Scientific Models, Inferences, and Results

### Permian-Triassic Extinction Models

The Permian-Triassic extinction event, which occurred approximately $252$ million years ago, resulted in the loss of over $90\%$ of marine species and $70\%$ of terrestrial species. Scientists have proposed different models to explain the primary cause of this mass extinction.

Model 1 (Volcanic Outgassing Model)
This model proposes that the extinction was triggered by massive, prolonged eruptions of the Siberian Traps, a large region of volcanic rock. These eruptions released vast quantities of carbon dioxide ($CO_2$) and sulfur dioxide ($SO_2$) into the atmosphere over hundreds of thousands of years. The resulting extreme greenhouse effect led to severe global warming and ocean acidification. The warming also depleted ocean oxygen, causing widespread marine anoxia that gradually suffocated marine life.

Model 2 (Bolide Impact Model)
This model proposes that the extinction was caused by the impact of a large asteroid or comet (a bolide). The collision instantly vaporized rocks, sending massive amounts of dust, pulverized rock, and sulfur aerosols into the stratosphere. This blocked sunlight, causing a rapid global drop in temperature ('impact winter') and halting photosynthesis. After the dust settled, the high levels of water vapor and vaporized greenhouse gases left in the atmosphere caused rapid global warming and acid rain, leading to a sudden, catastrophic collapse of ecosystems.

Which statement on the right correctly describes how each new scientific finding on the left supports or contradicts the proposed models?

### Models of Hawaiian Hotspot Volcanism

Hawaiian volcanoes are located in the middle of the Pacific Plate, far from plate boundaries. Two models explain the source of magma and the age progression of the Hawaiian-Emperor seamount chain, where volcanoes get older further northwest.

Model 1 (Deep Mantle Plume Model)
A narrow plume of hot mantle material rises from the core-mantle boundary (about $2{}900\text{ km}$ deep) to the crust. This plume is stationary relative to the deep mantle. As the Pacific Plate moves northwestward over this fixed 'hotspot,' decompression melting of the plume creates a chain of volcanoes. The source of magma is the deep mantle, which is rich in primordial helium ($^3\text{He}$) and contains higher concentrations of primitive trace elements compared to the upper mantle.

Model 2 (Shallow Plate-Tectonic Extension Model)
Magma rises from the shallow upper mantle (asthenosphere, less than $200\text{ km}$ deep) due to localized crustal extension (cracking) of the Pacific Plate. Tectonic stresses bend the plate, causing propagating fractures. Magma is not fed by a deep plume but is passive melting of the upper mantle drawn upward into the fractures. The northwestward age progression occurs because the stresses that cause fracturing propagate along the plate over time. The magma source is the recycled oceanic crust in the shallow mantle, characterized by normal ratios of helium ($^3\text{He}/^4\text{He}$) and depleted trace elements typical of the upper mantle.

***

A geologist is comparing the mechanisms and geochemical predictions of Model 1 and Model 2 for the origin of Hawaiian volcanism. Match each model-specific claim or prediction on the left with its corresponding underlying assumption or explanation on the right.

### Late Ordovician Mass Extinction Models

The Late Ordovician Mass Extinction (LOME), which occurred approximately $444$ million years ago, resulted in the loss of about $85\%$ of marine species. Scientists have proposed different models to explain the primary cause of this extinction event.

**Model $1$ (Glaciation/Cooling Model)**
This model proposes that the growth of the Gondwanan ice sheet triggered the extinction. The accumulation of ice locked up global water, leading to a rapid eustatic sea-level fall of over $100\text{ meters}$. This regression drained shallow, warm epicontinental seas, which hosted the vast majority of marine life. Furthermore, global temperatures plunged, and the cooling of tropical waters eliminated species adapted to warm climates. In this view, habitat loss due to sea-level drop and direct thermal stress from cooling were the sole triggers of the first extinction pulse.

**Model $2$ (Anoxia/Volcanism Model)**
This model proposes that large-scale volcanic eruptions from a large igneous province triggered the extinction. The eruptions released massive amounts of carbon dioxide ($CO_2$) and sulfur dioxide ($SO_2$) into the atmosphere, causing short-term acid rain followed by long-term global warming due to the greenhouse effect. Warming reduced the solubility of oxygen in seawater, and increased weathering washed nutrients into the oceans, causing widespread marine anoxia (oxygen depletion). Acidification of the oceans further prevented calcifying organisms from building shells. In this view, oxygen starvation (anoxia) and ocean acidification were the primary causes of the mass extinction.

