More on Cosmic Snowflakes,
In previous studies of aerogel impacts, three types of impacts have been noted.
1.long carrot-shaped tracks, of the type stardust at home is looking for.
2.Shallow craters, of unknown origin, where the crater is lined with glazed glass from the aerogel and no impactor material is present. Only the aerogel.
3.Shallow craters, with orbital trash clearly visible.
The current interpretation of what causes these is an ultra-high velocity impact where all impactor material is ejected from the crater. That impact energy alone is the determining factor in these impacts, and density is not a factor. That the two types of impacts 1) and 2) represent a continuum of the stardust impactor, related to energy.
Problems with this interpretation is that the higher energy impacts are making shallower craters which would not be true of impactors of the same density. As and impactor contacts the low density aerogel, momentum is exchanged (MV) and a high density impactor has higher mass at the impact site and will always continue into the depths of the aerogel before coming into contact with enough aerogel mass to stop it.
Second, the lack of impactor residue. For there to be no impactor residue found in the glazed glass coating of the crater, all the material would have had to have exited the aerogel from the direction it came, and none of the material would have mixed with the melted aerogel glass. This would mean that a significant portion of the aerogel glass must have vaporized and also exited in the opposite direction from where the impactor came. It would be the only way to insure that some of the impactor material did not remain in the glazed glass crater material.
Third, the ultra high velocity impactor theory can be disproved by examination of the shallow impact crater density. Comparing the density, mass to volume ration of a shallow impact crater to that of a nearby sample of aerogel, one might expect a measurable mass loss of the shallow impact crater samples. If this is not the case, it would disprove the ultra high impactor model. Statistically, of the many samples I would think there would be some mass loss due to the large amount of SiO2 that would have been required to eject all heavier atoms of an impactor. Although this might still be debatable and require additional work to prove.
A much better interpretation of these impact is that they are Low Density Cosmic Snowflakes.
A Low Density Cosmic Snowflakes model matches the observed impact data much better than the ultrahigh velocity impactor continuum model. Although, impact energy is the primary factor in how much damage (cratering) will be done to the aerogel, it is not the only important factor. Density of the impactor is also important, as it affect the distribution of energy during the impact. Distributing the energy over a larger surface area would lead to shallower impact craters as the momentum (MV) exchange between the aerogel and the impactor will occur at or near the surface. One would expect a half sphere impact crater would result from an impactor with a density less than or equal to that of the aerogel. A symmetric impact with half the sphere missing.
Second, Cosmic Snowflakes would most likely leave residue Hydrogen (H) and Oxygen (O) in the glazed glass of the crater. But, Oxygen is a component of aerogel (SiO2) so it would be impossible to tell the Oxygen in the aerogel's SiO2 and impactor's H2O. That is, unless the Cosmic Snowflake contains a different proportion of Oxygen isotopes than the aerogel. Additionally, Hydrogen is used in the manufacturing process. So any residual Hydrogen in the aerogel might be mask the Hydrogen from the Cosmic Snowflake.
Third, it explains the lack of impactor material at the impact site if the Low Density Cosmic Snowflake was recently formed. Virgin, or relatively pristine and new Low Density Cosmic Snowflakes would not have time to collect additional heavier atoms within the object. If there were a large portion of heavier atoms material in the Snowflake, some of these would mix and remain within the glazed glass portion of the impact crater. This isn't the case, no material has been found so therefore the Low Density Cosmic Snowflakes must have formed very recently.
Speculation on Cosmic Snowflakes.
Everyone knows that water has many unique properties that makes it very special and different from most other matter in the universe. Many of these properties stem from the fact that the hydrogen atoms are not evenly distributed around the Oxygen atom. This unique distribution of hydrogen leads to an unequal distribution of charge of a water molecule. One end being positively charged, the other negatively charge. Electromagnetic charge is a force that is orders of magnitude greater than gravity, so would most likely be involved in the process of aggregation of single or small numbers of molecules collecting in interstellar space to form larger objects.
So, it might be expected that water with it's somewhat unique electrically charged properties might actually play a roll in the formation of larger objects, beginning with Cosmic Snowflakes, dirty Snowball, Stardust, comets, asteroids and planets. Cosmic Snowflakes and Stardust, two distinctly different objects, but the same object in a continuum leading to bigger things. The pristine, virgin Cosmic Snowflake would be right at the beginning of the entire process and that would be an ongoing process today.
