ALMA2040: Submillimeter-wave Limb Sounding of Venus and Mars

The gist: ALMA2040 proposes using high-resolution submillimeter heterodyne spectroscopy to perform vertically resolved, long-term limb sounding of Venus and Mars atmospheres from the ground, bridging spacecraft snapshots with temporal and spatial evolution.

Scientific Rationale for Studying the Atmospheres of Venus and Mars

Studies of Solar System bodies provide a unique pathway to understanding the formation and evolution of the Solar System and the conditions that made Earth habitable Among the diverse research topics in planetary science, Venus and Mars are particularly valuable comparative laboratories for understanding the evolution of terrestrial planets Understanding the origins of these differences is essential for identifying the physical and chemical processes that govern atmospheric evolution, water distribution, and climate on terrestrial planets As the number of known exoplanets grows and proposed facilities such as the Habitable Worlds Observatory (HWO) enable more detailed studies of terrestrial worlds, Venus and Mars will provide essential benchmarks for interpreting distant planetary systems Venus and Mars are thus not only compelling targets in their own right but also key to understanding the diversity and habitability of terrestrial planets across the universe>

Role of Ground-Based Observations in the 2040s

Ground-based observations play a role beyond complementing spacecraft measurements Planetary atmospheres vary on timescales ranging from sub-hourly and diurnal cycles to seasonal and decadal changes Another major challenge is bridging the spatial scales sampled by spacecraft and groundbased observations Because groundbased observations typically have coarser spatial resolution than spacecraft measurements, they may probe different physical regimes, complicating direct comparisons between the two datasets> A major goal for planetary observations in the 2040s should therefore be to establish a coordinated observing system that links spacecraft measurements with ground-based monitoring in both space and time

Achieving Spacecraft-Comparable Observations from the Ground

ALMA2040 is expected to provide a new observational capability for planetary atmospheres by combining unprecedented angular resolution with the ability to conduct long-term, repeated observations from the ground Heterodyne spectroscopy at submillimeter wavelengths provides a powerful means of probing planetary atmospheres For Venus and Mars, molecular species such as CO, H2O, HDO, SO, and SO2 can be used to probe atmospheric circulation, transport, photochemistry, and the planetary water cycle<ref:2610.22233#pg2> A particularly transformative capability of ALMA2040 will be to combine this high spectral resolution with sufficiently high angular resolution to enable high-resolution limb sounding of planetary atmospheres In particular, achieving spatial scales comparable to those sampled by spacecraft limb sounders would allow localized atmospheric structures and chemical features identified by spacecraft to be followed from the ground over extended periods>

Characterizing Atmospheric Circulation in the Middle Atmosphere

A major advantage of limb sounding is its sensitivity to horizontal winds Submillimeter heterodyne spectroscopy therefore provides a unique means of directly measuring atmospheric winds on these terrestrial planets<ref:2610.22233#pg2> Before the advent of ALMA, CO Doppler-shift observations of the atmospheres of Venus and Mars were conducted using both single-dish telescopes (e.g., Lellouch et al. 1991, 1994; Rengel et al. 2008; Cavalie et al. 2008; Clancy et al. 2008, ´2012, 2015) and millimeter interferometers, including the Plateau de Bure Interferometer (PdBI) and the Nobeyama Millimeter Array (NMA) (Moreno et al. 2009; Moullet et al. 2012). These previous studies provided spatially resolved measurements of line-of-sight wind velocities over specific altitude ranges (around 100 km for Venus and 60 km for Mars), but with a relatively coarse vertical resolution of approximately 10−15 km<ref:2610.22233#pg2> ALMA2040 will extend this capability to vertically resolved, three-dimensional wind measurements> • Venus: A major outstanding question is the atmospheric circulation between approximately 70 and 130 km altitude A sub-solar-to-anti-solar (SSAS) circulation driven by the day–night thermal contrast is expected to contribute, but observations (Moullet et al. 2012; Clancy et al. 2012, 2015) and recent general circulation model (GCM) studies (e.g., Martinez et al. 2026) suggest a more complex circulation involving atmospheric waves, including gravity waves propagating upward from the cloud layer and transporting angular momentum> High-resolution measurements of the vertically-resolved wind fields, combined with GCMs, would provide a direct test of these mechanisms and new constraints on the origin and maintenance of Venusian super-rotation, and its role in mediating the seasonal climate> • Mars: The atmospheric circulation of Mars under relatively quiescent conditions is reasonably well characterized, but its response to global dust storms remains poorly understood> During these events, atmospheric temperatures can change by several tens of kelvin and the global circulation can be substantially reorganized> The high optical depth of dust prevents visible and infrared observations from probing the atmosphere within major storms, whereas submillimeter and millimeter observations are much less affected by dust<ref:2610.22233#pg2> ALMA2040 will provide vertical resolution, allowing us to determine how dust-induced heating modifies the circulation at different altitudes> CO will be the primary tracer for these Doppler-wind measurements on Venus and Mars Isotopologues such as 13CO and C18O provide additional constraints> In particular, combining 12CO and 13CO can help disentangle temperature from CO abundance> Because adjacent CO rotational transitions are separated by approximately 115 GHz, the current ALMA generally observes only one transition per Band, limiting simultaneous coverage of a broad altitude range<ref:2610.22233#pg2> ALMA2040, with coordinated observations across multiple Bands, could combine multiple transitions and isotopologues to retrieve vertically resolved wind and temperature structures over a broad altitude range>

