Primordial Asteroids, And The Stories They Are Telling

The main asteroid belt of our solar system — with almost two million asteroids a kilometer in diameter orbiting in the region between Mars and Jupiter.  There are billions more that are smaller. New research has identified the “family” of a primordial asteroid or planetesimal, one of the oldest ever detected.

Asteroid, we’ve long been told, started tiny in our protoplanetary disk and only very gradually became more massive through a process of accretion.  They collected dust from the gas cloud that surrounded our new star, and then grew larger through collisions with other growing asteroids.

But in recent years, a new school of thought has proposed a different scenario:  that large clumps of dust and pebbles in the disk could experience gravitational collapse, a binding together of concentrated disk material.

This process would produce a large asteroid (which is sometimes called a planetesimal) relatively quickly, without that long process of accretion.  This theory would solve some of the known problems with the gradual accretion method, though it brings some problems of its own.

Now research just published in the journal Science offers some potentially important support to the gravitational collapse model, while also describing the computational detection of a primordial family of asteroids some 4 billion years old.

Led by Marco Delbo’, an astrophysicist at the University of the Côte d’Azur in Nice, France, the scientists have identified a previously unknown family of darkly colored asteroids that is “the oldest known family in the main belt,” their study concluded.

The family was identified and grouped together by the unusual darkness (low albedo) of its asteroids’ reflective powers, a signature that the object has a high concentrations of carbon-based organic compounds.  This family of asteroids was also less extensively heated — having formed when the sun radiated less energy — and contains more water, making them potential goldmines for understanding the makeup and processes of the early solar system.

Artist depiction of a dusty disc surrounding a red dwarf.artist rendering of a protoplanetary dust disk, from which asteroid, planetesimals and ultimately planets are formed. NASA/JPL-Caltech/T. Pyle (SSC)

“They are from an original planetesimal and the location of these fragments tell us they are very, very old,” Delbo’ told me.  “So old that the original object is older than the epoch when our giant planets moved to their current locations.”  That would make this ancient asteroid family more than 4 billion years old, formed when the solar system was but 600 million years from inception.

By adding up the masses of the members of the asteroid family, the researchers could also come up with a size for the original planetesimal that gave birth to the asteroid family — at least 35 kilometers wide at its inception.

Some background:

What is termed our “solar nebula” is thought to have been a disk-shaped cloud of gas and dust that remained after the formation of the sun.  Just like a dancer that spins faster as she pulls in her arms, the cloud began to spin as it collapsed. Eventually, the cloud grew hotter and more dense in the center, with a disk of gas and dust surrounding it that was hot near the center but cool at the edges.

Marco Delbo is a researcher at the Italian Istituto Nazionale di Astrofisica (INAF) at present on leave at the Observatoire de la Côte d’Azur in Nice, France, with an External Fellowship of the European Space Agency (ESA).

Since these earliest days of the solar system, a vast collection of dust and later rocks of all shapes and sizes has been circling the sun, especially in the broad expanse of space between Mars and Jupiter.  This is both the material from which planets were formed, and also leftover material from the formation of the solar system.

There are many of these asteroids, or planetesimals, but they don’t carry much mass — all of them together roughly equaling that of our moon.

There are some 130 known “families” of asteroids.  The effort to understand the processes that created the asteroids has been enormously difficult because they have been broken and then broken again and again as they crash into each other.

But that is changing thanks to this discovery of the new family of “dark” asteroids.  Unlike the brighter, highly reflective asteroid families nearby, the population of dark asteroids’ orbits are more spread out, interpreted to mean that more time has passed since the asteroids formed.

Most asteroid families are thought to have formed about 1 billion years ago. By aggregating the sizes of the modern dark asteroids, researchers suggest their original planetesimals formed about 4 billion years ago, making this one of the oldest asteroid families in the main asteroid belt.

The scientists also determined that the dark family’s original planetesimals were no smaller than about 25 miles across.

This provides support for the gravitational collapse hypothesis, originated at Germany’s Max Planck Institute, by suggesting the oldest asteroids started out large, and then became smaller through collisions and other destructive forces happening in the ancient solar system.

The earlier and more conventional theory had the asteroids starting small and getting gradually bigger. This difference in hypotheses has been a hot topic among planetary scientists for nearly a decade.

This image, taken by NASA’s Near Earth Asteroid Rendezvous mission in 2000, shows a close-up view of Eros, an asteroid with an orbit that takes it somewhat close to Earth.  American and Japanese and European missions to study and scoop up material from asteroids are now on their way. The European Space Agency has also undertaken an asteroid landing mission and a joint NASA-ESA asteroid-ramming mission is under consideration. NASA/JHUAPL

These findings are not based on telescope viewing and measuring;  that was all done by NASA’s  Wide-field Infrared Survey Explorer in 2011.  The spacecraft took images of some 750 million objects, including millions of asteroids. 

