Astronomy

How are boulders formed on asteroids?

How are boulders formed on asteroids?


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For example: Are they soft landed meteorites? Or do they form from melted material during a violent impact? Or are they the result of some kind of erosion process?


It looks like you are asking about rubble piles, asteroids that are made up of a large number of different sized objects that are weakly held together by gravity. A few of the component objects are large, but most are very small (down to grains of sand).

By way of analogy, think of playing pocket pool. Rack the 15 target balls but leave the rack on. The cue ball bounces off when you strike the rack with the cue ball, but that's about it.

Now let's try again, but this time with the rack removed. To make the target balls a even more rubble pile-like, we'll use a piece of cardboard to add a bit of space between the target balls. Now something very different happens when the cue ball strikes the target balls. The balls are only loosely connected to one another. This makes them very good at absorbing the momentum of the cue ball and at distributing the energy and momentum throughout the rack. Given a low to moderate cue ball velocity, the collision will be close to purely inelastic.

And that's how you get a rubble pile. It's one inelastic collision after another after another. Over the course of 4.6 billion years, you have a pile of dust and sand with some rocks and a few large boulders mixed in.


Pieces Of Asteroid Vesta Found On The Surface Of Bennu

Asteroid Bennu, subject of NASA’s OSIRIS-REx asteroid sampling mission, is a big pile of rubble. The latest analysis from OSIRIS-REx suggests that the rubble might have not a single origin. It found evidence from asteroid Vesta, the second-largest body in the Asteroid Belt.

The researchers believe Bennu likely formed as a result of a collision between asteroids, one of which was a fragment of Vesta. When the scattered debris of the collision ended up forming Bennu, some of the Vesta rocks ended up on the surface of this small body. The discovery is reported in Nature Astronomy.

“We found six boulders ranging in size from 5 to 14 feet (about 1.5 to 4.3 meters) scattered across Bennu’s southern hemisphere and near the equator,” lead author Daniella DellaGiustina of the Lunar & Planetary Laboratory, University of Arizona said in a statement. “These boulders are much brighter than the rest of Bennu and match material from Vesta.”

It is possible that they formed from the parent body of Bennu but it is more likely they were pieces of Vesta. The boulders are made of pyroxene, which forms at high temperatures from the melting of rocky material, and can be 10 times brighter than the surrounding surface. Bennu's rocks are made from water-bearing minerals, so it's unlikely it, or its parent body, experienced high temperatures.

It's not unusual to see remnants of an asteroid splashed across the surface of another. NASA’s Dawn spacecraft saw craters on Vesta where different asteroids had collided, breaking away pieces and leaving dark material behind. Similarly, a large black boulder was seen by the Japanese probe Hayabusa on asteroid Itokawa. Just yesterday, a study revealed fellow pile-of-rubble asteroid Ryugu, visited by Hayabusa2, is also the product of a cosmic collision.

“Our leading hypothesis is that Bennu inherited this material from its parent asteroid after a vestoid (a fragment from Vesta) struck the parent,” said Hannah Kaplan of NASA’s Goddard Space Flight Center. “Then, when the parent asteroid was catastrophically disrupted, a portion of its debris accumulated under its own gravity into Bennu, including some of the pyroxene from Vesta.”

Asteroids moving through the Solar System interact with one another, and can over time end up in a collision. The gravitational interaction can also take them from the Asteroid Belt (between Mars and Jupiter) to near the Earth’s orbit, which is the case for both Bennu and Ryugu.

“Future studies of asteroid families, as well as the origin of Bennu, must reconcile the presence of Vesta-like material as well as the apparent lack of other asteroid types. We look forward to the returned sample, which hopefully contains pieces of these intriguing rock types,” added Dante Lauretta, OSIRIS-REx principal investigator. “This constraint is even more compelling given the finding of S-type material on asteroid Ryugu. This difference shows the value in studying multiple asteroids across the Solar System.”

OSIRIS-REx is scheduled to fly down to the surface of Bennu and collect a sample of soil next month, so stay tuned.


