APOD: The Missing Craters of Asteroid Itokawa (2014 Feb 09)

[APOD Robot]

The Missing Craters of Asteroid Itokawa

Explanation: Where are the craters on asteroid Itokawa? Missing -- unexpectedly. The Japanese robot probe Hayabusa approached the Earth-crossing asteroid in 2005 and returned pictures showing a surface unlike any other Solar System body yet photographed -- a surface possibly devoid of craters. The leading hypothesis for the lack of common circular indentations is that asteroid Itokawa is a rubble pile -- a bunch of rocks and ice chunks only loosely held together by a small amount of gravity. If so, craters might not form so easily -- or be filled in whenever the asteroid gets jiggled by a passing planet or struck by a massive meteor. Recent Earth-based observations of asteroid Itokawa have shown that one part of the interior even has a higher average interior density than the other part, another unexpected discovery. The Hayabusa mission returned soil samples from Itokawa which are also giving clues the ancient history of the unusual asteroid and our entire Solar System.

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[cairns.kevin]

Looks like it is in an active erosion phase, with fine particles being stripped away first.

[TraKtorman]

Soil on an asteroid?

[Boomer12k]

So, it is possibly two uneven clumps that have merged...also...I think I see two craters on the far right end...but it could just be the lighting.

A "Scraggy Potato" to be sure. Very interesting.

:---[===] *

[Nitpicker]

It seems this asteroid is just not attractive enough.

[hownow]

It seems this asteroid is just not attractive enough.

Funny ... but, maybe literally true? How big is this asteroid? Bigger than a breadbox?

[Nitpicker]

It seems this asteroid is just not attractive enough.

Funny ... but, maybe literally true? How big is this asteroid? Bigger than a breadbox?

According to:
http://en.wikipedia.org/wiki/25143_Itokawa

it is (in round numbers) 500 x 300 x 200 metres.

[neufer]

So, it is possibly two uneven clumps that have merged...

One of StratStafford's, no doubt: http://www.huffingtonpost.com/2013/04/1 ... 54324.html

[orin stepanek]

Maybe a recent volcanic ejection that has been in space for only a few thousand years?

[BDanielMayfield]

It seems this asteroid is just not attractive enough.

Well, maybe not to you Nitpicker … oh-oh, you were talking about gravitational attraction, weren’t you?

Scientifically this is a very interesting object. Here’s a link showing photos from other angles: http://planetarynames.wr.usgs.gov/images/itokawa.pdf

It’s interesting because, being so small, it must have a very low surface gravity, which also must vary quite a bit across its irregular surface. So if it were to spin too fast or if it was hit by something with significant momentum (Mass x Velocity) it would disintegrate.

So, gravitationally speaking, how attractive is Itokawa?

Bruce

[neufer]

So, gravitationally speaking, how attractive is Itokawa?

Any animal that can jump up 4 inches on Earth
can jump off Itokawa into outer space.

[BDanielMayfield]

So, gravitationally speaking, how attractive is Itokawa?

Any animal that can jump up 4 inches on Earth
can jump off Itokawa into outer space.

Right. So this represents a size or class of objects that are just barely able to attract and hold onto smaller objects. I wonder, how did all the rubble in this "rubble pile" come together in the first place, since each piece on its own would have proportionally weaker attractions?

Bruce

[BDanielMayfield]

Maybe a recent volcanic ejection that has been in space for only a few thousand years?

Or maybe a few million years? Not an inconceivable idea Orin. It's on an Earth crossing orbit, so the logical question is could a super-volcano on Earth have launched pumice (low density rock) into solar orbit?

Bruce

[geckzilla]

How do asteroids form at all? The can't all have come from volcanic eruptions. I imagine most start small with the electrostatic forces pulling together clumps of dust.

[Chris Peterson]

Soil on an asteroid?

Sure. "Soil" is a term commonly used to describe the surface layer of the regolith on bodies other than the Earth.

[neufer]

How do asteroids form at all? The can't all have come from volcanic eruptions.
I imagine most start small with the electrostatic forces pulling together clumps of dust.

