SPACE/COSMOS
A look inside stars and planets
Experiment confirms prediction about turbulence in rapidly rotating celestial bodies
Helmholtz-Zentrum Dresden-Rossendorf
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Complex flow patterns develop in the interiors of rapidly rotating stars. An international research team has now experimentally demonstrated for the first time a flow state characteristic of such celestial bodies. The artistic visualization shows a star characterized by turbulent structures.
view moreCredit: B. Schröder/HZDR
Since we cannot look into the interiors of stars and planets, we rely on lab experiments to replicate the physical processes that occur there. Led by the University of California, Los Angeles (UCLA), an international research team has now produced the first experimental verification of a theoretically predicted flow state that is deemed characteristic of the interiors of rapidly rotating celestial bodies. The Helmholtz-Zentrum Dresden-Rossendorf (HZDR) also participated in the study, which has been published in Physical Review Letters (DOI: 10.1103/pc8y-j7g8). The results provide a robust experimental basis for testing theoretical models of the processes that occur inside these celestial bodies.
Inside stars and planets, heat is transported by convection: Hot material rises while cooler material sinks. This generates turbulent flows that drive the Earth’s magnetic field and shape the dynamics of stars. According to theoretical predictions, these flows attain a special state inside rapidly rotating celestial bodies. The scientific community calls this ultimate state the diffusivity-free regime, in which large flows are determined almost exclusively by buoyancy and rotation while fluid properties such as viscosity or thermal conductivity become almost negligible. Many models of the interiors of stars and planets are based on the assumption that convection operates precisely within this ultimate regime.
A question that has been unanswered for decades
Yet until now, this had precisely been the issue: In classical lab experiments, thermal boundary layers form on the walls of the test vessels, affecting the flow to the point that its state remained hidden. For a long time, it was therefore unclear whether it was even possible to verify the theoretical predictions in an experimental setting.
The international research team achieved the decisive breakthrough with a rotating liquid-metal experiment using liquid gallium as the test medium and a special oscillating flow mode that occurs exclusively in liquid metals instead of the usual stationary convection. Unlike classical convection flows, this mode is not determined by the thermal boundary layers on the vessel walls, but by the temperature gradient inside the liquid, making it possible to create the conditions that models have long predicted.
“Our experiment demonstrated this theoretically predicted state in the lab for the first time, which greatly strengthens our confidence in the models we use to describe processes inside stars and planets,” says Dr. Jewel Abbate of UCLA, who conducted this research as part of her PhD studies.
A new pathway into the interior of stars
Dr. Tobias Vogt from HZDR’s Institute of Fluid Dynamics also participated in the experiments and measurements during two research stays at UCLA. To validate the experimental results, the researchers compared three independent metrics with the theoretical predictions: heat transport, flow velocity, and temperature fluctuations within the fluid. All three matched the models quantitatively. High-resolution numerical simulations further confirmed the results.
“What makes our work particularly compelling is the fact that theory, experiment, and numerical simulations now agree quantitatively, allowing us to confirm experimentally that the underlying physical models describe the observed heat transport very accurately. This gives us far greater confidence to apply these models to the interiors of planets and stars,” Vogt summarizes.
The results close a decades-old gap between theory and experiment, opening up new possibilities for investigating the dynamics inside stars and planets in lab experiments – for more reliable models of these fundamental processes in the universe.
Publication:
J. A. Abbate, Y. Xu, T. Vogt, S. Horn, K. Julien, J. M. Aurnou, “Diffusivity-Free Turbulence in Liquid Metal Rotating Rayleigh-Bénard Convection Experiments” in Physical Review Letters (2026) (DOI: 10.1103/pc8y-j7g8)
Further information:
Dr. Tobias Vogt
Institute of Fluid Dynamics at HZDR
Phone: +49 351 260 2451 | Email: t.vogt@hzdr.de
Prof. Jonathan M. Aurnou
Department of Earth, Planetary, and Space Sciences (EPSS)
University of California, Los Angeles (UCLA)
Email: aurnou@g.ucla.edu
Media contact:
Simon Schmitt | Head
Communications and Media Relations at HZDR
Phone: +49 351 260 3400 | Mobile: +49 175 874 2865 | Email: s.schmitt@hzdr.de
The Helmholtz-Zentrum Dresden-Rossendorf (HZDR) performs – as an independent German research center – research in the fields of energy, health, and matter. We focus on answering the following questions:
• How can energy and resources be utilized in an efficient, safe, and sustainable way?
