Planetary Geophysics

I. Introduction: Comparative Planetology

The field of planetary geophysics focuses on the comparative study of the physical characteristics and geological processes of various planets. Such a comparative approach is instrumental in understanding not only the formation and evolution of these celestial bodies but also their current states. By examining planets such as Venus, Earth, and Mars, scientists can develop models that explain planetary dynamics and anticipate geological phenomena.

Key Parameters for Planetary Comparison

To facilitate a thorough understanding of these planets, several key parameters are commonly compared:

• Mass (kg): This refers to the total amount of matter contained within the planet, measured in kilograms, which influences gravitational pull and atmospheric retention.

  • Venus: 4.87 × 10²⁴ kg

  • Earth: 5.97 × 10²⁴ kg

  • Mars: 0.642 × 10²⁴ kg

• Radius (km): The average distance from a planet's center to its surface, which affects gravitational strength and surface area.

  • Venus: 6052 km

  • Earth: 6373 km

  • Mars: 3396 km

• Density (Mg/m³): Density indicates how compact a planet’s material is, calculated by dividing mass by volume.

  • Venus: 5.2 Mg/m³

  • Earth: 5.5 Mg/m³

  • Mars: 3.9 Mg/m³

• Orbital Period (Earth days): This is the time required for a planet to complete a single orbit around the Sun and has implications for seasonal changes and climate stability.

  • Venus: 224.7 days

  • Earth: 365.2 days

  • Mars: 687.0 days

• Mean Distance from the Sun (km): This distance influences solar energy received by the planet, thereby affecting climate and atmospheric dynamics.

  • Venus: 1.082 × 10⁸ km

  • Earth: 1.496 × 10⁸ km

  • Mars: 2.279 × 10⁸ km

• Rotation Rate (Earth days/rev): The time taken to complete one rotation on its axis, impacting day length, weather patterns, and atmospheric circulation.

  • Venus: 243 days/rev (retrograde rotation)

  • Earth: 1 day/rev

  • Mars: 1.03 days/rev

• Surface Gravity (m/s²): This is the acceleration due to gravity experienced at the planet's surface, influencing atmospheric retention and geological processes.

  • Venus: 8.87 m/s²

  • Earth: 9.81 m/s²

  • Mars: 3.70 m/s²

• Surface Pressure (MPa): Measured in megapascals, this signifies the atmospheric pressure at the planet’s surface, crucial for understanding climate and potential for liquid water.

  • Venus: 9 MPa (extremely high, contributing to greenhouse effect)

  • Earth: 0.1 MPa

  • Mars: 0.001 MPa (very thin atmosphere)

• Surface Temperature (°C): Average temperature on the planet’s surface, affecting geological and potential biological processes.

  • Venus: 430 °C (due to intense greenhouse gases)

  • Earth: 10 °C (a range conducive to life)

  • Mars: -65 °C (cold, with some seasonal variations)

• Magnetic Moment (T m³): A measure of the strength of a planet’s magnetic field, indicating the presence of a molten outer core or other processes.

  • Venus: < 3 × 10¹¹ T m³ (weak magnetic field)

  • Earth: 7.5 × 10¹⁵ T m³ (strong magnetic field due to active dynamo)

  • Mars: < 2 × 10¹¹ T m³ (localized magnetic fields indicative of crustal remnants)

These parameters play a critical role in deciphering the internal dynamics and surface processes unique to each planet and are essential for developing robust models of planetary geology.

II. Venus: Surface Mapping and Challenges

A. Cloud Cover and Radar Imaging

Venus is notorious for its dense atmosphere and continuous cloud cover, primarily composed of sulfuric acid droplets. This thick atmosphere severely limits direct observations using optical instruments. Therefore, it is essential to use Synthetic Aperture Radar (SAR) technology, which allows scientists to effectively map the surface by penetrating the dense cloud layers.

