Before you begin this course, make sure you have completed the Course Orientation (see the Start Here! [1] menu at left).
In this lesson, you will begin a trek toward understanding the significance of water on Earth and its importance to a host of unique features on your home planet. You will also apply the scientific method along the way, think about how hypotheses are best evaluated, and get a chance to hone your skills of critical reading. Reading scientific articles and papers in a critical way is an invaluable skill. You will be doing this with your own students' work as well as with the published literature. Not everything you read, even in the scientific literature, is correct, or even well-reasoned. In Earth science, well-reasoned speculation is acceptable, as long as it is identified as such. Thus, scientific articles should present one or more clear, identifiable hypotheses and should evaluate those hypotheses using data collected for that purpose, presented in the paper along with other supporting information.
With the proliferation of "self publishing" on the Web, one can find all sorts of "bad science." How do we promote the "critical thinking and reading" skill to our students to allow them to sort the wheat from the chaff? Practice, of course! We will ask you to read selected articles, discuss them with the class, and provide data plots that support your views and points. We will also ask you to translate that science-speak into a product that is interesting and accessible to the lay person. Let's dive in!
By the end of Lesson 1, you should be able to:
Lesson 1 will take us two weeks to complete. Activities are due on Monday Jan 31st.
The chart below provides an overview of the requirements for Lesson 1. For assignment details, refer to the lesson page noted.
REQUIREMENT |
LOCATION |
SUBMITTED FOR GRADING? |
|
Activity 1: Quantification and Plot Analysis (1) Try to complete by Jan 21 |
page 4 | Yes - Your discussion board participation counts toward your overall class participation grade |
|
Activity 2: The "Habitable Zone" Discussion Experience |
page 5 | Yes - Your discussion board participation counts toward your overall class participation grade |
|
Activity 3: Critically Reading Scientific Literature and the Scientific Method
|
page 6 |
Yes - Comment Summary submitted to dropbox |
If you have general questions, please post them to our Questions discussion forum, which is linked under both the Lessons and Communicate tabs in ANGEL.
If you have something time sensitive and/or personal please send email to Mike [2] [3]
In our Astrobiology Research Center at Penn State, there is a room called “The Habitable Zone.” This whimsical name is a reference to a concept that has developed in the search for life on other planets. Of course, in “The Habitable Zone” room, there are comfortable couches, a coffee pot, computer connections and large screens for projection of computer images or teleconferences—all the ingredients for encouraging development of scientific intercourse, the lifeblood of the Astrobiology Research Center. A zone of habitability for life within a solar system has certain requirements too, including an optimal distance from a sun, optimal planet size and gravity, perhaps a magnetic field, and even the presence of a planet of large mass somewhere else in the solar system, among other characteristics. Some scientists speak of the “Goldilock’s Principle” for which everything needed to be “just right” for life to originate and prosper on Earth. We will explore this principle below, and you will need to discover what parameters are potentially important and why?
We know, of course, that the Earth is the only planet in our solar system that has water present in all three phases on its surface, and that oceans occupy about 71% of Earth’s surface area. Recent work suggests the presence of water oceans on Earth shortly after its formation (4.6 x 109 years ago), as early as 4.3 to 4.4 x 109 years ago. But from where did this water come? And why is there not abundant water on other planets today? Yes, we have good evidence for water in the subsurface on Mars, and water is a component of the Martian polar ice cap. Some scientists have suggested that water was once much more abundant on Mars’ surface—even forming large oceans.
