Water on Mars and a Common Misconception

I wanted to put together a short post about water stability on Mars because there is a common misconception that floats around in the scientific literature and in popular circles which can be misleading.

This is timely because of the latest discovery of water on Mars by Lujendra Ojha and others which showed the presence of hydrated phases associated with recurrent slope lineae (RSL's).   This suggested the presence of actual flows of liquid brine on or directly underneath the surface providing evidence for an active hydrological cycle on Mars today.

The purpose of this post is not to weigh in on the likelihood of this being real or not, but rather to talk about something closely related which is the stability of water on Mars. We all have been told that water is not stable on Mars -- if you placed a glass of water on the surface it would either freeze or boil depending on the temperature at the time. Under most Mars conditions it would boil until it froze, then it would sublimate away (move directly from ice to vapor without melting).

This basic understanding is pretty accurate, but the problem creeps in when one consults a stability diagram of water and starts plotting things.  I will use a recent post by the University of Copenhagen Centre for Ice and Climate as an example, but want to reiterate that this is a common misconception that is even present in some peer reviewed scientific literature.

Figure 1 from the blog post at the Centre for Ice and Climate showing the phase stability of water with pressure.



So upon quick inspection this figure looks correct. The atmospheric pressure on Earth is 1 atmosphere (or ~1 bar) and we all know the freezing point of water is 0 C and the boiling point is 100 C. But the Mars line looks really close the liquid water stability field suggesting that liquid water may be stable under only slightly different conditions...can this be true?

What happens on Earth when we spill some water on a table and let it sit there? According to what we see in this diagram, at 25 C we should be in the blue "liquid" stability field and therefore the liquid water should be stable. This means it should remain as liquid water. But we all know that this isn't true, the water eventually evaporates (and if it didn't we wouldn't have a hydrological cycle and things would suck).  So clearly at the surface of the water, different rules apply.

Here is the misconception, when we are dealing with an atmosphere, we should instead use the "partial pressure" of water vapor in the atmosphere to calculate the stability of water. At 25 C the highest amount of water that can be present in the atmosphere is ~31 mbars. This drops the line in Figure 1 way down towards the triple point. So while the water molecules sitting inside the liquid are experiencing a happy 1 bar (1000 mbar) experience of stability, the molecules at the surface are being sucked into the atmosphere because they are experiencing instability driven by the lower vapor pressure. But as things get colder, it becomes more likely that condensation can occur and we experience dew and other atmospheric phenomena (which I don't want to talk about because I will probably start saying inaccurate things since I really study rocks for a living).




Figure 2. Phase diagram of water showing Earth and Mars atmosphere regions.

Now looking at Mars. Figure 1 plots the Mars line on the total atmospheric pressure. So if there was a standing body of water or volume of ice, this is the stability it would feel. But the "partial pressure" of water vapor in the atmosphere is much much lower in the range of 0.1 mbars (~0.0001 atm). So if we plot this on a different diagram, we get something that looks like Figure 2.

Here we see that we can get ice formation on Mars from the atmosphere if it gets cold enough which is consistent with our observations and what we see in the polar regions. Likewise we also see that liquid water on Mars is not stable with regard to the atmosphere, and it isn't even close.

Multiple small missions as a pathway to mars sample return

This is the abstract I am submitting to the Mars Exploration Conference. I hope it mirrors what many people have thought, and stimulates some good discussion:

Introduction: Recent discoveries by the Mars Exploration Rovers, Mars Express, Mars Odyssey, and Mars Reconnaissance Orbiter spacecraft include multiple, tantalizing astrobiological targets from both past and present day Mars. The most desirable path to Mars Sample Return (MSR) should be an attempt to visit as many of these sites as possible to find the site most likely to provide the highest impact returned samples. Here we propose an MSR architecture where the first step (potentially flown in the 2018 opportunity) would entail a series of smaller caching missions to multiple landing sites on the Martian surface instead of one large flagship-class sample caching mission to a single site (Fig. 1). Such an architecture would preserve a direct pathway to MSR as stipulated by the recent Planetary Decadal Survey [1] and permits investigation of diverse scientific questions while simultaneously complying with the budgetary constraints that are likely to exist during this next decade.

