The problem the researchers tackled is known as base heating. As a rocket ascends, the hot exhaust plume radiates energy toward the aft structures of the vehicle, including the base region between clustered nozzles and the surfaces surrounding them. At sea level, the surrounding air constrains the plume and much of the heat is convected away, but at high altitudes the plume expands freely into the rarefied environment. Under these vacuum-like conditions the gas cools rapidly as it diffuses, yet the condensed particles suspended in the flow remain hot and continue to radiate. Understanding how much of that particle-driven radiation strikes the rocket base is critical for thermal protection design, particularly for solid rocket motors, whose propellants typically contain substantial amounts of aluminum powder to boost performance.

Simulating such a flow is notoriously difficult because the physics spans two very different regimes. The gas in a high-altitude plume is so rarefied that the continuum assumptions underlying ordinary computational fluid dynamics break down; molecular collisions become infrequent and the flow must be treated statistically. The research team therefore employed the direct simulation Monte Carlo method, using the SPARTA code, to model the rarefied plume field particle by particle, or more precisely molecule by simulated molecule. This approach, rooted in the molecular gas dynamics framework pioneered by Graham Bird, tracks representative molecules and their collisions to build up a statistical picture of the flow without solving the continuum equations at all.

The alumina particles, however, demanded a different treatment. Because they are far more massive than gas molecules and travel on largely ballistic trajectories punctuated by drag interactions with the thin gas, the team modeled them with Lagrangian particle tracking implemented in the open-source OpenFOAM toolkit. The two solvers were then coupled to an in-house radiative base-heating solver based on the discrete ordinates method, which computes how thermal radiation propagates through the particle-laden plume and how much of it is intercepted by the base region. This integrated computational framework, combining DSMC gas dynamics, Lagrangian particle dynamics, and radiative transfer, is one of the study’s central technical contributions, allowing each physical ingredient to be resolved with the method best suited to it.

The first major finding concerns what happens to the gas itself. The exhaust plume analysis revealed substantial diffusion and backflow under vacuum conditions: instead of forming a narrow, directed jet, the exhaust spreads laterally and a portion of it reverses direction, flowing back toward the vehicle. This expansion causes a pronounced drop in gas temperature. That cooling has a welcome simplifying consequence for the modelers, because the reduced gas temperature justified neglecting radiative heat transfer from the gas phase altogether. In other words, at high altitude the radiative threat to the rocket base comes almost entirely from the hot alumina particles, not from the glowing exhaust gases, a conclusion that sharply focuses where design attention should be directed.

With the gas-phase radiation set aside, the researchers systematically explored how particle characteristics shape base heating. Holding the propellant composition fixed, they varied the size of the alumina particles and computed the resulting radiative flux. The results reveal a strikingly nonlinear relationship. Radiative base heating increases rapidly as particle diameters move from the submicron range into the micron range, but beyond that transition the rate of increase diminishes markedly for larger particles. The physical explanation lies in the interplay between how much thermal radiation a particle emits, which scales strongly with its surface area and temperature, and how those particles are distributed and transported through the expanding plume. Small particles cool quickly and radiate inefficiently in aggregate, while the largest particles contribute less per unit mass than the intermediate sizes where the effect peaks.

The second axis of the study examined propellant composition. For a fixed particle size, the team varied the aluminum mass fraction in the propellant and found that base heating rises steadily as more aluminum is added. This makes intuitive sense once the chemistry is considered: aluminum powder burns in the motor to form aluminum oxide, so a propellant richer in aluminum produces a greater loading of radiating oxide particles in the exhaust. The finding carries a practical tension for rocket designers, because aluminum is added to solid propellants precisely to raise performance, yet every additional percentage point of aluminum buys a hotter radiative environment at the vehicle base. The new results quantify that trade-off in a form that thermal engineers can use during early design studies.

The authors are careful to frame the reliability of their numbers. Because formal benchmark validation of the modified radiation coupling procedure has not yet been completed, they caution that the predicted radiative-heating levels should be interpreted as engineering-level comparative sensitivity indicators for particle-driven base heating, rather than as fully validated absolute local heat-flux predictions. In practical terms, the study tells designers with confidence how base heating changes when particle size or aluminum content is changed, but the absolute magnitudes await experimental or high-fidelity benchmark confirmation. That honesty about validation status is a valuable feature of the work, distinguishing robust trends from numbers that should still be treated as provisional.

