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Headshot of Emily Lakdawalla

NASA's Mars Announcement: Present-day transient flows of briny water on steep slopes

Posted by Emily Lakdawalla

28-09-2015 14:26 CDT

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

NASA held a press briefing today to publicize a cool incremental result in the story of present-day liquid water on Mars. It concerns a paper published today in Nature Geoscience by Luju Ojha and several coauthors: "Spectral evidence for hydrated salts in recurring slope lineae on Mars." How big a deal is this story? Was all the pre-announcement hype justified? Is this just NASA discovering water on Mars for the zillionth time? What does this mean for things many space fans care about: life on Mars or future human exploration?

To very briefly summarize the story:

  • A few days ago, NASA issued a media advisory titled "NASA to Announce Mars Mystery Solved," teasing "a major science finding from the agency's ongoing exploration of Mars."
  • The names of panelists listed with the media advisory made it clear that recurring slope lineae would be the topic of the briefing; Lujendra Ojha in particular is a graduate student, most of whose work has been on these features, and who is presenting such work this week at the European Planetary Science Congress (PDF).
  • Recurring slope lineae are narrow streaks that appear seasonally on Martian slopes; they have been observed to form and fade in widespread locations on Mars. (Here's an earlier article on recurring slope lineae.)
  • Past work on slope lineae, with the HiRISE camera on Mars Reconnaissance Orbiter, showed that they form in warm seasons when temperatures reach 250-300 kelvins, which strongly suggested that a volatile species like water was responsible.
  • The newly published work involves data from the CRISM spectrometer on Mars Reconnaissance Orbiter, and shows spectral evidence for hydrated salts (minerals containing molecular water in their structures) during the times when the slope lineae recur.
  • The best mineral matches to the spectral data are magnesium perchlorate, magnesium chlorate, and sodium perchlorate.
  • The presence of perchlorate salts could lower the melting temperature of water at Martian conditions by 40 kelvins, making it much easier for water to melt.
  • This work is considered very strong evidence that at widespread locations on present-day Mars, conditions sometimes arise for brief flows of briny liquid water -- probably not rivulets, just spreading wetness in the soil.
  • The widely varying locations and geologic settings where slope lineae have been observed to form and recur make it difficult to identify a single mechanism for replenishing liquid water to drive the recurrent activity.
  • The science team including Ojha and Alfred McEwen favor deliquescence as the source: perchlorate salts adsorb water vapor from the atmosphere until enough water is available to form a liquid and dissolve the salts.

If you want to learn more about the announcement, you can check out articles on the story written by journalists who had access to the paper and were able to interview the scientists while it was under embargo: Marcia Dunn for the Associated Press, or Kenneth Chang at The New York Times, or Clara Moskowitz at Scientific American, or Ian Sample at the Guardian, or Irene Klotz for Discovery. Alex Witze tweeted a lot of background during the live briefing.

Recurring slope lineae (RSL) in Newton Crater, Mars

NASA / JPL / UA / Emily Lakdawalla

Recurring slope lineae (RSL) in Newton Crater, Mars
Recurring slope lineae are narrow (0.5-5 m wide), relatively dark-toned features that form on steep (25-40˚), southern-hemisphere slopes, and that appear in early spring, grow longer in the downslope direction during spring and summer, and fade during autumn and winter.

Over the weekend, the Internet whipped itself into a frenzy anticipating this story and what it might mean for life on Mars. At the briefing, science mission directorate associate administrator John Grunsfeld fed that, saying: "The discovery we're talking about today is most exciting because it suggests it would be possible for life to be there today." Personally, I don't think extant life on Mars is any more likely because of today's announcement than it was before. An incredibly salty, corrosive, transient water environment is not a very good place to look for life. I think a much more habitable environment is available in the thin films of water that Phoenix observed in the soil at its near-polar landing site. A less-accessible, but also less-radiation-fried and more-continuously-habitable place would be deep underground, where Mars' internal heat could keep groundwater liquid for very long periods of time.

So this research doesn't have a whole lot to tell us about life on Mars, I don't think. What this research is telling us is that we are beginning to understand Mars, and that's no small thing. What we have here is a solid incremental result. Scientists made an observation: huh, there are fresh-looking dark streaks on Martian slopes. They followed up that observation by looking for more of them in similar environments, and found them. They followed up those observations by looking again at the same spots, and found that some of them faded over time, and there were new ones. What do you know, active geology on Mars! They kept looking at the same spots over and over, and found a pattern: they grew during the warm months, and faded during cold months. That suggested a hypothesis: maybe you need warmth because you are melting a volatile component, specifically water. That suggested a test: look for chemical evidence of water. The present work is the result of that test, and the result was consistent with the hypothesis.