The proposed series of missions would be flown at every launch opportunity from 2018 until 2028 to find and identify samples for MSR. These five launch opportunities would allow for 5-10 smaller MER-class rovers to be sent to multiple landing sites on Mars to explore the major discoveries of gullies, sulfates, phyllosilicates, carbonates, and chlorides from potential high interest regions such as Valles Marineris, Mawrth Vallis, and Nili Fossae. In addition, 1-2 telecom and science capable orbiters would also need to be launched to maintain the communication infrastructure and continue high resolution remote sensing of the surface. The landers would share a common EDL system, mobility platform, and rover chassis to increase efficiency and control costs. The rover design would allow for modular or customizable competed science instrument packages (including the sample caching system) necessary to characterize the environment and identify potential high value samples at a given landing site. There are 8 reasons why a multiple mission approach to the identification and collection of a suitable sample would be more desirable than a single flagship sample caching mission:

 1. Scientific Return: Many of the major questions about Mars that have been raised during the past ~5 decades of Mars exploration [2] remain unanswered such as: how much warmer and wetter was Early Mars? When did this clement period occur and how long did it last? How did impacts and volcanism perturb early terrestrial planetary environments? Does the modern climate of Mars permit liquid water for life? Modern water seepages have been proposed and discoveries of sulfates, phyllosilicates, carbonates, and chloride salts made from orbit point to geographic and temporal diversity of past habitable Martian environments. Discerning environmental conditions and their relevance to life requires landed missions to establish an historical and geological context which has been shown to be difficult to do with orbital data. While we know enough now about Mars to return scientifically interesting samples, it is unknown whether they will offer breakthrough discoveries. Multiple precursor missions exploring multiple exciting landing sites will substantially increase the probability for breakthrough MSR.

2. Breakthrough Discoveries: Landed missions provide the highest potential for breakthrough discoveries such as identification of organic material, detection of a biosignature, and/or the discovery of something unpredictable. These details simply cannot be discerned from orbit. The effects of a breakthrough discovery are difficult to predict, but could result in increased budgetary support for the Mars program (as evidenced by the discoveries surrounding ALH 84001) as well as a strong public, scientific, and political support for further investigation. Future breakthroughs could even ignite the nation’s interest in human exploration missions to Mars.

3. Programmatic Risk Mitigation: Because of high costs for each component, a weakness in the current MSR mission architecture is that a single mission failure would endanger the entire program. The program is especially vulnerable in the initial stages where a sample has yet to be identified. Multiple missions provide redundancy, the capability of recovering from a mission failure, and building technical expertise. This redundancy could also loosen the lander safety requirements and open up a larger portion of the Martian surface to exploration.  

4. Synergy with Human Exploration: There is a wide range of objectives that need to be addressed by Mars surface missions as a precursor to human exploration. These do not often have high enough science priority to be included as SMD mission objectives because there are few opportunities for landed missions available. However, the large number of missions in this program could allow for synergistic investigations and/or missions to be conducted with HEOMD to landing sites important for accomplishing exploration objectives [2]. The scalability of the multiple mission approach would also allow this program to potentially take advantage of HEOMD resources (if available), such as launches of several spacecraft on the SLS rocket.

5. Budgetary Flexibility: Although multiple small missions may be more expensive to implement in aggregate than a single sample caching flagship mission on a decadal scale, the large cost and complexity of a flagship sample caching rover makes implementation very difficult (or impossible in the current budget) due to phasing and high demands on single year budgets. Thus a more sustainable program of smaller missions would provide substantial budget flexibility, spreading the costs over time and helping shield the overall planetary program from MSR budgetary pressures. This approach also offers a return to NASA's earlier successful philosophy of multiple missions to common targets (Mariner, Viking, Voyager, etc.). The scalability of this model will also allow the program to expand during periods of higher budget availability by simply adding additional missions. We argue that the economies of scale and flexibility realized from the small, multiple mission approach are vital to accomplishing the goals of NASA’s Mars exploration efforts.

