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About This Drill
ACT Science: Conflicting Viewpoints (Drill 2) is a Science practice drill covering Conflicting Viewpoints. It contains 5 original questions created by Brian Stewart, a Barron's test prep author with over 20 years of tutoring experience.
ACT Conflicting Viewpoints questions ask you to compare two competing scientific hypotheses and evaluate their support. In this drill, two scientists debate how the Moon formed, the Giant Impact Hypothesis versus co-accretion, requiring analysis of evidence, assumptions, and how new findings would affect each argument.
Questions & Explanations
Scientists 1 and 2
The Moon is Earth's only natural satellite and formed approximately 4.5 billion years ago, shortly after Earth itself. Moon rocks returned by the Apollo missions have allowed detailed geochemical analysis. Scientists agree on several key observations: the Moon has a mean density of 3.34 g/cm³ compared to Earth's 5.51 g/cm³; its iron core accounts for only about 1 to 3% of its total mass, far less than Earth's iron core at roughly 32%; and the oxygen isotope ratios of lunar rocks are nearly identical to those of Earth's mantle. The Earth-Moon system also has unusually high angular momentum relative to other planet-moon systems in the solar system. How the Moon formed from these conditions is debated.
Scientist 1
The Moon formed when a Mars-sized protoplanet, called Theia, collided with the young Earth approximately 4.5 billion years ago. At that time, Earth had already undergone differentiation -- its dense iron had sunk to form a core, leaving the outer mantle iron-poor. The giant impact ejected enormous quantities of mantle material into orbit, where it coalesced to form the Moon. This explains why the Moon is iron-depleted and has such a small core: it formed primarily from Earth's silicate-rich mantle, not from iron-rich deep material. The near-identical oxygen isotope ratios of Earth and Moon are explained by the fact that Theia was compositionally similar to Earth, and that the debris disk was thoroughly mixed with Earth's own mantle. The high angular momentum of the Earth-Moon system is a natural consequence of the oblique angle of the impact. Computer simulations of a giant impact reliably reproduce the Moon's observed mass and orbital characteristics.
Scientist 2
The Moon formed by co-accretion -- the simultaneous accumulation of Earth and the Moon from the same rotating disk of gas and dust surrounding the young Sun. Because both bodies formed from the same region of the solar nebula, they inherited the same chemical composition, which explains the nearly identical oxygen isotope ratios observed in Earth and lunar rocks without requiring any special impactor. Co-accretion is a natural extension of the same planetary formation process that built all the inner planets and does not depend on the unlikely coincidence of a precisely sized and angled impact. The hypothesis that a specific Mars-sized body called Theia struck early Earth rests on no direct physical evidence -- no remnant of Theia has ever been identified. The orbital and rotational characteristics of the Earth-Moon system can be accounted for by the dynamics of early solar system accretion without invoking a catastrophic collision.
Question 1. According to Scientist 1, what explains the Moon's disproportionately small iron core?
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A) The impact ejected material primarily from Earth's iron-poor mantle, after iron had already sunk to Earth's core ✓
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B) The Moon and Earth formed from the same nebular material, which happened to be iron-poor
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C) Iron from the Moon gradually migrated to Earth over billions of years through gravitational attraction, according to Scientist 1
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D) The Moon's lower gravity was insufficient to retain iron during its formation
Explanation: Scientist 1 states explicitly: "At that time, Earth had already undergone differentiation -- its dense iron had sunk to form a core, leaving the outer mantle iron-poor. The giant impact ejected enormous quantities of mantle material into orbit... This explains why the Moon is iron-depleted and has such a small core: it formed primarily from Earth's silicate-rich mantle, not from iron-rich deep material." Option B describes Scientist 2's position. Options C and D are not proposed by either scientist.
Scientists 1 and 2
The Moon is Earth's only natural satellite and formed approximately 4.5 billion years ago, shortly after Earth itself. Moon rocks returned by the Apollo missions have allowed detailed geochemical analysis. Scientists agree on several key observations: the Moon has a mean density of 3.34 g/cm³ compared to Earth's 5.51 g/cm³; its iron core accounts for only about 1 to 3% of its total mass, far less than Earth's iron core at roughly 32%; and the oxygen isotope ratios of lunar rocks are nearly identical to those of Earth's mantle. The Earth-Moon system also has unusually high angular momentum relative to other planet-moon systems in the solar system. How the Moon formed from these conditions is debated.