The study builds on a long lineage of plume radiation research stretching back decades, from early analyses of large solid propellant rocket plumes using DSMC in the 1980s, through finite-volume radiation analyses of rocket plume base heating in the 1990s, to more recent investigations of how the optical properties of alumina particles influence radiative base heating. What distinguishes the new effort is the tight coupling of three state-of-the-art tools, SPARTA for rarefied gas dynamics, OpenFOAM for particle tracking, and a dedicated radiation solver, into a single pipeline capable of answering parametric questions about particle size and propellant formulation. The work was supported by the National University Development Project of Jeonbuk National University, and the corresponding author, Man Young Kim, has previously published related predictions of radiative base heating from solid rocket exhaust plumes.

For the aerospace community, the implications extend across launch vehicles, upper stages, and missile systems that operate solid rocket motors in the upper atmosphere and beyond. As missions increasingly demand precise thermal margins rather than conservative overdesign, knowing that the submicron-to-micron transition in alumina particle size is the critical regime, and that aluminum loading drives heating upward, allows engineers to target both their propellant specifications and their thermal protection layouts more intelligently. The study also highlights the growing power of multiscale simulation, in which molecular-level gas kinetics, particle mechanics, and radiative transfer are stitched together to illuminate phenomena that neither wind tunnels nor flight tests can easily isolate. As validation matures, frameworks of this kind could become standard instruments for predicting the invisible thermal weather that surrounds every rocket climbing toward orbit.

Subject of Research: Radiative base heating from alumina particles in high-altitude solid rocket motor exhaust plumes

Article Title: Effect of Particle Size and Propellant Composition on Base Heating in High-Altitude SRM Plumes

Article References: Joo, H. C., Kim, J. S., & Kim, M. Y. (2026). Effect of Particle Size and Propellant Composition on Base Heating in High-Altitude SRM Plumes. International Journal of Aeronautical and Space Sciences. https://doi.org/10.1007/s42405-026-01252-1

Image Credits: AI Generated

DOI: 10.1007/s42405-026-01252-1

Keywords: solid rocket motor, exhaust plume, base heating, alumina particles, particle size, propellant composition, direct simulation Monte Carlo, rarefied flow, radiative heat transfer, discrete ordinates method, Lagrangian particle tracking, high altitude

The puzzle the researchers set out to solve has been staring planetary scientists in the face for decades. On Earth, planetary-scale circulation is generally assumed to homogenize atmospheric composition, smoothing out differences in the mixing ratios of trace gases and aerosols over synoptic timescales. Mars refuses to conform. Observations from orbiting instruments reveal enormous planetary-scale inhomogeneities in the distributions of dust, water vapor, carbon monoxide, and argon. Water vapor climbs steeply in the northern summer while the southern winter hemisphere stays comparatively dry. Carbon monoxide accumulates toward the winter poles as the gas freezes out at the cold polar surface. Argon, an inert tracer, collapses dramatically over the winter pole as carbon dioxide condenses out of the air and concentrates the remaining constituents. If global circulation is supposed to mix everything together, Mars did not get the memo.

To find out why, the team turned to a Lagrangian particle-tracking approach built on the ensemble Mars atmosphere reanalysis system, known as EMARS. Rather than examining the atmosphere from a fixed grid of points, Lagrangian analysis follows individual parcels of air as they are carried by the winds, hour by hour, through the reanalysis data. The researchers released vast numbers of virtual tracers, some seeded inside the Hadley cell and some seeded outside it, and tracked their displacements over 30 Earth days. They also performed sensitivity experiments in which the strength of the mean circulation and the strength of transient eddies—the chaotic, wave-like disturbances superimposed on the mean flow—were independently amplified, allowing the team to disentangle which component of the flow controlled the transport regime.

The result was unambiguous. Tracers launched within the Hadley cell tended to stay there, riding the cell’s coherent conveyor belt, while tracers outside it were largely excluded, unable to penetrate the circulation’s boundaries. The edges of the Hadley cell, in other words, act as transport barriers: surfaces across which material exchange is strongly suppressed, much like the walls of the stratospheric polar vortex on Earth that help preserve ozone-depleted air over Antarctica. At the same time, the interior of the cell functions as a pole-to-pole teleconnection pathway. Air rising in the rising branch over the warm hemisphere is carried aloft, crosses the equator in the upper branch, and descends over the opposite pole, delivering material—dust, water, chemistry—from one hemisphere directly to the other in a single coherent loop. The two poles of Mars, though separated by half a planet, are dynamically coupled in a way that mid-latitudes are not.