6. Cost Risk: The multiple mission approach would substantially drive down risks of going over budget since each mission would utilize many of the same systems and technology. This reduces the amount of development and testing required, while also reducing the amount of risk by utilizing proven spacecraft and operations systems. Furthermore the commonality of many of the spacecraft components would improve the reliability of cost models, driving down unpredictability, permitting greater cost control, and providing a stable platform for instrument developers.

7. International Cooperation: This new mission framework for MSR expands the scope of previous exploration and caching efforts allowing for a new alignment of US and European (and perhaps other) international Mars exploration goals. Under this concept, for example, any European spacecraft that explores a high priority site on Mars would be directly contributing to the stated goals of MSR as defined by the Decadal Survey. The same would apply for craft from other spacefaring nations.

8. Technology Development: The establishment of a high-heritage, well tested, and optimized spacecraft design would provide a stable platform to pursue technology development for instruments and system improvements leading toward MSR. The costs and risks incurred by testing new technologies would be mitigated by utilizing them on a well-characterized spacecraft system. The testing of more substantial technologies in preparation for future human exploration would likely fall outside of the MSR architecture described here necessitating additional missions outside of the ones described here.

Direct Pathway to Mars Sample Return: We argue here that the multiple mission approach to sample identification and caching is a more desirable alternative to the single large flagship model and thus provides an alternative MSR architecture. If such an approach can be implemented within available budget constraints, NASA would be able to pursue MSR in conformance with the intent of the Decadal Survey, even if not in the way that had been previously envisioned during the Survey's deliberations.

References: [1] Committee on the Planetary Science Decadal Survey N. R. C. (2011) Vision and Voyages for Planetary Science in the Decade 2013–2022. [2] MEPAG (2010) Mars scientific goals, objectives, investigations, and priorities: 2010.

Inspiring the Next Generation with Great Exploration

I want to discuss what I learned from this exercise of ranking great explorations in history. I really think NASA should do an exercise like this to better understand how to perform its mission well.

Basically I think that all of the missions listed as Tier 1 in my previous post are great because of a record or legacy that they left behind. There have been great explorations throughout history, but the ones that have the most impact leave behind an inspiring legacy.

Darwin is a very good example of this. His voyages on the Beagle would not be remembered if it wasn't for his book "Origin of Species" and future work laying the foundation for evolutionary biology. If he never writes the book or if no-one reads the book then the mission is meaningless.

The Hubble Telescope and Voyager missions returned incredible pictures that by themselves inspired the world and gave us new insight into the nature of the cosmos. These missions were designed for this purpose and their images are so great because they were the first to show in breathtaking color things that we had never before seen or imagined.

Alexander and Columbus did not produce any well known direct record of their explorations, but the nature of their exploration was so fundamental that it was recorded and transmitted by others. An interesting comparison here is with Leif Ericson who apparently preceded Columbus to North America by nearly 500 years. This is not nearly as well remembered because it was simply not recorded or transmitted. Imagine if Leif's story had somehow been transmitted throughout England and Europe? It may have changed the course of history.

Now, imagine that the Apollo missions did not benefit from the wealth of news coverage, magazine articles, tv shows, and movies that the exploration spawned. Would this have still have the same stature and influence as it does today? Walking on the Moon captured everyone's imagination back in 1969 but today would merit probably only a single news cycle.

Imagine the cultural tools we have available to record, document, capture, and imagine space exploration -- books, paintings, poems, movies, blogs, twitter, photography, music, etc... Now go and try and learn about NASA's space exploration - more often than not you have to rely on classroom activities, press releases, and press conferences. I've got nothing against scientists, but storytelling and inspiration aren't really our strong suits.