• How can malignant tumors be more precisely visualized, characterized, and more effectively treated?
• How do matter and materials behave under the influence of strong fields and in smallest dimensions?
To help answer these research questions, HZDR operates large-scale facilities, which are also used by visiting researchers: the Ion Beam Center, the Dresden High Magnetic Field Laboratory and the ELBE Center for High-Power Radiation Sources.
HZDR is a member of the Helmholtz Association and has seven sites (Dresden, Freiberg, Görlitz, Grenoble, Leipzig, Rostock and Schenefeld near Hamburg) with almost 1,500 members of staff, of whom about 700 are scientists, including 200 Ph.D. candidates.
Journal
Physical Review Letters
Roman Space Telescope equipped with technology from Heidelberg set for launch
Beyond probing the history of the universe, the NASA space telescope will test a new technique for examining exoplanets.
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The NASA space telescope “Nancy Grace Roman” (or Roman for short) is scheduled to launch into space from the Kennedy Space Center in the US aboard a SpaceX Falcon Heavy rocket at 13:26 (1:26 p.m.) CEST on 30 August 2026, following a construction period of around ten years.
view moreCredit: NASA/SVS
The NASA space telescope “Nancy Grace Roman” (or Roman for short) is scheduled to launch into space from the Kennedy Space Center in the US aboard a SpaceX Falcon Heavy rocket at 13:26 (1:26 p.m.) CEST on 30 August 2026, following a construction period of around ten years. One of the telescope’s primary missions during its operational lifespan of at least five years will be to investigate the history of the universe and the contributions of dark matter and dark energy to its expansion and the formation of the cosmic large-scale structure. In doing so, its measurements will complement those of the European Space Agency’s (ESA) Euclid space telescope, which has been pursuing similar objectives since 2023.
MPIA in Heidelberg makes important contributions
Additionally, Roman will discover a multitude of new exoplanets, utilizing among other equipment the Coronagraph Instrument (CGI). The Max Planck Institute for Astronomy (MPIA) in Heidelberg, NASA’s sole direct German partner, has played a key role in developing and constructing this instrument. Under the leadership of Oliver Krause, engineers and scientists at MPIA designed, developed, manufactured, and tested central optical elements for the CGI, known as the Precision Alignment Mechanisms (PAMs). The company von Hoerner & Sulger, based in the neighbouring town of Schwetzingen, supported the construction process.
“The CGI aboard the Roman Space Telescope is the most technically sophisticated device for optical observations ever operated in space for scientific research,” says Oliver Krause, head of the Infrared Space Astronomy Research Group at MPIA.
Furthermore, MPIA is participating in the development of software to process both technical and scientific data, as well as in preparing the observations. The measurements are coordinated by a NASA panel, the Community Participation Program (CPP), whose core team includes Oliver Krause as the local project lead. On behalf of MPIA, Wolfgang Brandner is responsible within the CPP for observations aimed at detecting gas giants in visible light, while Gaël Chauvin is involved in observation preparations.
Shortly after launch, CPP members will work around the clock on data evaluation during the telescope's commissioning. “However, thanks to the CPP's global distribution, with members in the US, Japan, and Europe, our data analysts can carry out their tasks during normal office hours,” says Wolfgang Brandner. During routine operations, all data will be made available to the public immediately following processing at the Roman Science Support Center.
New camera design for a scientific breakthrough
CGI is an innovative instrument design that will be tested to enable direct imaging and spectroscopy of exoplanets in tight orbits around distant stars. Its optical elements—masks, deformable mirrors, and sensors—will suppress interfering starlight, making the faint light reflected by the planets visible for research.
The objective of CGI is to image planets and circumstellar discs around nearby stars in visible light. This will allow the study of gas giants that are older, cooler, and orbit their host stars at closer distances than the hot, young planets previously discovered through direct imaging.
To this end, CGI combines two established observation techniques for the first time in space: coronagraphs and adaptive optics. Coronagraphs block out bright objects using specialized masks, revealing fainter celestial bodies nearby. However, the masks employed usually cause strong image artefacts around the masked stars. Therefore, astronomers using this method almost exclusively find gas giants similar to Jupiter that orbit at relatively large distances from their host stars.
