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What is an impact crater?

An impact crater is a hole excavated out of a surface (e.g. a planet, moon, asteroid, or comet) when a smaller mass moving at very high speed collides with it.

Cratering - First Impressions.

In the days following the encounter with comet Tempel 1 Peter Schultz commented on his first impressions of the data from the impact. Read >>

Creating a Crater - What a Blast!

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How do impact craters form?

When an impactor strikes a target, it has a great deal of kinetic energy (proportional to the object's mass and the square of its velocity). For example, the impactor spacecraft for the Deep Impact Mission has a mass of 370 kg, and will be traveling at a velocity of 10.2 km/s. This means its kinetic energy will be 19 gigajoules (GJ), which is about the equivalent of the amount of energy released by exploding 4.8 tons of TNT, or about the amount of energy used in an average American house in one month.

Physics tells us that the total amount of energy is conserved when two bodies strike each other. The energy, therefore, can't be lost, but only transferred. The large amount of energy goes into making several things happen:

• Some of the material from both impactor and target will be melted or even vaporized (the impactor is destroyed during the impact, but only an immeasurably small amount of matter will be lost through conversion to energy) by the tremendous amount of heat generated by the impact
• A great deal of the energy and momentum will go into moving the material, part of which is driven downward, the rest of which will be ejected from the crater site
• A shock wave will pass through both the impactor and the target
• Some endothermic chemical reactions (ones which require energy) may be driven to occur, if they can happen fast enough, before the heat dissipates

Let's look at the stages in which a crater forms. The following descriptions really would better apply to a large impact on a planetary surface, like Earth or the moon. After a general description, we'll discuss how this may differ in Deep Impact's collision with a much smaller cometary target.

Crater formation takes place in three stages:

1. Compression Stage: During this stage, the impactor punches a (relatively) small hole in the target, and a shock wave begins to pass through the target. This is when the impactor's energy is converted into heat and kinetic energy in the target, as the pressure generated by the impact is so great that even solid material can act somewhat fluid, and flow away from the impact site. There is very little material ejected up and out of the forming crater during this stage, although a plume of impact-generated vapor rapidly expands above the crater. This stage is very quick, lasting an amount of time on the order of the impactor's diameter divided by its speed at impact (D/v). For Deep Impact, this stage will last only around (1 m / 10200 m/s) = 0.0001 seconds (100 microseconds).

 Christiansen, E.H., Exploring the Planets, 2/E, �1995. Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey.

2. Excavation Stage: During this stage, the shock wave begun in the compression stage continues outwards through the material. A very interesting part of this, however, is the fact that this wave spreads out from a point below the surface of the target. As a result, the wave actually spreads upwards from the impactor, and sends some of the target material up and out from the impact site. This material is referred to as the "ejecta." Initially the ejecta forms a plume of hot vapor, melt droplets and fine debris. Then a cone-shaped "curtain" of material spreads upwards from the impact site. Some or all of this ejecta will land in the area surrounding the crater, forming an ejecta "blanket." The crater itself grows very large very quickly during this stage, and material at the lip of the crater folds over creating a rim. Fractures often spread down into the target from the crater site as well. This stage is longer than the compression stage, lasting an amount of time roughly equal to the square root of the diameter of the impactor divided by the acceleration due to gravity from the target. For Deep Impact, this stage will last around 300 seconds.

3. Modification Stage: During this stage, loose debris from the impact will tend to slide down the steep crater walls. Some loosened material may slip in sheets, forming terraces along the crater sides. In some craters, a central peak may form as some of the target material splashes back upwards at the initial point of impact. This stage lasts about the same amount of time as the excavation stage, although of course the crater can be further modified by erosion, later impacts, lava flows or tectonic activity for millions of years afterwards depending upon conditions on the target. For Deep Impact, this stage is not very important, since the low gravity on the comet will probably only cause some small amount of collapse near the rim. There will not be the uplift that can be seen in larger craters, so there will be no central peak.

What will the Deep Impact spacecraft detect during this collision?

The compression stage of the crater formation is over so fast for the Deep Impact collision that it is very unlikely that there will be any data gathered from that, although the brightness of the flash may provide information about the surface materials. Most of the imaging and spectroscopy will occur during the excavation stage, and probably the early modification stage.

