Tuesday, March 17, 2015

Mighty Throxeus

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

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.
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.

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, K√§ufl 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 

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