Deep Atmosphere of Venus Probe as a Mission Priority for the Upcoming Decade

This is a white paper submitted to the Planetary Science and Astrobiology Decadal Survey. The deep atmosphere of Venus is largely unexplored and yet may harbor clues to the evolutionary pathways for a major silicate planet with implications across the solar system and beyond. In situ data is needed to resolve significant open questions related to the evolution and present-state of Venus, including questions of Venus' possibly early habitability and current volcanic outgassing. Deep atmosphere"probe-based"in situ missions carrying analytical suites of instruments are now implementable in the upcoming decade (before 2030), and will both reveal answers to fundamental questions on Venus and help connect Venus to exoplanet analogs to be observed in the JWST era of astrophysics.


EXECUTIVE SUMMARY
The deep atmosphere of Venus is largely unexplored and yet may harbor clues to the evolutionary pathways for a major silicate planet with implications across the solar system and beyond. In situ data is needed to resolve significant open questions related to the evolution and present-state of Venus, including questions of Venus' possibly early habitability and current volcanic outgassing. Deep atmosphere "probe-based" in situ missions carrying analytical suites of instruments are now implementable in the upcoming decade (before 2030), and will both reveal answers to fundamental questions on Venus and help connect Venus to exoplanet analogs to be observed in the JWST era of astrophysics. DD001 Figure 1: Conceptual Venus deep atmosphere probe mission for the 2020's decade. Such a mission could employ parachutes within the cloud deck (50-70 km) to enable time for gas ingest and processing and then freely fall to the surface at 10-15 m/s as it images the surface in the Near Infrared (NIR) windows to permit compositional mapping while profiling trace gases down to the surface (in their environmental context).

Composition of the Venus Atmosphere: the Essential Next Step in Venus Exploration
The Venus atmosphere holds clues to its origin, evolution, and dynamics and how it reflects the history of putative past oceans and active volcanism [Baines et al., 2013;Bougher et al., 1989;Treiman 2007;Garvin et al., 2020]. The late-1970's measurements from Pioneer Venus (PVLP) were incomplete and did not offer the precision to measure the noble gases, especially Xenon and Helium [Lammer et al., 2020], leaving ambiguities in our understanding of the planet. The single mid-atmosphere D/H value (~150) was suggestive of a large water inventory that was lost [Donahue et al., 1982], but did not survey the variability in this key value from the top of the atmosphere to the near surface. No complete inventory of diagnostic trace gases was accomplished, especially for the deep atmosphere from ~16 km to the surface, where most (66%) of the atmosphere resides [Bougher et al., 1989]. The lapse rate (temperature as a function of altitude) is insufficiently constrained and represents a key variable for current models of the deep atmosphere, where dominant CO 2 is super-critical [Lebonnois & Schubert, 2017]. No systematic compositional cross-section as a function of altitude from the midatmosphere clouds to the surface has ever been achieved. Without definitive compositional measurements of the bulk and lower-most Venus atmosphere, essential boundary conditions for evolutionary models that seek to explain Venus as a "system" cannot be developed [Kane et al., 2019, Figure 2]. The composition of the near-surface atmosphere is needed to constrain the chemical alteration of surface materials and exchange of volatiles in the coupled atmosphere-surface rocks system [Zolotov, 2018[Zolotov, , 2019. Venus stands out as the least well-measured large atmosphere in the solar system (Lammer and others 2020), further limiting what our nearest neighbor planet can tell us about habitability of Earthlike planets and the broader workings of our solar system and planetary systems beyond [Kane et al., 2019;NAS Exoplanets Strategy, 2018;Way et al., 2016].

The history of habitability at Venus?
One of the most exciting emerging justifications for Venus cloud-deck compositional measurements are their critical role in assessing past or present habitability (and potential for biological activity). Cloud-deck microbial metabolism has become increasingly recognized as a significant venue for biology in a variety of Earth environments, including the stratosphere. Limaye and others (2018) summarized the case for scenario on present-day Venus, with specific indicator species such as phosphine (PH 3 ) as a detectable biosignature at Venus and in spectroscopy of exoplanets [Sousa-Silva et al., 2020]. This scenario implies the in situ detectability of biogenic trace gases within the environmentally habitable cloud deck (~50-60 km altitude) today. Chemical signatures dating for Venus' oceanic period (or   Figure 2: Greater understanding of Venus will provide higher fidelity simulations and data interpretation of what an exo-Venus might resemble to a future astrophysical observatory such as JWST or others planned for the 2030s versus the poorly-constrained Venus atmosphere (right), where most of the trace gas contents are uncertain, especially below ~45 km (i.e., below the clouds) [Kane et al., 2019]. A conceptual DEAP mission could survey details of the composition from 70 km to the surface to quantify what future transiting exoplanetary spectroscopy telescopes (JWST etc.) can evaluate beyond our solar system. Deep-atmosphere data is needed to constrain and validate models attempting to understand whole-atmosphere conditions of Venus-like exoplanets. more recently) would be detectable with suitably sensitive analytical instrumentation of the type that have conducted related investigations on Mars as part of the Curiosity rover (as an example) for the past 8+ years [Trainer et al., 2019]. Such instrumentation was largely non-existent 20 years ago but on the basis of investments that high sensitivity mass spectrometers (far exceeding the level of sensitivity enabled by remote sensing) on such missions as Curiosity (SAM) and Cassini/Huygens (INMS and GCMS), bringing such sensors to the "samples" throughout the Venus atmosphere (Figure 1) is now possible.

