A first for geochronology: ages from Mars

Remote sensing, including mapping of topographic elevation, and the recent exploits of three surface vehicles – the Spirit, Opportunity and Curiosity Rovers – have provided lots of data for a host of geological interpreters. Producing a time frame for Martian geological and geomorphological events has, understandably, been limited mainly to the use of stratigraphic principles. Various rock units and surface features can be placed in relative time order through simple stratigraphic principles, such as what sits on top of what and which features cut through pre-existing rock units or are masked by them. The most important guide up to now has been interpretation of the relations between impact craters and both rock units and other geomorphological features. The Inner Planets are assumed to have recorded the same variation through time of the frequency and energies of bombardment, and that has been calibrated to some extent by radiometric dating of impact-related rocks returned from the Moon by the crewed Apollo missions. Some detail of relative timings also emerge from some craters cutting earlier ones. The only other source of Martian ages has been from rare meteorites (there are only 114 of them) whose stable isotope compositions are different from those of terrestrial rocks and more common meteorites. By a process of elimination it is surmised that they were flung from Mars as a result of large impacts in the past to land eventually on Earth. The oldest of them date back to 4.5 Ga, much the same as the estimated age of the earliest crystallisation of magmas on Earth.

MOLA colorized relief map of the western hemis...
Colorised relief map of the western hemisphere of Mars, showing Valles Marineris at centre and the four largest volcanoes on the planet (credit: Wikipedia)

But all Martian stratigraphy is still pretty vague by comparison with that here, with only 4 time divisions based on reference to the lunar crater chronology and 3 based on evidence from detailed orbital spectroscopy and Rover data about the alteration of minerals on the Martian surface. Apart from meteorite dates there is very little knowledge of the earliest events, other than Mars must have had a solid, probably crystalline crust made of mainly anhydrous igneous minerals. This was the ‘target’ on which much of the impact record was impressed: by analogy with the Moon it probably spanned the period of the Late Heavy Bombardment from about 4.1 to 3.7 Ga, equivalent to the Eoarchaean on Earth. That period takes its name – Noachian – from Noachis Terra (‘land of Noah’), an intensely cratered, topographically high region of Mars’s southern hemisphere, whose name was given to this large area of high albedo by classical astronomers. Perhaps coincidentally, the Noachian provides the clearest evidence for the former presence of huge amounts of water on the surface of Mars and its erosional power that formed the gigantic Valles Marineris canyon system. The rocky surface that the craters punctured is imaginatively referred to as the pre-Noachian. A major episode of volcanic activity that formed Olympus Mons and other lava domes is named the Hesperian (another legacy of early astronomical nomenclature). It is vaguely ascribed to the period between 3.7 and 3.0 Ga, and followed by three billion years during which erosion and deposition under hyper-arid conditions formed smooth  surfaces with very few craters and rare evidence for the influence of surface water and ice. It is named, inappropriately as it turns out, the Amazonian.

Remote sensing has provided evidence of  episodes of mineral alteration. Clay minerals have been mapped on the pre-Noachian surface, suggesting that aqueous weathering occurred during the earliest times. Sulfates occur in exposed rocks of early Hesperian age, suggesting abundant atmospheric SO2 during this period of massive volcanicity. The last 3.5 billion years saw only the development of the surface iron oxides whose dominance led to Mars being nickname the ‘Red Planet’.

Curiosity Rover's Self Portrait at 'John Klein...
A ‘selfie’ of Curiosity Rover drilling in Gale Crater (credit: Euclid vanderKroew)

A recent paper (Farley, K.A. and 33 others plus the entire Mars Science Laboratory 2014. In Situ Radiometric and Exposure Age Dating of the Martian Surface. Science, v. 343, online publication DOI: 10.1126/science.1247166) suggests that radiometric ages can be measured ‘in the field’, as it were, by instruments carried by the Curiosity rover. How is that done? Curiosity carries a miniature mass spectrometer and other analytical devices. Drilling a rock surface produces a powder which is then heated to almost 900°C for half an hour to drive off all the gases present in the sample. The mass spectrometer can measure isotopes of noble gases, notably 40Ar, 36Ar, 21Ne and 3He. Together with potassium measured by an instrument akin to and XRF, the 40Ar yields a K-Ar age for the rock. A sample drilled from a fine-grained sedimentary in Gale Crater gave an age of 4.2 Ga, most likely that of the detrital feldspars derived from the ancient rocks that form the crater’s wall, rather than an age of sedimentation. The values for 36Ar, 21Ne and 3He provide a means for establishing how long the rock has been exposed at the surface: all three isotopes can be generated by cosmic-ray bombardment. The sample from Gale Crater gave an age of about 78 Ma that probably dates the eventual exposure of the rock by protracted wind erosion.

By themselves, these ages do not tell geologists a great deal about the history of Mars, but if Curiosity makes it through the higher levels of the sediments that once filled Gale Crater – and there is enough power to repeat the mass spectrometry at other levels – it could provide a benchmark for Noachian events. The exposure age, interesting in its own right, also suggests that sediments in the crater have not been exposed to cosmic-ray bombardment for long enough to have destroyed any organic materials that the science community longs for.