Abstract

Many small boulders with reflectance values higher than 1.5 times the average reflectance have been found on the near-Earth asteroid 162,173 Ryugu. Based on their visible wavelength spectral differences, Tatsumi et al. (2021, Nature Astronomy, 5, doi:doi:10.1038/s41550-020-1179-z) defined two bright boulder classes: C-type and S-type. These two classifications of bright boulders have different size distributions and spectral trends. In this study, we measured the spectra of 79 bright boulders and investigated their detailed spectral properties. Analyses obtained a number of important results. First, S-type bright boulders on Ryugu have spectra that are similar to those found for two different ordinary chondrites with different initial spectra that have been experimentally space weathered the same way. This suggests that there may be two populations of S-type bright boulders on Ryugu, perhaps originating from two different impactors that hit Ryugu's parent body. Second, the model space-weathering ages of meter-size S-type bright boulders, based on spectral change rates derived in previous experimentally irradiated ordinary chondrites, are 10 5 –10 6 years, which is consistent with the crater retention age (<10 6 years) of the

1-m deep surface layer on Ryugu. This agreement strongly suggests that Ryugu's surface is extremely young, implying that the samples acquired from Ryugu's surface should be fresh. Third, the lack of a serpentine absorption in the S-type clast embedded in one of the large brecciated boulders indicates that fragmentation and cementation that created the breccias occurred after the termination of aqueous alteration. Fourth, C-type bright boulders exhibit a continuous spectral trend similar to the heating track of low-albedo carbonaceous chondrites, such as CM and CI. Other processes, such as space weathering and grain size effects, cannot primarily account for their spectral variation. Furthermore, the distribution of the spectra of general dark boulders, which constitute >99.9% of Ryugu's volume, is located along the trend line in slope/UV-index diagram that is occupied by C-type bright boulders. These results indicate that thermal metamorphism might be the dominant cause for the spectral variety among the C-type bright boulders on Ryugu and that general boulders on Ryugu may have experienced thermal metamorphism under a much narrower range of conditions than the C-type bright boulders. This supports the hypothesis that Ryugu's parent body experienced uniform heating due to radiogenic energy rather than impact heating.


Creating Asteroids

Learn about the characteristics of asteroids.
Learn how asteroids are formed in our Solar System.

Participants will learn that asteroids are large boulders found in our Solar System orbiting the Sun by looking at images of asteroids and discussing in the classroom.

Participants will demonstrate how planetary bodies, including asteroids, are formed through the grouping of small particles using clay.

At the end of the session, when all the asteroids have been put to dry, revisit the questions and topics discussed in the introduction. Specifically, let the students explain:

What is an asteroid?
How are asteroids formed?
Students can explain this using the example of how they created their own model asteroid from clay.

  • Images of asteroids (provided)
  • Clay (a handful per participant)
  • Paint brushes
  • Paint
  • Table lining/protector

What is an Asteroid?

Asteroids are boulders orbiting the Sun, with sizes ranging from some hundred metres to several kilometres. An asteroid is called a meteorite if it hits the Earth. If it completely evaporates in the Earth’s atmosphere before crashing on the surface, it’s called a meteor. People usually refer to meteors as ‘shooting stars’. Most meteorites are composed of silicates or a mixture of iron and nickel. In the past, some huge meteorites have struck Earth. Sixty-five million years ago, almost 90% of animal species were eradicated (including dinosaurs) when a meteorite hit Yucatan, Mexico. Luckily, this happens very rarely! We owe this to Jupiter, which attracts many asteroids with its gravitational pull.


Asteroid Ida

How are asteroids formed?

Dust particles in the early Solar System collided, forming larger clumps, known as planetesimals. These could grow by attracting more dust with their gravitational fields some grew large enough to form the planets. Others remained, becoming the asteroids. Some of these asteroids collided with each other (and the early planets), fragmenting into smaller asteroids. Some of the collisions were slow enough that the asteroids merged, producing oddly shaped asteroids.

Where are asteroids located?

Many asteroids form large rings or belts around the Sun. There are two asteroid belts in our Solar System: the main belt (or simply called the asteroid belt) between Mars and Jupiter, with thousands of asteroids (see picture below), and the Kuiper belt, named after its discoverer, a disk-shaped region that extends outside of Neptune’s orbit and contains countless asteroids and many dwarf planets, of which Pluto is the most famous.

Most asteroids are found in the asteroid belt however, there are asteroids that are not in that orbit, and they are called Near Earth Objects (NEO). Sometimes these NEOs can reach our planet Earth.

