes, it could have been the source of the northern sea.

In any event, it’s worth speculating how an ocean on Mars would have fared during the Noachian aeon (4.5-3.5 billion years ago), the Martian period that coincided roughly with the Hadean (4.5-4.0 billion years ago) and Archean (4.0-2.5 billion years ago) aeons on Earth, as well as the system-wide cataclysm known as the Late Heavy Bombardment (4 billion years ago). Two different pictures are available for this period on Earth.

In the happier picture, a global ocean formed within a few hundred million years of the Earth’s cooling and persisted ever afterward. Although asteroids frequently struck during the Hadean, the impacts were insufficient to resurface the Earth, and even the Late Heavy Bombardment was never intense enough to boil away the ocean. Thus life could have formed early and survived the epoch of asteroid strikes. In the harsher picture, however, a global ocean formed quickly, but was constantly subjected to cataclysms from the sky. Between 4.5 and 4.1 billion years ago – the era when the northern sea billowed on Mars, according to Villanueva and colleagues – Earth was probably struck by one to four objects with diameters of 1000 km (620 miles) or more, and by three to seven with diameters of 500 km (310 miles) or more (Marchi et al. 2014). Each of the larger objects would have resurfaced the whole planet and sterilized the biosphere; each of the smaller ones would have evaporated all surface water. It’s difficult to reconcile the long-lasting Martian ocean proposed by Villanueva’s group with the cruel bombardment envisioned by Marchi’s group, since the impactors originated in the asteroid belt and would have hit Mars just as hard as Earth.

As I look forward to more research that might solve these mysteries, I want to suggest a name for the hypothetical Martian ocean, regardless of its ultimate relation to reality. Although Latin could provide Mare Boreale, the North Sea, or Oceanus Borealis, the Northern Ocean, I prefer to use a bona fide Martian source: the Barsoomian language, spoken by the imaginary inhabitants of Mars in a series of books by Edgar Rice Burroughs (1875-1950). Burroughs’ Martians preserved historical records of the existence of five seas on Barsoom, back in the days when a pale race known as the Orovar circumnavigated the planet and built ornate treasure cities. Of all these marinered seas, everyone agreed that the mightiest was Throxeus. What better name for the vast body of water proposed by Villanueva’s group?

Planisphere of ancient Barsoom, the imaginary version of Mars created by Edgar Rice Burroughs in the eleven novels of his John Carter series (originally published 1912-1941). The largest body of water is Throxeus, mighty ocean of the north. Credit: Oberon Zell and Ralph Aeschliman.

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References to Throxeus appear many times in the Martian series. Below is a typical example from The Chessmen of Mars (1922). Believe it or not, these words were spoken by a diamond-studded prince from the wealthy land of Gathol while attempting to flirt with a haughty, raven-haired princess of Helium:

“Your ancient history has doubtless told you that Gathol was built upon an island in Throxeus, mightiest of the five oceans of old Barsoom. As the ocean receded Gathol crept down the sides of the mountain, the summit of which was the island upon which she had been built, until today the bowels of the great hill are honeycombed with the galleries of her mines.”

All that remained of Throxeus in Burroughs’ universe were dead sea bottoms overgrown with ochre moss. But in the map shown above, which offers a glimpse of ancient Barsoom, Gathol is visible as an offshore island amid a still-thriving Throxeus. Now that we’ve found traces of that lost ocean, maybe it’s time to send a robot and see if the diamond mines are there, too.

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REFERENCES

Andrews-Hanna JC, Zuber MT, Banerdt WB. (2008) The Borealis basin and the origin of the martian crustal dichotomy. Nature Letters 453, 1212-1215.

Brasser R. (2013) The Formation of Mars: Building Blocks and Accretion Time Scale. Space Science Reviews 174, 11-25.

Burroughs ER. (1922) The Chessmen of Mars. New York: A.C. McClurg.

Hansen B. (2009) Formation of the terrestrial planets from a narrow annulus. Astrophysical Journal 703, 1131-1140. Abstract: http://adsabs.harvard.edu/abs/2009ApJ...703.1131H

Marchi S, Bottke WF, Elkins-Tanton LT, Bierhaus M, Wuennemann K, Morbidelli A, Kring DA. (2014) Widespread mixing and burial of Earth’s Hadean crust by asteroid impacts. Nature 511, 578-582. Abstract:  http://adsabs.harvard.edu/abs/2014Natur.511..578M

Marinova MM, Aharonson O, Asphaug E. (2008) Mega-impact formation of the Mars hemispheric dichotomy. Nature Letters 453, 1216-1219.