So if we want to inspire the next generation, or any generation for that matter, it seems NASA should utilize the tools that have always worked for inspiration: art. Without art to tell the story, space exploration rapidly becomes a bland, boring, and inaccessible for the regular person. You don't need some fancy web developer to make NASA more appealing to the younger generation, you need artists who can design content that appeals to that generation - video games, music, movies, books, magazine articles, and stories.

So I suggest all future NASA missions include artists as part of their EPO program. Not only that, but I think artists should have involvement in mission operations, inspiring photos to take etc...

This isn't a new idea, and it has been utilized on and off in several missions I know about. The Mars Phoenix mission allowed an author to have direct access to nearly every meeting and discussion and he has produced a book about it called "Martian Summer". Furthermore MSL involved James Cameron in the development of a camera for filming IMAX type footage on Mars. Unfortunately I think that was dropped for budgetary reasons. I think this type of artistic involvement could be developed to a much greater extent without increasing mission costs or compromising scientific results.

Greatest Explorations of All Time

So I recently became interested in the greatest explorations of all time because I was wondering if one could glean what particular elements were present during each one that made them especially great. So I set about making a list - and I've got some discussion about this which I'll post later.

My criteria was mainly one thing - How did the exploration in question affect the everyday lives of people of the world?

Since I'm not a historian I may have missed on a few of these, I also owe much to friends and coworkers who helped provide suggestions and arguments. Here is the list:

Tier 1 – Greatest Explorations

Apollo 11
Cultural and historical impact is huge and worldwide.

Christopher Columbus
The historical and cultural impact of this voyage is indisputable

Hubble Space Telescope
This borders on a different category, but did travel to orbit. Our first real step into understanding our universe.

Voyager Spacecraft
My personal #1, the pictures returned from these spacecraft continue to inspire and set the stage for exploration of the solar system. Pioneer should be lumped in here as well I think.

Alexander the Great
Reshaped Europe and Asia, both through conquest and scientific/cultural exchange.

Darwin
The most influential expedition in our understanding of where we came from. Although this also borders on a strictly scientific discovery (which is a different kind of exploration), but he did go somewhere relatively unexplored.

Odysseus
Whether or not he really existed and did what he claimed, the account of his travels is part of the foundation of western culture and represents the quintessential exploration.

Tier 2 – Great Explorations

Marco Polo
Credited with bridging east and west. Perhaps most famous because of the book written about his travels by an author after he returned.

Ibn Battuta
The Arab Marco Polo. Lived in the 14th century and travelled from Morocco all the way to China and back again. Also wrote extensively about his life.

Apollo 15
Possibly the most important scientific mission of the Apollo era. Also brought in the “grandeur”.

Viking
First visit to Mars which had captivated imaginations for decades. Probably ranked here because I’m biased.

Speke-Burton
Discovered the source of the Nile – a major discovery for the era. Also represents the Victorian exploration of Africa which had a huge impact on the culture at the time.

Ferdinand Magellan
First circumnavigation of the globe, although a Pyrrhic victory. Discovered the Magellan strait, and also discovered the international date line because their ship log suggested it was a day later than it really was after they returned.

Lewis and Clark
Probably put this here because we live in the US. Anyhow, a seminal exploration of the American west – also represents many others that came afterwards.

Napoleon in Egypt
He brought along scientists and photographers and rediscovered ancient Egypt in Europe. This ignited a cultural phenomenon.

Tier 3 – Human Achievement
(However usually didn’t result in substantial cultural change or scientific discovery)

Amundsen
First to reach south pole. Widely heralded and an incredible testament to pure exploration. Also Amundsen discovered the NW passage – a big feat for its time.

Yuri Gagarin
First to orbit the Earth – if I lived in Russia this would replace Apollo 11. Not sure how to weight these, but I don’t think both deserve to be in the top tier. I think the returned samples from Apollo 11 and science from them put it at the top.

