When New Horizons flew past Ultima Thule on New Year's Day, we got our very first
look at a classical Kuiper belt object. It was unlike anything we'd ever seen
before - a contact binary that resembled a dark red snowman. Those initial images
were incredible, but they were also low res thumbnails that were taken just as
New Horizons was approaching it. Those images had to be low resolution because
at six and a half billion kilometres from Earth,
New Horizons' download rate was less than one kilobit per second. But now the
closest images have been taken and they are at full resolution and they are
spectacular. This image was taken when New Horizons was just 5358 kilometers
from Ultima Thule. This image isn't just our closest look at Ultima Thule, it's a
window into the formation of our solar system four and a half billion years ago.
Welcome back to Launch Pad, I'm Christian Ready, your friendly neighborhood
astronomer. And here it is, the best image ever taken of Ultima Thule. In fact, this
image made the front cover of the May 17 issue of Science magazine, The image is
actually a composite of two different camera systems onboard the New Horizons
spacecraft - the Multispectral Visible Imaging Camera, or MVIC is responsible
for capturing the color information of Ultima Thule, while the Long-Range
Reconnaissance Imager or LORRI is a monochromatic camera but it has very
high resolution. So by combining them we get the color and details necessary to
really understand what's going on on this Kuiper belt object. Ultima Thule is
43 astronomical units from the Sun - that's 43 times farther away from the
Sun than Earth. It rotates once every 16 hours but its axis is tilted by 98
degrees and that means it's currently rotating almost face on to the Sun like
a propeller. The daytime side spends 149 years in sunlight while the nighttime
side spends the equal amount of time and darkness. But at this distance the
daytime side barely rises above 60 Kelvin - that's -352 degrees
Fahrenheit or -213 degrees Celsius. Either way you cut it it's cold. The
night side is even colder, reaching all the way down to just
30 Kelvin. Not only is its temperature cold, it's also dynamically cold. In other
words, it follows a circular orbit around the Sun in roughly the same plane as the
rest of the planets. For this reason, it's considered dynamically cold because
nothing ever happened to it that would fling it into a higher, more eccentric
orbit. That makes Ultima Thule a Cold Classical Kuiper Belt Object (CCKBO) because it
never experienced any perturbation from Neptune or anything else that would
fling it into a higher, more eccentric orbit around the Sun. There are hundreds
of thousands of Kuiper belt objects but they occupy a volume of space beginning
at Neptune's orbit at 30 astronomical units extending all the way out to 55
astronomical units so that's a lot of space and therefore the chances of there
being an impact between any two classical Kuiper belt objects is
effectively zero. This means Ultima Thule formed in its current orbit
four-and-a-half billion years ago and has remained undisturbed ever since.
Ultima Thule is a contact binary with the large and small lobes dubbed "Ultima"
and "Thule", respectively. There's no evidence of compression fractures or
other features that are typical as a result of high-speed collisions, so it
seems that these two objects merged together in a slow, gentle collision at
just maybe a couple of meters per second. Such environments were common in the
early solar nebula. Overall, the nebula was mostly gas and dust with the
protosun forming at the center. But throughout the nebula were pockets of
denser particles called "pebble clouds". Objects coalescing in these clouds would
have moved really slow relative to one another,
so low-speed collisions were easily possible. Ultima and Thule likely merged
together in such a low-speed environment four-and-a-half billion years ago. But
its overall shape turned out to be much stranger than it first seemed. Rather
than a snowman, Ultima Thule looks more like a walnut stuck to a giant space
pancake. This really started to become apparent as additional images were
downloaded and compared to each other. The best overall fitting shape model has
Ultima Thule at a roughly thirty kilometers in length 20 kilometers in
width and about ten kilometers in thickness. The
thickness measurement is the least certain because only a small fraction of
the nighttime side could be imaged. So the big question is 'how did Ultima
Thule end up like this?' Well one clue comes from its color, which
is red. But it's really more of a very dark red. Ultima Thule only reflects a
few percent of the sunlight that falls on it, so think more along lines of
potting soil except with a slight tint of red if you shine a bright light on it.
