Mission Update - January/February 2006
Water Ice Found on a Small Portion of the Comet's Nucleus
by Lucy McFadden
In a paper appearing in Science Express on Feb. 2, 2006, an article by Sunshine et al. reports on the Deep Impact science team's finding of a small area of water ice on the surface of Tempel 1. This is the first time that water ice has been observed on the surface of a comet. Past efforts with the near-IR spectrometer on Deep Space 1 mission flying past comet Borrelly and from the ground of comets far from the sun and not enshrouded with coma, have yielded no evidence of water ice on their surface.
As a comet approaches the Sun, it releases gas and dust in its immediate vicinity forming the coma and obscuring the nucleus from view unless spacecraft can get at close range. Deep Impact did just that. Imaging with the two cameras, the HRI and MRI showed small regions that were about 30% brighter than surrounding areas. After scaling the images to an average value of the nucleus, three discrete areas on the nucleus are brighter in the ultraviolet and darker in the near-infrared. When Co-Investigator Dr. Jessica Sunshine looked at the spectra in that region, after subtracting a thermal component, what was left was the spectral signature of water ice, in the form of absorption bands at 1.5 and 2.0 µm. Absorption bands at these wavelengths are diagnostic of water ice. The combination of the relative colors and the spectra make a powerful case that there is water ice at these specific locations on Tempel 1.
Given that the spectrometer has a two dimensional detector, it is possible to make a map of Tempel 1 at the wavelength of the ice absorption bands. That map shows that the bright regions in the UV are correlated with dark regions in the near-IR where water ice absorbs light. Since the visible images have a higher spatial resolution, we use those images to calculate the extent of ice on Tempel 1's surface. That turns out to be a small fraction of the surface, only 0.5%. Next, the temperature map is combined with the color map, showing that two of the three regions are colder regions of the nucleus. Stereo images show the largest area of ice to be a depression 80 meters below surrounding areas. Never the less, the temperatures in this region are 285 -295 K, significantly above the ~200K at which ice would sublimate in space at the location of Tempel 1.
What is significant is that the extent of this ice on Tempel 1's surface is not sufficient to produce the observed abundance of water and its by-products in the comet's coma. The team thus concludes that there are sources of water from beneath the comet's surface that supply the cometary coma as well.
Also important is that the particle size of the water ice, is greater than the icy grains in the coma, and is probably recondensed onto the comet's surface. It is therefore probably not a primary block of cometary material which would be called a cometesimal.
Science Results, the View from Ground and Space
by Ray Brown
Introduction
Is it all over but the shouting? Definitely, not. It is time to use the massive amount of data relayed back to Earth from the Deep Impact instruments. Analysis will likely go on for years.
This article is a digest of a set of papers prepared by the Deep Impact science team and collaborators and published in a special section of the October 14, 2005 issue of Science. Here we focus on background science, results and conclusions rather than data or analytical methods. In particular we study the results relating to our search for primordial ices.
K. J. Meech et al. present a summary of observations made world-wide and in space. They point out that observations began in 1997 and continued into 2005, and that, in 2005 alone, the campaign claimed the attention of 73 Earth-based telescopes at 35 observatories in addition to observatories based in space. It was "an unprecedented coordinated observational campaign."1
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Before, During and After with the Keck 2
The telescopes in Hawaii had the second best seat (after the flyby spacecraft) for watching the encounter. In their article, M. J. Mumma et al.2 describe their observations made with the 10 meter Keck 2 telescope high atop Mauna Kea in Hawaii.
As the stream of ejecta emerged from the crater its intensity increased due to the outflow of dust. and also its ability to emit light. The intensity of the total coma rose quickly for about 40 minutes after which it rose slowly and leveled off toward the end of observing time, about 2 hours after impact, see top curve in Fig. 2. Peak intensity of the coma was measured close to the nucleus. It also rose quickly but only for 15 minutes after which it declined to its pre-impact level about 90 minutes after impact, see red curve in Fig. 2.
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When scientists analyze spectroscopic data they often look at two quantities derived from their data. The column number is the total number of molecules in a column viewed by the spectrometer. The relative abundance of a molecule with respect to water is the column number of a molecule of interest divided by the column number of water. Relative abundances are important because they provide a way to compare conditions at different times, before impact with after impact for example.
The high dispersion spectrometer on the Keck 2 telescope captured the spectra of eight gases in the ejecta: water, ethane, hydrogen cyanide, carbon monoxide, methanol, formaldehyde, acetylene and methane. The chemical symbols for these are respectively H2O, C2H6, HCN, CO, CH3OH, H2CO, C2H2, and CH4.
Only water, ethane, methanol and hydrogen cyanide were measured both before and after impact. After impact the abundance of methanol and hydrogen cyanide remained unchanged. However, the abundance of ethane was enhanced either by a factor of 1.8 or 3.0 depending on the method of analysis. The different values may be due to pre-impact outgassing being computed in different ways.