Scientist 1
The Moon formed when a Mars-sized protoplanet, called Theia, collided with the young Earth approximately 4.5 billion years ago. At that time, Earth had already undergone differentiation -- its dense iron had sunk to form a core, leaving the outer mantle iron-poor. The giant impact ejected enormous quantities of mantle material into orbit, where it coalesced to form the Moon. This explains why the Moon is iron-depleted and has such a small core: it formed primarily from Earth's silicate-rich mantle, not from iron-rich deep material. The near-identical oxygen isotope ratios of Earth and Moon are explained by the fact that Theia was compositionally similar to Earth, and that the debris disk was thoroughly mixed with Earth's own mantle. The high angular momentum of the Earth-Moon system is a natural consequence of the oblique angle of the impact. Computer simulations of a giant impact reliably reproduce the Moon's observed mass and orbital characteristics.
Scientist 2
The Moon formed by co-accretion -- the simultaneous accumulation of Earth and the Moon from the same rotating disk of gas and dust surrounding the young Sun. Because both bodies formed from the same region of the solar nebula, they inherited the same chemical composition, which explains the nearly identical oxygen isotope ratios observed in Earth and lunar rocks without requiring any special impactor. Co-accretion is a natural extension of the same planetary formation process that built all the inner planets and does not depend on the unlikely coincidence of a precisely sized and angled impact. The hypothesis that a specific Mars-sized body called Theia struck early Earth rests on no direct physical evidence -- no remnant of Theia has ever been identified. The orbital and rotational characteristics of the Earth-Moon system can be accounted for by the dynamics of early solar system accretion without invoking a catastrophic collision.
Question 2. Which of the following observations would both Scientist 1 and Scientist 2 accept as accurate?
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A) The Moon's small iron core cannot be explained without invoking a giant impact
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B) A remnant of the impactor Theia has been identified in lunar rock samples
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C) The oxygen isotope ratios of lunar rocks are nearly identical to those of Earth's mantle ✓
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D) The high angular momentum of the Earth-Moon system can only result from an oblique impact
Explanation: The nearly identical oxygen isotope ratios are cited by both scientists -- they simply disagree about what this means. Scientist 1 uses it to argue that Theia was Earth-like and the debris disk mixed thoroughly with Earth's mantle. Scientist 2 uses it as direct evidence that Earth and Moon formed from the same nebular material. The intro paragraph also establishes this as an agreed observation. Option A states Scientist 1's position, not both. Option B is explicitly contradicted by Scientist 2, who states "no remnant of Theia has ever been identified." Option D is stated only by Scientist 1.
Scientists 1 and 2
The Moon is Earth's only natural satellite and formed approximately 4.5 billion years ago, shortly after Earth itself. Moon rocks returned by the Apollo missions have allowed detailed geochemical analysis. Scientists agree on several key observations: the Moon has a mean density of 3.34 g/cm³ compared to Earth's 5.51 g/cm³; its iron core accounts for only about 1 to 3% of its total mass, far less than Earth's iron core at roughly 32%; and the oxygen isotope ratios of lunar rocks are nearly identical to those of Earth's mantle. The Earth-Moon system also has unusually high angular momentum relative to other planet-moon systems in the solar system. How the Moon formed from these conditions is debated.
Scientist 1
The Moon formed when a Mars-sized protoplanet, called Theia, collided with the young Earth approximately 4.5 billion years ago. At that time, Earth had already undergone differentiation -- its dense iron had sunk to form a core, leaving the outer mantle iron-poor. The giant impact ejected enormous quantities of mantle material into orbit, where it coalesced to form the Moon. This explains why the Moon is iron-depleted and has such a small core: it formed primarily from Earth's silicate-rich mantle, not from iron-rich deep material. The near-identical oxygen isotope ratios of Earth and Moon are explained by the fact that Theia was compositionally similar to Earth, and that the debris disk was thoroughly mixed with Earth's own mantle. The high angular momentum of the Earth-Moon system is a natural consequence of the oblique angle of the impact. Computer simulations of a giant impact reliably reproduce the Moon's observed mass and orbital characteristics.