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An early magma ocean on Mars?

The division of the lunar surface into two petrological domains – ancient anorthositic highlands and younger basaltic maria – spurred the idea, as long ago as the early 1970s, that the early Moon had a deep ocean of magma at the surface, whose cooling caused fractional crystallization. Low density plagioclase feldspar, dominated by high-calcium anorthite and bytownite, floated to the surface to form the lunar anorthosites leaving a more mafic mantle from which the mare basalts formed by partial melting. The key evidence in support of this hypothesis lies in the rare-earth elements of the two terrains. Because plagioclase feldspar has a much stronger affinity to incorporate the element europium (Eu) than the other REEs, the lunar anorthosites are enriched in Eu compared with its related elements. If the highland anorthosites did form by fractional crystallisation the remaining magma that formed the lunar mantle would be depleted in Eu yet enriched in the remaining REE. Although there are no samples of the Moon’s mantle there are plenty of the mare basalts that formed when it partially melted, probably as a result of huge impacts around 3.8 billion years ago. They should have inherited dominant features of mantle geochemistry, and indeed they do show characteristic depletion of Eu.

Lunar Highlands, near Descartes Crater. Collec...
Lunar Highland anorthosite, collected by the crew of Apollo 16. (credit: Wikipedia)

The giant-impact hypothesis for the Earth-Moon system presupposes that such a cataclysm would have left much of the outer Earth in much the same molten condition and destined to fractionate in the same manner. There are geochemical hints from terrestrial rocks that do support such an idea. An important target for exploration of Mars has been to check if a magma ocean also existed early in its history. Of the various missions in recent years only two have the capacity to shed useful light on the issue: the US Mars Reconnaissance Orbiter and Mars Odyssey. Both orbiters carry more sophisticated remote sensing instruments than any circling the Earth. The first has the hyperspectral Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) that senses visible to short-wave infrared (VNIR) radiation, the other deploys  the Thermal Emission Imaging System (THEMIS) that captures different parts of the longer wavelength thermal infrared (TIR) spectrum emitted by surface materials. Both allow spectra of surface materials to be reconstructed and compared with the features of known minerals from the Earth and Moon.

Feldspars are highly reflective for the most part of  the VNIR range but show a shallow, broad absorption feature centred on a wavelength of 1.26 micrometres. Such spectra have been detected using CRISM from parts of the Martian surface in the highlands of its southern hemisphere (Carter, J. & Poulet, F. 2013. Ancient plutonic processes on Mars inferred from the detection of possible anorthositic terrains. Nature Geoscience, v. 6, p. 1008-1012). The authors, from Chile and France, acknowledge that the plagioclase-rich rocks occur only in small patches, unlike the vast tracts on the Moon, and also that on Earth anorthosites are known to have formed by a variety of processes from far smaller magma systems than a veritable ocean of molten rock. Feldspars also show spectral features in the TIR, though not so distinctive, both plagioclase and alkali feldspars being very similar. Moreover, THEMIS deploys sensor for only 10 thermal wavebands, compared with 544 on CRISM.  A team of US remote sensers (Wray, J.J. and 8 others 2013. Prolonged magmatic activity on Mars inferred from the detection of felsic rocks. Nature Geoscience, v. 6, p. 1013-1017) used both CRISM and THEMIS data. While noting resemblances to lunar anorthosites, they adopt a more cautious approach to the spectra and prefer the broad, ‘sack’ term ‘felsic rocks’. It seemed possible from their work that feldspar-rich magmas may have formed by partial melting of common andesitic crust noted from the Martian surface: high spatial resolution images of the occurrences bear some resemblance to outcrops of granitic rocks in arid environments on Earth. That is, there may be highly evolved rocks akin to terrestrial continental crust.

The interesting spectral observations on Mars can only be validated by actual rock samples. While rovers still operating on the Martian surface are well able to produce geochemical data that would petrologically characterise most rocks that they encounter, none of them is in a terrain suitable for resolving this particular issue. Yet, coincidentally, a meteorite found in West Africa shows hallmarks of having been blasted from the surface of Mars and sheds useful light on various hypotheses about the Martian crust http://earth-pages.co.uk/2013/11/21/a-glimpse-of-early-martian-crust/. It is a breccia that may represent the soil or regolith that accumulated from early impacts that shattered and melted surface materials, and it is extremely old: zircons yielded an age of 4428 Ma. The clasts set in a fine matrix consist of a variety of igneous rocks, none of which are anorthosites. Some are coarse grained, plutonic rocks containing both alkali feldspars and plagioclase, which match terrestrial monzonites; broadly speaking members of the granite family. Having formed from the ejecta of large impacts, such regolith materials represent the breadth of compositions across the planet and extending deep into its crust. This one suggests that anorthosites may have been rare on early Mars.