GOALS AND RELEVANCE
We define the comprehensive survey of the definitive composition, dynamics, and environment of the Venus atmosphere (~70 km to surface) as a primary science goal for Venus exploration in the next decade [V&V, 2011;VEXAG Goals, 2020]. Trace gases within the deepest atmosphere (i.e., ~16 km to the surface), D/H in water, as well as Xe and He are particular targets of interest due to their relevance to climate, history, putative biology, and to surface weathering regimes [Fegley et al., 1997;Zolotov, 2019]. Importantly, gradients in particular species across altitude bands (and hence as a function of pressure and temperature) provide insight into processes that connect the surface to the deep atmosphere over time scales relevant to major transitions [Weller & Kiefer, 2020;Way & Del Genio, 2020]. Thus, trace gas concentrations should be investigated across the entire sub-cloud atmosphere to dramatically extend current abundance data and improve understanding of the active mechanisms (thermochemical and photochemical reactions among gases, volcanism, chemical weathering, lateral variations) across the planet.
The comprehensive measurement of the Venus atmosphere is the logical next step from the perspectives of astrobiology and climatology [e.g., Kane et al., 2019;NAS Exoplanets Strategy, 2018]. Inventorying the distribution of altitude-dependent trace gases, as has been achieved at Titan and for Mars, is identified as a direct approach to detect volcanic activity and search for clues to major environmental transitions at regional to planetary scales. In-depth understanding of Venus' trace gases can help to determine the roles of known and unknown processes (e.g., volcanic, thermochemical (gas-gas, gas-solid), photochemical, biological?) in controlling the current atmospheric composition to better constrain and refine relevant models beyond the current state-of-the-art [ Table 1]. The updated 2020 VEXAG Goals state that determining sources and sinks of atmospheric trace gases are an essential objective for Venus, and in association with the New Frontiers "VISE" mission definition [V&V, 2011].
Significantly, definitive measurements of the bulk and trace atmosphere at relevant, representative altitudes (< 60 km) [Peplowski et al., 2020] can provide missing boundary conditions for evolutionary models, as emphasized by Kane and others (2019) and Way and Del Genio (2020). As the VEXAG goals explicitly state, our knowledge of trace gas sources, sinks, and abundance across the entire Venus atmosphere remains far too limited. Additional measurements from beneath the cloud-deck to the surface, including near infrared compositional and "topographic" imaging [Garvin et al., 2018[Garvin et al., , 2020, can connect the unknown trace gas contents and gradients to the local-to-regional geology of the surface in key regions that are themselves diagnostic of global-scale evolution in space and time [Weller & Kiefer, 2020], including complex ridged terrains. Picking up where Pioneer Venus Large Probe (PVLP) and the Soviet Venera and Vega landers left off, a deep atmosphere probe capable of profiling the trace gases from the clouds to the surface while measuring key environmental parameters and establishing ground truth in comparison to remote spectroscopy is amply justified.

SCIENCE OBJECTIVES
We define three primary science goals/objectives and associated measurements for quantifying the atmosphere of Venus from an in situ point of view in Table 1 for a deep atmosphere probe (DEAP).

Objective 1: Origin and Diversity of Atmosphere-bearing Planets
Venus' atmosphere is an unexplored reservoir for comparing evolutionary pathways for large-atmosphere planets, and for connecting results to ongoing and upcoming studies of exoplanets, including those accessible to the James Webb Space Telescope (JWST).

Objective 2: Evolution of Planetary Atmospheres and Habitability
Understanding the history of water and other volatiles (including those involving S) for Venus from multi-altitude measurements of D/H and noble gas isotopes has the potential for transforming models of Venus oceanic state in space and time.
Objective 3: Atmosphere/Surface Composition for Climate Relevance Compositional constraints on local to regional surface geology will address the role of water in both formation (e.g., the role of water in the petrology of felsic rocks), as well as erosional and sedimentary processes that may have operated as tectonic regimes migrated over time on Venus.

PRIORITY ATMOSPHERIC ENTRY TARGET SITE TYPES
As atmosphere-surface interactions are relevant globally across the surface of Venus [Zolotov 2018], there is a large degree of flexibility in choosing any site for entry-descent and atmospheric transect science with descent imaging for composition. A wide variety of highlands regions would be compelling from Maxwell Montes to isolated tesserae such as Alpha Regio and Tellus Regio [Gilmore et al., 2015]. Revisiting previously investigated regions (PV probes, Venera, Vega landers) could also be beneficial for  Glaze et al. (2017) and Garvin et al. (2020). Please see also VEXAG Goals (2020) and text for details. the purpose of building upon preexisting results. Given lack of chemical, surface texture, and lithology data for elevated highlands known as tesserae, having at least one deep atmosphere probe with nearsurface imaging and trace gas composition would be desirable [Glaze et al., 2017].