Why are asteroids important to study?

Asteroids are part of a group known as minor bodies. Asteroids, comets and meteoroids can provide valuable information about the evolution of our Solar System
Additionally, there are small bodies called meteoroids, remnants of the formation of the Solar System. These meteoroids can be as small as a grain of rice. Meteoroids constantly meet Earth, producing what we see on Earth as ‘shooting stars’.

Is it true that an asteroid can hit Earth?

An asteroid 10 kilometres in diameter hit the Earth 65 million years ago. This asteroid impact on the Earth is assumed to be one of the reasons why the dinosaurs became extinct.

You can also tell the students about comets. Comets are like dirty snowballs or icy lumps of mud. They consist of a mixture of ice (from water as well as from frozen gases) and dust. Like asteroids, comets revolve around the Sun. However, their orbits are strongly elongated compared to planets. That is, they occasionally get very close to the Sun, and at times they get very far away. When they cross a planet’s orbit, they could collide with it. This happened, for example, in 1994, when the comet Shoemaker-Levy collided with planet Jupiter and broke into pieces. When comets come close to the Sun in their orbit, the ice in their core melts and evaporates. This results in a beautiful tail, which can be clearly seen in the night sky if the comet passes close enough to the Earth.
In 2061, Halley’s Comet will once again come close to the Earth. It orbits our Sun once every 76 years. Remember to mark its arrival on your calendar!


Halleys Comet


Bullet Holes on an Asteroid

Craters and boulders are ubiquitous features of planetary surfaces. Craters on boulders on a near-Earth asteroid provide a unique opportunity to probe the time since the asteroid's departure from the main asteroid belt.

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The OSIRIS-REx mission recently accomplished its goal of obtaining a sample from the surface of the asteroid Bennu. The event marked nearly 2 years since the spacecraft first arrived at Bennu. My first impressions of Bennu were ones of wonder and anxiousness, as the surface of the asteroid was far more rugged than we had anticipated (Lauretta et al. 2019). As part of the mission's Regolith Development Working Group (RDWG, led by Kevin Walsh), I participated in the effort to find a spot on the asteroid that was covered in fine grained material (< 2 cm) that could be ingestible by the spacecraft's sample container (Bierhaus et al. 2018).

Searching for a sample

In the course of looking for a sampling site, we obtained unprecedented detail of the surface of an asteroid. In July 2019, we undertook an observing campaign called Orbital B (Fig. 1a), where we obtained 1 cm/pixel scale resolution images of the surface (DellaGiustina et al. 2018). In this configuration, the spacecraft orbits the asteroid in the terminator plane, or the plane that divides the day-lit side and the dark night side of the asteroid. The images that returned were shadowy scenes (Fig. 1b), as the illumination angle from the Sun to the surface is quite steep. It was difficult to make out small particles, but we could sometimes see the long pixel-wide shadows they cast. So, we knew that they should be somewhere on the surface. Maneuvers by the spacecraft later in the year would provide better illuminated and higher resolution images of the surface, making the presence of fine particles more apparent.

Figure 1. The Orbital B survey of Bennu returned unprecedentedly detailed images of the asteroid surface. a, Schematic illustrating the Orbital B 1‐km terminator orbit (adapted from DellaGiustina et al. 2018). b, An image taken on July 3, 2019 during the Orbital B campaign (credit: OSIRIS-REx/OCAMS/PolyCam). The images taken during Orbital B were shadowy scenes of the surface. In the same way that it's easier to make out craters on the Moon along its terminator, the illumination geometry in Orbital B images allowed us to more easily observe tiny craters on the surfaces of boulders.

What we saw in the shadows

The Orbital B images are beautiful, and revealed previously un-seen features of the boulders. It's like taking a walk in your neighborhood in the dead of night. You're going to notice different things compared to what you typically notice in the light of day. In these gloomy images of Bennu's surface, there would sometimes be a rock that would stand out from the shadows. When these stand-out rocks were flat, we noticed that they would be riddled with small holes (Fig. 2a).

Figure 2. Bullet hole craters were apparent on some of Bennu's flat boulders. a, A crop of the image shown in Fig. 1b, focusing on a boulder, roughly 2 m wide on its longest axis, riddled with tiny craters (Ballouz et al. 2020). b, The tiny impact craters on Bennu reminded this author of the bullet holes on the outside walls of buildings in his hometown of Beirut (credit: Samir Ballouz).