Determining why Mars behaves this way required a dimensional argument. The team compared the relative importance of the mean overturning circulation against eddy-driven mixing, and found that on Mars the mean circulation dominates overwhelmingly. This dominance, they showed, arises from the combined effect of two planetary properties: Mars rotates nearly as fast as Earth, which suppresses large-scale turbulent mixing by strengthening rotational constraints on the flow, while its atmosphere is vanishingly thin—roughly one hundred times less massive per unit area than Earth’s. The thin atmosphere means the circulation responds rapidly to thermal forcing, and the single-cell Hadley circulation that emerges in each solstice season is both stronger and more coherent relative to the eddies than its terrestrial counterpart. The outcome is a transport regime in which advection by the mean flow overwhelms diffusive eddy transport, the opposite of the balance that prevails on our own planet.

To test whether this regime is truly unique, the researchers repeated the same tracer experiments on Earth and Venus, using the ERA5 reanalysis for our planet and the Venus Climate Database for our inner neighbor. Earth, with its two-cell Hadley circulation, vigorous baroclinic eddies, and comparatively thick atmosphere, mixed tracers broadly across latitudes, erasing sharp compositional gradients on short timescales. Venus, whose atmosphere superrotates and whose eddy field dominates the meridional overturning, showed an entirely different pattern again. Only Mars, sandwiched between these two extremes, produces the distinctive combination of isolation inside a coherent cell and pole-to-pole teleconnection along its spine. The three terrestrial planets, made of similar materials orbiting the same star, thus host three qualitatively different atmospheric transport regimes.

The implications reach well beyond atmospheric dynamics as a curiosity. The isolation of material within the Hadley cell helps explain a suite of long-standing Martian observations: the striking latitudinal gradients in dust, water vapor, and carbon monoxide; the formation of a polar ozone layer driven by transport rather than local photochemistry, as earlier work by Montmessin and Lefèvre proposed; and the delivery of water and dust to the poles that ultimately becomes recorded in the polar layered deposits, the ice-rich archives of Martian climate history. If tracers cannot readily escape the Hadley circulation, then the compositional signal deposited at each pole reflects material that traveled along a well-defined dynamical pathway, not a well-mixed global average. That insight could sharpen interpretations of isotope ratios in polar ice, including the deuterium-to-hydrogen histories used to reconstruct how much water Mars has lost to space.

Perhaps the deepest consequence of the study is conceptual. The conventional view of planetary atmospheres holds that global-scale circulation is, above all, a mixing machine: it redistributes heat, momentum, and composition, and iron out inhomogeneities. The new results demonstrate that this is not a universal law but a regime-specific outcome of planetary parameters. Change the rotation rate, the atmospheric mass, or the balance between mean flow and eddies, and a circulation that mixes on one world can confine and channel on another. Planetary atmospheric dynamics, the authors conclude, can actively restrict redistribution, creating compositional reservoirs that persist in plain sight of one another. For Mars, that means the air above the tropics and the air above the mid-latitudes live in partial isolation, while the poles trade material as though connected by a private telegraph line.

For scientists preparing the next generation of Mars missions, the findings offer practical guidance as well. Interpreting measurements of water vapor, argon, carbon monoxide, or photochemical species requires knowing where a parcel of air has been and which dynamical basin it belongs to. Lagrangian tools of the kind released by the team—publicly available alongside the EMARS and ERA5 datasets—can now provide that context, converting snapshots of composition into narratives of transport. And as exoplanet astronomers characterize atmospheres of worlds with rotation rates and atmospheric masses far from those of the solar system’s terrestrial trio, the Martian lesson looms large: the same circulation pattern that on Earth homogenizes the air can, on a thin-atmosphere world, fence it off—and link its poles in one seamless loop.

Subject of Research: Atmospheric material transport by the Martian Hadley circulation

Article Title: Hadley circulation drives material isolation and pole-to-pole teleconnection on Mars

Article References: Fan, C.-S., Sun, C., Xie, Z., Luo, Y., Gu, L., & Fan, S. (2026). Hadley circulation drives material isolation and pole-to-pole teleconnection on Mars. Nature Geoscience. https://doi.org/10.1038/s41561-026-02090-2

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02090-2

Keywords: Mars, Hadley circulation, atmospheric dynamics, Lagrangian particle tracking, polar vortex, planetary atmospheres, dust transport, water vapor, Nature Geoscience, teleconnection, Venus, EMARS reanalysis