Day 3: Temperature Balance & Radioactive Dating
For the third class, we primarily discussed temperature-balance situations in order to continue exploring the possibility of water on Mars. We began with general questions regarding video lectures 1.06–1.15. “Have we been able to trace flow paths from source to depository?” As discussed in the videos, the answer to this is, “Yes.” We have seen evidence of precipitative flow with dendritic patterning, for which we have many terrestrial analogs. On Mars, we have also seen outflows that seem to originate from “piles of rubble,” as Professor Brown described them. We attribute these to subsurface ice, which may meet magma, melt, and subsequently cause the ground above it to collapse, freeing the water. But why wouldn’t these flow paths be the result of debris flows? Professor Brown tentatively answered that the elevation gradients of certain areas may not be high enough to instigate flow. While this answer is not very rigorous, it does, intuitively, make sense.
We then considered several temperature-balance problems.
In one problem, we considered a sealed chamber with a layer of ice at
–10°C and a vacuum in the rest of the chamber. In this case, the ice would sublimate until vapor pressure is reached.
In another scenario, we again had a sealed chamber with a layer of ice at
–10°C and a vacuum, but there is no energy transfer allowed between the chamber and the environment. In the previous problem, we considered that energy transferred into the chamber from the environment would provide the energy necessary for sublimation. What would happen in this case, where energy is not able to transfer? We determined that the ice will still sublimate, but it will get the energy required from the rest of the ice layer. This would cause the ice to become colder. It was not easy to immediately accept that the ice would spontaneously cool, but we can consider the changing entropy of the system (where the gas is at higher and, therefore, more favorable entropy) to encourage the spontaneity. The matter will transition, decreasing in temperature and increasing in pressure until it reaches equilibrium between solid and gas. If we think of a phase diagram, the system will start at a point “southwest” of the triple point (due to the lower temp and pressure), and end on a point on the line that denotes the equilibrium between solid and gas. The path that connects the start and end point will follow the conditions that the system undergoes as the ice sublimates and lowers in temperature.
Changing gears, we then broke into pairs and went over various forms of radioactive dating.
The first mechanism we discussed was the Uranium-Lead system, for which we know the half-lives (4.5 Ga for 238U ⟶ 206Pb and 0.704 Ga for 235U ⟶ 207Pb). When we consider zircon, a silicate mineral, we know that it contains only Zr, Si, and O. However, uranium is lithophilic and will insert itself into the lattice structure. When performing U-Pb dating on zircons, we can look at the ratio of Pb to U, knowing that the initial content of Pb was zero. Then, incorporating the half-life, we can easily determine the age.
Next, we discussed the Potassium-Argon system.
We know that potassium is very abundant and composes many igneous rocks (think, potassium feldspar) and that when these rocks form, gases, such as argon, are pushed out. So, similarly to the U-Pb mechanism, we have our product at an initial value of 0. Coupled with the half-life (1.2 Ga), we can determine the age fairly simply.
by Jiabin Liu
From previous lectures we agreed that carbon dioxide (CO2) was the primary source of the polar cap on Mars, which shows seasonal expanding and shrinking features just as the ones on Earth do. So the Mars polar cap is not mainly water ice. There is, however, evidence on Mars surface that shows water might have been present in liquid form in the past. These features include large outflow channels, deltas, and river networks. Several questions arose here.
First, could these channels caused by other fluids, like liquid CO2? The answer is very unlikely. If we look at the phase diagram of water and CO2 (Figure 1), we notice that CO2 only exists in liquid phase under high pressure in the temperature range of Mars. Mars’ atmosphere has a low pressure that cannot sustain liquid CO2.
Figure 1: Phase diagram for water and carbon dioxide (CO2). Taken from DocStoc.com published by user “mainskweeze”.
The second question is, could these channels be caused by debris flow? Many regions of Mars are covered by dust, but where the channels occur are not steep enough to generate debris flow. Much more energy is needed to drive a dust flow than to drive a liquid flow, so the channels are still more likely from water flow.
Finally, if we conclude that these channels are indeed water channels, what are the origins of these water? To answer this question, we look at rivers on Earth. If the rivers come from precipitation, they originate from locations with high elevations. We could trace the channels on Mars back and hypothesize where they start from.
We then discussed in groups about the volatility of water ice, which is important in understanding the fluids and ice on Mars in past and present. We focused on the condensation and sublimation of water and used simple cases to illustrate several points.
First, how do one calculate the sublimation rate? If we consider a case in which sublimation and condensation is in equilibrium, we could figure out the condensation rate first, and sublimation rate will just equate. Consider a slab of ice in equilibrium with air at 20 ºC. The condensation rate depends on how fast the gas molecules hit the ice, and this molecular speed is determined solely by its temperature. Also, the rate depends on the partial pressure of water vapor, or how much molecules are present in a unit volume. If the sublimation and condensation are not in equilibrium, it is easy to see that the rate will also depend on the difference between the partial pressure and the vapor pressure.
