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Matthew Chojnacki

What’s Seeping on Mars? Recurring Slope Lineae

Posted by Matthew Chojnacki

13-05-2014 9:53 CDT

Topics: explaining science, Mars, Mars Reconnaissance Orbiter, hydrology

Perhaps the most intriguing planetary dynamic process discovered in recent history was hidden in plain sight for over a decade. Recurring Slope Lineae (RSL) are narrow, dark-toned streaks that descend steep Martian slopes, beginning in higher-lying rocky outcrops. We have watched them grow, fade, and reappear every year by taking repeated orbital images. They grow fast, over time scales as brief as a week. Their seasonal behavior and preference for warm equator-facing slopes suggests that something volatile, like briny water, could be involved. For example, the crater below in central Valles Marineris shows how RSL follow the Sun, always forming on the warmest slopes. 

RSL follow the sun in crater on Melas Chasm floor

NASA / JPL-Caltech / University of Arizona

RSL follow the sun in crater on Melas Chasm floor
Sequence of 8 images showing RSL forming first on the north-facing slope (bottom), then switching to the south-facing slope (top). This change occurred close to when the subsolar latitude moved from the north to the south of the latitude of this crater. In other words, RSL are most active on the slopes that receive more direct sunlight. There was also an episode of inactivity and fading of RSL on the south-facing slope, when the air over this region was especially dusty (Ls 235-246). Dusty air makes the daytime temperatures cooler, which may have temporarily stopped the RSL activity.

RSL were first detected within isolated craters, but we now find them regularly along canyon walls, small mounds, landslide scarps, and other landforms. 

RSL site in Coprates Chasma

NASA / JPL-Caltech / University of Arizona

RSL site in Coprates Chasma
Oblique northeastward view of one mid-wall RSL site in Coprates Chasma (using HiRISE-derived topography). RSL areas are indicated by black arrows. (inset) A closer view of the RSL along the peak from view of the dashed arrow. HiRISE PSP_006480_1660.

Note: the shapes and seasonal behavior of RSL are not the same as mid-latitude contemporary gully formation, which is most likely driven by seasonal carbon-dioxide frost (dry ice).

Hidden in Plain Sight

RSL were hidden for many years despite frequent imaging of the walls and floors of Valles Marineris by the Mars Orbiter Camera on Mars Global Surveyor. The typical resolution of those data (about 3 meters per pixel, but in very narrow images) and infrequent repeat imaging of the surface kept the Recurring Slope Lineae undetected. It wasn’t until the advent of repeat fine-scale images (25 centimeters per pixel) from the High Resolution Imaging Science Experiment (HiRISE) onboard Mars Reconnaissance Orbiter that RSL could be discovered. Even then, steep canyon and crater slopes are complicated geologic sites where landslides and wind erosion hide these active features. When I was studying Martian sand dunes at the University of Tennessee, I constructed one of the first Digital Terrain Models (DTMs, derived from HiRISE stereo pairs) in Valles Marineris; I didn't notice RSL growing high on the slopes in the two stereo images!

RSL and darkening fans in East Coprates Chasm

NASA / JPL-Caltech / University of Arizona

RSL and darkening fans in East Coprates Chasm
Sequence of 6 images showing a small area of steep slopes with RSL activity (latitude 14.7 degrees S, longitude 304.6 degrees E). The sequence is in Mars Year 31, southern spring. Note that some of the fans that RSL flow over became darker and brighter over time. There are many other such fans in this region that darkened and brightened at the same times. Image 0.25 m/pixel scale; scene 480 meters wide.

Initial Discovery

It was another student, Lujendra (“Luju”) Ojha, also working on DTM production, who finally caught the small lineae on crater walls growing between images. After Luju raised a flag to the HiRISE Principal Investigator Alfred McEwen, the hunt was on to track down the origin of the strange lineae. They were first discovered in the mid-latitude southern highlands (published by Alfred and coauthros in Science in 2011), but are now known to be abundant in equatorial regions (announced in Nature this year), particularly within the deep canyons of Valles Marineris (work I presented at the Lunar and Planetary Science Conference this year).

