The Mars 2020 mission, with its Perseverance rover and the Mars Environmental Dynamics Analyzer (MEDA) instrument, presents a unique opportunity to study the Martian atmosphere in unprecedented detail. MEDA, the most comprehensive environmental station ever sent to another planet, provides measurements of various atmospheric parameters at Jezero crater, a location selected for its potential to reveal evidence of past habitability. This study focuses on the first 250 sols of MEDA's operation, spanning the northern hemisphere spring and early summer. The research aims to characterize the diverse meteorological phenomena occurring in the atmospheric surface layer (ASL) of Jezero crater, providing insights into the dynamic processes shaping the Martian environment. Understanding the Martian ASL is crucial for several reasons. First, it represents the region of most direct interaction between the atmosphere and the surface, significantly influencing energy and mass exchanges. Second, the ASL's hydrologic cycle places constraints on the photochemistry of surface and near-surface air. Finally, the ASL's dynamics, predominantly driven by radiative processes, are essential for understanding surface processes relevant to habitability and the interpretation of remote sensing data. Previous missions have provided valuable data, but MEDA's advanced capabilities, including simultaneous measurements at multiple heights and high temporal resolution, enable a more comprehensive understanding of the Martian environment than ever before. This research promises to enhance our predictive capabilities of numerical models, providing crucial context for other rover investigations, Ingenuity flights, and future sample return missions. The specific research questions addressed in this paper involve characterizing the diurnal and seasonal variability of temperature, pressure, wind, humidity, and dust.

Studies of the Martian atmosphere have a long history, beginning with the Mariner and Viking missions, which provided initial meteorological data. The Pathfinder mission's ASI/MET experiment added to this knowledge by providing measurements of temperature, pressure, and wind. Subsequently, the Phoenix mission's meteorological package expanded these measurements. The Mars Exploration Rovers' Mini-TES provided insights into atmospheric composition, and the Mars Science Laboratory's REMS instrument offered more detailed environmental data from Gale Crater. InSight's APSS further added to the understanding of the Martian atmosphere. These previous missions established a foundational understanding of the Martian atmospheric surface layer (ASL), but MEDA offers significantly improved capabilities. Existing numerical models, like the Mars Climate Database (MCD) and MarsWRF, provide valuable predictions for Martian weather and climate, offering a framework for comparison with in situ measurements. Previous research on the Martian ASL has highlighted the importance of radiative processes, albedo, net radiative flux, and thermal inertia in driving its dynamics. These previous findings provide a context for interpreting MEDA's high-resolution data and understanding the spatial and temporal variability of Jezero's meteorology. Studies have also explored the role of dust devils and convective processes, along with atmospheric tides, in shaping the Martian atmosphere's dynamic behavior.

The Mars Environmental Dynamics Analyzer (MEDA) instrument on the Perseverance rover is the focus of this study. MEDA autonomously acquires data on a regular and configurable basis, typically covering more than 50% of a Martian sol (sol is a Martian day). Sampling sessions, usually one hour long, alternate between even and odd hours every sol, allowing for complete characterization of daily and seasonal cycles every other sol. This study uses data from the first 250 sols of the mission (solar longitudes Ls = 6°–121°), covering the northern hemisphere spring to early summer. MEDA includes several sensors, including a thermal infrared sensor (TIRS), a radiation and dust sensor (RDS), a humidity sensor (HS), a pressure sensor (PS), and atmospheric temperature sensors (ATSs). The methodology involves quantifying all surface energy budget (SEB) terms in situ using TIRS and RDS data. Thermal inertia (TI) is derived by minimizing the difference between measured and simulated ground temperature diurnal amplitudes. Surface albedo is determined from downwelling and reflected solar flux measurements. A radiative transfer model (COMIMART) is used to convert fluxes to the 0.2–5.0 µm range. The heat conduction equation is solved using the SEB as an upper boundary condition for homogeneous terrains. The near-surface thermal profile is determined from simultaneous temperature measurements at four heights (surface, 0.85 m, 1.45 m, and ~40 m). Pressure fluctuations are analyzed to identify dynamical mechanisms using power spectral density analysis. Wind patterns are analyzed to investigate the influence of local topography and regional winds. Atmospheric dust properties, including optical depth (OD) and particle size, are derived from SkyCam and RDS data. Relative humidity (RH) and water vapor volume mixing ratio (VMR) are analyzed using HS data to investigate the diurnal and seasonal hydrological cycle. Pressure fluctuations are analyzed at various scales to identify gravity waves and baroclinic eddies. Detailed descriptions of the operational strategies, measurement procedures, and data analysis techniques for each sensor are provided in the paper's Methods section. COMIMART and UH/FMI SCM models were used for radiative transfer simulations, and the Levenberg-Marquardt procedure was used to estimate free parameters.

