Day 5: Hematite Hot Spot
by Angela Nan
Today, one of the topics we focused on the most was the Meridiani Planum and its puzzling history. We got a taste of the complexity of the problem in the discussion of minerals and morphology on Mars in the guest lectures, so we were interested in exploring the question ourselves.
The Meridiani Planum has been a location of interest ever since we saw that blip of hematite from the earliest mineral spectroscopy data. At first, the blip looked complete random; why would there be such a significant hematite deposit at that location and at that location only? On closer inspection, we could see that the local topography was a bit
different—the ground was much smoother and flatter than the crater-riddled neighborhood. This was not enough to explain why there was hematite there, however, so we looked into this question more in class.
We considered many possible explanations for the hematite's abundance and locality. From the videos, morphological evidence showed that the area had a history of sand dunes and groundwater seepage and that the two processes danced with each other over a large span of time, probably in the millions-of-years-old range. This suggested that the area went through periods of mass desertification and groundwater discharge, but this raised a bigger question: where did all the water come from and go to on such a large scale?
The first thing we considered was the location of Meridiani Planum. The Planum is a little patch of flat ground on the equator in the middle of a larger flat plane bordered with highlands to the south and Tharsis and Valles Marineris to the west. This puts the Planum in the Hesperian, which we know is the period characterized by global drying out and the rise of Tharsis with massive outflows toward the north. We figured that these events could be the source of water for the Planum. We already theorized that the volcanoes of Tharsis could have heated up groundwater or groundice and caused huge flooding events to its northwest. This could have brought periodic surges in groundwater to Meridiani Planum and formed the hematite concretions between periods dominated by sand dune formation.
Even though we looked at a bunch of different factors that could've been important in the origin story of the Blueberries, we haven't found out as much about it as we wanted to. For one, we still haven't really answered the question of why the hematite only appeared in that one region. Was there only material for hematite in that one place? If so, how and why? If not, where is the rest of the hematite? More fundamentally, did we come up with a correct history of the Planum? What are the other factors we need to consider in this story? All these and more questions will have to wait for now.
Much of today's class was occupied by a excited discussion about Meridiani Planum. As discussed in the lectures, Meridiani Planum is geologically fascinating because it boasts both hematite and sulfates, which contain clues to the history of water on the planet. The region first gained popularity when the Mars Global Surveyor's Thermal Emission Spectrometer (TEM) returned images of hematite concentration on Mars. Meridiani Planum stood out as a beacon of hematite, and a rover was promptly sent over to learn more.
Hematite concentration on Meridiani Planum, Mars. Taken by the Mars Global Surveyor's Thermal Emission Spectrometer (TEM). Image credit: NASA/JPL/ASU.
Meridiani Planum is now the home of the beloved and resilient Mars rover, Opportunity. During Opportunity's tenure on Mars, we have learned that Meridiani Planum is covered in sulfate-rich sedimentary layers and hematite "blueberries." But were did all of the sulfates and hematite spheres come from?
Sedimentary layers and blueberries cover Endurance Crater in Meridiani Planum, Mars. Image Credit: NASA/JPL/Cornell. http://marsrover.nasa.gov/newsroom/pressreleases/20040608a.html
The class quickly agreed that both water and a sulfur source are necessary for sulfate formation. Several students suggested the nearby Tharsis volcanoes might be the source of sulfur, but Prof. Mike Brown reminded us that sulfur is rather common on Mars. But the sulfates are not. Sulfates are unique because they require water to form. But wait.... Perhaps the volcanoes are important after all. The volcanoes may have provided enough heat to melt ground water ice, which may have flowed through the ground to Meridiani Planum. Sulfates are also seen in Valles Marineris, so this theory sounds plausible.
But wait.... The “blueberries” are only found in Meridiani Planum, not Valles Marineris. So what is the difference? Students threw out a variety of ideas. Meridiani Planum is near the equator. Does that matter? Is it’s elevation important? Did ice ages due to Mars’s changing obliquity have an impact? Prof. Mike suggests that Meridiani Planum’s flat slope is likely important. Water likely seeped in from the nearby water channels. But much of the region surrounding Meridiani Planum is also flat, and “blueberries” are only found in a localized area... There must be more to the story. As the end of class drew near, students anxiously raised their hands to offer more hypotheses.
Before we could conclude upon a resounding "maybe" to any of the questions above, we ran out of time and another class kicked us out of the classroom. We’ll just have to wait until next time....
Location of Meridiani Planum on Mars. Image Credit: NASA/JPL/ASU.
by Junjie Yu
Today we had a rich discussion on the Meridiani Planum, the landing site of the Mars Exploration Rover Opportunity. It is one of the two places that the early Mars rovers decided to check out.
If you zoom out the topographic map of the Mars, you can see the features of alluvial fans and the erosion of the crater rim. The Meridiani Planum is very flat and smooth on the surface. It implies the existence of flow water on Mars. But where does the water come from? I would call it nowhere since none of the alluvial features are well-developed and extend very far. It is very hard for us to imagine that groundwater was important here; however, groundwater precipitation is one possibility that formed the sedimentary rocks. Why do we say so? The Thermal Emission Spectrometer has indicated an anomalous concentration of hematite in this region. Hematite is the mineral form of iron oxide (Fe2O3).