As examples, we look at ice rinks where the temperature is kept at –7 ºC so that the ice is in constant cooling and will not melt. Also, ice cubes in freezers are kept at temperature around –10 ºC. When you open your freezer, moisture from outside air will add to the freezer and condense to cause frost on the freezer’s wall. Ice cubes inside, on the other hand, also sublime and condense to the wall, so they do not last very long in the freezer even if you do not open it.
Finally we talked about radioactive dating, the method scientists use to know the age of rocks. The three common kinds are (1) Uranium-Lead dating, (2) Potassium-Argonne dating, and (3) Samarium-Neodymium dating. They are useful in different ways, but the main idea is the same: the decay happens in a certain timescale (known as half-life), so we measure the ratio of the mother element to the daughter element to know how long a certain piece of material has been formed. These decays all have very long half-lives, so they are ideal in measuring ancient rocks. Just as how we do it on Earth, we can pick up rocks on Mars and use radioactive dating to know their ages.
Day 4: Martian History
by Alec Brenner
We began the day with a remanent magnetic anomaly map of the Martian surface. While Mars has no significant global magnetic field, it has strong, regional, crust-coupled variations in magnetic field direction and strength, which are actually stronger than those typically found on Earth. More intriguing is the pattern formed by the anomalies: broad, well-defined lineations of alternating field direction are present, and appear eerily similar to seafloor magnetic lineations produced on Earth around spreading centers. These lineations appear on the heavily cratered southern highlands (i.e., Noachian terrain), and roughly encircle the equator.
How did these pronounced lineations show up, though? After all, Mars shouldn’t have plate tectonics, which produce similar features on Earth. One possible formation mechanism invokes a (Noachian) spreading center in the southern hemisphere, producing the lineations, and a subduction zone in the (now recycled) Noachian northern hemisphere. If true, this means that not only did Mars have plate tectonics, but it also had a strong, periodically-reversing magnetic field just like Earth. But this theory might have issues, and the only way to test it would be to do high-resolution aerial or ground-based magnetic surveys to look for the center of the spreading ridge in the lineation data.
This launched us into a discussion of the implications of plate tectonics (or the lack thereof) on other planets, including Earth and Venus. On Earth, plate tectonics operate such that crust is divided into two main types, oceanic and continental. This split means that a histogram of elevations on Earth’s surface has a bimodal distribution, one peak for each type of crust. This double peak is a hallmark of plate tectonics, and is present on Mars to some extent (although it is significantly noisier due to impact cratering). On Venus, though, the distribution of elevations is approximately gaussian – that is, there is only one type of crust. This implies the absence of plate tectonics, and thus the absence of efficient convective heat output by the planet. This in turn requires that conduction be responsible for most of the heat output from Venus’ interior, but this process is so inefficient that radiogenic activity in the planet’s core produces heat, and more quickly than conduction can get rid of it.
What would this mean for Venus? If the core continues to heat up, does the planet simply “explode”? Not necessarily, but it comes surprisingly close. As one theory would have it, the surface of Venus periodically undergoes complete turnover. In a process similar to how lava lakes can turn over, a piece of crust heats up enough to expose the molten interior, and the crust founders (sinks) in a chain reaction which leaves the entire surface molten. By this theory, the surface of Venus might be as young as 100 Ma (Ma = “Megannum,” or a million years) as opposed to conventional thought that much of it dates to billions of years ago. The jury’s still out on this debate, as better data on the age of the Venusian surface are needed to make any conclusions.
It was at this point that we realized that none of the previous discussion had been scheduled for that day; we were supposed to be looking at Mars topography with MOLA data. We turned our attention for the last half hour to this task, and began by examining some of the features we had found in the past week’s homework set. One interesting set of features was the Adamas Labyrinthus region of Utopia Planitia: crisscrossing “channels” covered the surface, and looked not dissimilar to mud cracks. The eventual conclusion was that they were probably some kind of outflow channel network, perhaps on a tidal flat.
We also examined a set of outflow channels on the eastern Hellas Basin which appeared to have a slightly different morphology than this farther north, in that they appeared to start with lakes rather than groundwater seepage areas.
Finally, Prof. Brown pointed out a strange feature of craters in the northern plains. Specifically, many of them have prominent ejecta “aprons” that show up well in topography. These raised areas could be due to impacts on what Prof. Brown described as a “splooshy” surface (highly technical terminology), such as a shallow sea, as opposed to the rigid bedrock which predominates in the southern highlands. We ended class for the day here.
by Valerie Pietrasz
Today we discussed a series of open-ended questions for which we don’t really have satisfying answers.
The first question was about magnetism on Mars. We know that Mars currently does not have a noticeable magnetic field. However, if we look at an image of Mars’ crustal magnetism (We looked at this one in class.) we see clearly see the remnants of a magnetic field captured in the crust—and it turns out this record is stronger than the paleomagnetic record we see on Earth. Moreover, we see very distinctive stripes laterally across the planet. So what caused this?