I grew up in Beirut after the Lebanese Civil War. Most of the buildings in my neighborhood, including the one I grew up in, were riddled with bullet holes (Fig. 2b). They were just a part of the background for me, and I didn't think much of them when I was young. When I saw similar features on Bennu's boulders, it immediately clicked in my head that these little impact pits may have a similar origin. Of course, rather than gun-fire, these bullet holes must have been made by impacts by micro-meteorites. Similar features had been reported on the lunar samples returned by Apollo astronauts (Fig. 3, Hörz et al. 1975, 2020), and on the returned samples from the asteroid Itokawa (Nakamura et al. 2012). As we'll discuss later, the "bullet-holes" we see in Fig. 2a accumulated on that boulder in timescales of over a million years!

Figure 3. Impact features on solid rock were also recorded by the Apollo missions. a, One of the returned Apollo lunar samples that exhibit an impact pit (Hörz et al. 1975), 1-cm scale bar shown. b, Circular impact feature on the top surface of an exceptionally large lunar boulder (Hörz et al. 2020). This image was taken during the Apollo 17 mission.

An unprecedented view of an asteroid surface

However, just as one can "see things in the shadows" that aren't really there, we needed to verify that these were indeed impact craters. Fortunately, just as we were taking those gloomy pictures of Bennu, the OSIRIS-REx Laser Altimeter (OLA), which was contributed by the Canadian Space Agency, created 3-D maps of the surface at about 5 cm resolution. The OLA data gave us breath-taking views of the asteroid surface. OLA removed all ambiguity on whether a crater was really a crater or just a trick of the light. With the help of the Altimetry Working Group (led by Olivier Barnouin and Mike Daly), we measured the morphology of the largest craters we found on boulders (Fig. 4), and confirmed that they had shapes similar to craters created in impact experiments in the lab. We moved forward, confident in our interpretation.

Figure 4. OLA data gave an unprecedentedly detailed view of Bennu's surface. An animation showing the raw OLA point cloud data of a 10-m boulder with a 5-m crater. The colors highlight the topography of the boulder and the surrounding region (warmer colors are higher elevation). Credit: NASA/University of Arizona/CSA/York/MDA. York University gratefully acknowledges contributions from JHUAPL.

In search of time

With the help of the Image Processing Working Group (led by Dani DellaGiustina), we correctly registered images to a shape model of Bennu, and measured the sizes of the craters and their host boulder. We ended up measuring more than 600 craters on boulders. Based on the sizes of the craters relative to their host boulders, we came up with a method to estimate the impact strength of the boulders, which gave us an idea of what size impactors made those craters. We discovered that the counting statistics of these craters resembled that of near-Earth objects (Brown et al. 2002). So, we inferred that these craters on boulders must record Bennu's time in near-Earth space. Then, with the total number and sizes of the craters, we were able to measure the exposure age of the boulders--which we found to be 1.75 million years. This age is much smaller than the expected age of its surface (Walsh et al. 2019), and the disruption event that likely created Bennu itself more than 100 million years ago in the main asteroid belt (Bottke et al. 2015). The returned samples will give us a clearer idea of the accuracy of our measurements of the near-Earth exposure age of the surface and the strength of Bennu rock. If verified, we can more confidently use the techniques we developed here on other airless planetary bodies in the future.

Ron Ballouz

Postdoctoral Research Scientist, Lunar and Planetary Lab, University of Arizona


Are all asteroids the same?

No way! Because asteroids formed in different locations at different distances from the sun, no two asteroids are alike. Here are a few ways that they differ:

  • Asteroids aren’t all round like planets. They have jagged and irregular shapes.
  • Some asteroids are hundreds of miles in diameter, but many more are as small as pebbles.
  • Most asteroids are made of different kinds of rocks, but some have clays or metals, such as nickel and iron.