A geologist compiles several hypotheses regarding the environmental conditions and mechanisms that drove the Late Ordovician Mass Extinction. Match each hypothesis to the model (Model $1$, Model $2$, or both) that supports it.

Origin of Earth's Moon

Two models are proposed to explain the origin of Earth's Moon.

*Model 1* (Giant Impact Hypothesis)
The Moon formed when a Mars-sized protoplanet collided with the newly formed proto-Earth approximately $4.5$ billion years ago. The energy of this impact vaporized much of the outer layers of both bodies, ejecting a mixture of rocky mantle material and vapor into orbit around Earth. Over time, this debris cooled and accreted to form the Moon. Because the ejected material came primarily from the rocky mantles of the colliding bodies rather than their iron-rich cores, the Moon contains a very small iron core and has a bulk density ($3.3\text{ g/cm}^3$) much lower than Earth's ($5.5\text{ g/cm}^3$).

*Model 2* (Capture Hypothesis)
The Moon formed independently in another region of the solar system, where it accreted from material richer in silicates and poorer in iron. As its orbit brought it close to the newly formed Earth, Earth's gravitational pull captured the Moon into a permanent orbit. Because the Moon formed in a different region of space, its chemical composition reflects the environment of its origin rather than Earth's, explaining its lower bulk density and small iron core. However, the capture process was a rare event requiring precise speeds and angles to prevent the Moon from crashing into Earth or escaping its gravity entirely.

Which of the following statements best describes a fundamental difference in the hypotheses of Model 1 and Model 2 regarding the material from which the Moon formed?

### Models of Coronal Heating

The Sun's outer atmosphere, the corona, has a temperature of over $1,000,000\text{ K}$, while its surface, the photosphere, is only about $6,000\text{ K}$. Because heat normally flows from hotter to cooler regions, scientists have proposed two models to explain how the corona is heated from below.

* Model 1 (Wave Heating Model)
This model proposes that convective motions in the photosphere continuously shake magnetic field lines, generating magnetic waves called Alfvén waves. These waves travel upward along the magnetic field lines into the corona. As the plasma density decreases with height, the waves become unstable and dissipate their energy through friction and turbulence, transferring kinetic energy to the coronal plasma. This heating is continuous and occurs uniformly along the magnetic structures.

* Model 2 (Magnetic Reconnection Model)
This model proposes that convective motions in the photosphere slowly twist and shear the coronal magnetic loops. Over time, magnetic energy builds up in these twisted lines. When the stress exceeds a critical threshold, the magnetic field lines break and reconnect in explosive events called "nanoflares." These nanoflares release stored magnetic energy, accelerating particles and heating the plasma to temperatures exceeding $10,000,000\text{ K}$ in localized patches. The heated plasma then cools down back to typical coronal temperatures.

Based on the descriptions of Model 1 and Model 2, which of the following statements best describes a point of agreement and a point of disagreement between the two models?

Venusian Atmospheric Superrotation

The atmosphere of Venus rotates nearly $60$ times faster than the planet's solid surface, a phenomenon known as superrotation. Two models attempt to explain this phenomenon.

*Model $1$*
Superrotation is driven primarily by solar thermal tides. Solar heating of the dayside atmosphere creates pressure waves (thermal tides) that travel westward, matching the sun's apparent motion. These waves transfer horizontal momentum directly to the upper atmosphere, accelerating the zonal (east-west) winds. This model predicts that wind speeds are highest where solar heating is most intense (at the equator) and decreases significantly toward the poles. Zonal wind speeds are not significantly affected by latitudinal air transport (meridional circulation).

*Model $2$*
Superrotation is driven by a combination of global meridional (north-south) circulation cells and atmospheric eddies. Solar heating causes hot air to rise at the equator and move toward the poles in the upper atmosphere. As this air moves, conservation of angular momentum causes it to spin faster. Eddies (turbulent disturbances) then transport this momentum back toward the equator. This model predicts that meridional wind speeds are critical to maintaining zonal superrotation and that superrotation occurs at all latitudes, with high-speed jet streams forming at mid-to-high latitudes rather than just at the equator.

Based on the models provided, is the following statement True or False?

'Model $1$ supports the belief that the transport of momentum by global north-south (meridional) circulation cells is the primary driver of Venus's atmospheric superrotation.'

### Models of Eukaryotic Origin

Eukaryotic cells are characterized by membrane-bound organelles, such as mitochondria and chloroplasts. Biologists have proposed two primary models to explain how these complex internal structures first arose from simpler prokaryotic ancestors.

