INTRODUCTION

This article reviews and highlights portions of an article Deep Impact: Excavating Comet Tempel 1 written by the Deep Impact science team and published in Science as well as on-line by Science Express, 09/08/05. Here the focus is on results rather than the measurements and analysis needed to obtain them. This review describes: incidents relevant to the spacecraft, the ejecta that spewed forth from Tempel 1, the crater that was formed, Tempel 1's shape, and the topography of the surface.

GETTING THERE

Two interesting things happened to the impactor spacecraft on the way to Tempel 1's nucleus. One was that about 20 seconds before impact, the impactor collided with a dust particle. The impactor rotated so that its Impactor Targeting Sensor (ITS) was no longer pointed at the impact point. Fortunately, the attitude control system righted the spacecraft but ten seconds later the spacecraft collided with another dust particle and again the attitude control system corrected the spacecraft's orientation.

Fig A. Image of the surface about 30 sec before collision.

In addition, the optical quality of the last image of the impact point was degraded indicating that dust had abraded the lens of the Impactor Targeting Sensor. The effective resolution of the last image was about 3 meters per pixel.

The other interesting thing that happened was outbursts of activity from the comet nucleus. It is not unusual for a comet to do this. In the week before impact, the spacecraft's instruments observed periodic spikes in intensity at two separate areas on Tempel 1. As you might expect, the outbursts occurred at about local sunrise. Typically, the intensity rose rapidly over a 10 minute period and fell more slowly over an hour or more.

FLASHES AND THE PLUME

As the impactor entered the nucleus, or shortly thereafter, a brilliant flash, lasting less than two tenths of a second, appeared probably as the impactor and part of Tempel 1 vaporized. The first flash was followed by a second presumably originating deeper within the comet. The second flash was brighter still and it momentarily saturated some pixels in the instruments on the flyby spacecraft.

Fig B. Sequence of eight images depicting the development of the plume and ejecta curtain] shows the development of the ejecta curtain. Credit: NASA/JPL-Caltech/UM M. F. A'Hearn et al., Science 310, 258 (2005); published online 8 September 2005 (10.1126/science.1118923). Reprinted with permission from AAAS. Permission to reproduce.

Subsequently, an ejecta curtain of hot gas and dust emerged from the crater. The material in the curtain moved more slowly than material in the plume associated with the flashes.

As the curtain expanded out of the forming crater its peak brightness increased until it reached its maximum brightness after three to four seconds. Several bright rays are seen emanating from the somewhat asymmetric cone.

Material in the curtain, consisting of gas and dust, moved more slowly than the vaporized gas in the plume but nevertheless was traveling from five to seven kilometers per second. The gas consisted mostly of steam and carbon dioxide whose temperature was initially over 1000 degrees Kelvin. Room temperature is about 295 degrees Kelvin or 72 degrees Fahrenheit.

We didn't expect the success of one part of the mission (bright dust cloud) to affect a second part (seeing the resultant crater). But that is part of the fun of science, to meet with the unexpected. --Lucy McFadden

The solid component of the curtain consisted of fine dust about 1 to 100 micrometers in size. (Human hair ranges from 17 to 181 micrometers in size). Although tiny, there was a lot of dust and it significantly impeded the passage of light through the plume. The degree to which material impedes the passage of light through the column of the material is called its optical thickness, an often used term. The ejecta curtain became optically thin relatively slowly. Even after 13 minutes its optical thickness was still significant.

There was much more material in the curtain than expected indicating that the strength of the material lying within tens of meters of the comet's surface is very weak and powdery. Therefore the ejecting gas and dust are able to tear an unexpectedly great amount of material from the crater as it forms.

Tempel 1's gravity is not very strong and so it is not able to hold the ejecta back and so most of it is able to escape into space. However, some of the dust does fall back onto the surface of the comet.

Nevertheless the comet's gravity is strong enough, to prevent the last ejecta expelled from the crater from getting very far from the surface. Thus the bottom of the plume remains essentially in contact with the rim of the crater. It does not detach from the surface, a phenomenon that was looked for with great interest by astronomers. Astronomers say that the ejecta is gravity-controlled rather than strength-controlled.

THE CRATER

The depth and diameter of the crater are determined by several properties, mainly the strength of gravity at the surface of Tempel 1, and the strength (shear yield-stress) of the material comprising the crater wall. In the case of Tempel 1, the comet's gravity, although small, overpowered the shear strength of the ice-entrained dust that comprises its interior.

Estimates place the depth of the powdery surface layer at tens of meters. As the article in Science Express points out, one might calculate the size of the crater by making certain assumptions including that the density of the surface layers is the same as the density of the comet as a whole.

The article mentions that the shadow of the ejecta cone on the surface of Tempel 1 was greater than 300 meters across at its base even at early stages of excavation. That is much wider than expected. But a final size for the crater was not given as the team is still analyzing the data.

THE SURFACE REVEALED

Fig C. Composite of ITS images. Credit: NASA/JPL-Caltech/UM M. F. A'Hearn et al., Science 310, 258 (2005); published online 8 September 2005 (10.1126/science.1118923). Reprinted with permission from AAAS. Permission to reproduce.

Because Tempel 1 did not rotate a great deal during the observation period, only a bit more than half of the surface was observed. However, a variety of surface features were found. They include both smooth and rough areas, scarps e.g. steep slopes or cliffs, and circular features. The circular features are reported to be impact craters on the basis of the distribution of their sizes. The article reports that the impact craters have not been seen on other comets.

In addition to areas of varying roughness , there are two relatively large flat areas. A twenty meter high scarp partly borders one of them.

Fig D. Temperature Map of Tempel 1. Credit: NASA/JPL-Caltech/UM M. F. A'Hearn et al., Science 310, 258 (2005); published online 8 September 2005 (10.1126/science.1118923). Reprinted with permission from AAAS. Permission to reproduce.

How hot is it on Tempel 1? Researchers made a temperature map of the sunlit side. and found the hottest point nearly directly under the sun. It's 329 degrees Kelvin under the sun and 260 in the shade.

The coldest temperature is important because the temperature at which ices such as water, carbon dioxide and carbon monoxide turn directly into gas is below 200 degrees Kelvin.

As you might expect, Tempel 1 is in equilibrium with sunlight meaning that there are no internal heat sources as there are within some planets. And that the surface does not retain the heat from the sun for long, it heats up quickly and cools quickly.