Scientist 2
The Moon formed by co-accretion -- the simultaneous accumulation of Earth and the Moon from the same rotating disk of gas and dust surrounding the young Sun. Because both bodies formed from the same region of the solar nebula, they inherited the same chemical composition, which explains the nearly identical oxygen isotope ratios observed in Earth and lunar rocks without requiring any special impactor. Co-accretion is a natural extension of the same planetary formation process that built all the inner planets and does not depend on the unlikely coincidence of a precisely sized and angled impact. The hypothesis that a specific Mars-sized body called Theia struck early Earth rests on no direct physical evidence -- no remnant of Theia has ever been identified. The orbital and rotational characteristics of the Earth-Moon system can be accounted for by the dynamics of early solar system accretion without invoking a catastrophic collision.
Question 3. Which of the following findings, if confirmed, would most weaken Scientist 2's hypothesis?
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A) Computer simulations show that a giant impact can reproduce the Moon's current mass and orbit with high precision
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B) Studies of other solar system bodies that formed by accretion from the same nebular region show iron contents much closer to Earth's than the Moon's iron content is ✓
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C) The Moon's oxygen isotope ratios differ slightly from Earth's when measured with high precision instruments
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D) The Moon contains trace amounts of water ice in permanently shadowed polar craters
Explanation: Scientist 2's co-accretion hypothesis predicts that the Moon and Earth formed from the same nebular material in the same region, so they should have similar bulk compositions -- including iron content. The Moon's iron content is dramatically lower than Earth's (core 1-3% vs. 32% of total mass). If other bodies that genuinely co-accreted from the same nebular region have iron contents much closer to Earth's, this shows that co-accretion from shared material does NOT produce the iron-depleted Moon, directly undermining Scientist 2. Option A strengthens Scientist 1 but does not specifically weaken Scientist 2. Option C would actually challenge both hypotheses and create a new problem for Scientist 1. Option D (polar water ice) is unrelated to either formation hypothesis.
Scientist 2
The Moon is Earth's only natural satellite and formed approximately 4.5 billion years ago, shortly after Earth itself. Moon rocks returned by the Apollo missions have allowed detailed geochemical analysis. Scientists agree on several key observations: the Moon has a mean density of 3.34 g/cm³ compared to Earth's 5.51 g/cm³; its iron core accounts for only about 1 to 3% of its total mass, far less than Earth's iron core at roughly 32%; and the oxygen isotope ratios of lunar rocks are nearly identical to those of Earth's mantle. The Earth-Moon system also has unusually high angular momentum relative to other planet-moon systems in the solar system. How the Moon formed from these conditions is debated.
Scientist 2
The Moon formed by co-accretion -- the simultaneous accumulation of Earth and the Moon from the same rotating disk of gas and dust surrounding the young Sun. Because both bodies formed from the same region of the solar nebula, they inherited the same chemical composition, which explains the nearly identical oxygen isotope ratios observed in Earth and lunar rocks without requiring any special impactor. Co-accretion is a natural extension of the same planetary formation process that built all the inner planets and does not depend on the unlikely coincidence of a precisely sized and angled impact. The hypothesis that a specific Mars-sized body called Theia struck early Earth rests on no direct physical evidence -- no remnant of Theia has ever been identified. The orbital and rotational characteristics of the Earth-Moon system can be accounted for by the dynamics of early solar system accretion without invoking a catastrophic collision.
Question 4. Scientist 2 uses the nearly identical oxygen isotope ratios of the Moon and Earth as support for co-accretion. This argument depends on which of the following assumptions?