[b][color=#0000FF][size=100]Geckos can stick to walls because of Van der Waals forces[/size][/color][/b]

---

<<In physical chemistry, the van der Waals' force (or van der Waals' interaction), named after Dutch scientist Johannes Diderik van der Waals (23 November 1837 – 8 March 1923) , is the sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds, the hydrogen bonds, or the electrostatic interaction of ions with one another or with neutral molecules or charged molecules. The term includes:

force between two permanent dipoles (Keesom force)

force between a permanent dipole
and a corresponding induced dipole (Debye force)

force between two instantaneously induced dipoles (London dispersion force).

It is also sometimes used loosely as a synonym for the totality of intermolecular forces. Van der Waals' forces are relatively weak compared to covalent bonds, but play a fundamental role in fields as diverse as supramolecular chemistry, structural biology, polymer science, nanotechnology, surface science, and condensed matter physics. Van der Waals forces define many properties of organic compounds, including their solubility in polar and non-polar media.>>

<<Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.

A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger bodies. When the bodies reach sizes of approximately one kilometer, then they can attract each other directly through their mutual gravity, enormously aiding further growth into moon-sized protoplanets. This is how planetesimals are often defined. Bodies that are smaller than planetesimals must rely on Brownian motion or turbulent motions in the gas to cause the collisions that can lead to sticking. Alternatively, planetesimals can form in a very dense layer of dust grains that undergoes a collective gravitational instability in the mid-plane of a protoplanetary disk. Many planetesimals eventually break apart during violent collisions, as may have happened to 4 Vesta and 90 Antiope, but a few of the largest planetesimals can survive such encounters and continue to grow into protoplanets and later planets.>>

[Chris Peterson]

How do asteroids form at all? The can't all have come from volcanic eruptions. I imagine most start small with the electrostatic forces pulling together clumps of dust.

Asteroids aren't the product of volcanism. As you suggest, they started coming together just as all the planets did, because of electrostatic forces. Once you have particles in the centimeter range, gravity starts playing an increasingly important role. The asteroid belt is a region where resonances with Jupiter create zones of instability and metastability, so no planets formed, but some of the material that otherwise forms planets was left behind. That's one reason why investigating and sampling asteroids is so scientifically important.

[neufer]

Astronomy 201b: Interstellar Medium and Star Formation
Alyssa A. Goodman, Spring 2012-2013, Harvard College
Formation of planetesimals (Transcribed by Bence Beky)
More: here

<<How do tiny dust particles build up to create compact solids a kilometer in diameter? This is one of the major questions remaining in planet formation research and although much progress has been made, the first half of the book as yet to be written. Here, I will talk about some of the ideas for planetesimal formation.

Low collisions speeds, high binding energies and high energy dissipation during impact facilitate particle growth. The electrostatic and gravitational forces are both possible binding energies but the latter only become important when planetesimals are very massive. The electrostatic interaction involved is typically the van der Waals force; the possibility of charged grains has been discussed but not explored. A collision between two charged particles would have a higher binding energy, making sticking easier. An ice coating also helps!

Chambers (2010) discusses the laboratory observations that have helped us to understand this process, although it is difficult to know exactly what conditions were like in the protoplanetary disk. At very low speeds (much less than 1 m/s), collisions between micrometer-sized dust grains tend to result in the grains loosely sticking together, at intermediate speeds (on the order of 1-10 m/s) more compact aggregates are formed, while at high speeds the grains tend to rebound and growth does not occur. Now, we have conglomerations that have reached millimeter to centimeter sizes and the a different behavior is observed: a collision between a grain and a compact aggregate at speeds less than 10 m/s results in rebound, moderate-speed collisions result in growth of the aggregate, while collisions at high speeds can fragment it.

Growth of planetesimals gets increasingly difficult as sizes approach a meter because binding energies decline while relative velocities increase. According to Chambers (2010), with turbulence disruptive collisions between a meter-sized and a much smaller planetesimal are frequent because relative speeds of 100 m/s are often reached and it is difficult to get larger bodies. Youdin (2008) also discusses other issues (both theoretical and observation) with growing large bodies via collisions. Since we need kilometer-sized planetesimals to proceed with the second half of our story, we have a problem which is usually called the “meter-size barrier” in the literature.>>

[neufer]

Click to play embedded YouTube video.

