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  • richardmitnick 2:38 pm on December 5, 2017 Permalink | Reply
    Tags: , , , , Earth and the Moon, Planetesimals,   

    From SwRI: “Collisions after Moon formation remodeled early Earth” 

    SwRI bloc

    Southwest Research Institute

    Dec. 4, 2017
    Jonathan O’Callahan

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    SwRI scientists modeled the protracted period of bombardment after the Moon formed, determining that impactor metals may have descended into Earth’s core. This artistic rendering illustrates a large impactor crashing into the young Earth. Light brown and gray particles indicate the projectile’s mantle (silicate) and core (metal) material, respectively. Courtesy of Southwest Research Institute.

    A study has suggested that the young Earth was repeatedly pounded by objects the size of the Moon, which may explain the composition of rocks on our planet.

    Published in Nature Geoscience, scientists from the Southwest Research Institute in Texas looked at the period after a Mars-sized body hit Earth and formed the Moon about 4.5 billion years ago, known as the giant-impact hypothesis. That impactor was thought to be at least 6,000 kilometers (3,700 miles) across.

    Some of the pieces of rock from that collision, known as planetesimals, coalesced into the Moon. Others, we had thought, stayed in Earth orbit for about 100 million years before breaking apart or being scattered by gravity.

    However, this study suggests a much more dramatic process took place. The researchers say their model hints at “multiple subsequent impacts with the Earth by 1,500- to 3,000-km-diameter [930- to 1,860-mile] projectiles”, they write in their paper.

    “This is more violent than thought,” the study’s lead author, Dr Simone Marchi, told IFLScience. “Some of these planetesimals may have exceeded 1,000 kilometers [620 miles] in diameter, some were perhaps as large as the Moon itself.”

    We’d previously thought about 0.5 percent of our planet’s mass was made up of material from these planetesimals. However, the researchers suggest this may be two to five times greater than previous calculations.

    It all stems around something called siderophile elements. These are things that get absorbed into iron like gold, platinum, and iridium. Some of these were delivered to our planet after the Moon was formed, while others were either absorbed into our core or ejected into space.

    In order to explain the amount we observe today, we need more collisions. Thus, this paper points to the period after the Moon’s formation as the culprit, with more large planetesimals hitting Earth.

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    Animation of a Moon-sized object hitting our planet. Southwest Research Institute

    “We modeled the massive collisions and how metals and silicates were integrated into Earth during this ‘late accretion stage,’ which lasted for hundreds of millions of years after the Moon formed,” Dr Marchi said in a statement. “Based on our simulations, the late accretion mass delivered to Earth may be significantly greater than previously thought, with important consequences for the earliest evolution of our planet.”

    This also helps solve another quandary. Namely, the presence of isotopic anomalies in some rocks on Earth had suggested that our mantle was mixed more than we thought after the Moon formed. This latest research could explain how that mixing occurred, as our planet was repeatedly hit by other impactors.

    See the full article here .

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    SwRI Campus

    Southwest Research Institute (SwRI) is an independent, nonprofit applied research and development organization. The staff of nearly 2,800 specializes in the creation and transfer of technology in engineering and the physical sciences. SwRI’s technical divisions offer a wide range of technical expertise and services in such areas as engine design and development, emissions certification testing, fuels and lubricants evaluation, chemistry, space science, nondestructive evaluation, automation, mechanical engineering, electronics, and more.

     
  • richardmitnick 6:30 am on June 17, 2017 Permalink | Reply
    Tags: , , , , , Planetesimals, , Watermelon Dust is the Best Dust: Forming Planetesimals Near the Snow Line   

    From astrobites: “Watermelon Dust is the Best Dust: Forming Planetesimals Near the Snow Line” 

    Astrobites bloc

    Astrobites

    June 16, 2017
    Michael Hammer

    Title: Planetesimal Formation near the snow line: in or out?
    Authors: Djoeke Schoonenberg and Chris Ormel
    First Author’s Institution: Anton Pannekoek Institute for Astronomy, University of Amsterdam
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    Status: Published in A&A [open access]

    How is it possible for planets to exist? Even though we know planets must have formed from planetesimals that are tens of kilometers in size, the most basic models of protoplanetary disks have trouble forming planetesimals from the micron to centimeter-sized dust that populates these disks. For dust particles to grow into planetesimals, they need to be able to clump together enough to reach roughly the same level of concentration as the gas in the disk – which can be difficult since there is 100 times more gas than dust.

    A few months ago, I wrote an Astrobite describing a simple model that naturally achieves the conditions needed to form planetesimals in the inner disk, thereby offering a way to form planets in the inner solar system like Earth and Mars. However, this model leaves out the gas giant planets in the outer solar system like Jupiter!