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A) Oxygen isotope ratios vary randomly and cannot be used to trace the origin of planetary bodies
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B) The giant impact hypothesis predicts that the Moon and Earth should have identical oxygen isotope ratios
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C) Identical oxygen isotope ratios between the Moon and Earth could not result from a giant impact event ✓
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D) No other planets in the solar system have oxygen isotope ratios similar to Earth's
Explanation: Scientist 2's argument is: "identical oxygen isotope ratios prove same-source co-accretion." For this to be valid, the identical ratios must be evidence FOR co-accretion and AGAINST a giant impact. This only works if Scientist 2 assumes identical ratios could NOT result from a giant impact. Scientist 1 directly challenges this assumption by arguing that the impactor Theia was compositionally similar to Earth and that the debris disk was thoroughly mixed with Earth's mantle -- a scenario that also produces identical oxygen isotopes. Option A would undermine Scientist 2's own use of isotopes as evidence. Option B describes the opposite of Scientist 2's position. Option D is unrelated to the argument.
Scientists 1 and 2
The Moon is Earth's only natural satellite and formed approximately 4.5 billion years ago, shortly after Earth itself. Moon rocks returned by the Apollo missions have allowed detailed geochemical analysis. Scientists agree on several key observations: the Moon has a mean density of 3.34 g/cm³ compared to Earth's 5.51 g/cm³; its iron core accounts for only about 1 to 3% of its total mass, far less than Earth's iron core at roughly 32%; and the oxygen isotope ratios of lunar rocks are nearly identical to those of Earth's mantle. The Earth-Moon system also has unusually high angular momentum relative to other planet-moon systems in the solar system. How the Moon formed from these conditions is debated.
Scientist 1
The Moon formed when a Mars-sized protoplanet, called Theia, collided with the young Earth approximately 4.5 billion years ago. At that time, Earth had already undergone differentiation -- its dense iron had sunk to form a core, leaving the outer mantle iron-poor. The giant impact ejected enormous quantities of mantle material into orbit, where it coalesced to form the Moon. This explains why the Moon is iron-depleted and has such a small core: it formed primarily from Earth's silicate-rich mantle, not from iron-rich deep material. The near-identical oxygen isotope ratios of Earth and Moon are explained by the fact that Theia was compositionally similar to Earth, and that the debris disk was thoroughly mixed with Earth's own mantle. The high angular momentum of the Earth-Moon system is a natural consequence of the oblique angle of the impact. Computer simulations of a giant impact reliably reproduce the Moon's observed mass and orbital characteristics.
Scientist 2
The Moon formed by co-accretion -- the simultaneous accumulation of Earth and the Moon from the same rotating disk of gas and dust surrounding the young Sun. Because both bodies formed from the same region of the solar nebula, they inherited the same chemical composition, which explains the nearly identical oxygen isotope ratios observed in Earth and lunar rocks without requiring any special impactor. Co-accretion is a natural extension of the same planetary formation process that built all the inner planets and does not depend on the unlikely coincidence of a precisely sized and angled impact. The hypothesis that a specific Mars-sized body called Theia struck early Earth rests on no direct physical evidence -- no remnant of Theia has ever been identified. The orbital and rotational characteristics of the Earth-Moon system can be accounted for by the dynamics of early solar system accretion without invoking a catastrophic collision.
Question 5. Analysis of ancient lunar breccia samples reveals microscopic glassy spherules with internal structures produced only by extreme shock pressures -- features associated with hypervelocity impacts. This finding would most strongly support the hypothesis of:
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A) Scientist 2 only, because shock features indicate the Moon was struck by later meteorite impacts after it formed
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B) Scientist 1 only, because shock-metamorphism features in primordial lunar material are consistent with formation from a high-energy impact event ✓
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C) Both scientists equally, because all planetary bodies experience impact events during formation
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D) Neither scientist, because shock features in lunar rocks are already well-documented and expected
Explanation: The question specifies features found in primordial lunar breccia -- the ancient material making up the Moon itself -- not from later surface impacts. Extreme shock-metamorphism in the Moon's primordial rock is consistent with the Moon's material having experienced the catastrophic pressures of a giant impact during formation, supporting Scientist 1. Scientist 2's co-accretion hypothesis involves gradual accumulation with no catastrophic collision, so it does not predict shock features in the Moon's primordial material. Option A misreads the question -- it refers to the ancient breccia itself, not later cratering events. Option C is too broad; the question is specifically about what the finding implies for Moon formation hypotheses.