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Astronomical data making a cosmic impact

As the planet with the strongest gravitational pull, it's no surprise that many astronomers have likened Jupiter to a sort of cosmic vacuum cleaner. With a mass more than 300 times greater than Earth and roughly 1,300 times more volume, Jupiter experiences more impacts from comets and asteroids than any other planet in the Solar System.

Density isosurfaces showing the impactor and the shock structure around it during the breakup of an SL9-type impactor in the Jovian atmosphere. As the comet penetrates into deeper levels, it begins to flatten (or pancake) and break up into fragments. Image courtesy Csaba Palotai.

Researchers Csaba Palotai and Joseph Harrington at the University of Central Florida, US, and Donald Korycansky at the University of California at Santa Cruz, US, lead the Jovian Impact Research Group in investigating the various stages of asteroidal and cometary impacts in Jupiter's atmosphere.

By observing impacts and analyzing images the scientists are able to determine some of the impact circumstances, such as whether it was an asteroid or comet, and roughly the impact angle. They then plug this information into a set of numerical 3D simulations to extrapolate missing information and learn more about that particular impact.

As a comet dives into Jupiter, the atmosphere becomes denser; the comet heats up and its body, which is mostly ice, rips apart and releases a trail of hot gas into the atmosphere. This creates a bright flash the scientists can then observe to determine additional traits of impact including, size, depth, and angle. The models designed by the researchers allow them to change a number of different variables, helping them predict not only what future impacts on Jupiter may look like, but impacts on Earth as well.

The researchers however, were not thinking of simulation on February 15, 2013, as an asteroid entered Earth's atmosphere and exploded over the Chelyabinsk region in the Russian Urals. Many amateur videos from the region captured the asteroid streaking across the sky and exploding in a bright light. NASA reports that the fireball was brighter than the sun, releasing huge amounts of energy that shattered windows and sent loose objects flying. Initial estimates of the asteroid had it weighing in at 11,000 tons and measuring 15 to 20 meters across.

Lagrangian tracers in the Jovian model showing the path and thermal history of individual impactor particles, looking up from the terminal depth at an angle. Most of the material remains at depth and only a small fraction of the impactor gets ejected back in the low-density entry channel. Color represents the logarithm of temperature of the particles at a given location. Image courtesy Csaba Palotai.

"Other than the extensive damage and injuries, it was a remarkable occurrence," says Harrington. "That the event was caught so well on so many videos is really great for the science of impacts! We'll be able to verify our models on a well
observed event, or change them to match it."

"This is an historic event and something that has not happened on Earth in over a century," explains Palotai. "This is a real turning point in impact science. At this time we have access to data that wasn't even conceivable a century ago." His students are beginning to grasp the enormity of the impact and what they've witnessed. The last documented blast in Earth's atmosphere of comparable magnitude occurred over Siberia in 1908 - an era without digital photography, camera phones, or the popular Russian dash cam.

The scientists model Chelyabinsk-sized impactors on Jupiter, such as the 2010 and 2012 events, both thought to be in the 10 meter range. They also model larger impactors on Jupiter. However, when the group tried to model the 2010 impact they ran into a challenge. "Small impactors take a much longer time to run than big ones because the resolution has to be higher to get the same number of grid cells on the impactor, but the waves travel out just as far and fast. This means the timestep is that much smaller, and the compute-time scales as the fourth power of the resolution; one power for the timestep and then another power for each dimension," says Harrington.

"In squeezing the same number of grid points onto a smaller impactor, the size of each grid cell is reduced significantly, down to the equivalent of say a foot in length. As soon as you go down to this size while at the same time maintaining an extreme velocity, each computational step takes on order of 10-6 seconds - that's one millionth of a second. This means if you want to simulate one second of an impact you have to run the model for a million timesteps. You can begin to see how much processing power is needed," adds Palotai.

The researchers plan to model as much of the Chelyabinsk data as possible. Their work remains a critical piece of the preparedness puzzle and enables them to predict outcomes of both a catastrophic and benign nature. Participating in the Jovian Impact Research group are Travis Gabriel, Noémi Rebeli, Greta Chappell, Dylan Mueller, and Jarrad Pond. The US National Science Foundation and the NASA Planetary Atmospheres Program support the Jovian Impact study.

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