Model 1 (Autogenous Model)
This model proposes that eukaryotic cells evolved directly from a single ancestral prokaryotic lineage. Through the gradual invagination (infolding) and specialization of the cell's outer plasma membrane, internal compartments formed. This process led to the creation of the nuclear envelope, the endoplasmic reticulum, and eventually mitochondria and chloroplasts. Because all organelles were formed from the host cell's own membrane, this model assumes that mitochondria and chloroplasts should not possess their own distinct genomes, ribosomes, or independent translation machinery.

Model 2 (Serial Endosymbiotic Model)
This model proposes that eukaryotic cells arose through a series of symbiotic relationships between different prokaryotic species. First, an ancestral host cell engulfed free-living aerobic alpha-proteobacteria, which survived inside the host and eventually evolved into mitochondria. Later, some of these early cells engulfed photosynthetic cyanobacteria, which evolved into chloroplasts. Because these organelles were once independent, free-living organisms, the model predicts that mitochondria and chloroplasts should retain their own circular DNA, double membranes, and distinct $70\text{S}$ bacterial-like ribosomes, rather than the $80\text{S}$ eukaryotic ribosomes.

Based on the models provided, match each evolutionary statement or prediction to its corresponding classification.

Avian Migration Navigation

Many migratory birds travel thousands of miles annually, navigating with remarkable precision. Scientists agree that birds utilize Earth's magnetic field, but they disagree on the underlying physiological mechanism.

*Hypothesis 1*
Migratory birds rely on light-dependent chemical reactions in the retina to navigate. Light absorption excites cryptochrome proteins, generating radical pairs (molecules with unpaired electrons). The spin states of these radical pairs are highly sensitive to the orientation of Earth's magnetic field. This chemical signal is processed by the brain as visual patterns of light and shade, allowing the bird to 'see' the magnetic field. Consequently, magnetoreception is impossible in total darkness or under light wavelengths (such as red light) that fail to excite cryptochromes.

*Hypothesis 2*
Migratory birds rely on microscopic crystals of magnetite ($\text{Fe}_3\text{O}_4$), a magnetic mineral, located in specialized sensory cells in the upper beak. These crystals act like tiny compass needles that physically align with Earth's magnetic field lines. This alignment exerts mechanical pressure on cell membranes, opening ion channels and sending electrical nerve impulses to the brain. This mechanism is light-independent and operates equally well in light, total darkness, or under any wavelength of light.

Based on the description of Hypothesis 1, which of the following conditions is necessary for a migratory bird to navigate using Earth's magnetic field?

### Models of the Ediacaran Extinction

The Ediacaran biota were a group of multicellular, soft-bodied organisms that dominated Earth's oceans during the late Ediacaran period (roughly $635\text{ to } 541$ million years ago) and then abruptly disappeared. Scientists have proposed two models to explain their extinction.

Model 1 (Biotic Replacement Model)
This model proposes that the extinction of the Ediacaran biota was driven by the rapid evolution and diversification of early Cambrian metazoans (true animals). These new animals acted as "ecosystem engineers" that disrupted the benthic environments. Specifically, the evolution of motile burrowing organisms (bioturbators) destroyed the microbial mats on the seafloor that Ediacaran organisms depended on for food and stabilization. Additionally, the emergence of mobile predators and competitors actively outcompeted or preyed upon the immobile Ediacaran organisms, driving them to extinction.

Model 2 (Environmental Catastrophe Model)
This model proposes that the extinction of the Ediacaran biota was caused by a sudden, global environmental crisis, specifically a major ocean anoxia event (severe oxygen depletion in deep marine waters). The Ediacaran biota lacked specialized respiratory organs and relied on passive diffusion across their body surfaces, making them highly vulnerable to low oxygen levels. Under this model, the extinction of the Ediacarans occurred first due to these geochemical changes. The subsequent diversification and rise of Cambrian metazoans did not cause the extinction, but rather occurred because the demise of the Ediacarans freed up ecological niches and resources.

Based on the models, which of the following statements best describes how Model 1 and Model 2 differ regarding the relationship between the diversification of Cambrian metazoans and the extinction of the Ediacaran biota?