[Anthony Barreiro]

Astronomy 201b: Interstellar Medium and Star Formation
Alyssa A. Goodman, Spring 2012-2013, Harvard College
Formation of planetesimals (Transcribed by Bence Beky)
More: here

<<How do tiny dust particles build up to create compact solids a kilometer in diameter? This is one of the major questions remaining in planet formation research and although much progress has been made, the first half of the book as yet to be written. Here, I will talk about some of the ideas for planetesimal formation.

Low collisions speeds, high binding energies and high energy dissipation during impact facilitate particle growth. The electrostatic and gravitational forces are both possible binding energies but the latter only become important when planetesimals are very massive. The electrostatic interaction involved is typically the van der Waals force; the possibility of charged grains has been discussed but not explored. A collision between two charged particles would have a higher binding energy, making sticking easier. An ice coating also helps!

Chambers (2010) discusses the laboratory observations that have helped us to understand this process, although it is difficult to know exactly what conditions were like in the protoplanetary disk. At very low speeds (much less than 1 m/s), collisions between micrometer-sized dust grains tend to result in the grains loosely sticking together, at intermediate speeds (on the order of 1-10 m/s) more compact aggregates are formed, while at high speeds the grains tend to rebound and growth does not occur. Now, we have conglomerations that have reached millimeter to centimeter sizes and the a different behavior is observed: a collision between a grain and a compact aggregate at speeds less than 10 m/s results in rebound, moderate-speed collisions result in growth of the aggregate, while collisions at high speeds can fragment it.

Growth of planetesimals gets increasingly difficult as sizes approach a meter because binding energies decline while relative velocities increase. According to Chambers (2010), with turbulence disruptive collisions between a meter-sized and a much smaller planetesimal are frequent because relative speeds of 100 m/s are often reached and it is difficult to get larger bodies. Youdin (2008) also discusses other issues (both theoretical and observation) with growing large bodies via collisions. Since we need kilometer-sized planetesimals to proceed with the second half of our story, we have a problem which is usually called the “meter-size barrier” in the literature.>>

In other words, "this is an active area of research." English translation: "We don't know. Please give us money and we'll put the grad students to work running computer models." At least we know with reasonable certainty that lots of asteroids exist, so they must have formed somehow!

[Chris Peterson]

In other words, "this is an active area of research." English translation: "We don't know. Please give us money and we'll put the grad students to work running computer models."

There is nothing that we know with 100% certainty. In this case, the better translation would be "We don't know with much certainty, although we have some good ideas that need more testing." The "please give money" part is quite acceptable as provided, however.

[geckzilla]

In other words, "this is an active area of research." English translation: "We don't know. Please give us money and we'll put the grad students to work running computer models." At least we know with reasonable certainty that lots of asteroids exist, so they must have formed somehow!

You take a box of dust on a hyperbolic flight and watch it yourself. I saw a video of a guy doing this but, frustratingly, I can't seem to find it.

[Chris Peterson]

You take a box of dust on a hyperbolic flight and watch it yourself. I saw a video of a guy doing this but, frustratingly, I can't seem to find it.

What happens with dust is fairly well understood. As noted, the complexity comes when you get up into the meter-sized range, since these bodies ought to have typical velocities that result in collisions breaking them up, not allowing for further aggregation. And that's a size range that is pretty much beyond our ability to experiment with, outside of computer modeling. Whatever happens is probably dependent upon conditions in the presolar nebula that aren't naturally found anymore.

[BDanielMayfield]

Um, there is now a "404 not found” error on the last “Itokawa” link, which I don’t believe had a problem earlier today.

I was seeking info on what the returned soil samples showed. It would be (and would have been from) quite a blast if the samples showed an Earthly volcanic origin. I realize that notion is extremely unlikely however, and that, as Chris said, this rubble is almost certainly from the asteroid belt.

Bruce

[BobStein-VisiBone]

...craters might ... be filled in whenever the asteroid gets ... struck by a massive meteor.

So what you're saying is, Asteroid Itokawa might show evidence of a crater after all. One that is bigger than Asteroid Itokawa.