Three molecules have low sublimation temperatures and are therefore likely to be less prevalent in the nucleus. They are carbon monoxide, methane and ethane. It is suggested that because two of these, carbon monoxide and methane, were not measured in the coma before impact, they have been sublimated out of the near surface region by the heat of the sun. Keck, on the other hand, measured ethane before impact, and in greater abundance, after impact.
The View from Space, Rosetta, OSIRIS
On July 4th, ground based telescopes, large and small, all over the world were trained on Tempel 1. Meanwhile, the OSIRIS cameras on board the Rosetta spacecraft also recorded the impactor's collision. Rosetta is a European Space Agency craft en route to comet 67P/Churyumov-Gerasimenko. Results are reported in an article by H. U. Keller et al.3 Although the OSIRIS cameras also observed other gas components of the coma and ejecta, the report in Science focuses largely on the radical CN.
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A main objective of the Deep Impact mission is to discover differences between the near surface composition of Tempel 1 and the composition of its interior. To accomplish this, scientists on the Rosetta team analyzed the production of the radical CN. CN is a negatively charged ion based on a carbon atom and a nitrogen atom. It is produced when a more complex molecule, its parent molecule, decomposes. HCN, hydrogen cyanide, is probably a parent molecule of CN. Fig. 4 shows how the number of CN molecules rises with time.
CN molecules vs. time, from OSIRIS data.
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Through a series of steps that estimate the rate at which CN and HCN molecules are produced, the Rosetta team was able to estimate the total number of molecules of both CN-parents and CN itself. The number of water molecules was also determined and the ratio of CN parent-molecules to water molecules computed for conditions before, during and after the impact event. The ratios suggest there was a greater abundance of CN parent molecules in the impact produced ejecta cloud than in Tempel 1's coma prior to impact. That can be interpreted to mean that while the comet's normal outgassing originates near the surface, Deep Impact has excavated HCN-enriched material from farther below.
Origins and Classifying Comets
A clear, concise description of comet formation and distribution in the solar system appears in the article by Mumma et al. According to current thinking, there is a disk of comets and other objects lying in the plane of the solar system that flares out until it becomes a spherical shell enclosing the entire solar system. The disk, called the Kuiper belt, begins at about the orbit of Neptune and extends out for several hundred astronomical units, where an astronomical unit is about 93 million miles. Pluto is considered by some to be the innermost Kuiper belt object. The shell, called the Oort cloud, is humongous. It occupies, roughly, the region from 10 thousand to 50 thousand astronomical units from the sun.
We now turn our attention to how and where comets formed. It may be that gravity caused unevenly distributed gas and dust grains in a giant molecular cloud to draw together. Gases, such as water vapor, and carbon dioxide froze to become the pristine ice that we seek so much. The freezing ices coated colliding dust grains and cause them to stick together forming clumps of ice and dust. Then the frozen clumps, collided to form small comets called cometesimals, which in turn collided and form the large comets that we see today. We would very much like to know in what region or regions of the solar system comets formed. Current thinking is that some comets, such as Tempel 1, formed in the region beyond where Jupiter orbits today, were perturbed into the Kuiper belt and the Oort cloud by the gravity of the major planets and then perturbed again back into the inner solar system.
After comparing abundances of molecules in Tempel 1, Mumma et al. suggest that, to paraphrase slightly, Tempel 1 and most comets in the Oort cloud formed in the same region of the protoplanetary disk. They add that their suggestion is consistent with the idea that comets that have been perturbed into the Kuiper belt and Oort cloud comets originated in the outer giant planets' region of the protoplanetary disk.
We can classify comets by the reservoir from which they entered the inner solar system, Oort cloud or Kuiper belt. We can also classify them by the characteristics of their orbits. For example, Jupiter family comets can be classified by how close they come to the sun and how they behave when encountering Jupiter. When classified in this way, Tempel 1 is a Jupiter-family comet. However, Meech et al. point out that some molecules excavated by the impact have abundances consistent with abundances of typical Oort cloud comets. Those molecules are water, ethane, methanol, acetylene and hydrogen cyanide.
1. K. J. Meech et al., Science 310, 265 (2005); published online 8 September 2005 (10.1126/science.1118978).
2. M. J. Mumma et al., Science 310, 270 (2005); published online 15 September 2005 (10.1126/science.1119337)
3. H. U. Keller et al., Science 310, 281 (2005); published online 8 September 2005 (10.1126/science.1119020)
Deep Impact Wake-Up Status
by Mike A'Hearn
Dear DI Science Team,
This is just to let you know that Jenn Rocca called me yesterday evening [Feb 9] to say that the initial reports from the DI wake-up activity were looking good. She then sent out during the night her Flight Director's Report. Everything they have tested has been fully functional and in the states that were expected. Thus the spacecraft seems healthy for an extended mission.
This morning they will bring the spacecraft to point state, i.e. bring it out of the safe mode that it has been in since August.
