Despite all the observations of comets over the centuries and what we have learned from those observations, there is still much that we do not know.
Ray Brown, science writer at the University of Maryland writes about the science objectives of the Deep Impact mission and explains how the activity of making a crater in Comet Tempel 1 will lead to information to meet those objectives.
||What We Still Don't Know
|Comets have the most primitive, accessible material in
the solar system.
||We do not know what is hidden below the evolved surface
|Comets must become dormant.
||We do not know whether ice is exhausted or sublimation
|There must be many dormant comets masquerading as
||We do not know how to identify these
|We know more chemical and physical details than for
other small bodies.
||We do not know how to use these details to constrain
our models for comets.
|Abundance of gases in the coma is widely used to infer
the ices in the protoplanetary disk.
||We do not know the relationship between coma abundances
and those in the nucleus.
|Comets break apart under small stresses.
||Nothing is known about variation of material strength
The aim of Deep Impact is to answer as many of these questions as possible:
Where is the pristine material in comets?
Deep Impact's primary scientific theme is to understand the differences between the interior of a cometary nucleus and its surface. Cometary scientists are convinced, albeit on the basis of little or no observational data, that the surface layers of the nuclei are highly evolved. Numerous perihelion passages can lead to significant loss of ice from the outer-most layers; if comets are as porous as they are generally presumed to be, then this can lead to significant changes in the ice. Calculations by Prialnik and Mekler (1991), by Benkhoff and Huebner (1995), and by Klinger (1996), for example, all indicate that evolutionary effects are important at depths below one meter.
Image above: These models disagree on the depth of evolutionary effects (how deep the mantle will be) and predict opposite density gradients (what part of the mantle is denser). Image credit: NASA/JPL
Do comets lose their ice or seal it in?
It is thought that as many as half the near-Earth "asteroids" are actually dormant or extinct comets. There is clear evidence for dormancy or extinction of comets, but we have no way to be able to choose whether a comet is dormant (ice still present but unable to escape) or extinct (ice totally exhausted). We do not know whether the development of a mantle chokes off sublimation, thus trapping considerable amounts of ice in the interior, or if the mantle remains quite porous so that subsurface ice can sublime and escape freely. When the impactor makes a crater on the comet we will be able to determine dormancy or extinction. In the case of dormancy the impact will return the area to activity, whereas in the case of extinction no activity will be resumed.
What do we know about crater formation?
Crater growth rate and final crater morphology from the experiment will provide important clues for the nature of the upper surface of the comet. Laboratory experiments at hypervelocities reveal that the ejection angle depends on the porosity of the target: Highly porous materials result in higher angle trajectories (relative to the surface).
To best answer these questions, the Deep Impact team has created a list of science objectives that they must consider in the design and implementation of the mission:
1. Observe how the crater forms
2. Measure the crater's depth and diameter
3. Measure the composition of the interior of the crater and its ejecta
4. Determine the changes in natural outgassing produced by the impact