Nimmo F, Hart SD, Korycansky DG, Agnor CB. (2008) Implications of an impact origin for the martian hemispheric dichotomy. Nature Letters 453, 1220-1223.

Villanueva GL, Mumma MJ, Novak RE, Käufl HU, Hartogh P, Encrenaz T, Tokunaga A, Khayat A, M. D. Smith MD. (2015) Strong water isotopic anomalies in the martian atmosphere: Probing current and ancient reservoirs. Science Express 5 March 2015 / 10.1126/science.aaa3630 

Return to Mars

Santa Maria Crater, Mars, by the Opportunity Rover. Image credit: NASA

Sometime after my eighth birthday, a friend and I discovered a shelf of amazing old books in the back of our small-town library. They were fat, well-worn hardcovers, most bound in red but some in green, with old-fashioned lettering and eye-popping illustrations: scantily clad women, men fighting monsters, distant views of crumbling cities, even a dinosaur or two.

Some of the books were about Tarzan, whom we already knew well. Others were about journeys to the center of Earth, which we’d certainly heard of. Still others were about Mars – but not any version we’d ever dreamed of. Instead of little green men with bald heads zipping around in flying saucers, we saw beautiful ladies dressed like harem girls with feathers in their hair – sinewy bare-chested men waving long swords at freakish adversaries – and stone palaces with tapestries hanging from intricately carven walls.

All those amazing books were written by one man – Edgar Rice Burroughs – and most of them were illustrated by another – J. Allen St. John. The combination of the two was irresistible. After careful consideration of the pictures, I took a volume titled Thuvia, Maid of Mars to the librarian’s desk. (As it happened, the very first book of the Martian series, A Princess of Mars, was missing from the collection.) I believe my friend chose one of the stories about the Earth’s Core.

To our surprise, the librarian wasn’t pleased with our interest in Burroughs. She told us that these books were no longer recommended for children, which is why they had been stashed away in back. Nevertheless, considering that my friend and I were regular patrons who had long since graduated from Dr. Seuss, she let us check out our chosen volumes. Over the next several months we worked our way through a great many of those red and green books.

Illustration for Thuvia, Maid of Mars by J. Allen St. John

That was 50 years ago. The library has since burned down, and the books went with it. I haven’t seen that friend since high school. But my love for Burroughs’ Martian tales has never abated. So last night, when Disney’s adaptation of A Princess of Mars premiered at the local IMAX under its focus-group-approved title of John Carter, you can be sure I was sitting in one of the good seats.

Overall I enjoyed the movie. The Disney team wisely decided against updating the source material and kept the action embedded in its late 19th century context. They did an excellent job of capturing the atmosphere of Frank Schoonover’s original illustrations (Princess was one of the few stories that St. John didn’t do). As a result, the visual impact of the film is very strong. The six-limbed green Martians have been brought to life as only CGI can do it, and the vast, swirling battle scenes look like they leaped from the pages of the book, with John Carter hurtling over his enemies like Douglas Fairbanks on wires. I especially liked the glimpses of Helium and Zodanga, the battling cities of the red Martians; these are thoroughly exotic and otherworldly places, with an ancient/future look that is central to Burroughs’ vision. At times I even had the impression that director Andrew Stanton took some cues from Aelita, Queen of Mars, a Soviet science fiction film of 1924 that, while owing a large debt to Burroughs, also showcased the latest in Futurist design with its costumes and sets.

To my surprise, I was perfectly content with Taylor Kitsch in the title role. I might have picked somebody more obviously Southern and more convincingly military to play John Carter, Captain in the Army of the Confederacy (Josh Holloway and Ben Browder spring to mind), whereas Kitsch used to model for Abercrombie & Fitch. But in fact he brings a wry sense of humor to the part, and I liked his chemistry with Lynn Collins, who plays Dejah Thoris, the actual Princess of Mars. Physically Kitsch is perfect, with a lean, lithe body that resembles those of the men in St. John’s illustrations from the 1920s, rather than a Schwarzenegger-type steroid overdose. His athleticism is apparent even through the wires and big software.

Lynn Collins has a more thankless role as the perennial damsel in distress, but at least she gets to wield a sword with the best of them, and in keeping with her characterization in the original novel, we are constantly reminded that Dejah Thoris is a trained scientist. This new Dejah does depart from the original in one key area: she’s the one who first puts the moves on Carter, rather than the reverse. Mars has become much less puritanical since Burroughs’ day! I thoroughly approve.

