Terrestrial Planets Comparative Climatology (TPCC) mission concept

The authors and co-signers of the Terrestrial Planets Comparative Climatology (TPCC) mission concept white paper advocate that planetary science in the next decade would greatly benefit from comparatively studying the fundamental behavior of the atmospheres of Venus and Mars, contemporaneously and with the same instrumentation, to capture atmospheric response to the same solar forcing, and with a minimum of instrument-related variability. Thus, this white paper was created for the 2023-2032 Planetary Science Decadal Survey process. It describes the science rationale for such a mission, and a mission concept that could achieve such a mission.

terrestrial planets-as advocated by the Venus and Mars community advisory groups [8][9][10]. These comparisons are essential for understanding planetary habitability and the consequences of human activity. These lessons may then be extended to studies of extra-solar terrestrial planets, and their evolution and potential habitability.
Comparison of the current state of Venus, Mars, and Earth suggests distinctions in the evolutionary history of each of these terrestrial planets. Although each planet formed from the same proto-solar nebular elements, the apparent carbon and water histories are vastly different. Venus likely had enough water for a global ocean for billions of years [11], but which is now lost. Over time, Earth's carbon has become integrated within its crust. Mars has a primarily CO2 atmosphere and had liquid water in its past, but today is cold and dry.
Quantitative studies of the isotopic and noble gas composition of each planet's atmosphere can tell us how their paths diverged. Xenon (Xe) is of particular interest for understanding the origin and evolutions of the terrestrial planets. Ongoing investigations of Mars and Earth have provided some detail on this noble gas, but the measurements for Earth and Mars do not match measurements made elsewhere in the solar system and therefore indicate a major climate-changing event early in solar system history [12]. Measurements of Xe and other noble gases, such as neon (Ne) and argon (Ar) at Venus are either absent or of insufficient precision to contextualize Venus' evolution relative to Mars and Earth. Furthermore, previous D/H measurements at Venus are uncertain, yielding uncertainty in processes responsible [13] and the details of initial water delivery cannot be discerned. By improving these measurements, TPCC would elucidate the role of asteroid/primitive body delivery of water, clarify loss processes, and improve and equalize current understanding of Mars and Venus climate histories.
The climates of solar system bodies are driven by energy inputs from the Sun and atmosphere absorption and transport, and are modified by atmospheric aerosols and trace constituents. Climate models need explicit observations to refine and constrain how these processes combine to create the observed climatic state. Model-data comparisons (and interpretations) benefit from common observations, i.e., those obtained in a standardized manner at each target ( [7] and chapters therein) under similar conditions. Comparing the behavioral differences in atmospheric response given variation in input the variables of composition, atmospheric pressure and temperature, and solar radiation at Earth, Mars, and Venus allow in-depth study of how the same fundamental physics leads to diverse climatic states. Key processes include radiative forcing, condensation and vaporization, atmospheric dynamics, chemical cycles, atmosphere-surface interactions, and geochemistry [14]. By obtaining contemporaneous measurements at Mars and Venus, under similar and measured solar energy input conditions, TPCC would elucidate plausible paths to the varying climatic states observed in the terrestrial planets.

