On Monday, September 28th, NASA held a special press conference which, they had promised, would “solve” a “major” mystery about Mars.
As I noted in my Space Sunday update prior to the conference, the major speculation was that the US space agency would be discussing what are called recurring slope lineae (RSL) features on Mars.
RSLs have been the subject of intense debate and discussion since 2011, when an undergraduate called Lujendra Ojha published the first in a series of papers on their presence on Mars. In essence, they are ridges and rills which appear on the slopes of hills and craters, notably in the equatorial regions of Mars. The significance here being that on Earth, identical features are always the result of free-flowing water.
Given that it is known that Mars once supported liquid water on its surface, the presence of these features wouldn’t be that exceptional were they part of the ancient landscape. However, as the “recurring” in the title suggests, the Martian RSLs appear to be active – recurring frequently, sometimes on the seasonal basis. renewing and growing, with new ones also being periodically created.
Given the overall similarities between RSLs seen on Mars and those seen on Earth, particularly in Antarctica, the common belief has been that liquid water is responsible for the features on Mars. If true, then it would indicate two things.
The first would be that Mars would appear to have a subsurface water table of some description – which would be consistent with the idea that as the planet lost its atmosphere, whatever water remained on the surface may have retreated underground. The second is that it would seem to indicate that Mars is still in some way geologically active, with some mechanism at work forcing this water to the surface and creating these sudden, if short-lived outflows.
The NASA conference coincided with the publication of another paper in Nature Geoscience by Ojha and his colleagues. both pointed directly to water being the cause of the Martian RSLs. In particular, they both report that spectral analysis of some of the more recent and broader RSL channels shows they are rich in hydrated salts, which strongly indicates the presence of water. These salts are consistent with the chemical signatures of magnesium perchlorate, magnesium chlorate and sodium perchlorate.
This is significant because the presence of perchlorate deposits in water can work to prevent that water freezing solid in the kind of summer daytime temperatures – around -23C (-10F) – often experienced in the regions where these RSLs are found. Thus, if held in suspension, they would create a watery brine capable for fluid motion, and which, if released in significant enough amounts, could give rise to the RSLs prior to the water itself sublimating rapidly into the tenuous Martian atmosphere, leaving the hydrated deposits behind.
The conclusion is that it is indeed liquid water that is causing these RSLs on Mars, and that this water is in a liquid, rather than solid state, at least during certain periods, such that it can be forced to the surface.
However, all is still not entirely clear – something which tends to cast a shadow on the idea of a “mystery” having been “solved”. For one thing, if the RSL rills are below a certain width, they are entirely devoid of any hydrated deposits. This could mean that some other process is involved in their formation, which has yet to be determined. Further, the mechanism which is actually responsible for forcing the water to the surface a creating the outflow which result in these RSLs is still unknown.
Nevertheless, that the larger RSLs do now appear to be the result of water action on Mars tends to confirm the hypothesis that some of the Martian surface waters which one existed did indeed retreat underground as the climatic and atmospheric conditions changed. If so – and assuming microbial life did get a foothold on Mars – it is possible that those microbes “followed the water” underground as well – and might still be there.
While it is unlikely that such life would survive in the kind of briny mixture believed to be responsible for the RSLs, it could be present in latitudes closer to the Martian poles, where it has been shown that large amounts of subsurface water ice may well exist. And that’s an intriguing prospect for those planning NASA’s Mars 2020 rover mission and the European 2018 ExoMars rover, both of which are designed to seek out direct evidence for life having once arisen (and may still be present) on Mars.
Avalanche on Mars
As the news about the RSLs was still being digested, NASA released another remarkable picture captured by the HiRISE camera aboard the Mars Reconnaissance Orbiter (MRO): that of an icy avalanche in progress.
The avalanche, about 20 metres (65 ft) across, occurred in the “polar layers” of the Martian north pole, where sheets of carbon dioxide ice lay atop steep, rocky slopes at the edge of the polar region. Having been warmed by the Sun, a section of this layered deposit of dry ice broke away from the shelf and tumbled down the stepped slopes, and was captured by the HiRISE camera as it did so.
Such avalanches are not uncommon on Mars, and this is not the first time MRO has recorded one. However and more usually, it is the aftermath of such avalanches which are caught on camera, which features the dust cloud kicked-up by the ice and the material it has collected as it has proceeded down the slopes comes to rest at the foot of the polar shelf.
Curiosity Sends a Postcard
Throughout September, NASA’s Curiosity rover has been quietly continuing to make progress up the slopes of “Mount Sharp”, the huge mound of deposited material in the middle of Gale Crater. It has been passing through a region dubbed the “Stimson Unit”, a layer of sandstone, some of which is exhibits properties which suggests it has been altered by fluid action, possibly as a rise of ancient groundwater mixed with various chemicals.
At the end of September, the rover arrived at a location where more “ordinary” sandstone and the sandstone possibly altered by water action lay in close proximity to one another, offering an ideal opportunity for some comparative studies.
In the first part of the study, Curiosity was ordered to obtain samples from a sandstone rock dubbed “Big Sky”. These samples, which were gathered on September 29th, will be delivered to the rover’s on-board Chemistry and Mineralogy X-Ray diffractometer (CheMin) and the Sample Analysis at Mars (SAM) instrument suites, where they will be analysed.
This work will be undertaken as Curiosity turns its attention to the more “interesting” sandstone, which it will study using the ChemCam laser and its robot arm mounted Mars Hand Lens Images (MAHLI) and x-ray spectrometer. Assuming a suitable target can be found, an additional sample gathering exercise might also take place, allowing the sandstone to be fully analysed and the results compared with those obtained from the September 29th sample, hopefully providing a clearer indication of the processes at work in the area.
Ahead of all this activity, and at the start of September, Curiosity reached a vantage point offering a dramatic view out across the slopes of “Mount Sharp” towards which the rover is heading. As one might expect, the opportunity to take photos was too good to miss; and on Friday, October 2nd, NASA released a composite image of the view as captured by Curiosity, at the time.
The image, which has been white-balanced so that all the features appear as they would under normal sunlight on Earth, shows a stunning landscape which is breathtaking in its beauty. In the foreground, roughly 3 kilometres (2 miles) from Curiosity from the rover (lower part of the image) is a dark-tinted ridge rich with the iron oxide hematite, often associated with watery environments. Beyond this is an undulating plain rich in clay minerals leading to a series of buttes leading to the horizon, all high in sulfate minerals.
In a nod to the famous “wish you were here…2 postcards, NASA actually issued the image with “Greetings from Gale Crater, Mars” printed in to the right corner. Commenting on this, Curiosity’s Project Scientist Ashwin Vasavada said, “The only thing more stunning than these images is the thought that Curiosity will be driving through those lower hills one day. [So] we couldn’t help but send a postcard back to all those following her journey.”
All images and video: NASA JPL, unless otherwise credited.