Homework 6. October 19, 2011

Transcription

1 Homework 6 October 19, Chapter 4 2. What do we mean by the geological record? why is it important? The geological record is a collection of rocks and fossils that tell us about Earth s past. It is important because analysis of these rocks can help us to answer important questions about the development of our planet. 5. Explain the technique of radiometric dating and how we know it is reliable. Be sure to explain what we mean by a radioactive isotope, parent and daughter isotopes, and a half life. Radiometric dating is the technique of determining (or estimating) the age of an object by considering the kinds of radioactive decays that could have occurred in the atoms that compose it. The idea is that, if we have a decay process where atom A decays into atom B we can determine the age of the sample by measuring the ratio of the numbers of the two atoms. We can do this because we have a physical model of radioactive decay that tells us how this ratio changes with time. We know that, Then, we can solve for t to get A = (A + B)2 t/τ ln((a + B)/A) t = τ ln 2 In this model atom A would need to be a radioactive isotope, which is an isotope that undergoes some kind of radioactive decay. And, in this decay A and B would be called the parent and daughter isotopes respectively. The parent isotope is what we start with and the daughter is the decay product. Finally, τ is the half life, which is the time it takes for half of the sample to decay - half of the A s decay into B s. We know that this method is reliable for several reasons. First, we can perform a consistency check and make sure that the method gives us the same answer if we test the same rock with 2 different decay processes or test different samples that we assume must be the same age (like if we break a rock in half and test the 2 halves independently). Also, radiometric dating agrees with other methods of dating, like counting tree rings. 1

2 10. What were the heavy bombardment and the late heavy bombardment and what effect might they have had on life? Both were periods where the Earth experienced a large number of collisions with meteorites and other bodies in our solar system. The heavy bombardment occurred a few hundred million years after the formation of the solar system. We believe that it was caused by Earth colliding with other planetesimals in the path of its orbit that formed by accretion in the solar nebula. the late heavy bombardment was a spike in impacts that started 4.1 billion years ago and lasted 300 million years. This might have been caused by a change in Earth s orbit moving it towards groups of planetesimals that were previously far enough away from the planets to avoid collisions. Now, clearly these frequent collisions would have been dangerous to any life on Earth at the time. Although, even when the collision rate was at its highest we would only expect a large collision every million years or so. Then, the question is if these events were destructive enough to kill all life on the planet, essentially resetting the development of life. It seems like the answer is at least pretty close to yes. The book says that the largest collisions could have released enough energy to vaporize oceans and melt the Earth s crust. However, it is possible the some microorganisms living deep underground or at the bottom of the ocean could have survived. So, this means that life could never have evolved past this point during this time, but it could have existed and developed while the collisions were happening. 13. Describe how plate tectonics shapes important geological features of Earth, including mid-ocean ridges, continents, mountain ranges, rift valleys, and earthquakes. How did Hawaii form? Mid-ocean ridges are caused by tectonic plates moving apart (this is called seafloor spreading). Then, at the boundary between the plates molten rock from the mantle can seep into the ocean. When this rock solidifies it forms the ridges. Continents are formed from thick slabs of continental crust, which is less dense and more varied in composition than seafloor crust. This means that the continents must have formed gradually over time. Mountain ranges can be formed when two continental plates collide in such a way that they both get pushed upward. Rift valleys are thin patches of crust that are formed when continental plates move apart. Earthquakes happen most frequently when plates scrape against each other. Sometimes, if the boundaries of the plates catch the tension can build up until it is quickly released in the form of an earthquake. Hawaii was formed by the motion of the Pacific plate over a hot spot. The lava that erupts from the hotspot is what forms the islands. 2

3 18. What are ice ages, and what may cause them? What do we mean by snowball Earth periods, and how does Earth recover from them? Ice ages are periods in which the average temperature of the Earth drops a few degrees, which causes an increase in snowfall. They may be caused by variations in the rotation and orbit of the planet. In particular, changes in the tilt of the Earth s axis can make the seasons more or less extreme, which could account for the changes in climate. Snowball Earth periods are particularly long and cold ice ages in which the oceans could freeze and much of the Earth s surface can become covered with ice. Earth can recover from these periods because even if the oceans are frozen the inner sections of the Earth continue to produce carbon dioxide through volcanic activity. However, the frozen oceans would not be able to absorb the carbon dioxide the way liquid oceans can. So, the greenhouse effect would have gradually increased until the planet warmed up. 1.1 Does it make sense? 21. We can expect that if there are paleontologists a few million years from now, they will find the fossil remains of almost every human who ever lived. This does not make sense because most organisms do not fossilize, indeed it only happens under very special conditions. 22. Nearly all the rocks I found in the lava fields of Hawaii are igneous This makes sense because igneous rock is formed by volcanic activity. 23. The most common rock type in the strata of the Grand Canyon is sedimentary rock. This makes sense because we know that sedimentary rock is characterized by the formation of strata due to changes in the way sedimentation occurs over time. 24. Although EArth contains its densest material in its core, it s quite likely that terrestrial planets in other star systems would contain their lowest-density rock in their cores and their highest-density rock in their crusts. This does not make sense because it is the Earth s gravity that causes material at the core to be denser. We expect gravity to work the same way in other star systems so we would expect to see the same radial ordering by decreasing density. 25. If you ha a time machine that dropped you off on Earth during the Hadean eon, you d be quickly killed by a large impact. This is unlikely since we said above that even during the heavy bombardment periods There was only a large impact every million years or so. 3