Captain Cook
A seminal figure in global exploration

Edmund Hillary-Norgay
Incredible achievement even though Everest isn’t really the highest mountain…

Apollo 13
Incredible statement on the power of human endurance and innovation.

Sir Francis Drake
Pirate, Explorer, Hero. Was the first Englishman to circumnavigate the Globe.

Xenophon & the 10 thousand
This is from a famous account of a greek mercenary contingent stranded deep within the Persian empire and having to fight its way back home. Also primarily famous because of the written account.

Heyerdahl
Famous book, and an incredible journey that represents the real explorers who first crossed the pacific.

Shackleton
Incredible story – probably more famous because of the photos and accounts that survive.

Lindberg
Another classic accomplishment – 33 hours straight!

Leif Ericson
First western settlement of the North American continent – historical significance was perhaps foiled by global cooling?

Chinese Exploration – Admiral Zheng He
According to Wendell, if these expeditions hadn’t stopped, there may have been a Chinese armada at England in the 15th Century.

Tier 4 – Honorable Mentions

Ballard - Titanic

Jules Verne

Galileo to Jupiter

Magellan to Venus

Cassini to Saturn

Cortes – Mexico/Central America

Peary – First to the North Pole

Post – First to fly around the world

Challenger Expeditions – First dedicated naval expeditions to explore the oceans

John Wesley Powell

Stanley-Livingston

LaSalle
Mississippi River

Prince Henry the Navigator
Portuguese Explorer

Jacques Cousteau

Problems in Mars Science - Mysteries or Puzzles?

I recently came across an article written by Malcolm Gladwell in the New Yorker called "Open Secrets". This seems to be about the fall of Enron and the prosecution of Jeffrey Skilling, but it also discusses an interesting way of thinking about problems. The article sets forth a dichotomy - a problem can be viewed as a puzzle or a mystery. These terms are a bit counter-intuitive and describe concepts that don't quite jive with what you might think those words mean.

So let me define the terms here. In the article a puzzle is a problem which has a definite solution where new information will yield an obvious answer. A mystery is a problem where we may in fact have too much information and may not come to a satisfactory conclusion. Gladwell argues that it is critical that we identify the particular type of problem we face (puzzle vs. mystery) because if we approach a mystery like a puzzle we will utterly fail to address it adequately.

If I may, I'll offer this analogy to make this clearer. Suppose you have a 1000 piece puzzle but are missing 900 of the pieces. You can approach this problem in two different ways - One person may decide to go looking for the missing pieces - maybe they fell out of the box, maybe they got put into a different box etc. Another person may realize that finding all of the pieces is now a fools errand, but careful analysis of the pieces we do have could reveal most of what the puzzle depicts.

Now lets complicate matters further and imagine that instead of just missing 900 pieces, you also have 3000 pieces of other puzzles mixed together with the puzzle you are trying to solve. Now if you choose to go looking for the missing pieces you may in fact find pieces that fit another puzzle altogether instead of the puzzle you are interested in.

I think this approach is a really valuable way of looking at scientific investigation. Perhaps just as valuable as the scientific method. Of course the scientific method is a different way of looking at problem solving and provides a much more detailed methodology.

There are a lot of scientists who approach scientific problems by hunting for the missing pieces (piece hunters). This usually includes people who specialize in a particular technique (remote sensing, isotopic analysis, TEM, etc.). In their world, their technique will provide the key missing pieces because the measurement has never been done before on whatever sample they are analyzing. Of course once they have found the piece (made the measurement using their favorite technique) they need to figure out how it fits. But since their entire effort has been focused on just finding the piece, they frequently look for an easy connection and write their paper.

There are other people (piece thinkers) who instead approach scientific problems by just examining the pieces (data) that has already been collected. These people find interesting connections that haven't already been made, and thus can make discoveries without finding out anything new. Of course this is usually difficult to do, since despite my callous description of the piece hunters, they usually have made most of the connections already.