We don’t really have a definitive answer to this question at this time. Assuming that Earth and other planets accreted from a pre-existing solar nebula, possible sources of water on Earth could include capture of solar nebula gas (including volatile water vapor), adsorption of water from gas onto grains during accretion of these planets, accumulation and trapping of hydrous (water-bearing) minerals forming in the inner solar system or falling in from the asteroid belt, and impacts with comets and water-bearing meteors. Theories of the origin of water run the gamut from suggesting that all Earth’s water accumulated early in its history and, through various processes, was pooled into its vast surface oceans, to those that suggest importance for later water accumulation by repeated impacts of extraterrestrial objects. To be sure, Earth’s early accretion was a violent episode characterized by many impacts of “planetesimals,” from dust particles to objects as large as one tenth to one third of the mass of the accreting planet. Impacts ultimately provided sufficient energy to melt much of the earliest Earth, producing one or more “magma” (molten rock) oceans. At least one massive impact ejected material into Earth orbit, and this material subsequently accreted to form Earth’s Moon. We will revisit the Moon (at least in a virtual sense) and its significance to the oceans later in this course. Intense bombardment, referred to as the “late heavy bombardment”, ended about 3.9 x 109 years ago. Evidence for this includes the large lunar mare (huge basins) produced by these large impacts.
The mass of Earth’s oceans is about 1.4 x 1021 kg. (How do we know this? See Activity 1, problem 1). But there appears to be far more water in Earth’s interior, something between 10 and 50 oceans worth. It is likely that most of this water accreted within the Earth early in its history and that, in steady-state, some 5 to 10% remains on the surface in the ocean-atmosphere system. Although some evidence supports water delivery by later cometary or meteoritic (asteroidal) impacts, it is likely that surface water was accreted early and outgassed from within the Earth. Nonethless, some new observations of comets (comet LINEAR) provide new support for cometary origins of water on planets in the inner solar system.
See "A Taste for Comet Water. [5]"
Those two planets also likely accreted much water during their formation as well as having been bombarded by comets just as the Earth was.
In a poll of the readers of Astrobiology Magazine, a scientific journal, 41% rated liquid water as the key factor needed to make a planet habitable, followed by a combination of all other candidate elements [nutrient, water, oxygen, ozone, photosynthetic sources like sunlight, and carbon dioxide].
Also see "Life's Little Essential [6]" by Peter Tyson.
For background in this section you will need to read two articles:
So, the "Goldilock's Principle" postulates that everything was "just right" on Earth for life to originate and prosper. Did water play a role? Make a list of all the ways that water could be important to the evolution and continuity of life on Earth (think broadly). For example, if Venus and Mars once had water, and even oceans, why do they not now have them? Clearly, if life arose in the presence of water, that water would have to persist in order to sustain life. Could life have evolved on Mars? Where would you look for life on Mars today?
One of the key constraints on the accumulation of oceans at the Earth's surface and the origin and survival of the earliest life on Earth is the size and frequency of objects that impacted the Earth. Lunine (2006) summarizes the impact history on Earth (largely inferred from the preserved record of impacts on Earth's Moon; why not directly from the earthly record?). Note that in the first 0.3 billion years (4.5-4.2 Ga) after Earth's accretion, the frequency and size of impactors was such that multiple "sterilizing" impacts occurred. In addition, these impacts probably "blew away" any oceans that may have accreted early and created a "steam" atmosphere. Certainly, some water was lost from Earth's surface to space. Fortunately, sufficient water existed either through accretion or continued addition by comets and asteroids (section 1) that oceans could again accumulate on Earth's surface. But life could have originated multiple times and been erased from Earth's surface by these large impacts. However, some models suggest that some life might have survived if it had evolved in higher-temperature environments, such as hot springs systems. In contrast, Venus and Mars somehow lost much of their water (and/or were initially endowed with much less than Earth?) during their early history, leaving Earth in the Goldilocks zone, and, perhaps, prohibiting an origination of life and/or continuity of life at their surface.