Now for what I didn’t like. As I hinted before, I was already well-acquainted with Barsoom (as Burroughs called it) and various members of the family of Dejah Thoris before I ever got to read A Princess of Mars. So when I finally did read the book (I still have my crumbling paperback from Ballantine Books, printed in 1963), I was delighted by the way it took me step-by-step from Earth to Mars, filling in details about Carter’s early days on Barsoom among the green men of Thark that had been mentioned only in passing in the later books of the series.

Disney and Stanton chose a very different approach. They begin the movie in the midst of some wild Barsoomian action involving the men of Zodanga, and only later introduce John Carter. (Bad idea!) They also added an absurdly complex back story to explain how John Carter traveled to Mars and why Helium and Zodanga are at war. (Burroughs was content with Apache magic in the first case and plain old aggression in the second.) In so doing they took two minor characters from the original series – Sab Than, Prince of Zodanga, who is Dejah’s unsuccessful suitor and a complete nonentity; and Matai Shang, Holy Hekkador of the Holy Therns, who doesn’t even appear in Princess but fills a small supporting role in the next two novels – and goosed them into major villains. I would have been far, far happier with a movie in which Matai Shang did not appear at all, and certainly not in a bald, shape-shifting incarnation that bears no resemblance to anything Burroughs wrote.

Just as unfortunate is the excessively choppy way in which they handle John Carter’s days among the Tharks at the beginning of the story. Burroughs built a careful narrative of Carter’s rise through the ranks of the Tharkian chieftains, which was paralleled by the rise of his Martian friend, Tars Tarkas. Every time Carter killed a Thark, he took the dead warrior’s name, possessions, and rank, thus winning the right to live among the green barbarians as “a prisoner with power.” Similarly, Tars Tarkas begins simply as a chieftain of the Tharks, and only slowly rises to Jed (big chief) and finally Jeddak (emperor) at the end of the novel.

Martian flyers, from JOHN CARTER

But in the film, Tarkas is already Jeddak when the story begins, and we never learn why exactly the green Martians call our hero Dotar Sojat instead of John Carter (it’s because he killed two guys named Dotar and Sojat). Still worse, we get only the most garbled version of what for me was the most memorable part of the Tharkian chapters: the strange history of Sola, the only green Martian capable of love and compassion, and Sola’s forbidden relationship with Tars Tarkas. Although Sola is a character in the movie, and we do learn that she is Tarkas’ daughter, her story unfolds in a puzzling, disjointed way that is bound to confuse anybody who doesn’t know the book. Thus Stanton squandered an opportunity to tell an alien but highly moving story, and failed to flesh out two interesting characters who are so central to his plot.

I could continue listing all the ways in which Disney departs from Burroughs, but I’d sound even more like a geeky old curmudgeon than I already do. I’ll be content to make two final complaints. First, Disney was stupid to introduce Dejah Thoris to the audience before John Carter gets to meet her, so that we lose the opportunity to see her through his eyes. Second, a key scene in this version violates everything Burroughs tells us about Martian mores. Stanton shows us Tardos Mors, Jeddak of Helium, telling Dejah that she must marry a villain she hates in order to bring peace and save her city. I had to stifle a groan as I watched that happen.

Illustration for A Princess of Mars by Frank Schoonover

To quote from the novel: “Tardos Mors . . . has sent word that he and his people would rather look upon the dead face of their princess than see her wed to any than her own choice, and that personally he would prefer being engulfed in the ashes of a lost and burning Helium to joining the metal of his house with that of Than Kosis.” (Than Kosis = Jeddak of Zodanga, another detail Stanton jettisoned.)

Bottom line: I enjoyed the movie, but with mixed feelings. It’s very gratifying to see these beloved old visions of Mars brought to life, but Disney and Stanton undercut their own success by treating their source material in such a cavalier manner. In the 50 years since I first visited Barsoom, I’ve been present on opening weekend for all the intervening landmarks of science fiction cinema: 2001: A Space Odyssey, Star Wars, Alien, Blade Runner, Matrix, and Avatar – not to mention the be-all and end-all of epic fantasy, Lord of the Rings. This retelling of A Princess of Mars could have had an impact similar to those classic films, but the creative team decided instead to concoct an unnecessarily contrived and chaotic story that is unlikely to translate into an enthusiastic market share.

Peter Jackson understood that you need to stick to the book and pay attention to the hardcore fans. In the end, Andrew Stanton did not. I’m grateful to him for bringing us this gorgeous piece of cinema, but I strongly doubt that we’ll be seeing a sequel in my lifetime.

Still, I’d love to be wrong.

Taylor Kitsch & Lynn Collins as John Carter & Dejah Thoris