🌍Planetary Science Unit 6 – Planetary Atmospheres and Climates

Key Atmospheric Components

• Nitrogen ($N_2$) most abundant gas in Earth's atmosphere, making up ~78% of the air we breathe
• Oxygen ($O_2$) second most abundant gas at ~21%, essential for life and produced by photosynthesis
  • Ozone ($O_3$) forms a protective layer in the stratosphere, absorbing harmful UV radiation
• Water vapor ($H_2O$) variable concentration, typically 1-4%, plays a crucial role in weather and climate as a greenhouse gas
• Carbon dioxide ($CO_2$) minor component at ~0.04%, but a potent greenhouse gas driving climate change
• Argon (Ar) inert gas making up ~0.93% of the atmosphere, does not participate in chemical reactions
• Trace gases include methane ($CH_4$), nitrous oxide ($N_2O$), and chlorofluorocarbons (CFCs), which can have significant impacts on climate and the environment despite their low concentrations
• Aerosols solid or liquid particles suspended in the atmosphere, can be natural (dust, sea salt) or anthropogenic (smoke, pollution), and affect climate through scattering and absorbing radiation

Atmospheric Structure and Layers

• Troposphere lowest layer, extends from the surface to ~8-16 km, contains most of the atmosphere's mass and water vapor
  • Characterized by a decrease in temperature with height at a rate of ~6.5°C/km (lapse rate)
  • Most weather phenomena occur in this layer, including clouds, precipitation, and storms
• Stratosphere layer above the troposphere, extending from the tropopause to ~50 km, contains the ozone layer
  • Temperature increases with height due to absorption of UV radiation by ozone
  • Relatively stable layer with little vertical mixing, allowing for the formation of the ozone layer
• Mesosphere layer above the stratosphere, extending from the stratopause to ~85 km, characterized by a decrease in temperature with height
  • Coldest layer of the atmosphere, with temperatures reaching as low as -90°C at the mesopause
  • Noctilucent clouds, the highest clouds in the atmosphere, form in this layer
• Thermosphere layer above the mesosphere, extending from the mesopause to ~500-1000 km, characterized by a significant increase in temperature with height
  • Temperatures can reach up to 2000°C due to absorption of high-energy solar radiation by oxygen and nitrogen
  • Ionosphere, a region of charged particles, is located within the thermosphere and plays a crucial role in radio communication
• Exosphere outermost layer of the atmosphere, extending from the thermopause to ~10,000 km, where atoms and molecules can escape into space
  • Very low density, with particles rarely colliding and following ballistic trajectories
  • Transition zone between the atmosphere and the vacuum of space

Energy Balance and Greenhouse Effect

• Solar radiation primary source of energy for Earth's atmosphere and surface, with an average intensity of ~1,360 W/m² at the top of the atmosphere (solar constant)
  • ~30% of incoming solar radiation is reflected back to space by clouds, aerosols, and the Earth's surface (albedo)
  • ~70% is absorbed by the atmosphere and surface, warming the planet
• Terrestrial radiation emitted by the Earth's surface and atmosphere in the form of infrared (longwave) radiation
  • Earth's surface emits radiation as a black body, with a peak emission wavelength determined by its temperature (~10 μm for a surface temperature of 288 K)
  • Greenhouse gases absorb and re-emit terrestrial radiation, trapping heat in the lower atmosphere
• Greenhouse effect process by which greenhouse gases (e.g., $CO_2$, $H_2O$, $CH_4$) absorb and re-emit infrared radiation, warming the lower atmosphere and surface
  • Without the greenhouse effect, Earth's average surface temperature would be ~-18°C, instead of the current ~15°C
  • Enhanced greenhouse effect due to anthropogenic emissions of greenhouse gases is driving global climate change
• Radiative forcing measure of the change in energy balance due to a perturbation, such as an increase in greenhouse gas concentrations, expressed in W/m²
  • Positive radiative forcing (e.g., increased $CO_2$) leads to warming, while negative radiative forcing (e.g., increased aerosols) leads to cooling
• Climate sensitivity measure of how much the Earth's surface temperature changes in response to a given radiative forcing, typically expressed as the temperature change per doubling of $CO_2$ concentration
  • Estimated to be ~1.5-4.5°C per doubling of $CO_2$, with a best estimate of ~3°C
  • Depends on various feedback mechanisms, such as water vapor feedback and ice-albedo feedback