There is some evidence (what is it? See Lunine, 2006) for free water near Earth's surface as early as 4.4 Ga (the earliest known rocks extant on Earth) and fairly definitive evidence in rocks for large bodies of water (oceans?) by 3.6 Ga. Life may have arisen at that time, and there is reasonably strong evidence from structures and cellular features preserved in rocks that there were widespread mats of bacteria in shallow marine environments by about 3.3 Ga (Lesson 3 will entertain some hypotheses regarding the chemical composition of seawater). However, it took until nearly 0.54 Ga for multicellular marine animals to evolve. There is much speculation regarding the origin of life and why evolution took "so long" to allow more complex animals to exist. Little or no oxygen in the early atmosphere and oceans may have been a limiting factor, but there is disagreement regarding when the atmosphere-ocean system became "oxygenated." Available data indicate that some oxygen may have persisted in the atmosphere after 2.4 Ga, but more limited data may support an earlier timing for the "rise of oxygen." Note that oxygen can be considered a toxin to organisms that evolved in oxygen-deficient environments. Microbial organisms that once lived at the ocean surface would have been forced to seek refuge in oxygen-depleted environments below the seafloor when the oceans became oxygenated. Much work on this topic is going on in the Penn State Astrobiology Research Center as you read this.
Back of the envelope (BOTE) calculations are often useful to provide a perspective on the relative importance of a process or system mass balance (inputs vs. outputs). At times BOTE calculations are useful just to give one an idea how to approach a problem and to understand the relationships among the key parameters, and, perhaps, which ones need to be more precisely known. Scientists and others use plots to convey data relationships that are viewed as meaningful—perhaps to examine possible patterns or correlations that can provide insights into cause and effect.
We will use both in this course to help elucidate key ocean system details. So let's practice a bit. The exercise will also give us an idea of your facility with scientific notation and unit analysis.
Start: A BOTE calculation (it's simple, but let's step through it).
Question: What is the mass of water in Earth's oceans? How would you go about determining this from some basic information? In other words, what values/parameters do you need?
mass [m] is the amount of material that occupies a given volume, units in kilograms (kg) SI system); and m=volume (V) x density (rho), where density (rho) is the mass/unit volume (depends on the substance), given in kg/m3.
V=length(l ) x width(w) x height(h) (or depth(d)), or, because Area(A)=l x w, V(m3)=A (m2) x d(m).
To obtain the mass of ocean water, we would need the area of the ocean and it's average depth to calculate the volume of water, and a value for the average density of seawater. These numbers are known reasonably well and we can look them up in any oceanography textbook. A=362 x 1012 m2 (from 362 x 106 km2) and d(avg)=ca. 3800 m, so (you do the math)...
Vocean=1.375 x 1018 m3. Agreed?
The average seawater density is ca. 1037 kg m-3 (why did we use m-3?), therefore we have mseawater= 1.375 x 1018 m3 x 1037 kg m-3 =1.426 x 1021 kg. That's ca. 1.4 x 1018 t of seawater ( a metric ton=103 kg). Everybody see how we get here (and how to manipulate exponents and units)?
Part 1: A BOTE calculation for you to do.
Part 2: Plotting and Analysis (use your favorite plotting program, but produce an attractive plot with appropriate labeling).
Part 3: Read the postings by other EARTH 540 students. Respond to at least one other posting in each part. You may ask for clarification, ask a follow-up question, expand on what has already been said, etc.
You will be graded on the quality of your participation. See the grading rubric [11] for additional information.
During the next week, prepare a 250-word or so discussion posting for each of the 4 topic questions in the lesson introductions.
Your posting should provide a sense of the significance of and controversy surrounding each of the questions and evidence for or against key hypotheses. Your posting should be written for a non-scientist but communicate the flavor of debate, type of evidence and how it is used, etc.
You will be graded on the quality of your participation. See the grading rubric [11] for additional information.
Do comets still deliver substantial water to Earth? Could the oceans be growing year by year? Frank et al. (1986) think so. What is their evidence and how was it received by the scientific "establishment?" We will give you some experience in critical evaluation of hypotheses and data using a real-world example with great relevance to our topic. Their paper was, understandably, controversial, so there has been much discussion and evaluation of the data and conclusions. It's a great example of how the scientists vett their ideas through publication and receive feedback from their (not always kind) colleagues. It is also an opportunity to explore the "scientific method" a bit.