For smaller planets in tighter orbits, these unwanted effects must be reduced. For this reason, CGI additionally features an adaptive optics system, enabling a higher brightness contrast between stars and planets. This technology is typically found in ground-based telescopes, where it helps eliminate image degradation caused by atmospheric turbulence. For space-based cameras, however, the required processing power presents a new challenge.
Technology of maximum precision
The CGI’s design aims to detect a planet whose nearby host star is a billion times brighter—roughly corresponding to the contrast ratio between Jupiter and the Sun. Compared with current capabilities, this represents up to a thousandfold improvement. An integrated spectrograph will then enable researchers to analyze the atmospheric composition of these planets.
These goals require the PAMs, manufactured by MPIA, to guarantee exceptionally high precision and stability in the positioning of optical elements, such as filters, coronagraphs, and mirrors, over a period of several hours. During operation, the PAMs must not tilt by more than 40 milliarcseconds over an eight-hour timeframe (3.6 million milliarcseconds correspond to one degree). This is equivalent to the angular size of a human being in Los Angeles when viewed from Heidelberg.
Following a successful CGI mission, this technology could be further refined for future space observatories like the Habitable Worlds Observatory. Directly imaging an Earth analogue would then be within reach.
Background information
The Nancy Grace Roman Space Telescope (formerly WFIRST: Wide-Field Infrared Survey Telescope) was developed under NASA leadership. The telescope is named after astronomer Nancy Grace Roman, who directed NASA’s astronomical research programmes for decades. Among other achievements, she was responsible for the scientific planning of the Hubble Space Telescope. The 2.4-metre primary mirror is similar to the one used in the Hubble Space Telescope. For one of the two scientific instruments, the Coronagraph Instrument (CGI), the Max Planck Institute for Astronomy (MPIA) in Heidelberg constructed central optomechanical components.
For the translation from the German original, a language model was utilized in an intermediate step, with the output being editorially reviewed and corrected.
One of six flight models of the Precision Alignment Mechanisms (PAMs) for the Coronagraph Instrument (CGI), a camera aboard the Nancy Grace Roman Space Telescope. The PAMs position and stabilize the optical elements of the CGI during observations.
Credit
O. Krause / MPIA
The impact of thermocapillary convection on phase-change material melting process under varying gravity conditions
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Fig. 1. Sketch of the simulation domain with (A) aspect ratio (AR) = 2 and (B) AR = 10.
view moreCredit: Space: Science & Technology
With the accelerated advancement of lunar and Mars exploration programs, long-term human habitation on extraterrestrial bodies faces the severe challenge of extreme diurnal temperature variations. Phase change materials (PCMs), owing to their ability to store or release substantial latent heat during solid–liquid phase transitions while maintaining a nearly constant temperature, have become an ideal solution for space thermal control systems. However, in the microgravity environment of space, the ground-dominant natural convection tends to disappear, and thermocapillary convection may become the prevailing heat transfer mechanism; in low-gravity environments such as those on the Moon and Mars, the coexistence and competition between these two convective regimes remain unclear. Existing studies have predominantly focused on either Earth's gravity or pure microgravity conditions, with a notable scarcity of research on the coupled effects of thermocapillary and natural convection at intermediate gravity levels. The melting dynamics of two typical organic PCMs, succinonitrile and n-octadecane, under low-gravity conditions have not yet been systematically elucidated. Therefore, clarifying the roles of thermocapillary convection under varying gravity levels and container geometries holds significant engineering guidance for the design of efficient space thermal control systems.
In a recent study published in Space: Science & Technology, the research team led by Ruiz from the Universitat Rovira i Virgili in Spain systematically investigated the influence of thermocapillary convection on the melting process of PCMs through numerical simulations. The study focuses on succinonitrile and n-octadecane as model PCMs, subjecting them to a temperature difference of 40 K in two rectangular cavities of different aspect ratios to drive melting, while simulating four gravity environments: microgravity on the International Space Station, lunar gravity, Martian gravity, and Earth's gravity. The results indicate that the aspect ratio is the key factor determining the dominant convective regime. In a flat cavity with an aspect ratio of 10, thermocapillary effects dominate the melting process at all gravity levels, significantly accelerating melting and generating multi-cellular oscillatory flows. In contrast, in a cavity with an aspect ratio of 2, natural convection is substantially enhanced and, at higher gravity levels, can counteract thermocapillary effects and even reduce the melting rate. Succinonitrile exhibits a markedly faster melting rate than n-octadecane owing to its higher thermal conductivity; nevertheless, the two materials show consistent trends in the competition between thermocapillary and natural convection. The study points out that in low-gravity environments such as those on the Moon and Mars, the rational selection of cavities with a large aspect ratio allows thermocapillary and natural convection to act synergistically, significantly enhancing the melting rate of PCMs. This research provides important theoretical foundations and optimization directions for the design of thermal management systems for future lunar and Martian bases, as well as for space-based PCM experiments in orbit, offering significant engineering application value for supporting the development of thermal control technologies for long-term extraterrestrial habitation.
