New Frontiers of Extrasolar Planets: Exploring Terrestrial Planets

Grant-in-Aid for Scientific Research by the Ministry of Education, Culture, Sports, Science and Technology  Scientific Research on Innovative Areas (Research in a proposed research area) 2011-2015

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Beyond the Snow Line


The term “cosmoglaciology” was created in the 1980s when the NASA planetary probe Voyager observed the moons of the outer planets in our solar system and sent back many impressive photographs. Great variety was found in the four large moons of Jupiter, known as the Galilean Moons. Voyager found that the moon Io, which is closest to Jupiter, had a surface covered with sulfur and was hot enough that active volcanoes could be seen on it, while Europa, the next moon outwards from the planet, had a surface covered by ice and etched by quite unusual global faults. Farther out, observations quickly confirmed that Ganymede and Calisto were also bodies composed mostly of ice. Observations by the later Galileo probe discovered that there was an ocean inside Europa (a subsurface ocean), and the possibility of a subsurface ocean on Ganymede is also being discussed. Beyond Jupiter and even farther from the sun are worlds with surfaces of ice instead of rock – worlds that present a new area of study in the field of glaciology that was developed here on Earth. The glaciers, ice sheets, and ice flow laws of Earth are being applied to the study of these icy moons.
The term “snow line” has also emerged in theories of how the solar system was formed. It indicates the boundary of the area in the primordial solar nebula where water vapor could freeze and form ice. This snow line is thought to have been located close to what is now the asteroid belt. In the area of Jupiter, which was located beyond the snow line, ice was plentiful and this was the most important reason why its ice moons formed. At present, ice is still the primary component of all astronomical objects beyond the snow line except for the giant planets, and the abundance of rock and metals is half or less than that of ice.
In observations of exoplanetary systems as well, the snow line is an important boundary which characterizes the system structure. More and increasingly advanced observations related to these snow lines are expected in the future. In the area beyond an observed snow line, we are certain to find a world of ice similar to that in our solar system. Our research group is conducting studies concerning the origins and evolution of astronomical ice objects by reproducing the world of ice beyond the snow line. We are a research group that is active in the field of planetary glaciology which I mentioned at the beginning. We have created a cold room (Figure 1) that is an extremely powerful means of conducting reproduction tests using ice in this new academic field.



This cold room is an experimental space in a prefabricated room that is kept constantly at -15°C and where two researchers can enter and work. In our group, we put on winter clothes and are chilled together with the ice as we conduct our experiments. So why is it necessary to chill the people instead of just the ice samples when we conduct the experiments? There are several reasons. Our group conducts collision tests that reproduce collisions between astronomical ice objects, and these tests require relatively large ice samples. In particular, some of the cratering tests use large blocks of ice weighing 10 kg or more. While it is possible to use local cooling when the sample size is small, using a freezer or other means to locally cool such a large sample is very inefficient. Then there is the problem of recovering the collision fragments which are an important product of the collision tests. The distribution of collision fragment sizes and quantities is said to be a power-law distribution, but in order to investigate the distribution accurately we must recover even the smallest fragments. After a collision test, the collision fragments are scattered across the chamber, and if the chamber were at room temperature the fragments would quickly melt. This would make it impossible to study the collision fragments from our ice collision tests. The simplest way to solve this problem was to place the chamber inside a cold room. Therefore our group installed the vacuum chamber that we use for collision tests inside the cold room, making it possible to recover all the ice fragments, still in solid form, after they were scattered. Finally there is the problem of measurement. Measuring the weight of each individual fragment and counting the number of tiny fragments is impossible at room temperature because the ice melts too quickly. The only way to get such measurements is for the researchers to go into the cold room themselves. It is for these reasons that we created the cold room for ice collision tests and installed the collision testing vacuum chamber in it (Figure 1).



Here is an example of a collision test performed at low temperature. Figure 2 shows images of a collision between ice balls captured by a high-speed camera. 3 cm ice balls were collided at a relative speed of 83 m/s, and the images show that immediately after the collision the ice balls were covered with innumerable cracks that began from the point of impact. The pulverized ice balls appear to be overall crushed and fused together while ejecting small fragments at high speed in the horizontal direction on the collision plane. However in the end, nearly all the fragments are ejected in a direction perpendicular to the direction of the collision. The numerical simulations for reproducing the collision phenomena of this destruction are still in the development phase, and we need to perform these tests in order to identify the mechanisms involved and improve the model. Through this kind of cold room collision testing, we intend to identify the elementary physical processes of destructive collisions and crater formation. By applying the results to numerical models of planetary formation, we expect to contribute to an understanding of the astronomical body collision processes involved in planet formation and exoplanetary systems.

Figure 1: Cold room installed at Kobe University and the vacuum chamber used for collision tests in the cold room
Figure 2: Conditions of a destructive collision between ice balls. The size of the ice balls is 3 cm and the collision speed is 83 m/s. The numbers indicate the elapsed time after the collision.

Masahiko Arakawa (University of Kobe)

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