In the far reaches of the solar system, a small space rock shows us how giant planets arise.
Arrocot – the most distant and most primordial world ever visited by a human spaceship – now reveals its secrets in three new explorations.
These results may resolve some controversy about how planetesimals are formed – small rocky 'seeds' that turn into planets. And this process looks much 'softer' than previously thought.
“Arrocot is the farthest, most primitive, and oldest object ever explored by a spacecraft, so we knew it would have a unique history,” said Alan Stern of the Southwest Research Institute in Colorado and principal investigator for the New horizons'.
“The spacecraft gave us information on how planetesimals were formed, and we believe that the result marks significant progress in understanding the general formation of planetesimals and planets.”
The New Horizons probe visited Arrocot, formerly known as (486958) 2014 MU69 or Ultima Thule, last year as it departed from the Kuiper Belt.
At an average distance of 6.7 billion kilometers from the Sun and an orbital period of 293 years, Arrocot is the most distant single object in the solar system that astronomers have discovered.
So far from the Sun, out of the reach of harsh solar radiation, and with a stable orbit, Arrocot has been pretty much a time capsule since the solar system formed 4.6 billion years ago.
Last May, the first stream of research was published, detailing the initial results of this flight, based on only 10 percent of the data New Horizons was still sending home to Earth.
Various groups of scientists have discovered that Arrocot was once a binary object, the two halves of which softly fused together, although the processes that led to this were unclear; and that its surface was predominantly red, although what gave it that color was not known.
Now, after analyzing the data used in these original documents 10 times, some of the questions have been answered.
Typically, there are two competing theories about how planets are born.
According to a long-standing hierarchical model of planetesimal accretion, the building blocks of planets form when different parts of the solar nebula – the cloud of gas and dust that formed the sun and planets – dissipate.
On the other hand, the build-up model assumes that elements from the same area gradually and gently come together to form binary objects.
The latest data gives weight to the latest model.
According to the researchers, if the Arrocot were formed from pieces connecting from different parts of the nebula, evidence of collisions would be visible.
'There is no evidence,' the team wrote in their paper, 'about heliocentric, high-speed collisional evolution or any catastrophic (or sub-catastrophic) impact during its life cycle … Instead, we conclude that its two lobes came together at a low speed level, not more than a few meters per second and possibly much slower. '
This suggests that the two lobes formed in the same part of the solar nebula – a cloud of gas and dust that formed the sun and planets.
“Arrocot looks like this, not because it was formed by collisions, but in a more complex dance in which its composite objects slowly rotate around each other before uniting,” McKinnon said.
Second article by astronomer John Spencer of the Southwest Research Institute and colleagues studying the surface of the Arrocot. They confirmed that it is smooth and slightly cratered, which is in stark contrast to other objects in the solar system.
They also confirmed that the Arrocot did not have rings or satellites over 180 meters within a radius of 8,000 kilometers, or atmosphere, gases or dust, the presence of which would indicate a relatively recent collision. This indicates that Arrokot was not disturbed for a very long time.
But they also took a closer look at the Arrocot craters and found that the object's surface, about 4 billion years old, is almost as long as the solar system itself.
Overall, despite the lack of craters on its surface, the observed crater density is consistent with an age of about 4 billion years.
Finally, in the third article, astronomer Will Grundy of the Lowell Observatory and his colleagues studied the peculiar color of the Arrocot. The reddest naturally occurring material – the so-called 'infrared matter' – in the solar system can be found in the Kuiper Belt, and Arrocot is covered in it, but the exact nature of the material is unclear.
The team found the object to be uniformly cold and red, covered in methanol ice and complex organic molecules that they could not accurately identify based on the limited spectral data that New Horizons was able to collect. These molecules are likely to create the color red.
This not only confirms that organic molecules are the source of the infrared substance; the uniformity of color – as well as the age of the surface as determined by Spencer's team – also supports the conclusion that Arrocot was formed in a highly localized region.
“Arrocot has a physical characteristic that slowly collected together with 'native' materials in the solar system,” Grandi said. “An object like Arrocot would not have formed and looked the way it is in a more chaotic accretionary environment.”
There is probably not much data left about the Arrocot that the probe can send to Earth, so any future analysis should be based on what we already have. But it seems that these distant Kuiper belt objects could tell us a lot more about the birth of our solar system.
“This is not just a space potato. It's a wonderful world that has told us a wonderful story. '
The articles were published in the journal Science and can be found here, here and here.
Sources: Photo: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute / Roman Tkachenko