RSL site in Melas Chasma

NASA / JPL-Caltech / University of Arizona

RSL site in Melas Chasma
Melas Chasma RSL forming on a fan of fine-grained material in HIRISE enhanced color image ESP_034962_1670.

Ongoing Monitoring

Now that we know the properties of “typical” RSL-hosting terrain we can identify candidate sites in lower-resolution images from other instruments, including MOC, and the Context Camera on Mars Reconnaissance Orbiter (~6 meters per pixel, but much broader coverage than MOC or HiRISE). We are also finding them on older serendipitously located HiRISE images. We have found candidate sites to be widespread in places like eastern Valles Marineris (Coprates and Melas chasmata)! The problem is, we can only quantify slope lineae growth and raise these sites' status from "candidate" to "confirmed" using HiRISE data. Because of limited viewing opportunities and the camera’s narrow field of view, choices must be made by the HiRISE team: do we focus on a few sites to monitor regularly, or spread out the images in an attempt to confirm as many sites as possible? 

The HiRISE team regularly monitors several sites across Mars that show thousands of slope lineae forming in the same locations every year, when temperatures are warm enough for liquid water. But pure water should rapidly evaporate or even boil in the thin Martian atmosphere. In contrast, liquid brines that would form in the salty soils commonly observed at Martian landing sites have depressed freezing temperatures, and are much more stable in the harsh conditions of the Red Planet. A tempting terrestrial analog occurs in Antarctica during the warmest summer months. There, similar features, called "water tracks," form from salty brines on steep slopes.

The analog is compelling, but frequent HiRISE monitoring has not yet solved the mystery of how RSL actually form. One puzzle is that we don't know where the putative water that create RSL originates. Periodic melting of shallow ice, regional groundwater, deep ancient brines, or some atmospheric phenomena (e.g., deliquescence or vapor phase transport) are all possibilities, but all have problems. For example, shallow ground ice is a plausible water source at higher-latitude sites, but not at warmer equatorial locations. Within craters, a regional groundwater source seems like a possibility, but the hydrology doesn't work for RSL that begin within 100 meters of the tops of precarious peaks and ridges. Preexisting faults may could provide pathways from the deep subsurface for ancient brines, but how this would work in detail is not understood. Atmospheric deliquescence is a process by which salty soil absorbs water from the atmosphere when temperatures and pressures are sufficient (like at the Phoenix landing site and for Antarctic water seeps) and could explain special topographic occurrences. However, it isn’t clear that enough deliquescent water could accumulate in equatorial Mars to cause the widespread flows that we have observed.

RSL site in eastern Coprates Chasma

NASA / JPL-Caltech / University of Arizona

RSL site in eastern Coprates Chasma
Oblique northeastward view of one mid-wall RSL site in eastern Coprates (using HiRISE-derived topography).

Multiple working hypotheses are being tested for the genesis of the RSL. In addition to ongoing HiRISE monitoring, science teams are also working in the laboratory, measuring fluid interaction with Mars analog soils. These efforts won't just help us understand the unique phenomenon of RSL, but also contribute to our understanding of potential extraterrestrial life.

See other posts from May 2014


Or read more blog entries about: explaining science, Mars, Mars Reconnaissance Orbiter, hydrology


Doug Currie: 05/13/2014 09:15 CDT

Are there any of these linaea discovered yet in Gale Crater especially perhaps on the sides of Mount Sharp as it is near the Martian equator. I would be especially interested to know if there are any linaea that Curiosity could investigate. Could any of these linaea have low enough salt concentrations in the brine for some microbes to cope in them?

