Mars geology as seen through the lens of the Curiosity rover

January 2021

This month NASA's Curiosity rover celebrated its 3000th day on Mars. One Mars day, technically called a 'sole' is a bit longer than an Earth day, lasting about 24 hours and 40 minutes,so it's a long time now since Curiosity originally touched down on 5^th August 2012 in the Gale crater.

The rover has been exploring this same area ever since, slowly climbing the slopes of nearby Mount Sharp. During its eight-year mission Curiosity has produced a huge amount of data, allowing the scientists to have a close-up look at the geology of the Red Planet.

Many of the pictures which Curiosity's onboard camera has sent home aare of different rock formations. With its onboard toolkit, Curiosity has frequently drilled into particularly interesting rocks and analysed those samples inside its onboard geology lab. Curiosity is also equipped to analyse samples of the rock powder created by drilling and also to take samples of the Martian atmosphere.

Curiosity's internal lab consists of a suite of instruments known as Sample Analysis at Mars or SAM. There is a mass spectrometer which samples gasses from the atmosphere or those gasses released from solid samples by heating. The gas chromatography separates out individual gases which are then analysed by mass spectrometer as well as by a tunable laser spectrometer. The latter device measures oxygen and carbon isotope ratios in carbon dioxide (CO₂) and methane (CH₄) in the atmosphere which should allow the scientists to distinguish whether the samples are of geochemical or biological origin. Finally there is CheMin. CheMin is the Chemistry and Mineralogy X-ray powder diffraction and fluorescence instrument which analyzes the samples of drilled rock powder delivered to it via robotic arm.

So what type of rocks are there on Mars?

Using photographic evidence we see many similarities between the rocks on Mars and those on Earth. There are volcanic rocks which look very much like columnar basalt and scoria. Scoria is a dark-colored igneous rock with abundant round bubble-like cavities known as vesicles. Its colour ranges from black or dark gray to deep reddish brown. On Earth, scoria forms when magma containing abundant dissolved gas flows from a volcano or is blown out during an eruption.

The evidence of columnar basalt is interesting because of how it is created on planet Earth. When lava cools, basalt contracts and cracks or fractures. When contraction occurs at equally spaced points then a hexagonal fracture pattern will develop, but it may take other shapes. The fracture pattern that forms at the cooling surface tends to propagate down the lava as it cools, forming long, geometric columns. We know that on Earth water can play a role in the formation of columnar jointing in lava flows and if we extrapolate that pattern to Mars, this is further evidence to suggest that at one stage the Red Planet had surface water.

By the way, the columnar basalt and scoria photographs did not actually come from Curiosity but were taken by the Mars Reconnaissance Orbiter and Spirit rover (a robotic rover which was active on Mars between 2004 and 2010) before Curiosity arrived on Mars. Indeed, one of the functions of the Mars Reconnaissance Orbiter was to help choose the best landing site for Curiosity.

Shortly after landing inside the Gale Crater on Mars, Curiosity sent pictures of a rock outcrop that resembled shales found on Earth. Shale is a typical fine-grained sedimentary rock with a composition similar to mud. What differentiates shale from other mudstones is that shale splits readily into thin pieces along the lamination - a term described as 'fissile'. Lamination means that the rock is made of many thin layers.

In addition to shale, Curiosity also found and photographed a rock similar to what we earthlings call conglomerates - a classic sedimentary rock made up of rounded clasts filled with sand and mud which are cemented together. On Earth the cement is usually composed to calcite and quartz.

In 2015 Curiosity followed this up with pictures of mudstones inside the Gale Crater and images of an outcrop which looks like a cross-bedded sandstone from the lower slope of Mount Sharp. On Earth such stones require a large body of water to form and are found on the bottom of lakes, rivers or the ocean floor.

In 2012 Curiosity performed the first analysis of Martian soil using X-ray differentiation. This analysis showed the presence of several minerals such as feldspar, pyroxenes and olivine - a composition similar to the weathered basaltic soils of Hawaiian volcanoes. In 2013 the rover carried out the first sample drill into a rock called "John Klein". The first analysis of the portion of the drilled powder was done using SAM. The sample was first heated to 835C to release gasses from the sample which were then analysed using SAM's quadrupole mass spectrometer. The analysis showed the presence of water, carbon dioxide, oxygen, sulfur dioxide, and hydrogen sulfide.

Another of Curiosity's important finds in the Kimberley region of Gale crater was a rock containing high levels of manganese oxide (MnO). Rocks containing MnO require abundant water and strongly oxidizing conditions to form. The rock was analysed using Curiosity's Chemistry and Camera (ChemCam) instrument, which fires laser pulses from atop the rover's mast and observes the spectrum of resulting flashes of plasma to assess the targets' chemical makeup. Nina Lanza, a planetary scientist at Los Alamos national Laboratory in New Mexico, and a lead author of this study (ref) comments of the finding:

"The only ways on Earth that we know how to make these manganese materials involve atmospheric oxygen or microbes,... Now we're seeing manganese oxides on Mars, and we're wondering how the heck these could have formed? ...Microbes seem far-fetched at this point, but the other alternative -- that the Martian atmosphere contained more oxygen in the past than it does now -- seems possible. These high manganese materials can't form without lots of liquid water and strongly oxidizing conditions. Here on Earth, we had lots of water but no widespread deposits of manganese oxides until after the oxygen levels in our atmosphere rose."

Journal Reference:

Nina L. Lanza, et al. Oxidation of manganese in an ancient aquifer, Kimberley formation, Gale crater, Mars. Geophysical Research Letters (2016) vol. 43(14) pp.7398-7407 full article