NOTE: For this assignment, you will need to record your work on a word processing document. Your work must be submitted in Word (.doc) or PDF (.pdf) format so I can open it.
L1_Activity 3_AccessAccountID_LastName.doc (or .pages or .pdf)
For example, student Elvis Aaron Presley's file would be named "L1_Activity 3_eap1_presley.doc"—this naming convention is important, as it will help me make sure I match each submission up with the right student!
You will be graded on the quality of your participation. See the grading rubric [11] for additional information.
Various Web site with links to resources aimed at teachers and students:
Reading the technical/scientific literature:
Links to other Web sites:
Have another Web site on this topic that you have found useful? Share it in the Comment area below!
Don't see the "Comment" area below? You need to be logged in to this site first! Do so by using the link at the top of the left-hand menu bar. Once you have logged in, you may need to refresh the page in order to see the comment area below.
Reminder - Complete all of the lesson tasks!
You have finished Lesson 1. Double-check the list of requirements on the first page of this lesson ("Lesson 1" in the menu bar) to make sure you have completed all of the activities listed there before beginning the next lesson.
If you have anything you'd like to comment on, or add to, the lesson materials, feel free to post your thoughts below. For example, what did you have the most trouble with in this lesson? Was there anything useful here that you'd like to try in your own classroom?
Don't see the "Comment" area below? You need to be logged in to this site first! Do so by using the link at the top of the left-hand menu bar. Once you have logged in, you may need to refresh the page in order to see the comment area below.
Authors: Michael Arthur and Chris Marone, Department of Geosciences, College of Earth and Mineral Sciences, The Pennsylvania State University.
© 2014 The Pennsylvania State University
This courseware module is part of Penn State's College of Earth and Mineral Sciences' OER Initiative.
Except where otherwise noted, content on this site is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
The College of Earth and Mineral Sciences is committed to making its websites accessible to all users, and welcomes comments or suggestions on access improvements. Please send comments or suggestions on accessibility to the site editor. The site editor may also be contacted with questions or comments about this Open Educational Resource.
Links:
[1] https://www.e-education.psu.edu/earth540/l1.html
[2] mailto:maa6@psu.edu
[3] mailto:marone@psu.edu
[4] https://courseware.e-education.psu.edu/courses/earth540/priv/KastingsSciAm_2003.pdf
[5] http://www.astrobio.net/news/index.php?name=News&file=article&sid=141&theme=Printer
[6] http://www.pbs.org/wgbh/nova/origins/essential.html
[7] https://courseware.e-education.psu.edu/courses/earth540/priv/Lunine2006.pdf
[8] https://courseware.e-education.psu.edu/courses/earth540/priv/KnollandGrotzinger2006.pdf
[9] https://courseware.e-education.psu.edu/courses/earth540/priv/MenardSmithJGR1966.pdf
[10] https://courseware.e-education.psu.edu/courses/earth540/priv/OceanVolume.CharetteSmith.2010.pdf
[11] https://www.e-education.psu.edu/earth540/grading_rubric_problemsets
[12] https://courseware.e-education.psu.edu/courses/earth540/priv/FranketalGRL1986.pdf
[13] http://smallcomets.physics.uiowa.edu/pdf/
[14] http://smallcomets.physics.uiowa.edu/
[15] http://evolution.berkeley.edu/
[16] http://geology.com/teacher/
[17] http://www.faculty.english.vt.edu/Collier/stc/ch2.htm
[18] http://www.astrobio.net/pressrelease/1905/are-we-drinking-comet-water
[19] http://www.nasa.gov/mission_pages/mars/news/mgs-20061206.html
[20] http://astrobiology.ucla.edu/pages/res2a.html
[21] http://brembs.net/gould.html