The Sliding Surface of Mars
We’re all familiar with images of the surface of Mars, with the Tharsis volcanoes straddling the equator and the great gash of the Vallis Marineris just to the south. It’s a view seen in many orbital images of the planet, and one thought to have been more-or-less representative of the topography of Mars from the earliest times.
However, new studies by geomorphologists, geophysicists and climatologists led by a team of French scientists, suggest that the surface of the planet underwent a gigantic “tilt” of between 20 to 25 degrees some 3 to 3.5 billion years ago, drastically altering its appearance whilst also offering an explanation for one of the mysteries of Mars.
While a process known as variations of obliquity can cause a planet’s axial tilt to shift over large periods of time (Earth’s axial tilt of 23.4° is decreasing at the rate of about 47 minutes of arc per century, for example), this is not the cause of Mars’ shifting “face”. Rather it is the result of the massive Tharsis Bulge.
The largest volcanic dome in the solar system, Tharsis is a plateau some 5,000 km (3,125 mi) across and around 12 km (7.5 mi) thick, topped by the massive volcanoes of Tharis Montes: Arsia Mons, Pavonis Mons, and Ascraeus Mons. It formed over a period of roughly half a billion to a billion years, commencing around 3.7 billion years ago.
The French research suggests that as the Tharsis Bulge grew as a result of volcanic activity, so it gained considerable mass – perhaps a billion billion tonnes), which caused the crust and mantle of the planet to “slip” around the core, rather like turning the flesh of an apricot around its stone. Thus, Tharsis appears to have “dropped” to the equator from a latitude of around 20 degrees north, completely changing the face of Mars during its first 1 to 1.5 billion years of history – the time at which life might have arisen, if it arose at all.
While such a slippage had previously been suggested, notably through the work of Isamu Matsuyama of the University of Arizona in 2010, the French study is the first to offer definitive geomorphological evidence that this is the case. One of the major outcomes of the work is that it explains why Mars has huge and seemingly anomalous underground reservoirs of water ice located far from the poles. As the mantle and crust shifted, so they carried the frozen land which originally lay over the poles away from them, complete with the subsurface water and ice.
Overall, the study radically alters the generally accepted chronology of Mars, which has Tharsis forming before the before the widespread creation of rivers and water channels on Mars. now it appears that Tharsis formed at a time congruent with the existence of liquid water on Mars and the formation of river valleys and other water features. Thus, the volcanic activity on Tharsis may have actually contributed to the period of liquid stability on the planet.
The Methane Snows and Particle Clouds of Pluto
The New Horizons team has discovered a chain of exotic snowcapped mountains stretching across the dark expanse on Pluto informally named “Cthulhu Regio”, one of the minor planet’s more identifiable features, and which stretches almost halfway around Pluto equator, some 3,000 km (1,850 mi) in length and some 750 km (450 mi) across, with one end abutting the ice-covered flats of “Sputnik Planum” I’ve previously written about in my coverage of New Horizons.
The high-resolution images show a mountain range in approximately 420 km ( 260 mi) long, the highest slopes of which are coated with a bright material that contrasts sharply with the dark red of the more usual dark red colouring of the region (thought to be the result of dark tholins, complex molecules initially formed by the reaction of methane and sunlight high in Pluto’s atmosphere, coating much of “Cthulhu Regio”. Scientists believe the white material could be methane which has condensed out of Pluto’s tenuous atmosphere to form ice, coating the peaks, much as ice can condense out of cold air on Earth to form frost. There has even been speculation that the white material is the result of methane ice condensing as “snow” and falling across the peaks.