This research reveals diverse meteorological phenomena in Jezero crater during the first 250 sols of the Perseverance mission. Key findings include: 1. **Surface Energy Budget (SEB):** MEDA provided the first in situ measurements of the SEB on Mars, revealing the diurnal variation of all SEB components. The non-Lambertian behavior of surface albedo was characterized, which is crucial for interpreting orbital observations. 2. **Atmospheric Surface Layer (ASL):** Temperature measurements at four heights showed a transition between a stable night-time thermal inversion and a daytime convective regime. Large vertical thermal gradients were observed during daytime convection. 3. **Aerosol Concentration:** Multiple daily optical depth measurements indicated higher aerosol concentrations in the morning than in the afternoon. This observation is consistent across various sols and is attributed to both dust and water ice clouds. 4. **Wind Patterns:** Wind patterns were mainly driven by local topography, with a smaller contribution from regional winds. Daytime upslope currents from the southeast and nighttime downflows were observed, consistent with pre-landing model predictions. 5. **Hydrologic Cycle:** Diurnal and seasonal variability in relative humidity (RH) and water vapor volume mixing ratio (VMR) revealed a complex hydrological cycle. The maximum RH typically occurred in the early morning, with maximum VMR reached around midnight. A seasonal minimum in night-time VMR was observed near Ls = 70°, and a large increase in VMR was observed on the evening of sol 104, possibly due to dry air mass advection or local surface exchange processes. 6. **Pressure Fluctuations:** Analysis of pressure fluctuations revealed signatures of gravity waves and baroclinic eddies, including multi-sol waves during a season previously characterized by low wave activity. These findings are consistent with measurements made by InSight, and power spectra of temperature and pressure fluctuations showed similarities. 7. **Dust Devils:** The highest abundance of dust devils (DDs) detected on Mars to date was observed at Jezero. Pressure drops associated with DDs ranged from ~0.3 to 6.5 Pa, lasting from 1 to 200 s. DD diameters were estimated to range from 5 to 135 m, with rotational speeds of −4–24 m s⁻¹. A small number of DDs produced measurable albedo changes on the surface, observed through variations in the upward/downward radiation ratio measured with TIRS and RDS sensors. 8. **Atmospheric Dust Properties:** Analysis of RDS and SkyCam data revealed dust particle sizes ranging from ~1.2 to 1.4 µm, consistent with previous studies. The non-sphericity of these particles was assessed using a T-matrix approach. 9. **Clouds:** The presence of clouds, including water-ice clouds at altitudes around or above 40 km, was detected both during daytime and twilight. The diurnal evolution of thermal infrared aerosol OD, contributed by both dust and water-ice clouds, was observed throughout the sol, including at night, offering new insights into their interaction with the surface and the rest of the atmosphere. 10. **Thermal Tides:** Thermal tides were detected in both pressure and temperature data, with diurnal, semidiurnal, and terdiurnal components exhibiting significant variability, likely related to changes in atmospheric opacity caused by clouds and dust loading. The amplitude of these tides were compared to previous observations by other Mars missions.

The findings from this study significantly advance our understanding of the Martian atmosphere at Jezero crater. The in situ SEB measurements provide crucial ground truth data for refining and validating existing Martian climate models and improving remote sensing interpretation. The observations of the ASL reveal the complex interaction between radiative processes, surface properties (like albedo and thermal inertia), and atmospheric dynamics, highlighting the diverse responses of the Martian surface to solar forcing. The detection of high-frequency gravity waves and baroclinic eddies, along with the unexpected occurrence of multi-sol waves in a period normally characterized by low wave activity, underscores the dynamic nature of the Martian atmosphere and challenges existing understanding of atmospheric wave generation and propagation. The high abundance of dust devils observed at Jezero further emphasizes the unique characteristics of this region's surface and atmospheric conditions. The detailed characterization of the dust properties and the ability to track optical depth throughout the sol, including at night, represent significant advancements in our ability to monitor atmospheric processes. The insights into the complex hydrological cycle at Jezero provides further context for understanding the distribution and evolution of water on Mars. The findings are directly relevant to the Mars Sample Return mission, as they provide detailed information on the environmental conditions that samples are exposed to and can be used to better understand processes that may have affected the samples. Future work should focus on expanding the dataset to cover a wider range of seasons and explore the interaction between local and regional atmospheric dynamics.

This research, based on the first 250 sols of MEDA data from Jezero crater, provides a comprehensive and detailed characterization of the Martian atmospheric surface layer. The findings significantly improve our understanding of Martian meteorology, revealing a rich diversity of dynamic processes at various spatial and temporal scales, including diurnal and seasonal variations. This work enhances our understanding of the Martian climate and improves our ability to predict future weather events, as well as supporting the ongoing Mars Sample Return Campaign. Future research should extend the study over longer time scales to fully capture the annual cycle and focus on using coupled atmospheric and subsurface models to better constrain the observed behaviors.

The study's timeframe, limited to the first 250 sols of the Perseverance mission, only covers a portion of a Martian year, potentially limiting the full characterization of annual variations. The focus on a single location (Jezero crater) may not be representative of the Martian atmosphere as a whole. Although the MEDA instrument is highly advanced, uncertainties associated with its measurements are discussed in the Methods section and should be considered when interpreting the results. The assumptions made in data analysis, such as the homogeneity of terrains for thermal inertia calculations, could introduce some limitations.