Hematite is typically found in places where there has been standing water or hot springs, such as those in Yellowstone National Park in the U.S. If you take a look at the very earliest images provided by Prof. John Grotzinger, you can notice some small spherical things on the eolian sand dunes. We know they are wind-generated because the sandstones have cross-bedding, the characteristic feature of sand dunes. They look very similar to blueberries. The blueberries are actually hematites. The formation of hematite requires iron contained in the surface material and hot water that circulates in the region. The groundwater comes up and goes down, so that the hematite will precipitate out and grow as concretions. That is why we observed tree-ring like features inside of the blueberries as well as the outside layers. This observation is very important because it confirms earlier, geomorphic evidence for the existence of water on the surface of Mars in the geologic past.
Figure 1. Examples of eolian sand dunes with current-ripple cross-bedding (Grotzinger et al., 2005, EPSL).
Day 6: Ice Caps
Today we discussed the fluctuating surface pressure of Mars as recorded by two Viking landers. A graph of the surface pressure vs. time of year was displayed as we gathered into groups to identify reasons for the sinusoidal-like fluctuations (two peaks, corresponding to southern and northern hemisphere summers and two troughs, corresponding to southern and northern hemisphere winters). The source of higher surface pressures in the summers is likely due to the evaporation of CO2 ice caps. As temperature increases, Martian ice caps retreat, releasing CO2 into the atmosphere and increasing atmospheric pressure. As temperature decreases, Martian ice caps grow in size, capturing atmospheric CO2 and decreasing atmospheric pressure. The southern hemisphere winter corresponds to a deeper trough due to colder and longer winters (Mars is near aphelion, which means it is further away from the sun and moves slower). One Viking lander was at higher elevation than the other, as demonstrated by an overall lower atmospheric pressure (by 1 mbar). We performed a quick calculation to confirm that CO2 ice caps could in fact explain the trend in data collected by the Viking landers.
We wrapped up our study of Mars with a competition between the three groups. Each group was instructed to create proposal for a new rover mission to Mars that would answer questions about what it was like on Mars 4 billion years ago. The cost could not exceed ~$10 billion. Our group decided to send two small rovers to the Noachian region characterized by magnetic stripes: one rover on each side of the axis of symmetry. The rovers would be equipped with a dating instrument, an IR spectrometer and a seismometer. Moving away from the axis of symmetry in opposite directions, the rovers would date the rocks, measure the water content, identify the mineralogy and determine whether there is a distinct layer associated with rifting (through seismic refraction). If in fact the age increases as the rovers move away from the “rift,” it may be evidence for plate tectonics. This would suggest that there was abundant, non-intermittent water during the Noachian. Correlation between the change in climate, atmospheric composition and magnetic field with the end of plate tectonics may help us understand what happened on Mars ~4 billion years ago. If we do not find evidence for rifting, we would seek to explain the observed magnetic patterns – which is in itself a puzzling and interesting question as we have never observed such a striking pattern without rifting. After each group presented, it was announced that the government would not be funding a new rover mission due to the recession (even though our group insisted that there would be military applications). Of course, we rebelled, took over the “government” and funded all three Mars rovers (okay, not really).
by Jingyuan Li
For the last lecture on the Mars unit, we discussed the surface pressure of Mars, starting with the figure below. Viking Landers 1 and 2 are two spacecrafts which landed on different locations on Mars. The graph shows the surface pressure of Mars throughout one year, with 0 Ls (solar longitude) being the Northern Hemisphere spring equinox. This figure raises some questions. Why did the two spacecrafts record different surface pressures? What is the explanation for the fluctuation of the surface pressure throughout the year?
The answer to the first question can be found by thinking about the definition of surface pressure: the force of air above the ground divided by the surface area. Since Viking Lander 1 has a lower surface pressure than Viking Lander 2, there must be less air above Lander 1. Thus, Lander 1 is at a higher elevation than Lander 2.
The second question draws upon our knowledge of the polar caps on Mars. There are CO2 ice caps at both the north and south pole. In the summer, the surface temperature rises and causes the ice to sublimate, releasing more gas into the atmosphere. In the winter, the cooler temperature causes the CO2 to freeze, decreasing the amount of gas in the atmosphere. This explains the sinusoidal pattern in the surface pressure. Lastly, the two maximum points are not equal because the two polar ice caps are not equal in size.
The next thing we did was find an equation for the scale height using dimensional analysis. Scale height, H, is the vertical distance over which the pressure of the atmosphere changes by a factor of e. We are given the following variables:
kB, the Boltzmann constant, with units kg*m2*s–2*K
T, the temperature, with unit K
g, gravity, with units m*s–2
M, mass, with units kg
To solve for H, we need to put the other variables together so that we end up with the units of H (meters). The final equation is: H = (kB*T)/(M*g).