Mathilde, Gaspra, and Ida are three asteroids that have been imaged by NASA spacecraft. In this image, you can see that asteroids come in a variety shapes and sizes. Image credit: NASA/JPL


Vesta: A Differentiated Asteroid

Figure 3: Piece of Vesta. This meteorite (rock that fell from space) has been identified as a volcanic fragment from the crust of asteroid Vesta. (credit: modification of work by R. Kempton (New England Meteoritical Services))

Vesta is one of the most interesting of the asteroids. It orbits the Sun with a semi-major axis of 2.4 AU in the inner part of the asteroid belt. Its relatively high reflectivity of almost 30% makes it the brightest asteroid, so bright that it is actually visible to the unaided eye if you know just where to look. But its real claim to fame is that its surface is covered with basalt, indicating that Vesta is a differentiated object that must once have been volcanically active, in spite of its small size (about 500 kilometers in diameter).

Meteorites from Vesta’s surface (Figure 3), identified by comparing their spectra with that of Vesta itself, have landed on Earth and are available for direct study in the laboratory. We thus know a great deal about this asteroid. The age of the lava flows from which these meteorites derived has been measured at 4.4 to 4.5 billion years, very soon after the formation of the solar system. This age is consistent with what we might expect for volcanoes on Vesta whatever process heated such a small object was probably intense and short-lived. In 2016, a meteorite fell in Turkey that could be identified with a particular lava flow as revealed by the orbiting Dawn spacecraft.


Asteroids' Encounters with Earth

Asteroids that fall into the category of Near Earth Objects have orbits that bring them in close proximity with our planet, and a number of notable impact sites have been attributed to asteroids. Most famously, the Chicxulub crater under the Yucatán Peninsula in Mexico is the result of an asteroid impact that might have wiped out the dinosaurs. Another asteroid known as Tunguska didn't impact Earth, but exploded a few miles above the Podkamennaya Tunguska River on June 30,1908. A similar explosion made headline news in 2013, when the near-Earth asteroid dubbed Chelyabinsk exploded in air blast that left 1,500 Russians seeking medical attention.


Ryugu’s Rocky Past: Different Kinds of Rocks on Ryugu Provide Clues to the Asteroid’s Turbulent History

Researchers find evidence that asteroid Ryugu was born out of the possible destruction of a larger parent asteroid millions of years ago. Thanks to the Hayabusa2 spacecraft, the international team was able to study certain surface features in detail. Variations in the kinds of boulders scattered on Ryugu tell researchers about the processes involved in its creation. The study of asteroids including Ryugu informs the study of the evolution of life on Earth.

The asteroid Ryugu may look like a solid piece of rock, but it’s more accurate to liken it to an orbiting pile of rubble. Given the relative fragility of this collection of loosely bound boulders, researchers believe that Ryugu and similar asteroids probably don’t last very long due to disruptions and collisions from other asteroids. Ryugu is estimated to have adopted its current form around 10 million to 20 million years ago, which sounds like a lot compared to a human lifespan, but makes it a mere infant when compared to larger solar system bodies.

“Ryugu is too small to have survived the whole 4.6 billion years of solar system history,” said Professor Seiji Sugita from the Department of Earth and Planetary Science at the University of Tokyo. “Ryugu-sized objects would be disrupted by other asteroids within several hundred million years on average. We think Ryugu spent most of its life as part of a larger, more solid parent body. This is based on observations by Hayabusa2 which show Ryugu is very loose and porous. Such bodies are likely formed from reaccumulations of collision debris.”

Hayabusa2 captures images of unusually bright S-type rocks that stand out from the darker C-type material that makes up the bulk of Ryugu. Credit: © 2020 Tatsumi et al.

As well as giving researchers data to measure Ryugu’s density, Hayabusa2 also collects information about the spectral properties of the asteroid’s surface features. For this study in particular, the team was keen to explore the subtle differences between the various kinds of boulders on or embedded in the surface. They determined there are two kinds of bright boulders on Ryugu, and the nature of these gives away how the asteroid may have formed.

“Ryugu is considered a C-type, or carbonaceous, asteroid, meaning it’s primarily composed of rock that contains a lot of carbon and water,” said postdoctoral researcher Eri Tatsumi. “As expected, most of the surface boulders are also C-type however, there are a large number of S-type, or siliceous, rocks as well. These are silicate-rich, lack water-rich minerals and are more often found in the inner, rather than outer, solar system.”