MISSION STRATEGIES
The proven track record of successful noble and trace gas measurements at Titan (Huygens) combined with well understood techniques for comprehensive atmospheric characterization in Martian environments [Curiosity SAM: Trainer et al., 2019] make this a highly achievable mission concept for development in the next decade. A successful mission prioritizing atmospheric composition from ~70 km to the surface will require: (1) analytical instrumentation to measure gases with high sensitivity and signal-to-noise, (2) robust infrastructure to ingest, isolate, and process gases for measurement, (3) probe flight systems necessary to enable nadir-looking descent imaging for composition and topographic terrain analysis, and (4) entry systems to ensure an encapsulated probe flight system with suitable parachutes to enter and descend through the atmosphere over a surface region of interest at scales of ~350 km × 100 km (typical landing error ellipse at Venus from previous missions). On the basis of recent mission concept proposals and investments by spacefaring agencies, such a deep atmosphere probe flight system (Figure 1) with instruments and necessary avionics and telecommunications systems is implementable in the upcoming decade.

(1) Detection Instrumentation:
High-sensitivity noble and trace gas measurements, as have been demonstrated for over 8 years on Mars (and during probe descent at Titan) are available with flight-proven deep space experience, including Quadrupole Mass Spectrometers (QMS), Tunable Laser Spectrometers (TLS), and other varieties of gas-phase and aerosol analytical systems. Table 1 documents traceability to science goals. Science objectives require ~1 ppbv limits of detection, with high-precision isotopic analytical capabilities for key atmospheric species.
(2) Probe Infrastructure: Given past flight experience (Cassini's Huygens probe, and PV Large Probe), a deep atmosphere probe mission would require multiple redundant inlets for gas ingest and processing, necessary optical viewports (e.g., sapphire) for instruments such as nadir-pointing descent imaging systems, penetrations to permit atmospheric structure and environmental measurements (p, T, accelerations), and additional penetrations for S-band radio-frequency telecommunications systems for data relay to supporting spacecraft (for ultimate downlink to Earth). These components have been demonstrated in planetary atmospheres (Mars, Venus, Titan, and Jupiter) and represent a low-technical risk approach for Venus.

(3) Special Probe Flight System Support Requirements
Beyond the capability of environmentally isolating analytical instruments from the Venus atmospheric environment for up to ~1.5 hours of descent, high optical through-put windows for descent imagers and other possible sensors (spectrophotometers) require relatively large port diameter (tens of mm). Descent imaging for composition and topography has yet to be performed at Venus, but experience from DISR on the Titan Huygens probe [Soderblom et al., 2007] have demonstrated the potential of this approach. A nadir-pointed descent imaging system with near IR bands that permit surface radiance to be measured from below the clouds to the surface will permit discrimination of broad compositional trends ranging from high-silica rocks (felsic) to surfaces coated with alteration products such as hematite [Filiberto et al., 2020;Zolotov 2018], and weathered basalts. Figure 3 illustrates this potential at Venus, providing compositional assessment at scales from 5 m (at altitudes below ~2 km) to 100 m at higher altitudes (25 km). Combining compositional discrimination of endmember lithologies with 3D perspectives by processing of multi-frame, overlapping descent imaging [Garvin et al., 2018] will provide first-of-its-kind geological characterization tied to lower atmosphere trace gas chemistry which may be in disequilibrium [Zolotov, 2018;Lebonnois & Schubert, 2017], thus, bringing lander/rover scale observations to Venus without the requirement for safe landings as a precursor to New Frontiers-class lander missions being considered for the 2030s [Garvin et al., 2020].

CONCLUSION
Venus' atmosphere from the top of the cloud deck near 70 km to the surface presents a spectacular planetary laboratory that has remained largely unexplored. Ever since 1983 and the first prioritization of NASA planetary exploration missions [Morrison & Hinners, 1983], there has been a widelyrecognized need for a deep atmosphere probe to Venus, which was echoed in the two most recent planetary Decadal Surveys. The case for such a mission in the decade of the 2020's is now more urgent as exoplanetary observations and models point to Venus-analog planets being commonplace beyond our solar system [NAS Exoplanets Strategy, 2018].  Figure 3: High-sensitivity descent imaging can discriminate between end-member rock types; 8 km (LEFT) and 2 km (RIGHT) altitudes shown, illustrating how readily felsic rocks can be distinguished even with the Rayleigh scattering and blur due to the massive Venus atmosphere. Mapping felsic rock units in the Venus highlands (tesserae) at scales from 100 m down to 5 m over areas as large as 25 x 25 km² is possible from a DEAP mission using descent camera technologies that have been demonstrated at Mars and Titan, and may be related to the role of water in rock formation and erosion [Gilmore et al., 2015;Hashimoto et al., 2008;Filiberto et al., 2020].