Now, I don't think this dichotomy is a black and white thing. I think many of the piece hunters also spend a lot of time trying to understand the pieces already collected, and many of the piece thinkers also do their fair share of finding new pieces. Obviously, the most powerful science is done where both methods are combined and an investigation is targeted to find a specific missing piece. The problem for the piece hunters is that because of their technique specialty, they can only find certain types of pieces. If the targeted piece isn't the right kind, then they are out of luck.

Applying this whole concept to Mars, if we imagine Mars as a puzzle. We definitely have started with only a very few pieces, so it makes a lot of sense for us to focus our energies on obtaining new pieces to the puzzle. However in the last 10 years, this has resulted in a multitude of pieces - many of which don't fit into our specific Mars puzzle. So now we find ourselves in a position where piece thinking may prove more fruitful than piece hunting. In addition, I think it is time that the piece hunters sit down at the table and start putting more effort into understanding the pieces they've already collected, than simply collecting more for the sake of collecting more.

So switching back into the Gladwell terminology, I would say that Mars has changed in the past ten years from a puzzle to a mystery. More innovative thinking is needed to approach and identify the key scientific questions - and design better investigations.

Review of "Contemporaneous deposition of phyllosilicates and sulfates: ..."

This is a review of

Baldridge, A. M., S. J. Hook, et al. (2009). "Contemporaneous deposition of phyllosilicates and sulfates: Using Australian acidic saline lake deposits to describe geochemical variability on Mars." Geophysical Research Letters 36.

This paper came out a year before the Hurowitz et al. (2010) article that I reviewed above, and really deserves to be recognized for first discussing the topic of Fe hydrolysis as a source of acidity in martian groundwater systems, especially since the Hurowitz article does not reference it. This paper is one of the few good studies of a terrestrial analog. It is good because it uses the analog to make new and interesting hypotheses for Mars rather than studying an analog that seems similar to Mars and then not saying much of anything.

The paper examines the geochemistry of acid saline lakes in western Australia and notices that this geochemical behavior could also be happening on Mars. In the Australian lakes, saline reducing groundwater interacts with basement rocks to produce relatively Fe(II)-rich groundwater with neutral pH (6-8). However, when this water rises to the surface it becomes oxidized and is acidified through Fe hydrolysis reactions:

2Fe2+ + 1/2O2 + 5H2O ==> 2Fe(OH)3 + 4H+

The source of the acidity is either from diagenetic pyrite, sulfides in the basement rock, or oxidation of H2S. Note that none of these mechanisms create acidity through weathering of Fe-silicate minerals.

When this groundwater reaches the surface it mixes with fresh water from surface runoff and produces a series of minerals including kaolinite, sulfates, chlorides, opaline silica, Fe-oxides, jarosite, gypsum, etc.. The pH gradients created exist both with depth and laterally with the central portion of the lakes being higher pH along with the subsurface.

Applying this information to Mars, Baldridge et al. suggest that the Andrews-Hanna groundwater models would likely result in reduced neutral to high pH fluids - similar to the groundwater below the australian lakes. However, this groundwater may become oxidized and acidified as it rises to the surface if it contains enough Fe(II). They note that these environments may contain large geochemical gradients making it possible to precipitate phyllosilicates and sulfates contemporaneously.

This poses an interesting alternative to the "Bibring Hypothesis" which states that Mars went through an early phyllosilicate period with alkaline fluids then was later dominated by acidic sulfate forming fluids. I've always felt that this was way too oversimplified, and the Baldridge et al. paper clearly articulates a good reason why by showing that geochemical systems are complicated and it isn't crazy to have a single aqueous system capable of forming all of the minerals at the same time.