Two models are proposed to explain the heating of the solar corona, which is significantly hotter than the underlying photosphere:

* Wave Heating Model: Magnetohydrodynamic (MHD) waves, particularly Alfven waves, carry energy upward from the photosphere along magnetic field lines and dissipate this energy as heat within the corona. This model predicts continuous, uniform energy deposition without sudden localized temperature spikes.
* Nanoflare Model: Microscopic magnetic reconnection events (nanoflares) constantly occur in the corona, releasing magnetic energy that heats the local plasma to extremely high temperatures ($>10^7\text{ K}$) in brief, localized bursts. Both models assume that the coronal heating mechanism is intrinsically linked to solar magnetic fields.

Based on the models provided, match each experimental finding to the relationship it shares with the proposed models.

### Subglacial Lake Heating Models

Scientists are investigating the source of liquid water in Lake Vostok, a large subglacial lake located beneath the East Antarctic Ice Sheet. The air temperature above the ice sheet averages $-55^\circ\text{C}$, yet the water at the bottom of the lake remains liquid. Two hypotheses are proposed to explain this phenomenon:

Geothermal Hypothesis
The primary source of heat is geothermal energy radiating from the Earth's interior through the thin continental crust directly beneath the lake. This constant thermal flux maintains a water temperature of approximately $1^\circ\text{C}$ to $3^\circ\text{C}$ at the lake bed, preventing the water from freezing. Proponents of this hypothesis believe that the thickness of the overlying ice sheet ($4\text{ km}$) acts as an insulator, trapping this geothermal heat.

Frictional Hypothesis
The primary source of heat is friction generated by the movement of the ice sheet sliding over the rugged bedrock topography. As gravity pulls the massive ice sheet slowly downhill, kinetic energy is converted into thermal energy at the boundary between the ice and the rock. Proponents of this hypothesis believe that this sliding friction melts the bottom layers of the ice sheet, continuously feeding liquid water into the lake. They suggest that geothermal heat flux in the region is average and insufficient to melt the ice on its own.

Which of the following statements best describes a belief held by the proponents of the Frictional Hypothesis but NOT by the proponents of the Geothermal Hypothesis?

The Faint Young Sun Paradox

Standard astrophysical models indicate that during the Archean eon ($3.8$ to $2.5$ billion years ago), the Sun's energy output was only about $70\%$ to $75\%$ of its current value. Under these conditions, Earth's surface temperature should have been well below freezing, yet geological evidence confirms the continuous presence of liquid water oceans. Two models attempt to explain this paradox.

*Model 1*
Early Earth's atmosphere contained significantly higher concentrations of greenhouse gases, primarily carbon dioxide ($CO_2$) and methane ($CH_4$), than it does today. Active volcanism continuously released large quantities of $CO_2$ into the atmosphere. Additionally, because continental landmasses were small, the rate of silicate weathering (a chemical process that removes atmospheric $CO_2$ and stores it in crustal rocks) was extremely low. The abundance of these gases trapped outgoing infrared radiation, maintaining surface temperatures above freezing.

*Model 2*
The primary driver of early Earth's warmth was a lower planetary albedo (reflectivity), which allowed the Earth to absorb a larger fraction of the Sun's incoming radiation. During the Archean eon, continental landmasses occupied less than $5\%$ of Earth's surface, leaving the planet dominated by dark oceans that absorbed solar energy. Furthermore, the absence of land plants and certain marine organisms meant there were fewer biogenic aerosols to act as cloud condensation nuclei. This resulted in fewer, thinner clouds, allowing more solar radiation to reach and warm the surface.

Based on Model 2, which of the following statements best describes the hypothesis explaining how early Earth maintained liquid water?

Titan's atmosphere is rich in methane ($CH_4$), which is constantly destroyed by sunlight. Scientists propose different models to explain how it is replenished.

Model 1 (Episodic Cryovolcanism)
Methane is stored in methane clathrate hydrates within Titan's icy crust. This methane was incorporated during Titan's formation. Heat plumes from Titan's core periodically rise through the mantle, causing localized melting of the crust. This triggers episodic cryovolcanic eruptions that release large pulses of methane into the atmosphere. Replenishment is not constant; it occurs in brief, intense bursts separated by hundreds of millions of years.

Model 2 (Deep Serpentinization)
Methane is continuously produced in Titan's rocky core. Liquid water, circulating through the warm silicate core, reacts with olivine minerals in a process called serpentinization. This reaction produces hydrogen ($H_2$), which then reacts with carbon dioxide ($CO_2$) to synthesize new methane. This newly created methane continuously ascends through the liquid water ocean and the icy crust, escaping into the atmosphere via steady diffusion through tectonic fractures.