It's really a lovely example of the scientific process working, and it doesn't happen very often in planetary science. Our instruments are so coarse, our understanding so generally poor, that it's a rare thing to enjoy this cycle of observation, hypothesis, test, confirmation. Look at what's happening on Pluto this year: the photos are absolutely amazing but, for the most part, scientists have no idea how to explain what we see.

Of course, NASA didn't lead the story by saying "Cool New Incremental Result!" They said: "Mystery Solved!" and "Major Announcement!" Those phrases led to a weekend full of hype. They successfully got the mainstream media talking about science on Mars. As jaded and snarky as many of us are about NASA's frequent rediscovery of water on Mars, you can't deny that this a successful way to get space science stories into mainstream media. And it's no coincidence that today's science story meshes well with the politics of future Mars exploration, with the story NASA is trying to sell of scientific robotic exploration leading to future human exploration.

While I think the science is very cool, our new understanding that there may be extant liquid water appearing transiently all over Mars is actually not entirely good news for the future of Mars exploration. The reason why not is summed up in an article by Lee Billings for Scientific American: "Searching for Life in Martian Water Will Be Very, Very Tricky." The problem is one of planetary protection. We build all our spacecraft on a world that is just disgusting with life. Microbes bred on Earth can survive in every imaginable environment. We do our best to sterilize everything, but our best is not good enough; any spacecraft that departs Earth for Mars will necessarily carry stowaway microbes. How terrible would it be if we discovered microbes living on Mars, and then couldn't be sure that we didn't bring them to Mars from Earth?

NASA recognizes that the potential for contamination is a problem, so there is a Planetary Protection Office that is specifically charged with overseeing how missions avoid contaminating Mars with Earth biota. There are two main approaches. One approach is to sterilize the heck out of anything that will actually be touching Mars. That's why Curiosity's wheels were specially wrapped throughout its final assembly, and why it was such a scandal that the drill bits were handled after sterilization. The other approach is to avoid landing in any location where you might encounter -- or accidentally create, should you crash -- a present-day habitable environment where Earth microbes could thrive. For instance, current rules prohibit NASA from targeting a mission containing a hot radioisotope thermoelectric generator (such as Mars 2020) anywhere near a place where a failed landing might place that generator close enough to subsurface ice that the heat of the decaying plutonium could melt it.

But all bets are off once you send humans to Mars. There is absolutely no way to make a human clean of microbes. We are filthy with microbes, thousands and thousands of different species. We continuously shed them through every pore, every orifice, with every exhalation, and from every surface. True, almost all of our microscopic friends would fail to thrive in the radiation-baked, intensely cold and arid Martian environment. But life is incredibly tenacious. Sooner or later, humans will get to Mars; even if they die in the attempt, some of their microbial passengers will survive even the worst crash. Once we've put humans on the surface, alive or dead, it becomes much, much harder to identify native Martian life.

This is one of many reasons I'm glad that The Planetary Society is advocating an orbit-first approach to human exploration. If we keep our filthy meatbag bodies in space and tele-operate sterile robots on the surface, we'll avoid irreversible contamination of Mars -- and obfuscation of the answer to the question of whether we're alone in the solar system -- for a little while longer. Maybe just long enough for robots to taste Martian water or discover Martian life.

See other posts from September 2015


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


Richard: 09/28/2015 03:39 CDT

Today's news, while giving me chills... it's much like the announcement of frozen water at the Martian poles: "Uh, yeah, all that white stuff seen in telescopes for centuries? Yep! Water!" The idea of briny water, knowing what we know about Martian temps + ongoing changes observed with surface features that seemed to indicate liquids... it's not a huge shocker. The implications with atmospheric methane, however? Holy crap. Considering chemical reactions producing methane still require temps reaching over 300°C and water, this does not seem to explain that mystery as far as my amateurish knowledge is aware. I hope we realize moving forward that whatever "good stuff" (aka life) is *not* on the surface of Mars today, having no atmosphere or magnetic field offering protection. If life survives and thrives into the now, then it's surely under ground - where this liquid water could also easily reach/remain longer than on the surface. Just as the underground is expected to be colder than the surface? It would also keep such thermal energy from escaping; meaning in addition to protecting life from harsh radiation/etc., it's conceivable briny water is a constant somewhere below the surface. If life remains on Mars today, which the inconsistency and rarity of detected methane still indicates more strongly than anything else imo, it is *under* ground. Let us pray we don't waste trillions doing a manned exploratory search above it, only to come up empty. Still, what exceptional and entirely unexpected news! If *this* does not reinvigorate planetary science and space exploration, manned and unmanned alike, then our species is more tragic than I might have ever imagined. Let us hope this news is what kicks off the next great era in human development, at long last expanding our reach into the stars with all of our might. And it starts with an alternative to rocket propelled launches - space elevators, space fountains, maglev cannons... anything greatly reducing launch costs