New Measurement Needs: Background and Justification
Key new measurements would be made by the TPCC mission (Table 1-1). In particular, global wind and composition measurements will be made and used to explore the atmospheric dynamics and its regions of transition on both Mars and Venus.
No direct measurements of the global atmospheric circulation (winds) have been made on either Mars or Venus-an essential measurement needed for understanding climate forcing (Guzewich et al., white paper: [15]). Tracking the wind at Mars provides a direct measurement of how the atmosphere responds to forcing and how aerosols, vapor, and trace species are transported. Combined with concurrent measurements of temperature and aerosol abundance, a truly complete picture of the atmosphere and modern climate state can be painted. Measurements of surface energy balance across a wide spectral range-UV, visible, near-IR, and thermal-IR-would drastically improve our understanding of the energy flux into and out of the planet and how the atmosphere moderates that flux. Additionally, trace gas measurements would also be used to assess global climatology of key species of Mars from a polar or near-polar orbit-including isotopologues of oxygen in CO2 (e.g., 18 OCO) and carbon in CO (e.g., 12 CO and 13 CO). These species are important photochemical tracers for the atmospheric circulation (e.g., [16,17]); they are also important for understanding atmospheric escape, and in refining our understanding of the chemical cycles of CO2 in the martian atmosphere and how it is maintained over geological time (e.g., [13]). Other photochemically relevant species such as O3, NO, and NO2 would also be measured, providing important information about the water cycle (e.g., O3 is a tracer for water vapor saturation conditions; [18] and the astrobiologically-relevant planetary nitrogen cycles [19]. At Venus, global wind measurements would be used to improve numerical studies of the physical drivers of Venus' enigmatic atmospheric circulation patterns. Unlike Mars, Venus has had only a few prime missions to observe and measure its atmosphere and climate, which have been insufficient to address the objectives proposed here; Pioneer Venus, VeGa, Venus Express, and now Akatsuki. Regions of Venus' atmosphere include the sulfuric acid cloud layers at ~48-70 km, the mesosphere (70-90 km) and upper atmosphere (>90 km). In terms of dynamics, Venus' atmosphere can be split into four zones. At the cloud tops (~70 km), the mean atmospheric motion is dominated by the stable retrograde super-rotating zonal (RSZ) wind that is ~60-80 times faster than the planetary rotation. A meridional Hadley-cell circulation with the westward retrograde super-rotation dominates in the middle atmosphere. At altitudes between ~90 and 120 km the circulation is dominated by subsolar to antisolar (SSAS) winds, resulting from the pressure gradient produced by inhomogeneous heating from solar radiation; with the circulation transitioning from RSZ to SSAS between ~70 and 90 km [20]. Above 120 km, the SSAS is dominant but there is evidence of a residual RSZ wind-suggesting a blending of the circulation patterns. How each of these circulation patterns is produced and maintained is poorly understood, as is the exact mechanisms that govern interactions between the zones. For example, at altitudes >120 km, the blending may be driven by waves (e.g., [21,22]) and/or ion-neutral drag [23]. Additionally, while on average it seems that the equator-to-65°-latitude region moves in Hadleycell type fashion, above 65° latitude, a polar vortex linked to solid body rotation prevails [24,25]. Potential mechanisms for supporting cloud top level super-rotation include thermal tides transporting momentum upwards [26], redistribution of angular momentum via waves and mean circulation [e.g., 27,28], and topography-induced gravity waves [29]. Yet, it is known that a strong local time-dependent wind shear connects the regions extending from the cloud layers to the upper (>90 km) atmospheric regions. So, it is expected that such strongly variable conditions may easily break the stationary waves, releasing momentum, with a potential impact on the super-rotationbut at what altitude this phenomenon occurs and how it contributes to Venus' dynamic system is a critical unknown. Likewise, the altitudes and physical drivers of the transition from superrotation to SSAS in the 70-90 km region are not well characterized [30].
On Mars, the forcing of radiatively active water ice clouds has been shown to drive waves and tides in the atmosphere that affect the temperature structure of the Martian atmosphere globally [31]. As clouds are modulated by atmospheric tides [32,33], there is a feedback between clouds and tides that is not understood to date. The propagation of waves and tidal modes not only influences the temperature in the middle atmosphere but also the density of the upper atmosphere and transports energy from the lower to the upper atmosphere [34]. The transition region between Mars' middle and upper atmosphere (~80-120 km) is still largely unexplored.
Combining the synergistic wind observation and dynamic model studies with planned isotopic measurements would also provide the data needed to better understand the evolution of each atmosphere. On Mars, geologic evidence suggests that the atmosphere used to be considerably more massive and probably also warmer and wetter than today. D/H ratios show that a considerable part of this original atmosphere has been lost to space over geologic time [35], likely through photodissociation of water and escape of atomic H [e.g., 36]. Recent results [37] suggest that middle atmospheric water vapor can affect this process, even reducing the gap between estimates of integrated hydrogen loss and present-day escape rates. Currently, direct water vapor measurements at the key altitudes (60-80 km) are sparse and transport processes from the lower to the upper atmosphere are not well characterized. TPCC would acquire the data need to improve this characterization.
Likewise, TPCC-acquired O2 IR nightglow and NO UV nightglow emission detections would be used to investigate connections between energy, dynamics, and chemistry on Venus and Mars. These emissions result from the recombination of dayside photolysis products, which are dynamically transported to the nightside. Traces of the distribution of these emission with local time allows study of the feedbacks between the dayside and nightside chemistry and atmospheric dynamics relative to the solar input. Combining the nightglow measurements with measurements of temperature, winds, and chemical species would provide a comprehensive global picture of atmospheric dynamics at each planet. Although nightglow emissions were obtained at Venus during Venus Express [e.g., 38], and ongoing observations began at Mars by the MAVEN mission in 2014, nightglow emission datasets obtained at Venus and Mars within the same solar cycle do not exist. TPCC would support the direct comparison of atmospheric response to solar forcing relative to the solar cycle, removing uncertainty associated with solar cycle activity, representing a key advancement in our understanding of the atmospheres of these worlds.