Given the presence of S- as well as C-type rocks on Ryugu, researchers are led to believe the little rubble-pile asteroid likely formed from the collision between a small S-type asteroid and Ryugu’s larger C-type parent asteroid. If the nature of this collision had been the other way around, the ratio of C- to S-type material in Ryugu would also be reversed. Hayabusa2 is now on its return journey to Earth and is expected to deliver its cargo of samples on Dec. 6 of this year. Researchers are keen to study this material to add evidence for this hypothesis and to elucidate many other things about our little rocky neighbor.

“We used the optical navigation camera on Hayabusa2 to observe Ryugu’s surface in different wavelengths of light, and this is how we discovered the variation in rock types. Among the bright boulders, C and S types have different albedos, or reflective properties,” said Tatsumi. “But I eagerly await the analysis of the return samples, as this will confirm theories and improve the accuracy of our knowledge about Ryugu. What will be really interesting is knowing how Ryugu differs from meteorites on Earth, as this could in turn tell us something new about the history of Earth and the solar system as a whole.”

Ryugu is not the only near-Earth asteroid scientists are currently exploring with probes, though. Another international team under NASA is currently studying the asteroid Bennu with the OSIRIS-REx spacecraft in orbit around it. Tatsumi also collaborates with researchers on that project and the teams share their research findings.

“When I was a child, I felt the other planets were always out of reach. But with the power of the instruments on our spacecraft, the images are so sharp and clear it feels like you could almost touch the surface of these asteroids,” said Tatsumi. “Right now, I’m studying asteroids with giant telescopes in the Canary Islands. And one day, I hope to also explore icy comets and trans-neptunian objects such as dwarf planets. In this way, we may soon fully understand and appreciate how our solar system began.”

Reference: “Collisional history of Ryugu’s parent body from bright surface boulders” by E. Tatsumi, C. Sugimoto, L. Riu, S. Sugita, T. Nakamura, T. Hiroi, T. Morota, M. Popescu, T. Michikami, K. Kitazato, M. Matsuoka, S. Kameda, R. Honda, M. Yamada, N. Sakatani, T. Kouyama, Y. Yokota, C. Honda, H. Suzuki, Y. Cho, K. Ogawa, M. Hayakawa, H. Sawada, K. Yoshioka, C. Pilorget, M. Ishida, D. Domingue, N. Hirata, S. Sasaki, J. de León, M. A. Barucci, P. Michel, M. Suemitsu, T. Saiki, S. Tanaka, F. Terui, S. Nakazawa, S. Kikuchi, T. Yamaguchi, N. Ogawa, G. Ono, Y. Mimasu, K. Yoshikawa, T. Takahashi, Y. Takei, A. Fujii, Y. Yamamoto, T. Okada, C. Hirose, S. Hosoda, O. Mori, T. Shimada, S. Soldini, R. Tsukizaki, T. Mizuno, T. Iwata, H. Yano, M. Ozaki, M. Abe, M. Ohtake, N. Namiki, S. Tachibana, M. Arakawa, H. Ikeda, M. Ishiguro, K. Wada, H. Yabuta, H. Takeuchi, Y. Shimaki, K. Shirai, N. Hirata, Y. Iijima, Y. Tsuda, S. Watanabe and M. Yoshikawa, 21 September 2020, Nature Astronomy.
DOI: 10.1038/s41550-020-1179-z


Contents

The main piece of evidence for a lunar cataclysm comes from the radiometric ages of impact melt rocks that were collected during the Apollo missions. The majority of these impact melts are believed to have formed during the collision of asteroids or comets tens of kilometres across, forming impact craters hundreds of kilometres in diameter. The Apollo 15, 16, and 17 landing sites were chosen as a result of their proximity to the Imbrium, Nectaris, and Serenitatis basins, respectively.

The apparent clustering of ages of these impact melts, between about 3.8 and 4.1 Ga, led to postulation that the ages record an intense bombardment of the Moon. [5] They called it the "lunar cataclysm" and proposed that it represented a dramatic increase in the rate of bombardment of the Moon around 3.9 Ga. If these impact melts were derived from these three basins, then not only did these three prominent impact basins form within a short interval of time, but so did many others based on stratigraphic grounds. At the time, the conclusion was considered controversial.