Comprehensive Observations of Trace Species and Isotopologues

ALMA2040 will exploit this sensitivity together with high vertical resolution to systematically observe trace molecules and isotopologues that have so far been poorly characterized, providing new constraints on the atmospheric chemistry and water cycles of Venus and Mars<ref:2610.

1. textbfIncreased Vertical Resolution for Atmospheric Dynamics Mapping: The improved system can perform vertically resolved, three-dimensional wind measurements for Venus and Mars by combining high spectral resolution with high angular resolution to achieve spatial scales comparable to those sampled by spacecraft limb sounders. This capability allows researchers to test mechanisms such as the SSAS circulation driven by the day–night thermal contrast on Venus.

2. textbfLong-Term Temporal Evolution Tracking: The system can establish a long-term four-dimensional characterization of planetary atmospheres by repeatedly observing different local times, seasons, and solar-activity conditions. This enables the tracking of localized phenomena discovered by spacecraft over extended periods, bridging the gap between spacecraft measurements and the long-term evolution of planetary atmospheres.

3. textbf Enhanced Trace Species Constraint: The system can systematically observe trace molecules and isotopologues to provide new constraints on the atmospheric chemistry and water cycles. Specifically, it can constrain the Chemical Stability of CO2-Dominated Atmospheres by observing species like HCl, ClO, H2O, HO2, and H2O2 whose abundance profiles can be compared with photochemical models.

4. textbf Refined Water Loss Diagnostics: The system can monitor the vertical distribution and temporal variability of the HDO/H2O ratio to provide new constraints on the atmospheric processes governing the loss of water from Mars, particularly by observing its behavior before, during, and after such events like global dust storms.

5. textbf Improved Sulfur Chemistry Characterization: The system can precisely measure the vertical distributions of SO and SO2 from the lower to the upper cloud layers to constrain missing sulfur chemistry. This will allow researchers to monitor how the abundance of SO2 decreases by 4−5 orders of magnitude between the lower atmosphere and the cloud-top region.

6. textbf Optimized Instrumental Requirements for Routine Operation: The system is designed with a requirement for high sensitivity, high-spectral-resolution submillimeter spectroscopy at scale-height-level spatial resolution, across multiple frequency bands, in a routine and repeatable manner. This moves beyond achieving higher maximum angular resolution to enabling observations across multiple frequency bands in a way that is not restricted to specific array configurations.

Venus and Mars are unique laboratories for understanding the evolution and diversity of terrestrial planets. Despite their broadly similar rocky compositions, their atmospheres, climates, and surface environments have evolved into dramatically different states. Understanding the physical and chemical processes responsible for these divergent pathways is essential for interpreting terrestrial exoplanets and their habitability. By the 2040s, spacecraft missions will provide unprecedented close-up observations of Venus and Mars, while ground-based facilities can provide the long-term monitoring needed to place these snapshots in the context of atmospheric variability and planetary-scale circulation. ALMA2040 can uniquely bridge these two approaches by combining high-resolution submillimeter heterodyne spectroscopy with routinely accessible, extremely high angular resolution. In particular, high-resolution limb sounding would enable vertically resolved measurements of atmospheric winds, temperature, and trace species from the ground. An angular resolution of approximately 0.01" would allow spatial scales comparable to the atmospheric scale heights of Venus (around 5 km) and Mars (around 11 km) to be resolved. This capability would enable three-dimensional studies of Venusian upper-atmospheric circulation and Martian global dust storms, as well as systematic investigations of CO2 photochemistry, water loss through HDO/H2O fractionation, and Venusian sulfur chemistry and cloud formation. By repeatedly observing different local times, seasons, and solar-activity conditions, ALMA2040 would extend spacecraft measurements from snapshots to long-term four-dimensional characterization. It would therefore provide a unique observational framework for directly connecting spacecraft discoveries with the temporal and spatial evolution of planetary atmospheres.