Bob Ware: 05/15/2014 07:32 CDT

Hi Doug, In the blog April 14, 2014; "Interview with a Mars Explorer" by Bill Dunford I asked a similar class question. The response I received from the scientist Sarah Milkovich, which I pasted here is: Hi Bob, I don't think we'd drive over to any RSL, even if we were find some at a location that we could get to with the rovers. Spacecraft sent to Mars are cleaned to stricter standards depending on if they will be likely to encounter water ice (let alone liquid water), to avoid biological contamination. There are people thinking right now about what the presence of RSL mean for surface exploration of those locations. ***** So basically what I am understanding under the current rules, is that if we find it, we steer clear as in giving an RSL a wide clearance zone. Hopefully in the process we don't rove into another one and violate the conditions that Sarah Milkovich was alluding to.

Doug Currie: 05/15/2014 08:12 CDT

Hi Matthew and Bob; Bob thank you for your answer that clarified some of my questions. However since these features were found in some locations in 2011 and Curiosity was launched in 2012 couldn't NASA have put it through more stringent protection measures so it would be safe to investigate these RSLs if there were some in Gale Crater. I thought NASA's mission was to search for extraterrestrial life not keep away from the most promising locations of it. Also what do you think of Robert Zubrin's idea that spores from Earth to Mars could already have brought Earth bacteria to Mars and that Planetary Protection measures might be excessive. I wouldn't want to contaminate Mars but on the other hand I also don't want to avoid the most promising locations on Mars for life if there wouldn't be any significant additional risks in contaminating Mars by investigating these places. Any thoughts?

Bob Ware: 05/18/2014 01:07 CDT

Hi Doug, Transpermia (interplanetary transport of biological material) could have transported Martian material here which would make us the Martians or the reverse. Not knowing positively if both directions happened yet is a main reason for the extreme extra steps to try, yes try, to prevent the forward contamination (us to destination). Surveyor 3 was launched with forward contamination and we learned that after the APOLLO 12 mission to retrieve parts from her 3 years later, in November 1969. As a side note, that was not the APOLLO 12 mission objective. Therefore I think most likely we have already, unintentionally, forward contaminated Mars. We need to revisit our earlier spacecraft (that is all countries) and see what we did. Did our life mutate or simply survive without issue? One day we'll know but not in the near future. We need to retrieve the spacecraft samples to know possibly for sure. Should we jump right into an out gushing of some sort that we are now fully aware of? One one hand I say yes but what happens if we contaminate at sampling and the gushing is still moving downstream with our contamination? Are we poisoning Martian life to death or creating something entirely new that both ecosystems cannot handle in our current biological state? Most likely both ecosystems could cope without major a catastrophe happening since transpermia most probably happened at some point early on in our two planets history. This may seem like an exercise in futility or failing to seek out alien life but it actually is setting the table so when we do find life we can without doubt state it is alien, not Terran (us). Sure this puts stress on us who are very eager to see alien life before we die and I am one of them! I don't like the question "Are we alone?' That is to negative in my book. I like the question I ask, "Are we the first?" Your other point of 1st discovery in 2011 to launch in 2012. In that time length it is to late to modify a vehicle, do another checkout.

Bob Ware: 05/18/2014 01:17 CDT

sorry - blog character limitations. .... and decontamination then ship to the launch site. I am very eager to have then run to the 1st possible life sign site and snap pictures, micro and macro then sample away but that is where contamination rules (Planetary Protection) come into play. In this life quest we all need to understand that we really do not know what we are doing. We have just left crib side and we are now putting out fingers into ours mouths after having touched the floor. What are we getting into? We don't yet know. Mommy and Daddy think we are still sleeping soundly in our crib. As we Star Trek fans enthusiastically proclaim and believe, life is out there, let the adventure begin! That's my view on this entire situation. Anyone else agree or disagree?