As more data has become available, particularly from lunar meteorites, this theory, while still controversial, has gained in popularity. The lunar meteorites are believed to randomly sample the lunar surface, and at least some of these should have originated from regions far from the Apollo landing sites. Many of the feldspathic lunar meteorites probably originated from the lunar far side, and impact melts within these have recently been dated. Consistent with the cataclysm hypothesis, none of their ages was found to be older than about 3.9 Ga. [6] Nevertheless, the ages do not "cluster" at this date, but span between 2.5 and 3.9 Ga. [7]

Dating of howardite, eucrite and diogenite (HED) meteorites and H chondrite meteorites originating from the asteroid belt reveal numerous ages from 3.4–4.1 Ga and an earlier peak at 4.5 Ga. The 3.4–4.1 Ga ages has been interpreted as representing an increase in impact velocities as computer simulations using hydrocode [8] reveal that the volume of impact melt increases 100–1,000 times as the impact velocity increases from the current asteroid belt average of 5 km/s to 10 km/s. Impact velocities above 10 km/s require very high inclinations or the large eccentricities of asteroids on planet crossing orbits. Such objects are rare in the current asteroid belt but the population would be significantly increased by the sweeping of resonances due to giant planet migration. [9]

Studies of the highland crater size distributions suggest that the same family of projectiles struck Mercury and the Moon during the Late Heavy Bombardment. [10] If the history of decay of late heavy bombardment on Mercury also followed the history of late heavy bombardment on the Moon, the youngest large basin discovered, Caloris, is comparable in age to the youngest large lunar basins, Orientale and Imbrium, and all of the plains units are older than 3 billion years. [11]

While the cataclysm hypothesis has recently [ when? ] gained in popularity, particularly among dynamicists who have identified possible causes for such a phenomenon, the cataclysm hypothesis is still controversial and based on debatable assumptions. Two criticisms are that (1) the "cluster" of impact ages could be an artifact of sampling a single basin's ejecta, and (2) that the lack of impact melt rocks older than about 4.1 Ga is related to all such samples having been pulverized, or their ages being reset.

The first criticism concerns the origin of the impact melt rocks that were sampled at the Apollo landing sites. While these impact melts have been commonly attributed to having been derived from the closest basin, it has been argued that a large portion of these might instead be derived from the Imbrium basin. [12] The Imbrium impact basin is the youngest and largest of the multi-ring basins found on the central nearside of the Moon, and quantitative modeling shows that significant amounts of ejecta from this event should be present at all of the Apollo landing sites. According to this alternative hypothesis, the cluster of impact melt ages near 3.9 Ga simply reflects material being collected from a single impact event, Imbrium, and not several. Additional criticism also argues that the age spike at 3.9 Ga identified in 40 Ar/ 39 Ar dating could also be produced by an episodic early crust formation followed by partial 40 Ar losses as the impact rate declined. [13]

A second criticism concerns the significance of the lack of impact melt rocks older than about 4.1 Ga. One hypothesis for this observation that does not involve a cataclysm is that old melt rocks did exist, but that their radiometric ages have all been reset by the continuous effects of impact cratering over the past 4 billion years. Furthermore, it is possible that these putative samples could all have been pulverized to such small sizes that it is impossible to obtain age determinations using standard radiometric methods. [14] Latest reinterpretation of crater statistics suggests that the flux on the Moon and on Mars may have been lower in general. Thus, the recorded crater population can be explained without any peak in the earliest bombardment of the inner Solar System.

If a cataclysmic cratering event truly occurred on the Moon, the Earth would have been affected as well. Extrapolating lunar cratering rates [15] to Earth at this time suggests that the following number of craters would have formed: [16]

  • 22,000 or more impact craters with diameters >20 km (12 mi),
  • about 40 impact basins with diameters about 1,000 km (620 mi),
  • several impact basins with diameters about 5,000 km (3,100 mi),

Before the formulation of the LHB theory, geologists generally assumed that the Earth remained molten until about 3.8 Ga. This date could be found in many of the oldest-known rocks from around the world, and appeared to represent a strong "cutoff point" beyond which older rocks could not be found. These dates remained fairly constant even across various dating methods, including the system considered the most accurate and least affected by environment, uranium–lead dating of zircons. As no older rocks could be found, it was generally assumed that the Earth had remained molten until this date, which defined the boundary between the earlier Hadean and later Archean eons. Nonetheless, in 1999, the oldest known rock on Earth was dated to be 4.031 ± 0.003 billion years old, and is part of the Acasta Gneiss of the Slave Craton in northwestern Canada. [17]

Older rocks could be found, however, in the form of asteroid fragments that fall to Earth as meteorites. Like the rocks on Earth, asteroids also show a strong cutoff point, at about 4.6 Ga, which is assumed to be the time when the first solids formed in the protoplanetary disk around the then-young Sun. The Hadean, then, was the period of time between the formation of these early rocks in space, and the eventual solidification of the Earth's crust, some 700 million years later. This time would include the accretion of the planets from the disk and the slow cooling of the Earth into a solid body as the gravitational potential energy of accretion was released.