But both lobes are the same shade of dark red, and that's consistent with the
idea that they formed in the same cloud of material. The red color comes from
complex organic compounds called "tholins." Tholins are produced when smaller,
simpler organic compounds are broken apart due to exposure to sunlight. They
then recombine and form these larger, more complex macromolecules that have a
red color. Organic compounds are found on asteroids, moons, and even comets
everywhere we look in the solar system. But the process of tholin creation is
usually interrupted by some thermal event, such as when a comet approaches
the Sun and it really warms up. However, in the outer Kuiper belt, Ultima Thule
and other KBO's never really experienced any dramatic temperature fluctuations so
the red tholin goo just emerges and spreads all over the surface. In fact,
tholins may be the reason we don't see a lot of water in Ultima Thule's spectrum. It
could be that these complex molecules are are masking the water signature
somehow or maybe the water is simply underneath a layer of tholin goo. But
apart from water and tholins, the only other compound detected is methanol. No
carbon monoxide, no ammonia, no acetylene none of the high volatiles that are
common throughout the outer solar system are present. These molecules are abundant
on Neptune's moon Triton and on Pluto. But Ultima Thule is about a thousand times
smaller than those two other objects, so any volatiles that were there
probably have long since escaped to space. There was however one absorption
feature present in the spectra that has yet to be identified,
so I'm going with vibranium. Wakanda forever! Ultima Thule has an extreme
really complex geology with rolling hills, troughs, and some craters. The
largest is the Maryland crater on the Thule lobe. It measures seven kilometers
wide and two kilometers deep. Maryland is probably the result of an impact, albeit
a relatively gentle one. But the other craters are a lot smaller - no more than a
kilometer across. This suggests that while they could be impact craters, it's
also equally likely that they were formed by some other mechanism. For
example, there could have been a drainage or a collapse of the interior that would
have just opened up a crater, or maybe there were pockets of volatile ices that
sublimated once they were first exposed to sunlight. The larger Ultima lobe is
made up of eight separate units that appear to be smooshed together like
monkey bread. Each unit is five kilometers across. It's possible that
they each formed independently and then later merged somehow to become the
Ultima lobe. Such assembly models have been proposed to explain the structures
of comets like 67P Churyumov-Gerasimenko and 9p Tempel 1. But the individual units
making up these comets all have very different sizes. It's really not clear
why Ultima's units all seem to be roughly the same size. Meanwhile, the Thule lobe
doesn't have any evidence of clumping or assembly. It's possible that when
the Maryland crater formed, that event may have resurfaced the rest of the
Thule lobe, filling in and erasing any seams or gaps in between the pieces. The
brightest region is in the neck this may be due to fine particles that roll
downhill after the merger, or maybe they are larger chunks of ice that were
bashed up when the two objects merged together. But then again, why would they
merge together in the first place? Why wouldn't they just continue to orbit
each other? We're going to investigate that in a moment,
but first I want to thank my Patreon supporters for helping to make this
video possible. And I want to offer a special shout out
to my newest patron, Rock Howard. Thank you so much for your support and if you
would like to help keep this channel going for the price of a cup of coffee
every month, please make sure to head on over to my Patreon page. So far we've
uncovered a lot about the true nature of Ultima Thule,
but the big question is why would they merge together, and why would their long
axes be aligned the way they are? That's a very specific alignment, and it's not
something that would occur by random chance. But it is exactly what should
occur if the two objects were tidally locked to one another and merged very
slowly. Otherwise, the two lobes could merge into any random orientation. But if
they're allowed to approach each other slowly enough, then over time tidal
forces will line the two objects up along their long axes. Still, the Ultima
lobe remains unusually flat, and we've never seen anything quite like that
elsewhere in the solar system. But there are two small moons of Saturn that might
offer a clue. Atlas and Pan orbit very close to Saturn's rings and they've
acquired these really large skirts of icy particles from the rings that make
them kind of look like flying ravioli. These two moons are small and they
really don't have very strong gravitational fields so they're able to
build up these really tall walls of snow which turn out to be really effective
defenses against white walkers. Perhaps the individual pieces of the Ultima lobe
formed in the same manner, accreting snow and ice from the surrounding pebble
cloud and then later merging to become Ultima. So now we have a workable model
as to how Ultima could have formed and we also understand how tidal forces can
keep their long axes aligned together, but why are they actually a contact
binary? Why aren't they just tidally locked to each other orbiting a common
center of mass like other Kuiper belt objects? Well one of the big surprises
from the flyby was that no satellites were detected. That's really strange
because you would think that if there is a pebble cloud that collapses, there
should still be some pebbles around acting as satellites. Well maybe there
were satellites in the early days, but they escaped. As they did so, they would
carry away angular momentum and Ultima and Thule would slow down and lose
orbital energy until they eventually merged. Or maybe the lobes lost angular
momentum due to drag in the surrounding gas of the solar nebula. As the Sun
emerged, it blew out a powerful solar wind and later swept away that gas. It's
also possible that there were impacts that
occurred on the leading edges of both lobes, blunting their forward angular
momentum and allowing them to merge. It was probably a combination of
some or maybe all of these mechanisms that overtime dumped angular momentum
away from the two lobes allowing the tidal forces to take hold and bring the
two axes together in alignment as they gently merged at two meters per second.
In fact, you can model a two meter per second collision yourself by taking a
brisk walk into a wall. Or you could just do a computer simulation. This is a
simulation of about two hundred thousand particles which includes both gravity
and sliding friction. Blue particles experience no acceleration and red
particles experience the strongest. It turns out that the neck region
experiences the strongest acceleration and this may help explain why the neck
is so bright. Although these results were published in the May issue of Science
magazine, the manuscript was submitted at the end of February. At the time the New
Horizons team had only downloaded about 10% of the 50 gigabits of data collected
during the flyby. Since then the download total has risen to about 25% and that
includes the closest approached high-resolution images. There's also some
wide-field search data in the hopes of maybe finding some distant satellites of
Ultima Thule. If such satellites are found then that would be a game-changer
because it would allow the densities of the Ultima and Thule lobes to be measured.
That in turn would really help to answer a lot of open questions. Meanwhile, the
New Horizons spacecraft is in great health it has plenty of fuel on board
and a full set of working scientific instruments. It's speeding deeper into
the Kuiper belt, so when it's not transmitting data back to Earth, it takes
more images with its LORRI camera, hoping to find a possible target to fly past
next. Ultima Thule is telling us so much about the conditions of the early solar
nebula and how it formed. But its formation story is not unique.
In fact, most other planetesimals probably formed exactly the same way.
Some of those planetesimals are still around today as asteroids, comets,
and even small moons. But other planetesimals combined with yet more
planetesimals to become proto-planets, which in turn went on to become the
planets that we have today, including the one that you and I are on right at the
moment. You know, the day of the flyby I got to interview Dr. Alan Stern and some
other project scientists from the New Horizons team. It's really cool to go
back and watch those videos and see how much of what they initially thought held
up and how much of it has changed since. I'm gonna have a playlist of those
videos right here and I'll see you over there when we're done here. And please
make sure to subscribe and ring that notification bell so that you don't miss
out on any new videos. I'll see you guys next time and until then, stay curious my
friends.