However, I think the Baldridge et al. paper makes the same mistake as the Hurowitz et al. (2010) paper does. On the bottom of page 4 is the following quote:

The long aquifer flow paths would also promote dissolution of Fe-bearing volcanic glasses and silicates, thereby enriching the water in Fe2+ ions. As observed in australia, Fe2+ transported in solution would eventually oxidize and precipitate Fe3+ phyllosilicates and/or oxides, while generating acidity in the upward flowing waters.

Here the idea is that you can generate acidity by dissolving Fe(II) bearing silicate minerals. As mentioned in my post about the Hurowitz article, this reaction when viewed in total is pH neutral:
2FeO(pyx, ol) + H2O + 0.5O2 --> 2FeO(OH)

It is important to keep in mind that in the Australian lakes, and in geochemical models, the only way to generate acidity through Fe hydrolysis is if you have dissolved sulfide minerals or added acidity to the solution at some point. This is the requirement for the groundwater models of Mars. However since we do not show a large enrichment of Fe over Mg and Ca in the Meridiani sediments it seems unlikely that iron sulfides have been the source of the sulfur. So in order for the groundwater models to obtain acidity, they need to incorporate large amounts of SO2 into the aquifers during rainfall and runoff. Furthermore a large portion of this acidity must be neutralized through weathering of Fe(II) rich silicates rather than Ca or Mg-rich silicates which will permanently neutralize the acidity.

Review of "Early Mars hydrology: Meridiani playa deposits and the sedimentary record of Arabia Terra"

This is a review of:
Andrews-Hanna, J. C., M. T. Zuber, et al. (2010). "Early Mars hydrology: Meridiani playa deposits and the sedimentary record of Arabia Terra." Journal of Geophysical Research (Planets) 115: 06002.

This paper outlines a scenario in which groundwater movement on Mars results in massive sedimentation throughout Arabia Terra and elsewhere. The author uses geophysical groundwater modeling to constrain the possibilities of groundwater flow on Mars - and many of the areas the model predicts to accumulate sediment are observed to have large deposits of sediment. The correlation between the model prediction for Meridiani being one of the areas of high groundwater flow and the existence of the Meridiani sediments has convinced many people that a groundwater origin is the most likely explanation for the Meridiani sediments.

The work assumes a very arid Mars (drier than the Atacama) with very long groundwater residence times. It assumes higher rainfall in the equatorial region which then serves as the source of groundwater to the rest of Mars.

Overall I really enjoyed this paper. It deals with big topics and addresses them in a process oriented manner setting forth a clear testable hypothesis. However, I do not agree with the conclusions and feel that the paper misses many important points.

Geophysical Problems:

The paper assumes a Mars similar to today except warmer, with temperatures above freezing in the equatorial regions which . This fails to account for the fact that the obliquity of modern Mars is rare. In fact ancient Mars was more frequently at higher obliquity (>30deg) than not. A more realistic model should take into account the average obliquity of Mars during the late Noachian/early Hesperian. Of course, if one does take this into account, the equatorial regions no longer are the warmest spots on the planet and would be unlikely to be source regions for rainfall. Instead these regions would be cold and icy.

The paper argues that recharge of valley networks is necessary to explain their origin and thus precipitation and melting must have occurred in the low latitude region. However, it is clear that valley network formation has occurred in more recent times (most likely during cold, dry conditions) but did not result in massive sedimentation. Also it is unclear that the volume of water needed to carve the valley networks compares with the volume needed to deposit the hundreds of thousands of km3 of sedimentary material in Arabia and Meridiani.

Geologic Problems:

There are obvious problems with the assumptions made in the paper about permeability, porosity, and the lack of any regional groundwater barriers. There are also problems with the fact that some of the intracrater layered deposits are higher than the rims of the craters that they lie in the middle of. The paper argues that so much sediment accumulated that it filled the available basins, and the water table rose high enough to start sedimentation in the plains around the craters. But I want to focus on four other specific things here.

Firstly, the lack of playas. There are no discernable or identifiable playas anywhere to be found in this region. Given the huge amount of evaporite deposition, it would seem likely that in some areas playas would form. There are none to be found so far, and the lack of these playas means there is no direct evidence for evaporative environments.

