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
 |
Fig. 1: Map of Earth, showing the locations of observatories collaborating in the coordinated campaign (red dots).
Credit: NASA/ K. J. Meech et al., Science 310, 265 (2005); published online 8 September 2005
(10.1126/science.1118978). Reprinted with permission from
AAAS.
Permission to reproduce.
|
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.
 |
Fig. 2: Light curves obtained from the SCAM images (black) and from the spectral continuum (3.3 µm) in
individual spectra. All light curves show a rapid rise of intensity after impact. After its maximum, the peak spectral
intensity (red) falls rapidly to its preimpact value by UT 7:20. The total spectral intensity (blue) decays more slowly,
and the total coma intensity (black) plateaus.
Credit: NASA/ M. J. Mumma et al., Science 310, 270 (2005); published online 15 September 2005
(10.1126/science.1119337). Reprinted with permission from
AAAS.
Permission to reproduce.
|
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.
 |
Fig. 3: The Comet 9P/Tempel 1 as seen by the OSIRIS Narrow Angle Camera System on board ESA's Rosetta
comet-chaser spacecraft mission, on 30 June 2005, three days before the Deep Impact encounter. The distance between Rosetta
and Tempel 1 was 80 million kilometres. The stars seen in the image are elongated because the Rosetta spacecraft was
actively tracking on the moving comet while the image was acquired. Credit: ESA/OSIRIS consortium
|
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.
 |
Fig. 4: Number of impact-created CN molecules as a function of time. The data for apertures of different radii
(1 pixel = 31,200 km) are compared with models showing approximate lower (4 x 10 29) and upper limits
(6 x 10 29) for the number of parent molecules created at the time of impact.
Credit: NASA/ H. U. Keller et al., Science 310, 281 (2005); published online 8 September 2005
(10.1126/science.1119020). Reprinted with permission from
AAAS.
Permission to reproduce.
|
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.
References
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.
Mission Update Archive