B. Radar Power and Signal Attenuation

Understanding radar signal behavior is crucial to effective imaging:

• Radar signals lose energy based on distance—specifically, the power decreases proportionally to the square of the distance. This can be summarized as follows:

  • Radiated Power: Decreases as 1/r²

  • Returned Power: Deteriorates as 1/r⁴

  This implies that greater distances severely challenge the quality of radar data, requiring advanced technologies to enhance signal clarity and resolution.

• For SAR to produce accurate images, satellites must illuminate multiple surface areas effectively, which requires minimal angles between the radar beam and the horizontal ground surface.

• The energy source for radar systems like SAR typically comes from solar cells, which profoundly influences spacecraft designs. For example, the Magellan spacecraft operated with a notably limited power supply of just 200 Watts.

C. Principles of Synthetic Aperture Radar Imaging

SAR technology creates high-resolution images of planetary surfaces using the motion of the radar antenna over a target area. This method permits the processing of received radar signals in a manner that simulates a much larger antenna, thus enhancing image resolution.

Key Concepts of SAR Imaging:

• Along-Track Resolution: Achieved by analyzing the Doppler shift of frequency in the radar echo, which arises due to the relative motion between the satellite and the surface.

• Cross-Track Resolution: Determined by the time delay between when a radar pulse is transmitted and when the reflected echo is received. This delay is directly proportional to the distance between the radar and the surface.

• The brightness levels captured within each resolution element are directly proportional to the amount of energy that is backscattered by the surface features being imaged.

D. Image Artifacts in SAR Data

Despite the usefulness of SAR, various artifacts can arise, impacting the accuracy of the images:

• Velocity Mispositioning: When a target experience a velocity component along the radar beam, its position may appear distorted in the SAR images.

  • Example: In SAR images, vehicles on the Golden Gate Bridge can appear as if they are driving over water because of this velocity effect.

• Overlay Effects: These occur when steep slopes create timing discrepancies in returned echoes. Reflected signals from the top of a slope arrive before those from the base, causing the peak to seem closer than it is.

  • For instance, this distortion may result in extreme misrepresentation such as confusing the top of a U-shaped valley in Alaska with the glacier below it.

E. The Magellan Mission

The Magellan spacecraft marked a significant milestone in the study of Venus, employing SAR to produce extensive, detailed maps of the planet's surface. Here are some details about the mission's operations:

• The spacecraft imaged the surface, recording data onto magnetic tape. Subsequently, it would rotate to point toward Earth to transmit the collected data.

• This process was systematically repeated during the two years of orbit, transmitting new data every three hours, leading to comprehensive understanding of Venus's surface features, including geological and topographic information surpassing that of Earth at the time.

F. Surface Age and Tectonics

The surface age of Venus is relatively youthful, estimated to be between 300 million to 500 million years (0.3 - 0.5 Ga). Notably, this age exhibits surprising uniformity across the planet, suggesting a lack of significant geological evolution over time. Unlike Earth, Venus shows minimal evidence of plate tectonics, indicating a unique geological history and surface dynamics.

III. Venus: Surface Features and Processes

A. Impact Craters

Venus is replete with impact craters, many of considerable size:

• For instance, the Mead crater measures approximately 275 km in diameter.

• Unlike Earth, Venusian craters remain largely unweathered due to the absence of liquid water on the surface, which otherwise facilitates erosion.

• The absence of smaller craters on Venus can be attributed to the disintegration of small meteorites within the dense atmosphere before they reach the ground.

• Upon impact, the resultant debris is often fluidized due to atmospheric compression, allowing it to flow significant distances from the impact site.

B. Volcanic Flows

The geological landscape of Venus features extensive lava flows, often large and not directly linked to tectonic structures:

• Many of these flows possess low viscosity, likely consisting of basaltic material, conducive to extensive flow across the surface.