First, this study focuses on the melting behavior of phase change materials (PCMs) under different gravity environments, with particular attention to the coupled effects of thermocapillary and natural convection. With the advancement of lunar and Mars exploration programs, the extreme diurnal temperature variations on extraterrestrial bodies pose severe challenges to thermal control systems, and phase change materials, owing to their constant-temperature heat storage and release characteristics, have become an ideal solution. However, under microgravity conditions, natural convection is weakened or even eliminated, and thermocapillary convection may become the dominant heat transfer mechanism; yet the competition between these two convection modes in low-gravity environments such as those on the Moon and Mars remains unclear. The study selects two typical organic PCMs—succinonitrile (Pr=23) and n-octadecane (Pr=56)—the former possessing higher thermal conductivity and faster melting, while the latter serves as the working fluid for the MarPCM experiment on the International Space Station. Simulations are conducted in rectangular cavities with a fixed length of 8 cm and aspect ratios of 2 and 10, respectively; a temperature difference of 40 K is imposed on the left and right walls to drive melting, and the upper surface is set as a free boundary to apply thermocapillary forces (as shown in the computational domain setup of Fig. 1). The accuracy of the numerical method is validated by comparison with published results; the comparison shows that the flow field structure obtained in this study is in excellent agreement with that in the literature under identical conditions, confirming the reliability of the solver.
Second, the study reveals the significant influence of gravity level and container aspect ratio on the melting dynamics. The liquid fraction evolution curves (Fig. 2) show that succinonitrile consistently melts faster than n-octadecane owing to its higher thermal conductivity, and thermocapillary effects serve to accelerate or modulate the melting process in both materials. In the flat cavity with an aspect ratio of 10, thermocapillary effects accelerate melting at all gravity levels, enabling the solid–liquid interface to reach the cold wall more rapidly, while the characteristic time for the liquid fraction to reach 95% is also substantially shortened. In the cavity with an aspect ratio of 2, the situation is more complex: in the early stages of melting, thermocapillary flow accelerates interface advancement; however, as the melt grows, natural convection progressively strengthens and flows in the direction opposite to the surface flow, thereby reducing the melting rate in the later stages at higher gravity levels. Fig. 3 summarizes the specific values of the two characteristic times under the four gravity scenarios, clearly demonstrating that thermocapillary effects are most pronounced under microgravity and low-gravity conditions, with their relative contribution gradually diminishing as gravity increases.
Finally, the study provides an in-depth elucidation of the melting mechanisms under different conditions through visualization of the flow and temperature fields, as well as the temporal evolution of the convective contribution factor. In the cavity with an aspect ratio of 2, pure natural convection gives rise to stable single-vortex or double-vortex structures; pure thermocapillary convection, in contrast, generates complex multi-cellular flows; when both mechanisms coexist, a distinctive three-zone structure emerges—comprising a bottom natural-convection vortex, a surface thermocapillary vortex, and an intermediate transition zone—with natural convection gradually becoming dominant as gravity increases (Fig. 4). In the flat cavity with an aspect ratio of 10, natural convection is significantly weakened due to the restricted height, and thermocapillary effects still dominate in the mixed convection regime, resulting in substantial deformation of the melting front and sustained oscillations under low-gravity conditions (Fig. 5). Fig. 6 presents the temporal evolution of the thermocapillary factor, which reaches a peak rapidly in the early stage of melting and then gradually declines, with its value decreasing as gravity increases. The study concludes that in low-gravity environments such as those on the Moon and Mars, a rational choice of containers with a large aspect ratio enables thermocapillary and natural convection to act synergistically in accelerating melting. This finding offers important engineering guidance for the design of thermal management systems for future lunar and Martian bases, as well as for space-based phase-change material experiments in orbit.
Journal
Space: Science & Technology
Article Title
The Impact of Thermocapillary Convection on Phase-Change Material Melting Process under Varying Gravity Conditions