Finally, we want to calculate the surface pressure change on Mars if the ice caps sublimate. For the Northern polar ice cap, the diameter is 1200 km, the thickness of the ice is 2 m, and the density of the ice is 1600 kg/m3. We first find the mass of the CO2 in the ice cap:
density = mass/volume
1600 kg/m3 = M/(πr2), where r = 600 km.
This gives us M ≈ 3.5 *1015 kg.
Now we can calculate for the surface pressure. Start with the ideal gas law, PV = nRT. If we replace the R (gas constant) with kB and divide by volume on both sides, we get: P = n* kB*T, where n = number density (number of molecules per volume unit).
Now plug in H*M*g = kB*T (from our first equation): P = n*M*g*H.
Combining n, M, and H we get:
P = ncol,dens*g, where ncol,dens is the mass column density.
The mass column density is just mass/volume, so our final equation for pressure is: P = (M*g)/(4πr2 ).
Plugging in g = 3.7 m*s–2 (gravity of Mars), r = 3.39*106 m (radius of Mars), and the mass we calculated earlier (M ≈ 3.5 *1015 kg), we get a final answer of P ≈ 0.927 mb.
Looking back at our original figure (and knowing that the Landers are in the Northern Hemisphere), the difference between the Northern summer and winter is approximately 1 mb, which is basically the same answer we just calculated, using crude approximations.
For the last half hour of class, we brainstormed some ideas about how we could find out what Mars was like during the Noachian period (around 4.1 to 3.7 billion years ago). Each of the three groups were given $10 billion and told to propose their own mission to Mars. We ended class by presenting each of our group's ideas and voting on the best one.
Today was your last day on Mars. Our first mission was to explain the variation in atmospheric pressure from the Viking Lander data in Figure 1. Some notable features are the two maxima and minima in atmospheric pressure. What could explain this variability? The polar caps are mostly made out of CO2 ice and condensation and evaporation actually can have an effect on the partial pressure of CO2, but is this increase CO2 enough to explain the variation?
Figure 1. Atmospheric Pressure vs. Ls for Viking Lander 1, bottom curve, and Viking Lander 2, top curve. Ls shows the location of Mars around Sun with Ls = 0 as spring equinox for the Northern Hemisphere. Source: Smith, M.D. (2008). Spacecraft observations of the martian atmosphere. Annu. Rev. Earth Planet. Sci. 36:191–219
Put in perspective, the 3 mbar variations observed in Figure 1 is only a fraction of the about 1 bar atmospheric pressure at earth’s surface. To tackle this question we first thought about how Mars’ atmosphere look like and how does it varies with altitude. Obviously, the atmospheric pressure had to decrease with altitude. However, it was unlikely to be linear. Below is the sketch we decided on.
Figure 2. This is sample sketch of Mars’ atmospheric pressure (as of function of the initial pressure) vs altitude (m). I cheated a little bit because I drew this plot using approximate values from our proceeding calculation. I used Equation 1 and assumed an e-folding length of about 6000 m. In class, we had the same curve without the exact values.
Exponential decay of atmospheric pressure made sense because we want approach zero atmospheric pressure as we enter space. Therefore,
Patm=P0 e(–x/τ). (Equation 1: Atmospheric pressure as a function of P0, pressure at the surface, x, altitude, and τ, the e-folding length.)
What could affect pressure? (Or more precisely what could affect tau). Temperature (which is proportional to the energy of the gas), gravity, and CO2 particle mass.
Using dimensional analysis and the fact thatτ, the e-folding length, had units of length we derived Equation 2: τ ∝(kb T)/(mparticle g). (Equation 2: τ, the e-folding length, is proportional to kb T , which is related to the energy of the gas, g, acceleration of gravity on Mars, and the mass of a CO2 particle. kb is the Boltzmann constant and T is temperature in Kelvin. )
Next, we were given that every winter 2 m of CO2 ice (density of 1600 kg/m3) melts and evaporates through a region that is 1200 km in diameter around the North. This vital information then allowed us to calculate the equivalent CO2 partial pressure.
The first equation that came to mind was the ideal gas law (Equation 3).
P V = N kb T. (Equation 3: Ideal gas law where N is the number of particles and T is temperature. Since we’re looking at ice sublimating we can use T = 145 K.)
Assuming that the atmosphere instantaneously equilibrates, we set our volume that the gas can occupy equal to the surface area of mars times the e-folding length. V ≈ SAMars τ. (Equation 4: Approximate the volume the CO2 gas can fill as the surface area times the e-folding length.)
Combining Equation 2, 3, and 4, we solve for atmospheric pressure.
P ≈ (mice g)/SAMars. (Equation 5: Assume N * particle = mice and g is at Mars’ surface.)
Thus our order of magnitude estimation shows that P ≈ 1 mbar and the variability on Mars’ atmospheric pressure can likely be explained by CO2 ice volume changes during the summers of the North and South Poles.
I hope you get a chance to hear about our second mission of the day. We designed our own missions to explore the biggest unknown question: What was the Noachian like? But, I guess it didn’t matter in the end because the “committee” (a.k.a. Professor Mike Brown) decided that Congress canceled the mission.