Torbj??rn Larsson: 05/18/2014 02:00 CDT

Surveyor 3 is a bad example of possible crosscontamination, since it is a contested claim. [ ] However, crafts that have landed on Mars with or without (an early Russian craft) planetary protection protocols have transported more or less viable spores there. The less stringent protocols allow them for practical purposes. And as I write this I'm not sure if the heavier ones are able to kill of extremophiles, the protocols where developed before they were known. That is likely beside the point. Crosscontamination is likely mostly a problem for some experiments and for some biotracers, not for detecting exoplanetary life as such. Fossil finds moot the question, extant life can be sequenced to look for divergence time (if any planets were crosscontaminated early on), and it is extant biotracers/experiments that may run the risk of confusion from recent crosscontamination.

Torbj??rn Larsson: 05/18/2014 02:04 CDT

I should say that the _idea_ of planetary protection protocols were invented before extremophiles were known, AFAIU. (60s vs 70s.) But they may have changed to take care of that, as the thermal and pressure limits of extremophiles and their spores have soared.

Doug Currie: 05/18/2014 04:02 CDT

Thanks for your good answers Bob and Toby. Although I would like to see more progress in the search for life on Mars it is good to have some caution to avoid contamination but in view of that how much work is being done to protect interplanetary probes especially to Mars' surface more thoroughly from microbes including extremophiles? Also how might we tell whether some possible Martian life is contamination from Earth or is some indigenous Martian life such as not based on DNA and is there any planned mission to Mars from the US or Europe with that capability? Also I would still like to know how salty or briny the water in the RSLs is and what kind of Earth or possible Martian microbes it could support.

Doug Currie: 05/18/2014 05:08 CDT

Sorry Torbj, I didn't read your name carefully enough. I meant your correct name Torbj.

Bob Ware: 05/18/2014 07:40 CDT

Hi Torbj -- I was not aware of the Surveyor 3 bio results. Thank you and thanks for providing the article with the supporting links. Doug -- if you have not read it, please do so. It does not remove contamination concerns but we know have enough of an understanding so we know how to better deal with the issue than we did back on the "good old days."

Paul McCarthy: 05/19/2014 12:47 CDT

Re: "Robert Zubrin's idea that spores from Earth to Mars could already have brought Earth bacteria to Mars and that Planetary Protection measures might be excessive". Also, modern genetic sequencing techniques could easily distinguish any existing Martian lifeforms from introduced ones ("extant life can be sequenced to look for divergence time", above). The chance of sequences being indistinguishably similar is effectively zero. So, yeah, Planetary Protection measures are WAY too excessive. In addition, there's plenty of research that when an intact existing ecosystem is otherwise unperturbed, introduced species do not routinely dominate, and even when a particular introduced species does do well, I don't believe there's any example where it then eliminated all components of the pre-existing ecosystem!! These "insurances" will be magnified a thousand-fold in, or by, the incredibly alien environment of Mars, for which there's no remotely overall-similar Earth analogue, so that any introduced Terran microbe will start at a huge, huge adaptive disadvantage compared to any putative native Martian organism, which, by definition, will have EXQUISITELY tuned it's genetics to the incredibly specific, alien conditions, over several billion years!

Doug Currie: 05/19/2014 12:37 CDT

Hi Paul, those were some good insights why microbes from earth would have difficulty getting established on Mars. Bob, I went to and read the articles about Surveyor and the possible Earth contamination. I found it quite instructive that there weren't very good precautions so it could have received Earth contamination when it was returned to Earth but also found something to keep in mind that there weren't that much bacteria and only of one type that wouldn't be as much as likely if it was contaminated by Earth microbes after returning to Earth. I would still like to hear from someone how salty the probable briny water of the RSL is and what life it might be able to sustain.

Bob Ware: 05/22/2014 08:15 CDT

Hi Doug --- That is an interesting point you mentioned about upon return of the variety count of bacteria (one type) and not that many specimens of it. I'd also like to hear from an astrobiologist on her/his opinion of your question of what type of organism could survive in that environment.

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