Deep Impact revealed hidden comet cavities
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NASA's "Deep Impact" blast revealed that hidden cavities filled with pressurized gas hid within the comet Tempel 1. 
In 2005, an 816-pound impactor smacked into the 4.7-mile-wide comet at 23,040 miles per-hour. NASA's Stardust mission recently flew by the comet to see that the crater had ended up measuring about 190-feet wide.
The blast threw up a cloud observed by the Deep Impact spacecraft and 80 observatories on Earth, which obscured a close view of the impact crater, notes the new study led by Sergei Ipatov of Catholic University in the Monthly Notices of the Royal Astronomical Society.
The study looks at the speed of the particles ejected from the comet after the impact to discuss the interior properties of Tempel 1.
Results of our studies showed that there was a local maximum of the rate of ejection at (time) ~ 10 seconds with typical projections vp of velocities on to the plane perpendicular to the line of sight of about 100–200meters per second (220 to 440 mph). At the same time, the considerable excessive ejection in a few directions (rays of ejecta) began, there was a local increase in brightness of the brightest pixel and the direction from the place of ejection to the brightest pixel quickly changed by about 50?. In images made during the first 12 s and after the first 60 s, this direction was mainly close to the direction of the impact.
Noting that the outburst after the impact lasted more than a half-hour, the study concludes the impact unearthed cavities containing chemicals capped by the comet's crust.
The outburst ejection could have come from the entire surface of the crater, while the normal ejection was mainly from its edges. The 'fast' outburst could be caused by the ejection of material from the cavities that contained the material under gas pressure. The 'slow' outburst ejection could be similar to the ejection from a 'fresh' surface of a comet and could take place long after the formation of the crater.
Analysis of observations of the DI cloud and of outbursts from different comets testifies in favour of the proposition that there can be large cavities, with material under gas pressure, below a considerable fraction of a comet's surface. Internal gas pressure and material in the cavities can produce natural and triggered outbursts and can cause splitting of comets. The upper edge of the cavity excavated at te ~ 10 s could be located a few metres under the surface of Comet Tempel 1.
Other comets similar to Tempel 1 may contain such cavities, the study suggests.
See photos of: NASA
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