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Space Topics: Asteroids and Comets

Comet 9P/Tempel 1

Click to enlarge > The nucleus of Tempel 1 Credit: NASA / JPL-Caltech / UMD

Thanks to the Deep Impact mission, Tempel 1 is now one of the two best-studied comets in the solar system.  The other one is Halley's comet, famous for returning every 76 years to spread its tails across our skies.  Tempel 1 is not at all like Halley.  It's not big, or impressive, nor was it even well known before the Deep Impact mission.  Instead, Tempel 1 is a fairly typical member of the Jupiter family of comets, selected for study by Deep Impact because it was in a convenient location in the sky and was considered a representative comet.

Tempel 1's Orbit

Orbital Period: 5.51 years
Perihelion: 1.51 Astronomical Units (near the orbit of Mars)
Rotation rate: 1.71 days (unusually slow for a comet)
Due to close approaches to Jupiter, Tempel 1's orbit changes frequently.

What Deep Impact Learned About Tempel 1

Basic Facts

The results of Deep Impact's observations of Tempel 1 generally agreed with what was published about Tempel 1 before the encounter.  Even after the encounter, though, many of these numbers describing Tempel 1's physical state have big error bars on them.  It would take an orbital mission like Hayabusa or NEAR to produce less uncertain measurements.  One of the values that differed most from before the encounter to after the encounter was the size of the nucleus.  Before the encounter, it was thought that Tempel 1 was very elongated, like Itokawa.  But the pictures showed clearly that Tempel 1 is lumpy but not elongated.

Size: About 7.6 by 4.9 kilometers (4.7 by 3.0 miles), or an average diameter of 6.0 +- 0.2 kilometers (3.7 +- 0.1 miles)
Local force of gravity: 50 +34/-25 milligals
Mass: 7.2 x 10¹³ +4.8/-3.8 x 10¹³ kilograms (1.6 x 10¹⁴ +1.1/-0.84 x 10¹⁴ pounds).  For comparison, Tempel 1 loses about 10⁹ kilograms of mass every time it approaches the Sun, or about 0.001% of its total mass.  For another comparison, the impact tossed about 10⁶ kilograms of material off the comet, 0.000001% of its total mass, or 0.1% of the amount that it loses during every 5.5-year orbit.
Density: 670 +470/-330 kilograms per cubic meter (in other words, it could be anywhere from 340 to 1,000, which is less dense than water ice)

What Makes Up the Surface of Tempel 1?

Click to enlarge > Map of Tempel 1 This map of tempel 1 was composed of many images from both the flyby and impactor spacecraft. The resolution of the image is higher near the impact site (large white arrow). Arrows "a" and "b" point to unusually smooth regions. The tiny arrows point to a scarp at the edge of smooth region "a." The scale bar at lower right represents 1 kilometer (0.6 miles). The arrows at the upper right point to the Sun and celestial north. Credit: NASA / JPL-Caltech / UMD

The pictures of Tempel 1's nucleus show a lumpy-shaped object whose surface doesn't look very much at all like the other comets that have been seen up close (Wild 2 and Borelly).  The surface seems to be divided into several "facets," relatively flat areas that meet at mountainous angles. 

Some of these facets are covered with circular markings.  The Deep Impact science team has mapped the locations and sizes of the circular markings and concluded that their range of sizes compares well with the range of sizes one would expect if the circular markings were impact craters.  Lots of bodies in the solar system are covered with impact craters, but no comet has ever been known to show a set of circular features that could be impact craters.  Wild 2 also has depressions on it, but the ones on Wild 2 have a much more uniform size than one would expect if they were impact craters.

Other facets are very, very smooth.  Some of the smooth areas have at one side a steep cliff or "scarp," 20 meters (66 feet) high.  It's hard to construct a cliff like that except through some process of slumping or removal of material.  The Deep Impact science team says that the scarp looks like it was made by the removal of a 20-meter layer of material, "exhuming" (or unburying) a previously hidden surface.  But what could make the surface so smooth to begin with -- and what process would remove the material -- are not clear.

Although the pictures show some areas that look brighter than others, in fact Tempel 1 is unbelievably uniformly dark and gray.  Everywhere on its surface, even in the "brightest" spots observed by Deep Impact, it reflects no more than about 6% of the light that hits it; on average only 4% of the light is reflected.  In fact, despite the fact that comets are supposed to be icy bodies, Deep Impact saw absolutely no evidence for any ice on the surface of the nucleus.  Instead, it appears to be covered with very, very fine, dark dust.  Individual dust particles are only 0.5 to 1 micrometer (20 to 40 millionths of an inch) in diameter.