The aerogel experiment itself is a good model for how things aggregate in interstellar space. If the existence of Cosmic Snowflakes can be proven, these Snowflakes would act very similarly to the aerogel. Forming a large low density Cosmic Snowflake vastly increases the probability that a given mass of water would be impacted by some smaller piece of Stardust. Just as the aerogel slows the particle, so would the Cosmic Snowflake. Instead of forming a crater of glazed glass, it would form a crater of glazed solid water in the impact crater. Capturing the impactor particles mass, while becoming more dense itself.
So, as everyone continues to look for the Stardust tracks, also look for surface impacts as they might be the Cosmic Snowflakes and Dirty Snowballs that lead to a better understanding of the origins of ...
References, with relevant text include on the subject of the shallow impact craters in aerogel
Orbital Debris Collector (ODC)
http://stardustathome.ssl.berkeley.edu/ ... e_id=37336
compared to figure 4,
shows a good image of a surface crater that is almost identical to the one found in the stardust aerogel
Morphology of Impact Features in Space Exposed Aerogel,
Macroscopic examination of all surfaces revealed the presence of various impact features: (1) classical carrot-shaped tracks, (2) relatively shallow pits that are poorly understood and have no experimental analogue, and (3) white flakes that are embedded into the aerogel surface.
Even first-order inspection of these collectors with the unaided eye revealed a wide variety of impact features, ranging from slender and deep penetration “tracks” to relatively shallow, hemispherical “pits”. The latter have no experimental equivalent [2, 3, 4]. We note that such pits were observed on earlier aerogel collectors exposed on EURECA [Brownlee, pers. comm.], as well as Shuttle [Westphal, pers. comm.], yet they remained largely undocumented. Tracks in the ODC collectors display typical aspect ratios of L/D > 20, with the length (L) measured from aerogel surface to track tip, and depth (D) is maximum track diameter at any depth below the surface. Pits, on the other hand, exhibit an L/D < 2. Features of 2 > L/D < 20 are present as well, uggesting that they may be transitional between the slender track and the shallow pit. In this report, we suggest that a morphologic continuum exists among all features, and thus, an evolutionary sequence that is largely controlled by impact velocity.
Figure E represents the typical, shallow pit with an almost hemispherical shape. We consider these morphologies to represent a continuum with the slender track and the shallow pit constituting the endmembers. The transitional nature between deep tracks and shallow pits is also manifested by subtle changes in the appearance of the cavity walls. The interior cavity walls vary from a feathered, micro-fractured and somewhat “dull” appearance, to highly translucent, as if glazed, the latter suggestive of melting. Most long cavities assume the feathered look at depth. The majority of pits have the ighly transparent, glazed interiors and no feathering. Some of the larger pits, however, may develop substantial, spike-like, concentric fractures, and is the reason Westphal [pers. comm.] refers to such features as “hedgehogs”.
We consider the diversity of impact features in space-exposed aerogel to be largely the product of impact velocity, rather than of projectile physical properties, such as density or compressive strength. This hypothesis rests on a number of arguments:
(1) The shallow pits are not low-velocity features. We recognize low-velocity impacts by co-orbiting human-waste particles on ODC. Such particles produce pervasive and characteristic microfracturing and crushing of aerogel; the resulting depressions also contain copious amounts of “projectile”.
(2) The shallow pits are not caused by low-density projectiles. We have extensive experience with low-density, “fluffy” impactors from experiments employing compressed cocoa powder at 3 to 7 km/s. Such impactors result in shallow depressions, but the latter contain numerous, parasitic tracks; their interiors are not glazed, and they contain copious amounts of projectile.
(3) The molten interiors of pits require high velocities. While we cannot specify the velocity, the molten interiors argue for higher velocities compared to unmolten, fractured and “feathery” cavity walls.
(4) The pits contain no impactor residue. We have analyzed numerous impact features produced by waste-water and all contain measurable K, Na, and Cl, as a minimum, and pure water-ice seems unreasonable. This leaves high velocity and associated vaporization of the impactor as the most plausible cause.
(5) The ODC pits have no experimental analog, despite considerable variability of experimental impact conditions in aerogel [2, 3, 4]. Typical, non-porous silicate or metal projectiles make deep tracks, akin to A and B, at 3 – 7 km/s. Conclusions: The above observations and arguments strongly suggest that the pits are the result of very high-velocity impacts and that the morphologic continuum of impact features in space exposed aerogel is largely a function of impact velocity. The pits are essentially the equivalent of space-produced micro-craters that do not contain projectile residues. As a consequence, we conclude that there is a velocity dependent limit in the utility of aerogel to soft-capture hypervelocity particles.