Delbo’ and his team used computer models to search for groups of related asteroids spread within a V-shaped region. This V pattern is what one would expect from a single object that fragmented into pieces, and the wider the V-shape the older the objects.

Their asteroid family features rocks averaging 7.15 miles in diameter, and are found across the entire inner part of the main asteroid belt. The family has 108 members  and counting, with the largest of which the largest being asteroid 282 Clorinde, which is about 26 wide.

“Each family member drifts away from the center of the family in a way that depends on its size, with small guys drifting faster and further than the larger guys,” Delbo said.  “If you look for correlations of size and distance, you can see the shapes of old families.”

But that wasn’t all.

“By identifying all the families in the main belt, we can figure out which asteroids have been formed by collisions and which might be some of the original members of the asteroid belt,” said Southwest Research Institute astronomer Kevin Walsh, a coauthor of the Science article.

“We identified all known families and their members and discovered a gigantic void in the main belt, populated by only a handful of asteroids. These relics must be part of the original asteroid belt. That is the real prize, to know what the main belt looked like just after it formed.”

These primordial objects had to have formed differently from those belonging to the newer families. They were the original inhabitants and were present in the inner asteroid belt before anything else.

ranging from 21 to around 93 miles across, their size matches up with predictions from theoretical models of how large original asteroids might have been 4 billion years ago, when they initially formed.

In other words, their age and size supports the gravitational collapse theory of asteroid formation.

An artist’s concept depicts a distant hypothetical solar system, similar in age to our own. Looking inward from the system’s outer fringes, a ring of dusty debris can be seen, and within it, planets circling a star the size of our Sun. This debris is all that remains of the planet-forming disk from which the planets evolved. Planets are formed when dusty material in a large disk surrounding a young star clumps together. (NASA)

To put these findings into a larger context, I asked Elizabeth Tasker, astrophyscist at the Japan Space Agency and the Earth-Life Science Institute in Tokyo, to explain further.  She is the author of the soon-to-be released book, “The Planet Factory,” which deals extensively with these issues.  First is her take on the logic of gravitational collapse:

“In the gravitational collapse model, the pebbles and small boulders around 1m-ish in size concentrate in one region of the protoplanetary disk. This concentration initially happens because nothing is ever perfectly homogeneous, but it grows because having a group of rocks together helps mitigate the gas drag.

Elizabeth Tasker an associate professor in the Department of Solar System Science at ISAS /JAXA (the Japanese space agency.) Her research focuses on exploring galaxy, star and planet formation using numerical simulations.

This grows until eventually its combined mass is enough that their total gravity finally becomes a big enough force to bind them together into a planetesimal. This doesn’t happen until you have a serious chunk of mass, so the result is always a big planetesimal tens to hundred of kilometers across (about the size of Ceres). A smaller group of rocks wouldn’t have enough total mass to produce the gravitational force needed to collapse.”

And now why the Delbo’ paper is important:

“The formation of our own solar system is the key to understanding the properties of exoplanets around other stars. For example, if we truly want to find another habitable world, we need to understand how the Earth acquired and kept its oceans, developed a protective magnetic field and a sizeable moon, while Venus and Mars did not.

“A problem we face is that the early planet-forming action happened 4.6 billion years ago. We can build models, but how do we tell which one is correct when this all happened so long ago?

“Marco Delbo’ and his team have identified a holy grail; an observational signature that can be used to constrain the myriad of formation ideas we are imaginative enough to create.”


Ceres, Asteroids And Us

Ceres, as imaged by the spacecraft Dawn on a high altitude orbit 900 miles from the surface. The several bright spots on the asteroid have been of particular interest to scientists and are believed to contain salts and ice. The image is mosaic formed from a series of images.  (NASA/JPL-Caltech)

For most of us, asteroids exist primarily as a threat.  An asteroid that landed around the Yucatan peninsula, after all, is generally considered to have set into motion the changes that resulted in the elimination of the dinosaurs.

Other large in-coming asteroids laid waste to swaths of Siberia in 1908, dug the world’s largest crater (118 mile wide)  in South Africa long ago, and formed the Chesapeake Bay a mere 35 million years past.  And another large asteroid will almost certainly threaten Earth again some day.

There is, however, a reverse and possibly life-enhancing side to the asteroid story, one that is becoming more clear and intriguing as we learn more about them where they live.  Asteroids not only contain a lot of water — some of it possibly delivered long ago to a dry Earth — but they contain some pretty complex organic molecules, the building blocks of life.