MIchael Paine: 09/28/2015 04:29 CDT

Today’s announcement by NASA of flowing (very salty) water on Mars strengthens my case for the success of transpermia - Earth rocks with live microbes reaching liquid water on Mars: "This paper presents a first-pass attempt to estimate the probability [that microbes] have reached pockets of liquid water near the surface of Mars. The probability is based on a sequence of fortunate and somewhat unlikely events: 1. The microbes survive the trip between Earth and Mars aboard a meteoroid 2. The microbes survive the entry through the Martian atmosphere and the hard landing on the surface of Mars 3. Portions of the meteoroid bearing dormant microbes land on water ice and sink to where there is sub-surface liquid water 4. Some of the microbes colonise the habitat It should be noted that many of the values in this analysis are speculative…” See also , published after my paper.

Dave Moon: 09/28/2015 04:29 CDT

Isn't a big reason why the scientific process working doesn't happen very often in planetary science that we so rarely send a probe to the same place twice? Mars is one of very few bodies where follow-up on previous observations is possible.

Laura: 09/28/2015 05:10 CDT

Please be patient with me here. To me, "perchlorates" are the reason I can't drink water from the wells near my house and instead have to drink water imported by the Metropolitan Water District. I'm not really understanding what perchlorates are other than detritus from rocket fuel. Are there good perchlorates and bad perchlorates? Why are we excited about perchlorates on Mars and upset about them in wells? Sorry for my ignorance.

Douglass: 09/28/2015 05:46 CDT

Well said about filthy meatbag bodies and teleoperation. But get with it. To human spaceflight advocates these days, "exploration" is all about filthy meatbag bodies, and (filthy) feet on rocks. The reflexive opposition to such exploration telepresence this harkens back to what we call "historical exploration" where, in order to be an explorer, one has to look like Columbus or Lewis and Clark. You know, beards, swords, funny hats, and all that. But our technologies now allow us to achieve real "presence" on Mars, without actually having our filthy meatbag bodies on the surface. Let's all grow up and embrace these new technologies. But in order to have real-time presence, we've got to send people close -- ideally into orbit around Mars. So human spaceflight is still profoundly important to even modern exploration.

DrMorbius: 09/28/2015 09:12 CDT

John Grunsfeld seems like a good guy, but it is dismaying that a fly-boy is running the Science Directorate. Why can't this Directorate be run by a scientist, preferably a planetary geologist? And, of course, Grunsfeld has a hidden agenda, human spaceflight to Mars. Grunsfeld and NASA HQ are desperate to create a renewed interest (i.e. more money) in human spaceflight and this is the latest angle. At least this is not as ridiculous as the asteroid capture stunt, but it is still just another angle to get more funds for manned spaceflight and I suspect that NASA HQ is way more interested in getting more pork for manned infrastructure than actually getting astronauts to Mars.

Paul McCarthy: 09/28/2015 10:29 CDT

Granted, "tele-operat(ing) sterile robots on the surface", from orbit, is a vast scientific and financial improvement over "filthy meatbag bodies" tramping all over the crime-scene at truly astronomic cost. But, honestly, how does it really compare with operating the same (continuously improving) robots from Earth??? Ok, so maybe operations from orbit could be, say, five times as efficient in terms of the use of the robotic surface assets. But what on earth is the cost, relative to JPL-based human operatives, of placing significant human crews in long-term, radiation-hardened, Mars orbit??? We are certainly talking about at least 100-fold or 1000-fold greater costs!! So it will never be the case that orbital tele-operations come near Earthbound tele-operations in justification. And even less so, of course, for direct human surface operations. Just get used to it -- and spend a fraction of the sum sending ever-better and more frequent exploratory craft, until we have the answers we desire.