• An example includes the Mylitta Fluctus lava flow field, which extends roughly 900 km long and spans 450 km wide, showcasing the sheer scale of volcanic processes that have occurred on the planet.

C. Pancake Domes

Pancake domes are remarkable geological formations found on Venus:

• These circular, dome-like structures typically measure about 25 km in diameter and rise up to 750 m above the surrounding terrain.

• Geologically, they are interpreted as thick lava flows that erupted from openings in relatively flat areas.

• The intricate fractures atop these domes indicate a cooled exterior layer that was subjected to stretching due to ongoing lava movement beneath it.

• Models based on the dynamics of viscous flow assist in understanding the behavior and formation of these intriguing geological features.

D. Channels

Venus exhibits long, channel-like features that may result from molten lava:

• One such channel extends over 7,000 km, exceeding the length of the Nile River, marking it as the longest known channel in the solar system.

• Interestingly, these channels possess characteristics similar to terrestrial rivers, including meandering and cut-off bends; however, they are less sinuous, running in both uphill and downhill directions, suggesting geological activity post-channel formation.

E. Coronae

Coronae are large, circular geological features of internal origin:

• The largest among these is the Artemis Chasma, with a diameter reaching approximately 2100 km.

• They consist of a ring of concentric structures encircling a central area characterized by fractures and volcanic features.

• The formation of coronae is believed to be associated with the upwelling of hot mantle materials, known as plumes or diapirs, within the planet's interior. Some interpretations suggest that Artemis Chasma could also represent zones of intense compression and underthrusting due to their unique circular formations.

IV. Venus: Gravity, Topography, and Interior

A. Gravity Field Measurement

The gravity field of Venus was effectively measured using the Magellan spacecraft through a technique known as Doppler tracking that analyzes shifts in the spacecraft's orbit.

• By understanding the variation in gravitational pull across the surface, scientists gain insight into the planet's internal structure and dynamics. This vital information adds depth to our knowledge of Venus's geological history.

B. Convective Support of Topography

The geological features observed on Venus are significantly supported by mantle convection:

• Unlike Earth, where isostatic compensation predominates, some large topographic structures on Venus are sustained due to the dynamics of mantle convection.

• This process necessitates that the viscosity of Venus's mantle is at least ten times greater than that of Earth's mantle. The high viscosity can likely be attributed to the limited presence of water within Venus's mantle.

C. Admittance and Elastic Thickness

• Admittance (Z): The admittance of a planet is calculated through Fourier transformation of topography and gravity data, yielding the ratio of gravity anomalies to topographic heights across varying wavelengths. This metric is crucial for understanding lithospheric strength. The formula for calculating admittance is as follows:

  • [ Z(k) = \frac{2π(ρ_c - ρ_w)G}{1 - \exp(-kt_c) / [1 + Dk^4/g(ρ_m - ρ_c)]} ]

  • Where:

    • k is the wavenumber (k = 2π/λ, λ is wavelength)

    • ρ_c is the density of the crust

    • ρ_w is the density of the fluid above the crust (significantly impacts stability)

    • G is the gravitational constant

    • t_c is the crustal thickness

    • D accounts for material properties of the lithosphere

    • g entails gravity acceleration

    • ρ_m represents mantle density

• Elastic Thickness (T_e): This refers to thickness of the lithosphere that supports elastic stresses. Estimating elastic thickness involves fitting theoretical models to observed admittance data. For Venus, the elastic thickness is approximately 30 km, which aids in comprehending the lithosphere's strength and thermal condition under high surface temperatures (approximately 430 °C).

V. Mars: Seismic Exploration and Interior

A. InSight Mission

NASA's InSight lander, which successfully touched down on Mars on November 26, 2018, heralded a new era in Martian geophysical research.

• This mission is equipped with an array of geophysical instruments, the most significant of which is a highly sensitive seismometer designed to detect seismic activity on the planet.

