Overview
Citation: Widemann, T., Smrekar, S. E., Garvin, J. B., et al. (2023). Venus Evolution Through Time: Key Science Questions, Selected Mission Concepts and Future Investigations. Space Science Reviews, 219(7), 56. https://doi.org/10.1007/s11214-023-00992-w
This is a comprehensive review that synthesizes open questions about Venus’s evolution and details the coordinated fleet of missions that will address them in the 2030s.
Motivation
Venus serves as a natural laboratory for understanding terrestrial planet habitability and evolution. While Earth and Venus share similar mass and bulk geophysical properties, they followed radically different evolutionary paths. Venus is the only spatially resolvable, Earth-sized world that allows us to monitor geophysical envelopes (atmosphere, surface, interior) to support long-term evolutionary models. Major gaps remain regarding the stability of water reservoirs, the transition from a potentially habitable state to the current greenhouse state, and the nature of current geological activity.
Contribution
The paper provides a roadmap for Venus exploration by:
- Synthesizing key science questions across four domains (comparative planetology, primordial history, surface processes, and interior-atmosphere coupling).
- Detailing the instrument suites and science goals of three selected missions: VERITAS, DAVINCI, and EnVision.
- Identifying technology gaps and future mission concepts required to fully answer the habitability question.
Key Science Questions
The paper organizes open questions into four primary domains.
Comparative Planetology and Exoplanets
The Venus Zone is defined as the orbital region where an Earth-sized planet is more likely to be a Venus analog than an Earth analog. Understanding Venus directly informs the interpretation of exoplanet observations.
Magma Ocean Duration: Venus may lie at a boundary defined by magma ocean cooling times:
- Type I: Short-lived magma ocean (~1 Myr), allowing water condensation (Earth-like).
- Type II: Long-lived magma ocean (~100 Myr) due to high insolation, leading to desiccation via hydrodynamic escape.
Rotation Rate: Slow rotation is critical for maintaining temperate conditions in the Venus Zone via cloud-albedo feedback. This has implications for habitability assessments of tidally locked exoplanets.
Accretion and Primordial History
Impact History: Did Venus suffer a moon-forming giant impact? The absence of a moon challenges assumptions about early large-scale melting events.
Differentiation: Determining the timing of silicate/metal differentiation (core formation) via Hf/W chronometry is essential to constrain the accretion phase.
Volatile Delivery: Did volatiles arrive via solar nebula, asteroids, or comets? Xenon isotopes are key to detecting cometary contributions.
Surface Processes and Resurfacing
Two competing resurfacing models exist:
- Catastrophic: A massive pulse of volcanism ~1 Ga ago followed by quiescence (suggested by random crater distribution).
- Equilibrium: Continuous resurfacing where craters are modified gradually.
Tesserae Terrain: Complex, highly deformed tectonic terrains that may represent the oldest surface rocks. High-emissivity data suggests they may be felsic (silica-rich), potentially analogous to Earth’s continental crust formed in the presence of water.
Active Volcanism: Evidence includes variable SO₂ levels, emissivity anomalies at hotspots (Idunn Mons), and young lava flows.
Interior and Atmosphere Coupling
Tectonic Regime: Venus lacks plate tectonics but has deformation zones. It may be in a “stagnant lid” regime or a transitional state.
Noble Gases: Abundances and isotopes (Ne, Ar, Kr, Xe) track atmospheric loss and outgassing history.
Water Loss: The D/H ratio indicates water loss, but does not uniquely constrain when or how fast it happened.
The New Fleet of Missions
A synergistic fleet of three selected missions (plus international partners) will address these questions in the 2030s.
VERITAS (NASA Orbiter)
Primary Goal: Global mapping of topography, rock type, and active deformation.
Key Instruments:
- VISAR (X-band Radar): Global DEM (250m), imagery (30m), and interferometry (RPI) to detect cm-scale surface deformation.
- VEM (Emissivity Mapper): 6 near-IR bands to map surface iron content (felsic vs. mafic) through atmospheric windows.
Science Target: Determine if Venus has “continents” (felsic tesserae), active volcanism, and subduction-like features. VERITAS provides the global geophysical map and target identification.
DAVINCI (NASA Probe/Flyby)
Primary Goal: In situ chemical analysis of the deep atmosphere and descent imaging.
Key Instruments:
- VMS (Mass Spectrometer): Noble gases (specifically Xenon isotopes), trace gases, and D/H ratio.
- VTLS (Tunable Laser Spectrometer): High-precision isotopes of H, S, C, O.
- VenDI (Descent Imager): Near-IR imaging of Alpha Regio tesserae at <100m resolution to provide “ground truth” for orbital emissivity.
Science Target: Definitive atmospheric origin/evolution, history of water, and nature of tesserae. DAVINCI provides the chemical “ground truth” and high-res “spot check” of tesserae.
EnVision (ESA Orbiter)
Primary Goal: Holistic view from inner core to upper atmosphere, focusing on activity and geological history.
Key Instruments:
- VenSAR (S-band Radar): Polarimetric imaging and stereo topography.
- SRS (Subsurface Radar Sounder): Unique capability to penetrate the subsurface (up to 1km depth) to map stratigraphy, buried craters, and tesserae edges.
- VenSpec Suite: Spectroscopy (IR and UV) to link surface activity to atmospheric gas variations (SO₂, H₂O).
Science Target: Characterize the sequence of geological events, subsurface layering, and atmospheric-interior coupling. EnVision provides targeted, multi-scale geological analysis and subsurface sounding.
International Partners
Venera-D (Russia): Orbiter + Lander.
- The lander focuses on surface X-ray/Gamma-ray analysis (mineralogy) and surviving 2-3 hours.
- Includes an aerial platform (balloon) for cloud layer analysis.
Shukrayaan-1 (India): Orbiter.
- Features a polarimetric radar (VSAR) and potentially a low-frequency subsurface sounder.
Future Concepts and Technology Gaps
To fully answer the “habitability” question, investigations beyond the current fleet are required.
Long-Lived Surface Landers
Challenge: Electronics cannot survive Venus surface temperatures (470°C) for long periods.
Solution: High-temperature electronics (SiC, GaN) and battery technology.
Science Goal: Seismology. Measuring “Venusquakes” is the only way to definitively resolve the core state and interior structure.
Aerial Platforms (Balloons)
Environment: The cloud layer (50-60 km) is the “habitable zone” (20°C, 0.5 bar).
Science Goals:
- Long-term monitoring of atmospheric circulation and chemistry.
- Aerial Seismology: Detecting infrasound generated by groundquakes from the air (mechanical coupling is 60x stronger on Venus than Earth).
Sample Return
Concept: Skimming the upper atmosphere (<120 km) to collect noble gases and returning them to Earth for high-precision laboratory analysis.
Synergies with Exoplanet Science
Observations of Venus-like exoplanets (e.g., TRAPPIST-1 system) by JWST provide the statistical context for Venus’s divergent evolution. The upcoming decade represents a coordinated campaign:
- VERITAS provides the global geophysical map and target identification.
- DAVINCI provides the chemical “ground truth” and high-res “spot check” of tesserae.
- EnVision provides targeted, multi-scale geological analysis and subsurface sounding.
Understanding Venus allows us to interpret spectra from Venus analogs around other stars, making Venus exploration directly relevant to the search for habitable worlds beyond our solar system.