4 26. If there were no plate tectonics on Earth, our planet would be far too hot to have liquid oceans. This does not make sense. The Earth is not cooled by plate tectonics. The book says that venus may have no plate tectonics because the drying out of it s crust due to the evaporation of its oceans may have made the crust more resistant to fracture. However, the cause and effect is reversed in that case. 27. Without the greenhouse effect, there probably would be no life on Earth This makes sense, without the greenhouse effect the Earth would be too cold for liquid water on average. 28. If nitrogen were a greenhouse gas, our planet would be far hotter than it is. This makes sense because nitrogen is the most abundant gas in the atmosphere. 29. We can learn a lot about Earth s early history by studying the Moon This also makes sense. The Earth and the moon would have experienced some of the same effects purely due to their proximity. However, the moon is less dynamic than the Earth so it is less likely that the evidence of these effects would have been erased. For example, The moon retains many craters on its surface because of its lack of geological activity. Studying these craters gives us an idea of the history of impacts on Earth. 30. Science can never determine with confidence the times or sequence of events that occurred millions or billions of years ago. This is simply not true. While it is often difficult to get information about events in the distant past we have already seen several examples of science doing just that. 1.2 Mulitple Choice 36. Earth has retained a lot of internal heat primarily because of its... (c) relatively large size. 38. Earth has far less atmospheric carbon dioxide than venus because... (c) Earth s carbon dioxide is locked up in carbonate rocks. 51. Carbon dating. The half life of carbon-14 is about 5700 years To do these problems we will use our formula for radiometric dating above with τ = 5700 years. However, we will make a slight change. Since none of these problems deal with the quantity of the decay product, only with the quantities of carbon-14 at different times, we will express the formula in terms of the original quantity of carbon 14 and the quantity at a later time. To do this we just need to note that in the above formula the original quantity is given by 4

5 A + B. We will write the original quantity as N and the later quantity as n. Then the formulas are n = N2 t/τ (1) t = τ ln(n/n) ln 2 (a) To do this part we use equation 2 and the fact that here n/n =.77 t = 5700 ln(1/.77) ln 2 = 2150 years (b) This is the same question but with n/n =.062 t = 5700 ln(1/.062) ln 2 (c) Now, we use equation 1 with t = 100τ. = years n N = = So, carbon dating isn t used in these cases because it would be too hard to make a precise measurement of how much is left. 2 Chapter 6 1. What are the three lines of fossil evidence that point to an early origin of life on Earth? Discuss each line and what it it tells us about when life arose. What are the implications of an early origin for the possibility of life elsewhere? Stromatolites are rocks with a particular layered pattern. This structure is very similar to that displayed by groups of micro-organisms alive today. So, scientists believe that these rocks are fossilized remains of ancient life similar to the micro-organisms alive today. Analysis of these rocks seems to indicate that life is at least 3.5 billion years old. Microfossils are fossilized single cells. They also suggest an origin at least 3.5 billion years ago, although these finding are somewhat controversial because of the difficulty in identifying a microscopic fossil. Other, more definitive findings suggest life at least as old as around 3 billion years. Isotopic evidence comes from analyzing the ratios of isotopes of certain elements in rocks. This works because living organisms sometimes prefer certain isotopes of elements over others when incorporating those elements (2) 5

6 into their internal structures. Then, the fossils of these organisms will reflect this preference, setting them apart from rocks not formed from living creatures. This evidence suggests an origin for life at least 3.8 billion years ago. If we assume that what happened on Earth is typical, and not a freak coincidence, then it seems to imply that life forms readily when conditions are right. So, it is possible that life is pretty common in the universe. 4. What was the Miller-Urey experiment, and how did it work? Why is its relevance now subject to scientific debate? How else might Earth have obtained the organic building blocks of life? The Miller- Urey experiment was an attempt to find out if organic molecules could have spontaneously formed from inorganic material on the early Earth. The experiment worked by filling a series of flasks with chemicals thought to be abundant in the early Earth (water, methane, etc). Then, an electric shock is applied in one flask to provide energy for chemical reactions. After a week, it was found that organic material, including amino acids, had indeed formed. However, since the experiment was performed scientists have become more unsure about what chemicals were really present on the early Earth, and in what abundances. So, it is possible the the original experiment did not accurately represent conditions on Earth. It is possible that organic material was introduced to Earth in another way. One possibility is that organic materials were formed in chemical reactions near undersea volcanoes. It is also possible that organic molecules formed elsewhere and were transported here by meteorites. 6. Briefly summarize current ideas about the sequence of events through which life may have originated on Earth. What role(s) might clay or other inorganic materials have played? 1. organic molecules like amino acids are formed as was discussed in the previous question. 2. Larger organic molecules like RNA are formed from the smaller building blocks. These reactions could have been facilitated by inorganic materials like clay. Structures also form that enclose strands of RNA, these are called pre-cells. 3. Eventually, self-replicating RNA forms within the pre-cells. Natural selection ensures that those strands of RNA best at replicating dominate. 4. The natural selection continues until simple living organisms are formed. 5. DNA evolves from RNA and becomes the dominant hereditary molecule because of its advantages over RNA. Then, life continues to evolve until we reach the level of complexity seen today. 6