Weather Patterns and Climate Systems

• Hadley cell large-scale atmospheric circulation pattern in the tropics, driven by solar heating and the rotation of the Earth
  • Characterized by rising motion near the equator, poleward flow aloft, descending motion in the subtropics, and equatorward flow near the surface (trade winds)
  • Responsible for the formation of the Intertropical Convergence Zone (ITCZ) and the subtropical high-pressure belts
• Ferrel cell mid-latitude atmospheric circulation pattern, driven by the interaction between the Hadley and Polar cells
  • Characterized by rising motion in the mid-latitudes, poleward flow aloft, descending motion in the high latitudes, and equatorward flow near the surface (westerlies)
  • Associated with the formation of mid-latitude cyclones and anticyclones
• Polar cell high-latitude atmospheric circulation pattern, driven by cold air sinking over the poles
  • Characterized by descending motion over the poles, equatorward flow near the surface (polar easterlies), and rising motion in the subpolar regions
  • Plays a role in the formation of the polar front and the polar jet stream
• Monsoons seasonal wind patterns that reverse direction between summer and winter, driven by differential heating between land and ocean
  • Examples include the Asian monsoon, the African monsoon, and the North American monsoon
  • Associated with heavy rainfall during the summer months and dry conditions during the winter months
• El Niño-Southern Oscillation (ENSO) coupled ocean-atmosphere phenomenon characterized by fluctuations in ocean surface temperatures and atmospheric pressure across the equatorial Pacific
  • El Niño phase associated with warmer-than-average ocean temperatures and weaker trade winds, leading to increased rainfall in the eastern Pacific and drier conditions in the western Pacific
  • La Niña phase associated with cooler-than-average ocean temperatures and stronger trade winds, leading to increased rainfall in the western Pacific and drier conditions in the eastern Pacific

Atmospheric Dynamics and Circulation

• Coriolis effect apparent force arising from the Earth's rotation, causing moving objects (including air and water) to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
  • Magnitude of the Coriolis force depends on the latitude and the speed of the moving object
  • Plays a crucial role in the formation of large-scale atmospheric and oceanic circulation patterns, such as the Hadley cells and the jet streams
• Geostrophic balance balance between the pressure gradient force and the Coriolis force in the upper atmosphere, resulting in a flow parallel to the isobars (lines of constant pressure)
  • Geostrophic wind speed depends on the magnitude of the pressure gradient and the latitude
  • Helps explain the formation and behavior of large-scale atmospheric features, such as the jet streams and the subtropical high-pressure belts
• Thermal wind relationship between the vertical shear of the geostrophic wind and the horizontal temperature gradient in the atmosphere
  • Warm air to the right of the wind in the Northern Hemisphere, and to the left in the Southern Hemisphere
  • Explains the formation and strength of the jet streams, which are characterized by strong vertical wind shear and large horizontal temperature gradients
• Rossby waves large-scale atmospheric waves that propagate westward relative to the mean flow, driven by the variation of the Coriolis force with latitude (beta effect)
  • Play a crucial role in the transport of energy and momentum in the atmosphere, and in the formation of large-scale weather patterns, such as blocking highs and cut-off lows
  • Characterized by a wavelength of several thousand kilometers and a period of several days to weeks
• Atmospheric tides global-scale oscillations in the atmosphere, driven by the gravitational pull of the Moon and the Sun, as well as by solar heating
  • Diurnal (24-hour) and semidiurnal (12-hour) tides are the most prominent, with amplitudes of several meters in the lower atmosphere and several kilometers in the upper atmosphere
  • Can influence weather and climate patterns, particularly in the tropics and the mesosphere

Comparative Planetology

• Terrestrial planets (Mercury, Venus, Earth, Mars) characterized by solid, rocky surfaces and relatively thin atmospheres
  • Atmospheres are secondary, formed by outgassing from the interior and modified by surface processes (e.g., weathering, volcanism)
  • Composition and thickness of the atmospheres vary widely, from the thin, $CO_2$-rich atmosphere of Mars to the thick, $CO_2$-dominated atmosphere of Venus
• Giant planets (Jupiter, Saturn, Uranus, Neptune) characterized by thick, hydrogen-helium atmospheres and no solid surfaces
  • Atmospheres are primary, captured directly from the solar nebula during planetary formation
  • Composition of the atmospheres is similar across the giant planets, but the relative abundances of trace compounds (e.g., methane, ammonia) vary
• Atmospheric escape processes by which atmospheric gases can be lost to space, including thermal escape (Jeans escape), nonthermal escape (e.g., photochemical escape, sputtering), and impact erosion
  • Importance of atmospheric escape depends on factors such as planetary mass, distance from the Sun, and the presence of a magnetic field
  • Played a crucial role in the evolution of the atmospheres of the terrestrial planets, particularly Mars and Venus
• Runaway greenhouse effect scenario in which a planet's atmosphere becomes increasingly opaque to infrared radiation, leading to a rapid and irreversible increase in surface temperature
  • Thought to have occurred on Venus, resulting in its current high surface temperature (~460°C) and thick, $CO_2$-dominated atmosphere
  • Potential risk for Earth if anthropogenic greenhouse gas emissions continue unabated
• Habitable zone range of orbital distances around a star where liquid water can exist on a planet's surface, given sufficient atmospheric pressure
  • Depends on factors such as the star's luminosity, the planet's mass and atmospheric composition, and the presence of greenhouse gases
  • Inner edge of the habitable zone is determined by the onset of a runaway greenhouse effect, while the outer edge is determined by the condensation of $CO_2$ into clouds and the formation of $CO_2$ ice