Model 3 (Tidal Sublimation)
Titan accreted a vast reservoir of methane ice directly into its outer crust during formation. No new methane is currently being produced. Instead, gravitational interactions with Saturn generate tidal forces that flex Titan's crust. This tidal heating is concentrated in the crust, causing solid methane ice to sublimate (change directly from solid to gas). The gas escapes through porous ice, providing a steady, gradually declining release of primordial methane into the atmosphere.

Match each model of Titan's methane replenishment to the statement that best represents its hypothesis regarding the origin and release mechanism of the methane.

Martian Soil Activity Models

During the 1976 Viking Lander missions, scientists observed chemical activity when Martian soil samples were mixed with a liquid nutrient solution. Two scientists proposed competing hypotheses to explain this observation.

*Scientist 1*
The soil activity is biological. Martian soil contains microscopic organisms that metabolize organic nutrients. When these organisms consume the nutrients, they release carbon dioxide ($CO_2$) gas. Heating the soil to $160^\circ\text{C}$ kills these organisms, which is why preheating the soil prevents gas release.

*Scientist 2*
The soil activity is inorganic. Highly reactive oxygen-carrying chemicals, such as superoxides, are present in the soil due to exposure to intense ultraviolet radiation. When mixed with the liquid solution, these superoxides chemically oxidize the nutrients, releasing $CO_2$. Heating the soil to $160^\circ\text{C}$ thermally decomposes the superoxides, rendering them unable to react with nutrients.

According to Scientist 1's hypothesis, what is the primary agent responsible for the release of carbon dioxide gas from the soil mixture?

Researchers study the electrical resistance of four different wire samples as a function of temperature. The resistance $R$ (in ohms, $\Omega$) of a metal wire at temperature $T$ (in $^\circ\text{C}$) is modeled by the linear relationship:

where $R_0$ is the baseline resistance at the reference temperature $T_0 = 20^\circ\text{C}$, and $\alpha$ is the temperature coefficient of resistance (in $^\circ\text{C}^{-1}$).

The baseline resistance and temperature coefficients for the four wire samples are shown in the table below:

Arrange the wire samples in order of their predicted electrical resistance at $120^\circ\text{C}$ from lowest resistance to highest resistance.

Deep-sea hydrothermal vents support complex ecosystems in the complete absence of solar radiation. Scientists propose two models to explain how primary producers (microorganisms) at the base of these food webs obtain energy.

*Model 1*
One group of scientists hypothesizes that primary producers near hydrothermal vents utilize the geothermal radiation (specifically infrared light) emitted by superheated vent fluids as their primary energy source. They believe that these microbes possess specialized pigments capable of capturing low-energy geothermal photons to drive a unique form of geothermal photosynthesis.

*Model 2*
Another group of scientists argues that geothermal radiation is too weak to support the high biomass observed at vents. They hypothesize that primary producers rely entirely on chemical energy. According to this model, the microbes obtain energy by oxidizing dissolved inorganic compounds, such as hydrogen sulfide (_\text{H}_2\text{S}), and use this energy to convert carbon dioxide (_\text{CO}_2) into organic molecules.

Based on the models provided, is the following statement true or false?
"According to Model 2, the primary producers at deep-sea hydrothermal vents utilize specialized pigments to capture geothermal infrared photons as their primary energy source."

Martian Recurring Slope Lineae (RSL)

Recurring Slope Lineae (RSL) are dark, narrow streaks that appear on steep, warm Martian slopes during late spring and summer, fade in winter, and reappear the following year. Three scientists discuss the mechanism responsible for these features:

*Scientist 1*
RSL are caused by the seasonal flow of liquid brine (salty water). The Martian regolith contains hygroscopic salts (such as perchlorates) that absorb water vapor from the atmosphere in a process called deliquescence. During the warmer seasons, these salts absorb enough moisture to dissolve into liquid brines, lowering the freezing point of water and allowing liquid to flow downslope, darkening the soil.

*Scientist 2*
RSL are dry granular flows, or mini-avalanches of sand and dust, requiring no liquid water. The streaks appear on slopes that are at or near the angle of repose (the steepest angle at which granular material remains stable). Seasonal heating by sunlight increases the temperature of the dark dust particles, causing expansion and removing thin layers of adsorbed atmospheric gas between grains. This destabilizes the dust, causing it to flow downslope and expose darker subsurface material.

