Ultima Thule latest: an ancient red worldlet, dancing lobe to lobe
Findings from New Horizons continue to shed light on the Kuiper Belt Object. Richard A Lovett reports.
Details about the bizarre shape of a worldlet known as Ultima Thule, 6.5 billion kilometres away, are producing a breakthrough in understanding how the solar system formed, scientists say.
When NASA’s New Horizons spacecraft flew by on 1 January 2019, its first views of the Kuiper Belt Object revealed something that looked like a snowman, composed of two lobes, connected by a narrow neck.
But as later views showed it from different angles, scientists realised that the two “balls” making up the snowman weren’t spheres, but were flattened and elongated – more like deformed biscuits or hamburgers than meatballs.
Now researchers say that it appears that they are also very precisely lined up with each other.
“The two lobes have axes: a long axis, a short axis and an intermediate axis,” says William McKinnon of the Washington University in St Louis, US.
The researcher presented his findings at the fiftieth annual Lunar and Planetary Science Conference, held this week at The Woodlands, Texas.
“It appears that these axes are remarkably well aligned, certainly to within 10 degrees [of each other] and probably even better than that. It is very improbable that this would arise completely by chance.”
This unusual alignment, he says, is an indication that the two bodies that merged to create Ultima Thule didn’t collide randomly.
Instead, they were in orbit around each other with tidal forces from their mutual gravitational attraction causing them to line up with each other. “[Then] they merge and stick on the long axis ... end to end.”
Other evidence shows that when the two objects merged, they did so very gently. “You don’t see signs of violence or stress fractures,” says Alan Stern from the Southwest Research Institute (SWRI) in Boulder, Colorado, the New Horizons principal investigator.
McKinnon adds that the collision probably occurred at a gentle two metres per second – roughly the speed of a brisk walk.
All of this, he adds, lends support to a model of planet formation in which the building blocks formed from the gravitational collapse clusters of pebble-sized particles, rather than from things crashing into each other, randomly.
“The evidence we have from Ultima Thule fits with this picture and not the other,” McKinnon says.
Adds Stern: “That’s a huge step forward.”
Meanwhile, other scientists are studying other aspects of the worldlet, looking to find additional clues to its history.
Colour images, for example, show that it is very red. Most likely, says Carly Howett, also of the SWRI, this is caused by the effect of billions of years of sunlight on organic materials on its surface, in a process that slowly converts them into a class of compounds sometimes called tholins.
“The colouring is consistent with the idea that we’re looking at a primordial object,” she says.
Her SWRI colleague, Silvia Protopapa, adds that other spectroscopic features indicate that there is also water and methanol. That latter, she says, is also indicative of Ultima Thule’s primordial nature.
“It’s in comets and around protostars,” she says.
Other details are emerging about the object’s surface geology. It has at least one large crater that appears to be an impact crater, plus a chain of pit craters that might be from subsidence into a subterranean crack, says Kirby Runyon, a postdoctoral researcher at Johns Hopkins Applied Physics Laboratory in the US.
There may also be sublimation pits, formed when super-volatile compounds such as carbon monoxide burst out from below ground, leaving crater-like indentations on the surface.
There are also rolling hills, lumps, troughs, and even signs of avalanches.
“Even though the gravity is only 1/100,000 that of Earth, things still fall downhill,” Runyon points out.
In addition, the neck between the two lobes appears to be bent to one side. “That may be indicative of how those two lobes slowly collided,” Runyon says.
Stern adds that a lot more data is yet to come. “Most of the data from the flyby is still on the spacecraft,” he says.
Once the rest of the data is returned, sometime in late summer of 2020, he says, New Horizons will continue to study the interplanetary medium in the Kuiper Belt and to turn its onboard telescope onto whatever other Kuiper Belt objects happen to be close enough to be visible.
But it also has still has manoeuvering fuel, offering the prospect of another pass.
“We have more fuel on board than it took us to target this flyby,” Stern says.
There’s just one problem: it was hard enough to spot Ultima Thule from Earth, so that the spacecraft could be directed toward it.
“[And] as we get father and farther out, the objects are fainter and fainter as seen from Earth,” Stern explains.
Most likely, he says, New Horizons will have to search for its next target using its own telescope – hoping that it spots something in time to reach it with its remaining fuel.
“We probably only have the fuel for one,” Stern cautions, “[and] we may have to be lucky to get that one.”