Climate Evolution and Change

• Faint young Sun paradox observation that the Sun's luminosity was ~30% lower during the early history of the solar system, which should have resulted in a frozen Earth, but geological evidence suggests the presence of liquid water and relatively warm temperatures
  • Resolved by the presence of higher concentrations of greenhouse gases (e.g., $CO_2$, $CH_4$) in the early Earth's atmosphere, which provided additional warming
  • Highlights the importance of atmospheric composition in regulating planetary climate over geological timescales
• Snowball Earth hypothesis proposes that the Earth's surface was entirely or nearly entirely frozen during several periods in its history, most notably during the Neoproterozoic era (~720 to 635 million years ago)
  • Triggered by a runaway ice-albedo feedback, in which the growth of ice sheets led to an increase in the Earth's albedo, further cooling the planet
  • Recovery from a snowball state requires the buildup of atmospheric $CO_2$ through volcanic outgassing, which eventually leads to a strong greenhouse effect and rapid melting of the ice sheets
• Paleoclimatology study of past climates and their changes over geological timescales, using a variety of proxy data (e.g., ice cores, tree rings, sediment cores, fossil records)
  • Provides insights into the natural variability of the Earth's climate system, as well as the mechanisms and feedbacks that drive long-term climate change
  • Helps constrain the sensitivity of the climate system to various forcings, such as changes in solar irradiance, volcanic eruptions, and variations in the Earth's orbit (Milankovitch cycles)
• Anthropogenic climate change current and projected changes in the Earth's climate system due to human activities, primarily the emission of greenhouse gases from fossil fuel combustion and land-use changes
  • Characterized by a rapid increase in global average surface temperature, rising sea levels, changes in precipitation patterns, and more frequent and intense extreme weather events
  • Mitigation efforts focus on reducing greenhouse gas emissions through the adoption of clean energy technologies, energy efficiency measures, and carbon sequestration techniques
• Climate models numerical simulations of the Earth's climate system, based on the physical, chemical, and biological processes that govern the interactions between the atmosphere, oceans, land surface, and cryosphere
  • Used to study the response of the climate system to various forcings, such as changes in greenhouse gas concentrations, solar irradiance, and land use
  • Provide projections of future climate change under different emission scenarios, which inform policy decisions and adaptation strategies

Observational Techniques and Data Analysis

• Remote sensing acquisition of information about the Earth's atmosphere and surface from a distance, using instruments on satellites, aircraft, or ground-based platforms
  • Passive remote sensing relies on the detection of naturally emitted or reflected radiation, such as visible, infrared, or microwave radiation
  • Active remote sensing involves the emission of a signal and the detection of its reflection or scattering, as in the case of radar or lidar
• Radiometry measurement of the intensity and spectral distribution of electromagnetic radiation, used to study the energy balance and composition of the atmosphere
  • Radiometers can be designed to measure specific wavelength ranges, such as the visible, infrared, or microwave portions of the spectrum
  • Examples include the Advanced Very High Resolution Radiometer (AVHRR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra and Aqua satellites
• Spectroscopy study of the interaction between matter and electromagnetic radiation, used to determine the composition and structure of the atmosphere
  • Absorption spectroscopy measures the attenuation of radiation as it passes through the atmosphere, revealing the presence and concentration of various gases and aerosols
  • Emission spectroscopy measures the radiation emitted by the atmosphere itself, providing information on its temperature and composition
• Data assimilation process of combining observations with numerical models to produce an optimal estimate of the state of the atmosphere or climate system
  • Involves the use of statistical methods, such as Kalman filtering or variational analysis, to minimize the difference between the observations and the model predictions
  • Helps improve the accuracy and reliability of weather forecasts and climate projections by continuously updating the model state with new observations
• Satellite missions dedicated spacecraft designed to study specific aspects of the Earth's atmosphere, oceans, land surface, or cryosphere
  • Examples include the Orbiting Carbon Observatory (OCO) series for measuring atmospheric $CO_2$ concentrations, the Global Precipitation Measurement (GPM) mission for studying global precipitation patterns, and the Gravity Recovery and Climate Experiment (GRACE) for monitoring changes in the Earth's gravity field due to mass redistribution in the atmosphere and oceans
  • Provide global coverage and long-term, consistent datasets that are essential for understanding and monitoring climate change and its impacts