ScienceNotFiction: 09/29/2015 03:10 CDT

The Protoplanet History of Mars and Ceres Shared A Common Theme This is a tale of two brothers. Mars, the big brother, had a bigger and heavier molten core to start with, which had attracted many large asteroids (included water, olivine, serpentine and silicate rich ones) from the belt. Its iron composition was much less than Earth, and the tiny brother Ceres had even lesser iron to begin. But they both received a lot of salt water that had covered their entire surfaces. This ocean layer had reacted with the molten core for many millennia forming an array of silicates and metallic salts. The difference was that Ceres had received much more salt water than Mars by volume ratio. Because of its tiny size and low gravity, Ceres underwent an exo-crust evolution which preserved a substantial volume of salty liquid inside. Mars having greater gravity, much of the precipitants from the salt water reactions had fallen toward the core forming a denser crust underneath the ocean. Stalactite and stalagmite caves could be all over Mars, especially near the equator belt. Mars ocean layer would be at least several kilometer deep. The growing layer of settling precipitants would eventually cease the core-mantle reactions starting from the polar regions. With seismic activities due to crust-cooling, small portion of the ocean would drained into the mantle cavities forming pockets of underground salt water lakes and salt crystal caves. While ocean evaporation entering the final phase on Mars, the concentrated anhydrates and chlorides would further absorb the remaining moisture on the surface. These highly corrosive substances would oxidize (breakdown) most organic materials or organisms giving CO2 in the atmosphere. Frequent lightnings on the surface of Mars would convert chlorides to perchlorates over time, aborbing the remaining liquid H2O on the surface. Find out the fate of Ceres from comments under Dr. Rayman's DAWN blogs...!

Seraph: 09/29/2015 03:12 CDT

I need some data on the power draw from the Mars Reconnaissance orbiter. Would anyone be able to provide me with the Watts per hour and per "day" this NASA spacecraft's onboard systems draw? Much appreciated

Antonio: 09/29/2015 03:41 CDT

"How terrible would it be if we discovered microbes living on Mars, and then couldn't be sure that we didn't bring them to Mars from Earth?" PCR.

sepiae: 09/29/2015 03:57 CDT

Excellent summary - as expected :) I'm not 100% in agreement concerning the implications, mainly because of what separates appearance from confirmation. Sure, in any case, they do know how to put on a good show, and they had my pulse going there for a while. One thing, you wrote 'incredibly salty, corrosive, transient water environment is not a very good place to look for life.' The idea of how water has to be exactly to be viewed as potential for supporting life should of course cover a little range. The water in which life began evolving here on Earth wasn't quite the water coming out from our faucets either. The last point is a necessary reminder for the impatient me, especially the very ending. And I just love this phrasing: 'We build all our spacecraft on a world that is just *disgusting* with life.' :)

suilleabhan: 09/29/2015 04:24 CDT

Is this clever PR by NASA taking advantage of this weeks blockbuster release of 'The Martian' I was cyncial reading this in the paper but good to get a look into the real science behind the announcment. Thanks Emily, the best place on the interenet for keeping up to date with all things involving space exploration.

dougforworldsexplr: 09/29/2015 12:37 CDT

what priority is going to be made now in finding if these RSLs transient briny water flows come from aquifers just below the ground at the top of the crater or canyon walls that could contain purer water that would be more likely to support possible microbial life (and also be better shielded from radiation on the surface). Also what effort is being put forward by the Curiosity or upcoming Mars landers or rovers to be thoroughly decomtaminated and sent near some of these RSLs or source aquifers to search for microbes and then if they are not found there pave the way for astronauts on Mars since there would then be much less likelihood there is indigenous Martian microbial life?

Douglass: 09/29/2015 03:53 CDT

"But, honestly, how does it really compare with operating the same (continuously improving) robots from Earth???" The way to look at it is communication delays back and forth to Mars are at best of order ten minutes. That compares with the communication delays from the human brains to a connected limb of about 200 milliseconds. So that means a human sense of "presence" on Mars from the Earth will be about 3000 times worse than it would be for humans on or nearby Mars. That's pretty damning. So get in your car and drive to the store with 10 minutes of time delay. How long would that take, do you suppose? Yes, radiation in Mars orbit is a handicap, but it's not really a lot better on the surface, and it's very unlikely that such expeditions are going to bring bulldozers with them that they can use for piling up regolith. Basing humans in orbit on a martian Moon would be advantageous in this regard. Autonomous systems are getting a lot better, but it will be along time before one could achieve any measure of what we consider "human presence". We're not talking about humans on Earth versus humans at Mars. We're talking about humans on the martian surface versus humans in orbit around Mars. The latter would be significantly more affordable. But yes, the human space flight folks are in a desperate pickle here. We really, really, really want to send humans to Mars. But if we do, what 's the right way to use them there?