Our first suggestion was solar wind: it was much stronger earlier in our solar system’s history, and with a lack of a magnetic field on Mars, this could have caused the crustal imprint. However, Mars probably did have an atmosphere at this time that would have protected the planet from the solar wind as it sputtered away. So this is unlikely.
The next proposal was plate tectonics—although we believe that plates are not moving around on Mars now, they may have been in the past. This would imply a convecting mantle capable of generating a magnetic field, like on Earth, that would leave an imprint on a cooling crust. Moreover, plate tectonics would account for the fancy stripes we see—we could have spreading plates that capture the field symmetrically around the spreading center. So this is more likely. However, we do not have much more compelling arguments for plate tectonics on Mars, so we cannot confirm this.
Next, we discussed histograms of elevation on the rocky planets in our solar system. On our lovely Earth, this histogram has two peaks: at the level of the abyssal plains of our oceans and just above sea level, on our continents. (Looky here!) This is because we have two types of crust, oceanic and continental, that are in and equilibrium due to plate tectonics.
Interestingly enough, if we look at one of Mars, this histogram looks very similar: although a little messier, it also has two similar peaks at similar elevations. Wait, is this more evidence for plate tectonics in the past? Hmmm.
However, if we look at the histogram of Venus, we see instead a Gaussian curve with a single peak in the middle. This implies a lack of plate tectonics that creates the peaks we see on Earth and Mars. Unfortunately for Venus, plate tectonics is a planet’s primary mechanism of cooling its insides. Volcanoes, conduction, and radiation all contribute to cooling, but they are all terrible at it. This is part of the reason Venus is so freaking hot.
(After class I also found this diagram to play with if you’re more curious)
Lastly, we discussed a couple more unusual features on Mars’ surface courtesy of the third homework problem from week 1. First, we looked at portions of the northern basin (specifically around lat, long (105º, 37º)). There, we noticed a few interesting cracks that look like they could be very large mud cracks…or more drainage channels. More interestingly, we noticed that the craters in the area looked “splooshy”: they had a raised ring of sediment surrounding them, whereas craters found in the southern highlands do not. But what made these craters splooshy? Meteorites impacting an ocean full of water would cause a sploosh. However, the age of the outflow channels suggest the oceans existed during the Hesperian period—and the craters are Amazonian. So probably not. What about ice in the surface layer that melted upon impact? If we had that, then we would probably see splooshes in the craters in the southern highlands… which, unfortunately, we don’t.
Another inconclusive discussion.
Finally, we looked at Hellas Basin (85º, –33º). We found what looked like two large outflow channels that looked like they could be drainage into a lake. However, we could not find any other channels or other clues that this was a case…so again, maybe not.
Then we ran out of time and class was over.
Lecture today was mostly centered on discussion about questions students had. The three primary topics were:
1. How did the magnetic features on Mars arise?
2. How can we tell if plate tectonics have occurred on a planet?
3. Why do craters during the Amazonian appear different than most craters on Mars?
Examining the magnetic features of Mars, we can see a distinct set of alternating bands. On Earth, this would be attributed to spreading sea floor ridges. When basalt oozes up from the mantle, it preserves the magnetic field present at the time of cooling, giving a series of alternating bands that match the reversals of Earth’s magnetic field. However, we are not sure if plate tectonics occur on Mars, and the areas where subduction zones might be present (areas where crust is re-melted to make room for the new basalt) are obscured by newer Amazonian features. The class voted on whether they were spreading centers or not and the class was basically split in half, which just goes to show you that scientific understanding in this area is not complete! One way the hypothesis could be tested is to date the stripes and see whether the date was progressively younger towards the middle of the features. That would be pretty definitive evidence of spreading centers.
One way to tell if plate tectonics might have been occurring on another planet is to map the frequency of different elevations. On Earth, the distribution has two “humps,” corresponding to the two different kinds of crust. The first is oceanic crust which is denser, thinner, and richer in basalt. Continental crust is thicker and more buoyant (which is why I’m not writing this post from underwater) and primarily made of rocks like granite. This distribution indicates Earth has plate tectonics. On Venus, which does not have plate tectonics, the distribution is approximated by a Gaussian or one “humped” shape (like a bell curve) because all the crust is basically the identical. Interestingly, on Mars we also see a curve with two “bumps,” which may provide evidence that it once had plate tectonics.
Finally, we looked at some interesting craters in the flat, northern region of the planet on the MOLA Color map. These particular craters looked more “splooshy” then the sharp edged, well-defined craters in the older, southern region. Maybe this change in appearance had to do with a difference in the composition of the rock being impacted? We briefly discussed the idea that subsurface ice could affect the appearance of a crater.
Finally, a fun fact: There is a middle school in Nebraska called “Marrs Magnet,” which complicates Google searches on this topic.