2024-04-06 12:10:27
Jupiter’s satelliteseuropaIt is believed that there is a vast ocean inside, but it has not been possible to confirm this directly because the surface is covered with thick ice. So, how thick is this ice shell?
A research team led by Shigeru Wakita of Purdue University conducted an operation by the National Astronomical Observatory of Japan on the premise that the basin topography, which is formed by layered ring structures formed by collisions with celestial bodies, is related to the thickness and hardness of the ice shell. We performed a simulation on a “calculation server”. the result,Europa’s multi-ring basins cannot be explained unless the ice shell is at least 20 km thick.I understand that. The results of this study are important because they provide fundamental information about Europa’s structure, which is poorly understood.
[▲ Figure 1: An imaginary diagram of a large-scale collision that would create a multi-ring basin (Credit: Brandon Johnson (AI generated))]
■How thick is Europa’s ice?
So far, only one celestial body, Earth, has been found to have a large amount of liquid water on its surface. However, underground in icy bodies whose surfaces are covered with ice, the ice melts and turns into liquid water.internal sea” is widely believed to be widespread. Its size is said to be several to ten times larger than the Earth’s oceans.
There are many candidates for celestial bodies that are thought to have an internal ocean, including Jupiter’s satellites.europa” and Saturn’s moon Enceladus, there are some celestial bodies that are almost certainly expected to have internal oceans. However, since the surfaces of these celestial bodies are covered with ice, no confirmed examples have been found.
Not much is clearly known about this question, “How thick is the ice shell?” There have been a number of attempts to estimate the thickness using observational data obtained during close exploration by spacecraft and simulations using computer models, but initial research suggests that if it is thin, it will be several hundred meters thick, if it is thick, it will be thicker. It was extremely wide, several hundred kilometers wide.
This situation is improving as research progresses, and for example, for Europa, by incorporating small craters that are thought to be affected by the thickness of the ice shell into the calculation model, the thickness has been narrowed down from several kilometers to around 10-odd kilometers. It is. However, with previous models, it was difficult to distinguish between a “thin ice shell that is hard and difficult to break” and a “thick ice shell that is brittle and easy to break,” making it difficult to clearly determine the thickness.
■Estimating ice thickness from multi-ring basins
[▲Figure2:ImageofTyreoneofthemulti-ringbasins.Thediameterisapproximately149km(Credit:NASAJPL&ASU)]
In order to improve this situation, Dr. Wakita and his research team are trying to improve this situation bymultiple ring basinWe conducted a simulation using a topography of about 100 km in diameter called “. A multi-ring basin is a landform in which ring-shaped landforms are layered concentrically, and is thought to have been created by a collision with a large-scale celestial body that was large enough to break through the ice shell.
The thickness and hardness of the ice shell are expected to have a major influence on the conditions for the formation of multi-ring basins, but it is difficult to simulate the topography formed by the involvement of hard and soft materials. In fact, at first it took as much as a month to run one simulation, but with subsequent innovations we were able to improve the system to the point where over 100 calculations could be performed within a realistic amount of time.
[▲ Figure 3: Photo of the calculation server operated by the National Astronomical Observatory of Japan (Credit: National Astronomical Observatory of Japan)][▲Figure4:AnexampleofsimulationresultsTheblackdottedlineintheenlargedimageontheupperrightrepresentsthefaultthatoccursintheiceshellandwhenviewedfromaboveitcanbeobservedasamulti-ringbasin(Credit:ShigeruWakitaetal)
In this research, we will use the “calculation server” and a numerical collision calculation code that simulates celestial collisions “iSALE” to find the thickness and hardness of the ice shell that best reproduces the multi-ring basins that exist on Europa.
the result,Multi-ring basins cannot form unless the ice thickness is at least 20 km.This has been shown. We also found that ice is composed of hard and hard-to-break layers and brittle and easy-to-break layers, and that if the brittle parts are too brittle, multi-ring basins will not form. This structure of the ice shell is consistent with the assumption that an internal ocean exists, so the multi-ring basin indirectly indicates the existence of an internal ocean.
It is not known when Europa’s multi-ring basins were formed, and it is possible that the thickness of the ice shell is different between when the multi-ring basins were formed and in modern times. However, Europa’s surface is constantly being updated, and it is thought that the craters will disappear after 20 to 200 million years. This is a fairly short period of time geologically, and it shows that the estimated thickness of the ice shell at the time of the formation of the multi-ring basin is likely to apply to modern-day Europa.
■The thickness of ice also affects its own life.
The reason why celestial bodies like Europa, which are thought to have internal oceans, is attracting attention is because it is predicted that they may harbor unique life forms. So, even if unique life forms exist, by what route are the various elements and molecules that make up life forms supplied? The source could be material coming from outside, such as comets, or decomposition products of the ice shell, but the amount of supply also depends on the thickness of the ice shell.
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・Is there little oxygen supplied to Europa’s ocean? Research based on Juno data (March 19, 2024)
In this study, we were able to set a lower limit for the thickness of Europa’s ice shell, at least 20 km, but no upper limit has been set. The data for estimating Europa’s ice shell is mainly based on observation results from the Jupiter probe Galileo, which was launched by NASA (National Aeronautics and Space Administration) in 1989, and the reason is that there is a general lack of data. There is also. NASA is planning to launch a new spacecraft, Europa Clipper, in October 2024, so further observational data may improve this situation.
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