Click to enlarge > Temperature map of Tempel 1 Temperature maps of Tempel 1 were acquired throughout Deep Impact's approach to the comet using its infrared spectrometer. The hottest spot is the sub-solar point, and the coldest spots are in shadow. The correlation of insolation to temperature suggests that Tempel 1's thermal inertia -- its ability to conduct and store heat -- is very low. Source Credit: NASA / JPL-Caltech / UMD

Click to enlarge > Deep Impact: Pre- and post-impact spectra of Tempel 1 The X axis shows wavelength from 2 to 4.5 microns, (near- to mid-infrared). The Y axis is radiance; the higher the peaks, the more photons were measured at that wavelength. The red line represents the spectrum of the comet before the impact happened. It's very flat with very few features in it, trending "uphill" toward the right because it's a black body radiating dully in the thermal infrared, which is to the right, off of the graph. The green line represents the spectrum of the comet and ejecta after the impact. Each peak is a "spectral feature," which a spectroscopist can read like a book. Peaks at certain wavelengths -- even the shapes of peaks -- can identify certain molecules or atoms. The blue line is a first attempt at a "model fit." The "model" consists of a weighted average of spectra of the individual component materials that one would expect to find in the comet. The fit to the observed spectrum is good in the region she marked "H2O" (water) and "CO2" (carbon dioxide). But a middle hump, marked "C-H," has no corresponding hump in her model. There are so many different compounds that contain C-H bonds that could be responsible for part of the radiance in that peak that the problem is too poorly constrained, and the spectroscopist chose not to attempt to model the composition of the comet from that data. Credit: NASA / JPL-Caltech / UMD

Click to enlarge > Context image for Tempel 1 spectral measurement This context image shows Tempel 1 from one of the cameras. The blue dots represent the line along which the spectrometer was taking measurements. The point at which the above spectrum was captured is marked in red. It was looking at plain ordinary comet material, but milliseconds after the impact, a puff of superhot ejecta wandered into its field of view. Credit: NASA / JPL-Caltech / UMD

That doesn't mean that Tempel 1 doesn't have any ice; there's plenty of ice visible in Tempel 1's coma.  And Deep Impact found that there is a good reason that no ice is visible on the surface.  Using an infrared camera, Tempel 1 mapped the temperature of the visible surface.  The temperature was up to 329 Kelvin (56 Celsius, 133 Fahrenheit) on the sunlit side, and was as cold as 260 Kelvin (–13 Celsius, 9 Fahrenheit) on the nighttime side.  So, everywhere that Deep Impact could see, the comet was hotter than the temperature at which ices of water, carbon dioxide, and carbon monoxide would sublimate.  In other words, any ice present at Tempel 1's surface would quickly turn into a gas.  It's only at some depth below the surface that temperatures stay cold enough all the time that the Sun can't heat the comet enough to evaporate the ice.

What's Inside Tempel 1?

The main point of the Deep Impact mission was to excavate material from the interior of the comet—material that could possibly represent pristine components left over from the formation of the solar system.  Deep Impact did succeed in making a spectacular impact and throwing material from the surface, as well as some material from the inside.  But figuring out which material is from the surface, which is from the inside, and what it's all made of is a puzzle that could take years to solve.

The coma of Tempel 1 was observed for a long time from Earth and space before the impact, and it contained gases that are typical for comets: simple compounds like water, hydrogen cyanide, and carbon dioxide.  Remote observations also caught the telltale signs of more complex materials: spectral emissions from the carbon bonds found in organic chemicals (CH, C₂ , and C₃ ) and nitrogen-containing compounds (NH₃ and CN).

After the impact, the chemical signature changed.  Suddenly, many more complex compounds could be discerned within the spectral measurements.  Deep Impact saw the amount of water and carbon dioxide go up by a factor of 10.  And the amount of organic material that Deep Impact saw went up by a factor of 20.  Deep Impact scientists have not yet figured out which organic materials were present.  They could include formaldehyde (H₂CO), methanol (CH₃OH), and/or methyl cyanide (CH₃CN).

In fact, Deep Impact's spectrometer instrument won't be able to solve the problem on its own.  But the impact was thoroughly observed by most of Earth's large telescopes, some of which were equipped with instruments that should eventually be able to answer these questions.  Telescopes in Hawaii, including the Keck II and Gemini North observatories on Mauna Kea, were uniquely positioned to watch the impact itself (most other telescopes were on the wrong side of Earth to see Tempel 1 during the impact).

The Keck II telescope on Mauna Kea saw the same burst of organic materials that Deep Impact did and was able to make a few firmer identifications of what chemicals they were.  They saw methanol and methyl cyanide, along with acetylene (C₂H₂), ethane (C₂H₆), and possibly methane (CH₄).  Gemini North telescope was focused on Tempel 1's dust and saw a burst of silicate minerals including olivine and pyroxene, two of the most common rock-forming minerals on Earth and all the other terrestrial planets.  To all observers, the burst of new species subsided quickly, leaving Tempel 1 looking pretty much the same a few days after the impact as it did before the impact.

What was most surprising about the impact was the fluffiness and dustiness of the material that was excavated.  That fluffiness may mean that Tempel 1 really hasn't changed a lot since it first coalesced from tiny, fluffy dust particles in the solar nebula, and that Deep Impact found what it was looking for: a relic from the formation of the solar system.