TPCC Mission Concept Relevance
The TPCC mission guiding science goals are to (1) understand Venus' evolutionary history for comparison with Mars and the Earth, and (2) elucidate the driving forces and mechanisms of global circulation at each planet. These goals directly address cross-cutting themes and priority questions identified in the National Academies CAPS report [39] Table A.1 including: (i) building new worlds (evolution of inner planet atmospheres); (ii) investigating Planetary habitats (Mars/Venus evolution); and (iii) identifying Workings of Solar systems (roles of physics, chemistry, geology, and dynamics in driving planetary atmospheres). Since 1989, no new US-led mission has been dedicated to Venus. Although US-led Mars and internationally-led Venus missions have been active, neither the mission duration nor the payloads of these missions sufficiently overlapped to complete investigation of solar forcing contemporaneous at each planet. And, lacking any in-situ elements for the active Venus missions, detailed study of water history at Venus remains unresolved. The TPCC mission would include contemporaneous study of the atmospheric responses to solar conditions, and would provide the critical remote and in-situ data needed to investigate (1) the origin and diversity of terrestrial planet climates; (2) the relationship between the climate evolution and the origin and evolution of life at each planet; and (3) the processes that control climate on Earth-like planets as prioritized by Vision and Voyages ( [40] p. 5-3 to 5-5).
The applicability of the high-fidelity comparative investigations enabled by the TPCC mission to both inner solar system evolution and exoplanet terrestrial analog studies is a key advantage and potentially saves cost compared to sending multiple large spacecraft as envisioned by other mission concepts [e.g., 41,42]. Additionally, the TPCC concept addresses 12 VEXAG objectives and 7 MEPAG objectives included in the past and recently updated Goals and Exploration Roadmap Documents [8][9][10]. This concept would also contribute to the comparative climatology set of objectives advocated in the white paper by McGouldrick et al. [43], as well as complete almost all objectives outlined in the white paper by Brecht et al. [44]. Finally, the TPCC mission concept addresses exoplanet exploration questions identified by Showman et al. [45].

TPCC Mission Concept: Advantages and Details
Because the TPCC mission would study two planetary bodies with the same payload, standardized measurements would be obtained at each target. This would decrease the complexity (propagated error) included in the data analysis, facilitating a higher fidelity determination of the impact of the Sun on individual components in each climate system, including radiative feedback, dynamics, cloud formation rates, and atmospheric chemistry. The Venus portion of the TPCC concept is a hybrid of the Multi-Platform Mission Option B and Option C concepts defined in [10], and given the global access to winds, temperature, and aerosols would actually accommodate many of the goals associated with the variable altitude cloud level aerial platform.
The cost drivers identified are: a long mission duration with navigation in/out of orbit at two planets, sophisticated instruments ensuring that the science goals for each planet are met, a descent probe deployed into the Venus atmosphere, and possibly two spacecraft. The technology readiness levels for elements of the TPCC mission are high. The proof-of-concept instruments (Table 1-1) are all TRL 9 with the exception of the sub-mm sounder, which is TRL 5 but could be brought to TRL 6 with very modest cost/schedule, and the descent probe. Despite its powerful ability to provide fundamental climate measurements, sub-mm sounding is a tool that has never been used at Mars and Venus, despite being envisioned in multiple studies and proposals [e.g., 46,47,42].