What exactly will be seen during the excavation stage is a matter of some debate, and is actually one of the questions that this mission will answer. There are some certainties. It is known, for example, that impacts at this velocity always form circular craters, so we know the crater will be circular. We also know that there will be some ejecta, since that follows along with the impact.

It is uncertain, however, what size and type of crater will Attributes. There are three likely scenarios that the crater formation can take. In the first situation, the crater formation is governed mostly by the gravity of the comet nucleus (known as a "gravity-dominated" process). In this case, the ejecta cone spreads outwards at an angle of around 45�-50� from the surface of the comet. The cone's base remains attached to the comet nucleus. The majority (roughly 75%) of the material will fall back down onto the surface of the comet, forming a large-diameter ejecta blanket. In this model, the crater may be as large as a football stadium (around 200 meters in diameter), and 30-50 meters deep.

The second possibility is that the more dominant resisting force of the crater formation is the strength of the material (known as a "strength-dominated" process). In this case, the ejecta cone will be at a higher angle (around 60�). The cone's base will detach from the crater, and may detach from the comet entirely. Less material (around 50%) will fall back to the surface of the comet in this scenario, yielding a smaller ejecta blanket. In this model, the crater will be much smaller, on the order of 10 meters or less. The predictions of the volume of ejecta produced then differ by roughly a factor of 1000 (103).

A third possibility is that the comet material is so porous that most of the impactor's energy and momentum are absorbed in the process of compression and heating (known as a "compression-dominated" process). Since so much energy is used in compression, there is less available for excavation, and results in a much smaller diameter crater than expected. The crater will be deep, but produce a very small ejecta cone.

The problem in determining the characteristics of the crater lie in the fact that it is uncertain exactly what a comet is made of. So, scientists have run simulating experiments on Earth in a laboratory, and attempted to scale upwards to the conditions to be experienced on the comet. They use this scaling to produce mathematical models of impacts using computers. Modeling with different target materials leads to these different possible scenarios.

Which is why this mission is so important. Since it is impossible to simulate this impact completely on Earth, it is impossible to be absolutely certain what will happen. But, after all, if we could predict it for certain, why would we be doing this? The point to an experiment is to learn information you didn't know before.

What will we learn from how the crater forms?

The cratering process will help reveal what type of material makes up the nucleus (or at least the outer layer), and therefore how the comet formed and evolved. If the crater turns out to be gravity-dominated, this lends evidence to the theory that the comet's nucleus consists of porous, pristine, unprocessed material, and that the comet formed by accretion. If, however, the crater turns out to be strength-dominated, then this suggests that the material of the nucleus is processed somehow, resulting in a comet which can hold together better under impact. This would mean that it is not the pristine, untouched material of accretion. It's also possible that the initial crater formation will be strength-dominated, suggesting a processed outer shell to the nucleus, but that the bulk of the crater is gravity-dominated, suggesting that the impactor has punched through this outer shell into the pristine material below.

Another hopeful outcome from the crater formation is that observing how the radius of the ejecta plume and the velocity of the plume base change over time will allow for an estimation of the nucleus material's density. Since the comet's volume is known, as estimate of density allows for an estimate of the comet's mass.

References:

• Christiansen & Hamblin, Exploring the Planets, Prentice-Hall, 1995 (pp. 76-77)
• Melosh, Impact Cratering: A Geologic Process, Oxford University Press, 1989 (pp. 46-47)
• Schultz, Anderson & Heineck, Impact Crater Size and Evolution: Expectations for Deep Impact, Lunar and Planetary Science XXXIII, 2002
• Housen, Does Gravity Scaling Apply to Impacts on Porous Asteroids?, Lunar and Planetary Science XXXIII, 2002

Written by Don Wiggins, Maryland Space Grant Consortium Summer Intern, Chemistry Teacher, Governor Thomas Johnson High School, Frederick, MD

Related Resources:

Videos:

 Pumice Impact Test (Top View) - High-speed image sequence viewed from above of a high velocity impact test into highly porous target of fine dust particles. Pumice Impact Test (Side View) - High-speed image sequence viewed from the side of a high velocity impact test into highly porous target of fine dust particles. (QuickTime, 1.08 MB) (MPEG, 628 KB) (QuickTime, 9.89 MB) (MPEG, 3.86 MB)

Designing Craters: What will it take to make the right kind of crater in Comet Tempel 1? And - what will it look like as we observe it?

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