B. Marsquake Data

The InSight mission has facilitated the recording of over 1,300 marsquakes:

• Notable among these is the largest recorded marsquake measuring toward magnitude 5, showcasing the planet’s ongoing geological activity. Seismology findings contribute significantly to our understanding of Martian geology and seismic dynamics.

C. Challenges of Single-Station Seismology

Operating only a single seismometer on Mars presents unique challenges:

• Locating marsquakes is complicated due to the reliance on data from this single instrument, limiting the potential for employing traditional multi-station seismological techniques.

  • Differential Travel Times: Understanding the difference in arrival times between direct P-waves (primary waves) and S-waves (secondary waves) provides data essential for estimating distance to the quake source.

  • Back-Azimuth: The determination of the direction from which seismic waves originated (back-azimuth) presents complexities. It can be estimated through polarization analysis of the waves from the 3-component data received by the seismometer.

D. Marsquake Locations

Analysis of seismic activity indicates most marsquakes occur approximately 30° away from the lander, predominantly associated with the Cerberus Fossae graben system. Some recent seismic signals have been traced further from the lander, including the expansive Valles Marineris region, indicating considerable geological differences across Martian terrain.

E. Interior Structure

The seismic analysis conducted through the InSight mission provides insight into Mars's interior structure:

• Crust: Estimated to be approximately 50 km thick.

• Lithosphere: The lithosphere’s thickness is projected to range between 400 to 500 km, informing us about Martian geological history and potential past tectonic activity.

F. Core Size Estimation

Analysis of S-wave reflections at the core and P-wave behaviors allow estimates of the size of Mars’s core:

• Based on reflections and diffracted waves, the core radius of Mars is estimated to be approximately 1830 km, which is towards the upper limits of previous predictions, confirming significant internal processes.

G. Seismic Wave Phases

In identifying marsquakes and interpreting seismic data, various seismic wave phases are analyzed:

1. P-waves: Primary compressional waves; fastest and travel through solid and liquid.

2. S-waves: Secondary shear waves; unable to travel through liquids.

3. PP-waves: Primary waves reflecting off the surface.

4. PPP-waves: P-waves reflecting twice at the surface.

5. ScS-waves: S-waves reflecting at the core-mantle boundary.

6. Pdiff-waves: P-waves diffracting along the core-mantle boundary.

• By studying the travel times and wave behaviors, researchers can extract vital information about the interior structure, geological processes, and seismic history of Mars.

H. Meteoroid Impacts

InSight has also detected meteoroid impacts:

• The locations of these impacts have been validated through orbital imaging, confirming the methods used for estimating marsquake locations.

• These impacts have been correlated with surface wave detections, further enhancing understanding of Martian geological processes.

VI. Key Differences and Implications

A. Venus

• The absence of plate tectonics leads to unique mechanisms of heat dissipation, suggesting a distinctive geological evolution compared to Earth.

• The universally dry lithosphere affects the planet's strength and thermal properties.

• Uniform surface age indicates that episodic resurfacing events have occurred, highlighting a stagnant geological history.

B. Mars

• Evidence for active seismic activity suggests ongoing geological processes that shape the Martian landscape today.

• The layered structure comprising crust, mantle, and core can be systematically studied using seismic data, revealing key details about its geology.

• Detection of meteoroid impacts facilitates validation of current seismic location techniques and provides insights into surface dynamics.

C. Earth

• Characterized by active plate tectonics, which leads to dynamic landforms, earthquakes, and volcanism.

• Variability in surface age showcases a more complex evolutionary history compared to Venus and Mars.

• The presence of liquid water and a robust atmosphere significantly influence both surface and interior processes, providing essential conditions for life.

VII. Further Reading

For a comprehensive understanding of planetary geophysics, we recommend exploring review articles and detailed studies, such as those by Nimmo and McKenzie (1998) and Smrekar et al. (2018), which delve into the volcanism, tectonics, and interior structures of Venus and other planetary bodies.