7 7. Briefly discuss the possibility that life migrated to Earth. Also discuss the possibility that Earth life might have migrated to other worlds, and the implications of migration to the search for life elsewhere. We know that it is not uncommon (in terms of the time scales relevant here) for meteorites to form on one planet when a large impact causes them to fly out into space. Then, these meteorites can travel to another planet in the solar system, occasionally quickly enough for microbes within the meteorite to survive the trip. Furthermore, it seems that the interior of the meteorite is protected enough that neither the initial impact that ejects the meteorite nor the entry into our atmosphere would be lethal to microbes. Then, it is conceivable that life could travel from other planets in the solar system to Earth. Now, given the above we can reason that it is quite likely that microbes from Earth have traveled to nearby planets given the abundance of life on Earth. This means that if we were to discover life on another world it might be difficult to determine if it evolved there or migrated from here. 14. Summarize the history of the oxygen buildup as it is understood today, and describe key mysteries that still remain. When did oxygen reach current levels? There was very little oxygen in the atmosphere at first. Then, we believe that cyanobacteria began producing oxygen about 2.7 billion years ago. However, it appears that oxygen levels in the atmosphere did not really begin to increase until about 2.35 billion years ago. Then, the concentration of oxygen in the atmosphere gradually increased until it was high enough to support complex life. It appears that oxygen did not reach near the current level until about 1 billion years ago. What is still mysterious is why it took 350 million years from the time cyanobacteria started producing oxygen until it started to appear in the atmosphere. One explanation is that oxidation of rocks and minerals was able to use up all of the oxygen produced in this time. Another mystery is why oxygen buildup proceeded so slowly. 2.1 Would you believe it? 23. We discover evidence of life, in teh form of a particular ratio of carbon-12 to carbon-13, in rock that was originally formed in sediments and is 3.9 billion years old This is reasonable since we have already discovered rocks like this that are almost as old. 24. We discover an intact fossil of a eukaryotic cell, with a cell nucleus, that is 3 billion years old. This is also reasonable. The oldest evidence of cell nuclei that we have seen is about 2 billion years old, so it would be quite a find. However, the book states that nuclei do not fossilize well so it is possible that they existed much earlier. 7

8 25. We discover a preserved, 3.5 billion-year-old microfossil that apparently had a genome genetically just like that of many modern animals. This is somewhat reasonable because we know that since DNA replication wasnt as sophisticated back then the rate of mutation was much higher. So, it is possible that much of the genomes in modern animals had already evolved. 26. We discover clear evidence that life arose on a high mountaintop, not in the oceans This is not terribly plausible because the environment on mountaintops in the early Earth probably would not have been hospitable to early life. 27. We discover a fossil of a large dinosaur that lived approximately 750 million years ago. This is not plausible because there were several mass extinction events between then and when we know the dinosaurs existed. In all likelihood any dinosaurs that existed then would have been wiped out in one of them. 28. We discover that, contrary to present belief, oxygen was abundant in Earth s atmosphere eat the time when life arose. The problem with this is that oxygen would have been highly toxic to early life forms. 29. We discover a crater from the impact of a 10-kilometer asteroid that dates to about 2500 years ago An impact from an asteroid of that size would have probably wiped out much of the life on Earth, which clearly did not happen. 30. We discover an asteroid about 3 kilometers across that is on a collision course with Earth. Unfortunately for us, this isn t very farfetched. There are a lot of meteors out there and one can easily be heading towards the Earth. 31. We find fossil remains of an early primate that lived about 50 million years ago and was, from all appearances, identical to a modern gorilla. This would be difficult to believe. The earliest primates didn t come about until just a little before that time, so it is unlikely that they would have looked exactly like gorillas, especially considering that gorillas diverged from small monkeys between 30 and 40 million years ago. 32. The first life created in the laboratory has an RNA genome, rather than a DNA genome. Well, this is how it happened on Earth, so it makes sense that this is how it would happen in the lab as well. RNA is much simpler than DNA so it would be much easier for life to evolve to make use of RNA, even if ultimately DNA wins out. 8

9 2.2 Quantitative Problems 49. bacterial evolution The book said there would be about bacteria after 1 hour. Dividing by 1 million (10 6 ) have bacteria with mutations. This means that many mutations could arise quickly. If some of them were resistant to a drug and the others were wiped out, the few drug resistant bacteria could then repopulate the bottle after an hour, and most of the new bacteria would share the drug resistance (it s possible some would mutate and lose it). 9