TomHopp: 09/29/2015 04:46 CDT

Thanks, Emily, for your evenhanded reporting of this mixture of science and hype. I like Ojha's simple finding of perchlorate salts in the lineae. But the hype-sters have gone way over the top. Where did they get "flowing" water from? Not Ojha's data. I live in mountainous country where avalanches rumble down chutes and spread over talus slopes all winter long. Those Martian lineae are shaped exactly like the avalanches I see at home. As I recall, years ago when the lineae were first reported, dust avalanches were among the proposed causes. The evidence is right there before our eyes, but NASA folks seem to have talked themselves out of it in favor of what everybody wishes were there. And regarding deliquescence, it is an extreme case of a simpler concept, hygroscopic absorption of water vapor. Most materials that are hygroscopic take on little water and do not become liquids--they just gain weight. So, imagine perchlorate-coated dust on a steep slope. If seasonal moisture in the air is absorbed, the dust particles will get heavier. And this may in turn cause seasonal slope failure and avalanches. No running liquid water is needed. Sorry. I don't buy their argument.

vincecate: 09/29/2015 11:45 CDT

I think the case for flowing water is flawed. Looks like solid CO2 or solid H2O.

ScienceNotFiction: 09/30/2015 12:52 CDT

The Most Probable Native Life Forms on Mars - Arthropods This class of creatures could easily adapted to the present day condition of Mars. They have an efficient food storage organ and a very low metabolic rate. Their inactive lifestyle enables them to survive long periods without food, some can survive 6 to 12 months of starvation. Hidden under rock crevasses and caves from intense radiation exposure, they get enough water from H2O condensation on their body at night and can survive under extreme temperatures between −31 to 50 °C. There could be two major families distinguishable by color and size. One family would be living on the surface near rocky terrains. They are much smaller with dark camouflage skin patterns. The other family would be living in subterranean caves where the abundance of salt water has preserved the last remaining microbes and marine life forms. They are much larger as well as paler in skin pigment with fluorescence, and possibly eyeless. They can detect darkness and light only. They can survive in briny water and the skin would be able to absorb H2O and filter oxygen at the same time. They are very sensitive to vibration, an important predatory ability over other organisms. Imagine an astronaut stumbled upon the surface crust of their underground habitats, they will sense his presence immediately. During the last phase of ocean evaporation on Mars, the intense radiation would trigger widespread genetic mutations on many species. Arthropods with size as big as a giant spider crab dwelling underneath the Martian surface is a real possibility. If we are to deploy astronauts on Mars, we need to provide them not just radiation shields, but also appropriate weapons to deal with such encounter while not destroying their habitat. Killing even a single creature could bring extinction on that specie. If our goal of exploring Mars is to find new life, killing an indigenous alien creature there would be the biggest irony in human history.

ScienceNotFiction: 09/30/2015 01:21 CDT

Just look at what mankind has done to Galápagos Islands over the past 500 years. Filthy meatbag bodies are not my biggest concern, radiation and corrosive salts will take care of the organic issues. How to deal with the contingency of encountering a life-threatening situation from fierce predatory creatures is the biggest issue here. To kill or not to kill, that is the question ? What about our astronauts getting contaminated with exotic parasites and bacteria from these alien creatures, or even poisoned by their toxic body fluids? Earth already has plenty of super bacteria and viruses that we are unable to deal with. It will be a big gamble being the first one set foot on Mars !!! What kind of scientific achievement can we accomplished there? At least Ceres presents a potential for major scientific breakthrough. If we know the internal physics of Ceres, we can build similar core reactor of miniature size in space orbit for housing millions of humans as space colonies. It provides warmth, light, water, oxygen, hydrogen, electricity and even a magnetic field. This is what I mean a "scientific breakthrough."