*Scientist 3*
RSL are caused by the discharge of shallow subsurface aquifers. Underneath the Martian surface, thin lenses of water ice exist. During the peak of summer warmth, geothermal heat combined with seasonal solar heating melts these ice lenses. The resulting fresh water breaches the surface, flowing down the slopes and darkening the regolith before rapidly evaporating into the thin Martian atmosphere.

Based on the passage, match each scientist to the primary source or mechanism they hypothesize is responsible for the formation of Recurring Slope Lineae (RSL).

Grand Canyon Formation Models

The Grand Canyon in Arizona is one of the most prominent geological features on Earth. Geologists have debated how and when the canyon was carved. Two models describe the potential mechanism and timeline of its formation.

*Model 1 (Catastrophic Flooding)*
The Grand Canyon was carved rapidly, likely within a few weeks to months, approximately $15,000$ years ago. Near the end of the last glacial period, large prehistoric lakes (Lake Hopi and Lake Bidahochi) were trapped behind natural sediment dams on the Colorado Plateau. As climate warming caused rapid ice melt, these lakes overflowed, breaching the dams. The sudden, catastrophic release of millions of acre-feet of water cut deeply through the soft sedimentary rock layers, creating the canyon in a brief geological event.

*Model 2 (Incremental Erosion)*
The Grand Canyon was carved slowly and steadily over $5$ to $6$ million years by the Colorado River. The process began when the Colorado Plateau underwent gradual tectonic uplift, increasing the river's gradient and flow velocity. Over millions of years, the river's water, carrying abrasive sediments like sand and gravel, acted as a slow saw, cutting downward through the rising plateau. Weathering, landslides, and tributary erosion slowly widened the canyon walls to their current dimensions.

According to Model 1, which of the following events was directly responsible for initiating the carving of the Grand Canyon?

Deep-focus earthquakes occur at depths of $300\text{ km}$ to $700\text{ km}$, where high temperatures and pressures are expected to cause mantle rocks to deform plastically rather than fracture. Two scientists propose different hypotheses to explain the initiation of these deep earthquakes.

Scientist 1
Deep-focus earthquakes are caused by *dehydration embrittlement*. Subducting tectonic plates carry hydrous minerals (such as serpentine) deep into the mantle. As temperature and pressure increase with depth, these minerals undergo dehydration reactions, releasing liquid water. This pressurized fluid fills microscopic pore spaces, reducing the effective normal stress on pre-existing faults. The reduction in friction allows the rock to suddenly slip, generating an earthquake.

Scientist 2
Deep-focus earthquakes are caused by *transformational plasticity*. The mantle mineral olivine ($\text{Mg}_2\text{SiO}_4$) undergoes a phase transition to a denser polymorph, wadsleyite, at a depth of approximately $410\text{ km}$. Within cold subducting slabs, this transition is delayed, creating a metastable region of olivine. Under high shear stress, small zones of metastable olivine undergo a rapid, runaway transformation into wadsleyite. This phase change weakens the shear strength of the rock along narrow shear zones, causing localized instability and sudden displacement.

Based on Scientist 1's hypothesis, which of the following conditions must be met for a deep-focus earthquake to occur at a depth of $500\text{ km}$?

Exoplanet Atmosphere Models

Two scientists discuss the source of nitrogen ($N_2$) in the atmosphere of Kepler-186f, a newly discovered terrestrial exoplanet.

*Scientist 1*
The exoplanet's atmospheric nitrogen is entirely primordial, having been delivered by carbonaceous chondrite meteorites during planetary accretion $4.5$ billion years ago. The nitrogen was subsequently released into the atmosphere via early volcanic outgassing. Because the exoplanet's surface temperature has remained below $250\text{ K}$, there is no active tectonic recycling or geochemical process capable of returning nitrogen to the mantle or releasing new nitrogen from the crust. Thus, the total amount of nitrogen in the atmosphere has remained constant since the planet's formation.

*Scientist 2*
The exoplanet's atmospheric nitrogen is not primordial but is continuously replenished. Kepler-186f experiences ongoing tidal heating, keeping its mantle partially molten and driving active subduction zones. Nitrogen is continuously drawn into the mantle via plate tectonics and then released back into the atmosphere through arc volcanism. Furthermore, solar wind strip-mining slowly removes nitrogen from the upper atmosphere, meaning that without this active tectonic cycle, Kepler-186f would have lost its atmospheric nitrogen within $1$ billion years of formation.

Consider the following statement: According to Scientist 1, the volcanic outgassing of nitrogen on Kepler-186f occurred only during the early history of the planet and is not an ongoing process.

Based on the models provided, is this statement True or False?