The latest chapter in the asteroid saga is being written about Ceres, the largest asteroid in the solar system and recently declared to also be a dwarf planet (like Pluto.)

Using data from NASA’s Dawn spacecraft, a team led by the National Institute for Astrophysics in Rome and  the University of California, Los Angeles identified a variety of complex organic compounds, amino acids and nucleobases  — the kind that are the building blocks of life.  The mission has also detected signs of a possible subsurface ocean as well as cryovolcanos, which spit out ice, water, methane and other gases instead of molten rock.

“This discovery of a locally high concentration of organics is intriguing, with broad implications for the astrobiology community,” said Simone Marchi, a senior research scientist at Southwest Research Institute and one of the authors of the paper in Science. “Ceres has evidence of ammonia-bearing hydrated minerals, water ice, carbonates, salts, and now organic materials.”

He said that the organic-rich areas include carbonates and ammonia-based minerals, which are Ceres’ primary constituents.  Their presence along with the organics makes it unlikely that the organics arrived via another asteroid.

In an accompanying comment in the Feb. 16 edition of Science, Michael Küppers of the European Space Astronomy Center in Madrid makes the case that Ceres might once have even been habitable.

The paper provides “the first observations of organic material on Ceres, confirming the presence of such material in the asteroid belt,” he writes. “Furthermore, because Ceres is a dwarf planet that may still preserve internal heat from its formation period and may even contain a subsurface ocean.”

Illustration of the minor bodies in the inner part of the Solar System, including Jupiter trojans and the main asteroid belt. These objects are byproducts of planet formation and have key information about that process. Detecting them in extrasolar systems may help us to understand the early evolution of planetary systems. (NASA)

Asteroids are as ancient as the solar system, some 4.6 billion years old.  They are the leftovers from the planet formation process that took place in the disk around the very early sun —  pieces of rock that didn’t become parts of planets or moons and weren’t otherwise smashed to bits.

Both their ages and their compositions have made asteroids increasingly interesting to space scientists studying how the solar system came to look and behave as it does.  The result has been a suite of missions to asteroids organized by NASA, the Japanese Aerospace Exploration Agency (JAXA), the European Space Agency, the Russian space agency Roskosmos, and the China National Space Administration.

Many of the missions include substantial collaboration between different national space agencies.   The Dawn effort, for instance, has major European involvement. NASA’s OSIRIS-REx mission to the asteroid Bennu and the Japanese Hayabusa2  mission to Ryugu each have three co-investigators from the other agency — a first and an advance from NASA’s more traditional participating scientist program.  Both spacecraft are now on their way, will spend months on their destination asteroids, and are designed to bring home samples (in 2020 for Hayabusa2 and 2023 for OSIRIS-REx.)

NASA also approved two additional asteroid missions earlier this year.  The first mission, called Lucy, will study asteroids, known as Trojan asteroids, trapped by Jupiter’s gravity. The Psyche mission will explore a very large and rare object in the solar system’s asteroid belt — an asteroid made of metal. Scientists believe it might be the exposed core of a planet that lost its rocky outer layers from a series of violent collisions. Lucy is targeted for launch in 2021 and Psyche in 2023.

Why so many asteroid missions?

I put the question to Harold C. Connolly Jr. of Rowan University, mission sample scientist for OSIRIS-REx and a co-investigator for the mission.  He answered by email from Japan, together with Shogo Tachibana of Hokkaido University, who is a principal investigator of the sampling device and the sample analysis of Hayabusa2.  Both are co- investigators for the others’ sample analysis efforts.

“The science is really driving the interest,” they wrote. “There now exists broader understanding that asteroids are time capsules to the past and can help illuminate the origin of Earth-like planets and potentially even the materials and conditions that lead to the origin of life.

“The target asteroids of both missions are a treasure box of the earliest time period of the solar system, with such riches as prebiotic compounds (precursors to life-building organics) preserved in them.”

In Japan, the Hayabusa2 mission is also a follow-on to the hugely popular original Hayabusa mission, which returned with grains from the asteroid Itokawa in 2010. Despite enormous difficulties and the failure of its lander, the spacecraft brought back enough sample to tell scientists that the asteroid was four billion years old, at one time was exposed to temperatures of 800 degrees centigrade, and much more.

Hayabusa inspired so much interest in Japan that it led to not only the follow-on mission but also three movies, including one with star actor Ken Watanabe.

Hayabusa2 launched in 2014 and is scheduled to land on the asteroid Ryugu in 2018, as shown in this artist rendering. The asteroid is believed to be rich in gases and organic compounds. (JAXA/Akihiro Ikeshita)

In a phone conversation, Küppers of the European Space Astronomy Center expanded on the scientific importance of asteroids.