    Fortunately, beyond the snow line at about 2 AU, the disk will get cold enough for water vapor to condense into solid ice. It is already widely accepted that this extra ice will enhance the concentration of solids enough to form planetesimals and in turn, planets in the outer disk. Yet, the most detailed models of this scenario leave out enough relevant effects that we cannot reliably determine the impact of this extra ice.

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    Rough sketch which shows the sharp increase of solid surface density at the snow line, by a factor of ~3-4. [https://ay201b.wordpress.com/the-snow-line-in-protoplanetary-disks/]

    Djoeke Schoonenberg and Chris Ormel, the authors of today’s paper, set out to improve our understanding of whether the snow line can trigger the formation of planetesimals by creating a more rigorous model that better captures the dynamic structure of the disk and of the dust grains themselves.

    Model Upgrades with a Side of Fruit

    Schoonenberg and Ormel develop a steady-state model of how the ice beyond the snow line evaporates as it moves inwards and how some of the water vapor inside the snow line condenses as it moves outwards.

    My favorite part of their model is that they factor in the structure of individual “icy dust” grains. Many studies of protoplanetary disks often leave out what the mass in the disk is made of – instead, only tracking a distribution of density across the disk or nondescript particles. However, not only do Schoonenberg and Ormel describe each icy dust grain as 50% silicates (the typical composition of a regular dust grain) and 50% ice; but they also establish how the silicate “seeds” in each grain are divided up. As shown in Figure 1, a dust grain can either have a single silicate core like an avocado (their “single-seed” model), or it can have many smaller seeds evenly distributed through the grain like a watermelon (their “many-seeds” model).

    The authors run separate models for each type of dust grain. They then solve for the enhancement of ice and the solid-to-gas ratio to see if these values reach a high enough concentration to form planetesimals.

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    Figure 1. Structures of icy dust grains. In the avocado model, each grain has a single silicate core surrounded by a shell of ice. In the watermelon model, each grain has a bunch of smaller silicate seeds evenly distributed through the ice. When an avocado dust grain evaporates, it slowly loses only its ice before leaving its core behind. When a watermelon dust grain evaporates, it slowly loses both its ice and its silicate seeds together as it drifts inward. Adapted from Figure 1.

    Forming Planetesimals Early and with Watermelon

    Besides varying the structure of the dust grains, the authors also experiment with different disk viscosities, dust particle sizes, and a few other variables. In particular, they find that higher viscosities are best-suited for producing planetesimals. Since these higher viscosities are more likely to occur early on in a disk’s lifetime, this suggests that disks can form planetesimals right away!

    In the single-seed “avocado” model with the optimal higher viscosities, Schoonenberg and Ormel find that the concentration of ice can be enhanced by a factor of 3 to 5 just beyond the snow line (see Figure 2). Interestingly with the many-seeds “watermelon dust” model, they find that this enhancement can double! This occurs because in the single-seed model, only the extra ice from inside the snow line contributes to the enhancement. However, in the watermelon model, the small size of the watermelon seeds plays a key role. Since the seeds are so small, they get carried by the gas that makes up most of the disk. When some of the gas from inside the snow line moves outward, some of the watermelon seeds follow it outward. As a result, many of these watermelon seeds end up captured in icy dust grains and also contribute to the extra ice, which doubles the enhancement.

    Ultimately, the authors expect the ice enhancement in an actual disk to be in-between the results from the avocado and watermelon models due to the fact that both structures of dust grains are plausible, and also because other dust grains may be a “hybrid” of both fruits and have a large silicate core as well as additional smaller silicate seeds in the outer icy shell.

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    Figure 2. Solid-to-gas ratio near the ice line (blue vertical line). Solids are ice plus dust. The many-seeds model safely exceeds the threshold (orange horizontal line) to form planetesimals. Since the amount of solids is just an average at a given radius, the single-seeds model should also be able to form planetesimals. Adapted from Figure 5.

    Summary

    The ice enhancement of about ~7 that the authors find is ten times lower than the enhancement of 75 found with a simpler model, emphasizing the importance of considering the more intricate details of the problem! Thankfully, this enhancement is still high enough for the concentration of large dust grains to reach the concentration of gas and produce planetesimals (see Figure 2). More importantly, the authors find that the best conditions for forming planetesimals happen soon after a star and its disk are born, supporting observational evidence that giant planets in the outer disk can form quickly.

    Lastly, Schoonenberg and Ormel expect the cm-sized ice pile-up near the snow line to be detectable by radio (cm-wavelength) telescopes. Seeing this feature in a real disk would be the best test for finding out how well we understand planetesimal formation near the snow line.

    Featured Image Credit: Luis Calçada

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
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