How Ultima Thule Is Like a Sticky, Pull-Apart Pastry

How Ultima Thule Is Like a Sticky, Pull-Apart Pastry

HOUSTON — To Kirby Runyon, a planetary scientist at the Johns Hopkins Applied Physics Laboratory, the small distant object that NASA’s New Horizons spacecraft flew past on the first day of this year reminds him of monkey bread.

For people not familiar with this sweet pastry, monkey bread consists of balls of dough that are piled into a pan. As they bake, the balls combine into a larger whole. It is a structure that may help explain the process that turned bits of dust and gas into planets in the early solar system.

New Horizons traveled some 4 billion miles to take a close-up look at this 22-mile-long world in the solar system’s icy Kuiper belt beyond Neptune; it is officially designated as 2014 MU69 and nicknamed Ultima Thule. Planetary scientists have never been able to do so for an object this distant, a small shard that has been frozen and almost unchanged since it formed 4.5 billion years ago.

Ultima Thule was not as picturesque as New Horizons’ first target — Pluto, in 2015 — but it was full of surprises. The shape turned out to be unlike anything seen in the solar system. It was not just a simple ball, but two objects that at some stage touched and stuck together, like a snowman. Then the scientists discovered that the two lobes of Ultima Thule were not spherical, but more like lumpy pancakes. The larger lobe is also flatter than the smaller one.

The New Horizons team presented its newest findings on Monday at the Lunar and Planetary Science Conference in The Woodlands, Tex., a suburb of Houston.

At a news conference, Dr. Runyon of the mission’s science team described how lines on the surface of Ultima Thule appeared to delineate distinct clumps that might have once been smaller bodies.

“Just like Ultima Thule, they come together to form something wonderful,” Dr. Runyon said.

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That is a change from older ideas of planetary formation: that objects called planetesimals grew slowly and steadily in size. Ultima Thule suggests an alternative scenario in which a bunch of similarly sized objects merged together over a shorter interval of time.

Ultima Thule is very dark, reflecting just 7 percent of the light that hits it, but the scientists are still figuring out the origin of some patches that are somewhat less dark as well as spots that are even darker. They also are studying which depressions on the surface were carved out by collisions with smaller objects and which were the result of other processes, like the escape of gases from the object’s interior.

With the flattened shape of the two lobes of Ultima Thule, another surprise was that the rotational axis of each were nearly parallel to each other. William B. McKinnon of Washington University in St. Louis, a co-investigator on the mission, said the finding strongly suggested that the two lobes formed out of one rotating cloud of icy particles.

When the two finally touched, the collision was slow, about 2 meters per second, or 4.5 miles per hour.

“Which, if you take a brisk walk into a wall, you’ll find out what that’s like,” Dr. McKinnon said. “But I don’t recommend you do that.”

S. Alan Stern, the principal investigator for New Horizons, said he was surprised that the spacecraft has not discovered any moons around Ultima Thule. Although objects in this part of the solar system are comparatively small, a large fraction of them — perhaps a third or more — form in pairs orbiting each other.

However, the dynamics that caused the two lobes of Ultima Thule to move closer to each other might have ejected any other objects in the original cloud.


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