Secondly, the intracrater sediment mounds are almost universally sitting in the very middle of their respective craters. They have been differentially eroded around the edges of the crater leaving the central mound intact. If the water cementing these deposits was sourced from below, one would imagine that the areas of strongest cementation would be near the water source at the bottom of the crater and around the edges. Thus the sediment would grown from the outside inward. The reverse is true here, and it is unclear why the central portions of the basin should be so much more erosionally resistant than the outer edges.

Thirdly, the dips of these materials are too steep to account for deposition via liquid water. Instead layers frequently drape topography. This would be consistent with eolian reworked material - but then it would require all of the material to be reworked by eolian processes. This subverts the main mechanism for sedimentation argued for in the paper where it suggests that eolian materials are cemented in place by rising groundwater. If they are subsequently reworked by eolian processes, then it is not clear why they would remain in the same place and not be redistributed at the next shift in martian climate.

Fourthly, it seems that erosion is neglected in this model during the crucial time period in which the sediments are being deposited. Calculated deposition rates neglect any loss via erosion. If eolian reworking is indeed a part of the model, then it seems very likely that erosion could have been significant. Estimates of erosion at Meridiani at this time are around 0.8x10-5 m/yr (Golombek et al. 2006, Hynek and Phillips, 2001). This is on the same order magnitude as the deposition rates especially later in the model, and might be sufficient to reduce the deposition rate to zero.

Geochemical Problems:

The first problem here is the source of sulfur. If primitive martian rocks contain on average 1800 ppm sulfur, then you need to completely leach 100 times the volume of rock to get the equivalent volume of sulfate-rich material (20% Sulfate). It seems at such low concentrations, that in order to get enough sulfate, the amount of basalt weathering needed would create a super-alkaline solution or at least buffer the pH at neutral values. The salinity values used in this model assume Earth-like weathering processes to collect and concentrate salts. Without large sedimentary evaporite deposits from ancient oceans it seems unlikely that a brine with 80% of the salinity of sea water could be formed simply by interaction with basalt - no matter how long that interaction took place. Also, if it did, then it wouldn't be a sulfur-rich brine. In fact Na-Cl should play a much more dominant role.

The second problem is the bottom-up style of weathering. If the sediments were laid down by a rising water table, it is essential that sediments previously deposited would be saturated with water as the water table rose. Thus the most soluble components would be redissolved and redeposited on the surface. This repeated process would result in stratification of the deposit by solubility with the topmost portions being entirely composed of the most soluble minerals. This is the opposite of what we see at Meridiani, and it also is not indicated by remote sensing results from Arabia. Instead it seems that the sediments are well mixed with clastic material and the most soluble minerals are intermixed with the less soluble ones.

The third problem is acidity - since the basaltic aquifer should not yield acidic solutions - This is dealt with in the previous blog post - along with the Fe-oxidation theory published by Hurowitz et al..

Climate Problem:

Finally, the age of the Meridiani deposits place their formation at the end of the Noachian into the early Hesperian (Hynek and Phillips, 2008). Thus sedimentation ceased near 3.5 Ga. This is substantially after the dynamo shut down and loss of the magnetic field which was likely ~4.3 Ga. Atmospheric loss processes were well underway, and climate models have had a difficult time simulating any martian climate that is warm enough to host liquid water on it surface at any time (see Tian et al. 2010). This period also postdates the late heavy bombardment which likely sloughed off much of the martian atmosphere. Thus it becomes very difficult to argue that conditions on Mars were warm and wet for any prolonged period during this time.

The best argument for warm/wet conditions come from the existence of valley networks in the region, and evidence for surface flow of water within the Meridiani bedrock. However, there have been multiple arguments made that suggest valley networks do not require warm/wet conditions - and there are several examples of late Hesperian and early Amazonian valley networks that most likely did not form in a warm/wet climate.