Later calculations showed that the rate of collapse and cooling depends on the size of the rocky body. Scaling this rate to an object of Earth mass suggested very rapid cooling, requiring only 100 million years. [18] The difference between measurement and theory presented a conundrum at the time.

The LHB offers a potential explanation for this anomaly. Under this model, the rocks dating to 3.8 Ga solidified only after much of the crust was destroyed by the LHB. Collectively, the Acasta Gneiss in the North American cratonic shield and the gneisses within the Jack Hills portion of the Narryer Gneiss Terrane in Western Australia are the oldest continental fragments on Earth, yet they appear to post-date the LHB. The oldest mineral yet dated on Earth, a 4.404 Ga zircon from Jack Hills, predates this event, but it is likely a fragment of crust left over from before the LHB, contained within a much younger (

The Jack Hills zircon led to something of a revolution in our understanding of the Hadean eon. [19] Older references generally show that Hadean Earth had a molten surface with prominent volcanos. The name "Hadean" itself refers to the "hellish" conditions assumed on Earth for the time, from the Greek Hades. Zircon dating suggested, albeit controversially, that the Hadean surface was solid, temperate, and covered by acidic oceans. This picture derives from the presence of particular isotopic ratios that suggest the action of water-based chemistry at some time before the formation of the oldest rocks (see Cool early Earth). [20]

Of particular interest, Manfred Schidlowski argued in 1979 that the carbon isotopic ratios of some sedimentary rocks found in Greenland were a relic of organic matter. There was much debate over the precise dating of the rocks, with Schidlowski suggesting they were about 3.8 Ga old, and others suggesting a more "modest" 3.6 Ga. In either case it was a very short time for abiogenesis to have taken place, and if Schidlowski was correct, arguably too short a time. The Late Heavy Bombardment and the "re-melting" of the crust that it suggests provides a timeline under which this would be possible life either formed immediately after the Late Heavy Bombardment, or more likely survived it, having arisen earlier during the Hadean. Recent studies suggest that the rocks Schidlowski found are indeed from the older end of the possible age range at about 3.85 Ga, suggesting the latter possibility is the most likely answer. [21] More recent studies have found no evidence for the isotopically light carbon ratios that were the basis for the original claims. [22] [23] [24] It has been suggested, that life could have been transported off the Earth due to impacts and return and 'reseed' life after the world has recovered after a global impactor, thus not only restarting evolution, but also potentially confer a particular biological effect that enhances the stress capacity of the collected microbial organisms and thus their survival capacity. [25]

More recently, a similar study of Jack Hills rocks shows traces of the same sort of potential organic indicators. Thorsten Geisler of the Institute for Mineralogy at the University of Münster studied traces of carbon trapped in small pieces of diamond and graphite within zircons dating to 4.25 Ga. The ratio of carbon-12 to carbon-13 was unusually high, normally a sign of "processing" by life. [26]

Three-dimensional computer models developed in May 2009 by a team at the University of Colorado at Boulder postulate that much of Earth's crust, and the microbes living in it, could have survived the bombardment. Their models suggest that although the surface of the Earth would have been sterilized, hydrothermal vents below the Earth's surface could have incubated life by providing a sanctuary for heat-loving microbes. [27]

In April 2014, scientists reported finding evidence of the largest terrestrial meteor impact event to date near the Barberton Greenstone Belt. They estimated the impact occurred about 3.26 billion years ago and that the impactor was approximately 37 to 58 kilometres (23 to 36 miles) wide. The crater from this event, if it still exists, has not yet been found. [28]


Watch the video: ASTEROIDEN - die Bedrohung aus dem Kosmos. SPACETIME Doku (September 2022).