He said that Ceres research has already determined that asteroid most likely was formed further out in the solar system and then migrated in.  This conclusion flows from the observed presence of geological features and minerals on the surface that require the presence of water to form.  Closer-in asteroids are believed to have had any water baked out of them, strongly suggesting that Ceres was once further from the sun.

That asteroidal (and cometary) water plays an important role in the history of Earth.  “The oceans on Earth certainly could have been filled with water, and organic compounds, from asteroids like Ceres,” Küppers said.  Different kinds of water have different isotopic signatures, and the water signature on Earth is very much like that detected in some asteroids and comets.

The Dawn spacecraft has already visited the large asteroid Vesta on its mission, and found minerals formed in water, a geology with steep cliffs and landslides, and the presence of an enormous crater at one of the poles.  For Vesta, as for Ceres, a primary goal of the Dawn mission was to map the asteroid in various ways and with substantial precision.  The overall goal, however, is to explore the conditions and processes found on worlds as old as the solar system.

While Vesta is a described as a “protoplanet” because of its size, Ceres is considered to be a dwarf planet (as well as an asteroid) because it has sufficient mass and gravity to be rounded like a planet.  Vesta, and the other asteroids, are not.  Itokawa, below, is considerably smaller than Ceres or Vesta, and so has been rounded far less.

Ceres, the largest asteroid in the solar system, features areas with concentrations of shiny, white material.  Scientists have described them as likely to be salts and ice.  The dwarf planet contains about one third of the mass in the asteroid belt between Mars and Jupiter, yet it is still dwarfed in size by our moon.  The more detailed of the two images was taken by Dawn from 3,200 miles away. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA


NASA’s Dawn spacecraft captured this image of the asteroid Vesta while in orbit on July 18, 2011. The view looks across Vesta’s cratered and heavily-scarred south pole from a distance of about 6,500 miles. Vesta is the last remaining rocky protoplanet of the kind that formed the terrestrial planets. Numerous fragments of Vesta were ejected by collisions one and two billion years ago that left two enormous craters occupying much of Vesta’s southern hemisphere. Debris from these impacts has fallen to Earth as meteorites —  a rich source of information about Vesta. (NASA/JPL-Caltech/UCLA/MPS)


Dust samples from the asteroid Itokawa, visited by Hayabusa in 2005, were the first sample returns from the surface of an object beyond the moon, and it became a legendary mission in Japan because it had been plagued with so many problems. During the cruise phase, Hayabusa was hit by a solar flare, which damaged its solar panels, and then later in the cruise one of the reaction wheels failed. The lander Minerva also failed but, not to be deterred, the Hayabusa spacecraft itself was sent to land on the asteroid and collect samples.  There was a loss of communications and the probe was told to abort and fly back to a safe altitude. JAXA mission control later learned that Hayabusa had, unknown to them, reached the surface and kicked up some dust that made it into the sample container. (JAXA)


What was planned to be the biggest NASA asteroid mission is the Asteroid Redirect Mission.  It was proposed as the first robotic mission to visit a large near-Earth asteroid, to collect a multi-ton boulder from its surface, and to then redirect it into a stable orbit around the moon. Once in orbit around the moon, astronauts would explore it and return with samples in the 2020s.

The proposed mission was driven by science, but also was part of NASA’s plan to advance the new technologies and spaceflight experience needed for a human mission to the Martian system in the 2030s.  What’s more, some space scientists are concerned about the possibility of a large asteroid heading our way, and they want to develop techniques for just slightly changing an in-coming asteroid’s path so it would miss Earth.

Many in Congress were never excited by the asteroid re-direct plan, and the future of the mission remains quite uncertain.

But the part of the mission involved with learning more about asteroid pathways and how they might be changed is still, at least indirectly, alive.

That’s because the asteroid Bennu, the destination for OSIRIS-REx, is one that often comes close to the Earth.  (The acronym, by the way, stands for the Origins Spectral Interpretation Resource Identification Security Regolith Explorer.)

As explained on the NASA OSIRIS-REx webside, “Bennu is a B-type asteroid with a ~500 meter diameter. It completes an orbit around the sun every 436.604 days (1.2 years) and every 6 years comes very close to Earth, within 0.002 astronomical units (the term used to describe the distance from the sun to Earth.) These close encounters give Bennu a high probability of impacting Earth in the late 22nd century.”

Some put that probability of an Earth impact considerably lower, but it is nonetheless a sobering thought given the destruction that asteroids have periodically inflicted on the planet.