sepiae: 09/30/2015 09:09 CDT

3 thoughts in regards to bugs from Earth (and pls. keep in mind that I'm a lay-cuttlefish): given that it won't change soon that we're disgusting about and that it isn't likely that we'll find a means to completely sanitize an astronaut besides, you know, setting her/him on fire, and if we don't manage to detect any microbial life from orbit or via rover, nothing about that would be any different even if we waited a little longer, would it. Also, if microbial life managed to evolve on Mars we'd most probably still be dealing with differences compared to what we'd bring with us, even if origins would be mutual. Mars is after all a super-Tepui in a sense, separation would have been long and far. It ought to be possible to determine even the finest of these differences. Even strains of virus can differ between neighboring regions. Finally, even though life is indeed persistent, as far as we can tell the constant and unimpeded radiation has rendered the surface sterile, so what life there could be could not last for long when exposed. This would go as well for any freeloading microbes on gloves, spacesuits and equipment, would it not? Or to put it a little flippantly, maybe they should leave it all out to bake in the sun for a while before they start digging. The 2nd and 3rd thoughts combined may imply that, while we should by all means be triple-cautious, it should be possible to avoid the mistake of misidentifying our bugs for Martians. There should be a way to first make an inventory of what one carries about, just to narrow it down. A little restraint before calling home then might do the rest. Let me know if I'm being naive here.

sepiae: 09/30/2015 09:46 CDT

@ ScienceNotFiction: SNF, it is highly, highly unlikely that, should we ever come in contact with extraterrestrial bacteria or viruses, we'd be in any danger. Microbes evolved along with us - or rather we evolved along with them. What we call illness requires a relationship that needed to evolve (just btw., the number of species of virus vastly outnumbers that of all other organisms - combined, only a handful causes us troubles). When we talk about a 'new' bug, we normally mean a new strain. But it needs to have familiarized itself with us, or at least a closely related cousin (i.e. another mammal). I was trying to make this point above: any microbe evolving on another planet, even if there might have been common origins with what scurries about on ours (--> transpermia), is likely to be fundamentally different compared to what we have here. Hence chances are very good that they'd be harmless to us, because they haven't had a chance to familiarize with us, nor we with them, in an evolutionary sense. It makes for some thrilling SciFi (--> 'The Andromeda Strain'), and there's always the *other* chance as well, but it is a LOT smaller.

jimmars: 09/30/2015 11:40 CDT

Emily, thanks for your insights about liquid water on Mars. The streaks that grow larger as the temperature warms are not the same as the dark slope streaks that were found decades age. The dark streaks are far larger. It might be useful if we can find the whole extend of the recurrent slope streaks. They can be only a few meters across, so they do not easily, and quickly stand out as other features (like dark slope streaks). This may be a place were NASA could enlist the help of the public in looking for these changing features in pictures that we already have in our data.

jimmars: 09/30/2015 11:49 CDT

MRO can gather and store about 1600 watts/hour, but it only uses 40% of that at most which is 640 watts/hr.

ScienceNotFiction: 10/01/2015 12:58 CDT

Sepiae: On Earth, many solitary desert snakes, reptiles and arthropods were evolved to be extremely venomous. Neurotoxic and cytotoxic venom can kill a human in minutes. The saliva of Komodo dragons contains at least 50-80 different bacteria. The chelipeds of crab-like arthropod may cut through space suit materials. If their mouth can shoot out highly corrosive liquids such as perchloric acid, the astronaut's space suit may not last for more than 4 hours using the best anti-corrosive materials. It would be a nightmare scenario for the astronauts carrying limited water supply. "Aliens" the movie in real live.

Claudio: 10/01/2015 02:26 CDT

As an evolutionary biologist I can't help but thinking that if there's life on Mars is going to be so different from life on Earth that we would be able to tease martian microbes apart relatively easily. Either they are based on the same chemistry of living things here, and thus we can reconstruct their evolutionary history through their DNA sequences (it's called molecular phylogenetics) and see that they form a separate branch of the tree of life ( or an entirely new tree, or they are so different biochemically that we would be able to recognize them as martians by some relatively simple analysis of their chemical composition. Even if transpermia occurred, the natural selection that operates on Mars habitable environments very likely is quite different than on most if not all environments on earth, and I would expect martian microbes to show a suite of adaptations lacking in all earth microbes. Said that, successful adaptation of earth microbes to Mars environments would be catastrophic and such a risk can't be taken lightly

Alfred McEwen: 10/06/2015 06:02 CDT

Great article, as usual. But Emily wrote "An incredibly salty, corrosive, transient water environment is not a very good place to look for life." Just because we found small perchlorate residues does not mean that the flowing water was so salty, and the temperature data suggests that it isn't so salty in many places.

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