Exploring the Exosphere: A Comprehensive Guide for Science Students

The exosphere is the outermost layer of a planetary atmosphere, where the atmospheric gases are so thin that the individual gas molecules move in ballistic trajectories. This layer is characterized by its temperature, density, and composition, which can vary significantly depending on the planet and its position in its orbit.

Measuring the Exosphere: Incoherent Scatter Radar (ISR) and Spacecraft Missions

Incoherent Scatter Radar (ISR)

One way to measure the exosphere is by observing the density of neutral atoms and ions using incoherent scatter radar (ISR) data. This method involves fitting models of ion thermodynamics and chemistry to the measured ion temperature data. The ISR technique provides valuable insights into the exosphere, but it has some limitations, such as the lack of global coverage and the dependence on models for reconstructing height profiles of the thermosphere.

The ISR technique relies on the scattering of electromagnetic waves by free electrons in the ionized upper atmosphere. The scattered signal contains information about the electron density, electron and ion temperatures, and ion composition. By fitting these measurements to models of ion thermodynamics and chemistry, researchers can derive the density and temperature of the exospheric neutral atoms and ions.

One of the key advantages of the ISR method is its ability to provide continuous, high-resolution measurements of the exosphere over a wide range of altitudes. This allows for the study of temporal and spatial variations in the exospheric properties, which can be crucial for understanding the complex interactions between the exosphere and other atmospheric layers.

Spacecraft Missions: MAVEN

Another way to measure the exosphere is by observing the density of exospheric hydrogen using spacecraft missions, such as the Mars Atmosphere and Volatile Evolution (MAVEN) mission. The MAVEN mission has provided quantitative estimates of the hydrogen exosphere with nearly complete temporal coverage, revealing order of magnitude seasonal changes.

The MAVEN mission has utilized several techniques to study the exosphere:

1. Exospheric Hydrogen Density Measurements: The MAVEN spacecraft has measured the density of exospheric hydrogen by observing the density of Energetic Neutral Atoms (ENAs) produced by charge exchange between solar wind protons and exospheric hydrogen. This method has allowed MAVEN to derive the exospheric hydrogen distribution and escape rates.

2. Inferred Fraction of Solar Wind Protons Converted to ENAs: The MAVEN mission has measured the inferred fraction of solar wind protons converted to ENAs outside the Martian bow shock, which varies from ~0.5 to 5% over the mission. These measured values compare favorably to the 1 to 3% predicted by previous studies.

3. Constraints on Exospheric Structure and Hydrogen Escape Rates: The MAVEN mission has placed constraints on the exospheric structure and the escape rate of hydrogen from Mars by comparing the measured column of exospheric hydrogen upstream of the Martian bow shock with Chamberlain-type exosphere models. The mission has observed a strong peak in column density slightly after perihelion, approximately at southern summer solstice.

The MAVEN mission’s comprehensive measurements of the Martian exosphere have provided valuable insights into the complex interactions between the exosphere and other atmospheric layers, as well as the processes that govern the escape of atmospheric gases from the planet.

Factors Affecting the Quantitative Accuracy of Exosphere Measurements

It’s important to note that there are various factors that may affect the quantitative accuracy of the exosphere measurements obtained using both ISR and spacecraft missions:

1. Solar Wind Variability: The solar wind can significantly influence the exosphere, and its variability can introduce uncertainties in the measurements.

2. Inaccurate Determination of Bow Shock Location: The location of the planetary bow shock, which separates the solar wind from the planetary magnetosphere, can affect the accuracy of the measurements.

3. Inaccurate Collision Cross Sections: The collision cross sections used in the models for charge exchange and other processes can introduce uncertainties in the derived exospheric properties.

4. Charge Exchange with Other Species: Charge exchange with species other than atomic hydrogen in the exosphere can complicate the interpretation of the measurements.

5. Contributions from Neutral Solar Wind: Contributions from neutral solar wind produced by charge exchange with interplanetary hydrogen can also affect the measurements.

6. Particles on Satellite Orbits: The presence of particles on satellite orbits can introduce additional uncertainties in the measurements.

7. Departure from Chamberlain-type Exosphere: If the exosphere departs significantly from the Chamberlain-type model, the comparison with these models may not provide accurate constraints on the exospheric structure and escape rates.

Despite these challenges, the basic measurement techniques used in both ISR and spacecraft missions are generally simple and robust, and they do not even require accurate absolute sensitivity calibrations. By carefully considering these factors and incorporating them into the data analysis and modeling, researchers can obtain more reliable and accurate estimates of the exospheric properties.

Exospheric Composition and Structure

The composition and structure of the exosphere can vary significantly depending on the planet and its position in its orbit. For example, the Martian exosphere is primarily composed of hydrogen, with smaller amounts of other species such as oxygen and carbon dioxide.

The exospheric structure can be described using the Chamberlain model, which assumes that the exospheric particles follow ballistic trajectories and are in thermal equilibrium with the underlying thermosphere. This model provides a framework for understanding the distribution of exospheric particles and their escape rates.

One of the key parameters in the Chamberlain model is the exobase, which is the altitude at which the mean free path of the atmospheric particles becomes larger than the scale height of the atmosphere. Above the exobase, the atmospheric particles are no longer in thermal equilibrium with the underlying layers, and their motion is dominated by ballistic trajectories.

The exobase altitude can vary significantly depending on the planet and the atmospheric conditions. For example, on Earth, the exobase is typically located at an altitude of around 500-600 km, while on Mars, it can range from 200 to 300 km, depending on the season and solar activity.

Exospheric Escape Processes

The exosphere plays a crucial role in the escape of atmospheric gases from a planet. The thin, rarefied nature of the exosphere allows the atmospheric particles to undergo ballistic trajectories, which can ultimately lead to their escape from the planet’s gravitational field.

There are several key processes that govern the escape of atmospheric gases from the exosphere:

1. Thermal Escape: Atmospheric particles with sufficient kinetic energy can escape the planet’s gravitational field if their velocity exceeds the planet’s escape velocity. This process is known as thermal escape and is influenced by the temperature of the exosphere.

2. Nonthermal Escape: In addition to thermal escape, there are various nonthermal escape processes, such as sputtering, photochemical escape, and charge exchange, that can also contribute to the loss of atmospheric gases from the exosphere.

3. Atmospheric Stripping: The interaction between the solar wind and the planet’s magnetic field can lead to the stripping of atmospheric gases from the exosphere, a process known as atmospheric stripping or solar wind erosion.

Understanding the relative importance of these escape processes and their dependence on factors such as solar activity, planetary magnetic field, and atmospheric composition is crucial for modeling the long-term evolution of planetary atmospheres and their potential for supporting life.

Conclusion

The exosphere is a complex and dynamic layer of a planetary atmosphere, and its study requires a multifaceted approach. The combination of incoherent scatter radar (ISR) data and spacecraft missions, such as MAVEN, has provided valuable insights into the temperature, density, and composition of the exosphere, as well as the processes that govern the escape of atmospheric gases.

By understanding the factors that affect the quantitative accuracy of exosphere measurements and the underlying physical and chemical processes, researchers can continue to refine our knowledge of this critical layer of planetary atmospheres. This knowledge is essential for understanding the long-term evolution of planetary atmospheres and their potential for supporting life, as well as for developing accurate models of space weather and its impact on human and robotic exploration.

References:

1. Halekas, J. S., et al. (2017). Seasonal variability of the hydrogen exosphere of Mars. Journal of Geophysical Research: Space Physics, 122(5), 5383-5400.
2. Yi, Y., et al. (2022). Exospheric Temperature Measured by NASA‐GOLD Under Low and High Solar Activity. Journal of Geophysical Research: Space Physics, 127(7), e2022JA030277.
3. Halekas, J. S., et al. (2016). Mars exosphere and ionosphere: current understanding and future prospects. Space Science Reviews, 205(1-4), 259-315.
4. NASA’s GOLD Mission Reveals New Insights About Earth’s Space Weather Environment: https://www.nasa.gov/feature/goddard/2018/nasa-s-gold-mission-reveals-new-insights-about-earth-s-space-weather-environment
5. MAVEN Science Goals: https://lasp.colorado.edu/home/maven/science/science-goals/