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Today, physicist Eugene Bagashov continues his analysis
of the scientific missions to the asteroids Bennu and Ryugu respectively.
As Eugene explained in the previous episode,
one of the hypotheses scientists have proposed to explain a number of asteroid anomalies
is that many asteroids are essentially "rubble piles."
In fact, when asteroids have occasionally displayed dramatic
and completely unexpected comet-like tails,
the explanation astronomers have offered is that the so called rubble piles
simply began spinning faster and faster, hurling material into space.
However, as Eugene explains as he continues his discussion,
the latest scientific data offers little support for the rubble-pile asteroid hypothesis.
Let us now talk about the other issue, that is the spin of these asteroids.
Currently, Ryugu has a spin period of 7.63 hours and Bennu of 4.29 hours.
But here the interesting stuff begins.
One thing that the researchers from Hayabusa2 Mission keep referring to
is the hypothetical long-term trend in slowing down of the spin of Ryugu.
In that way they are able to, more or less, explain on the basis of the rubble pile model,
the surface slopes of the asteroid and also the distribution of boulders on its surface.
And they attribute this hypothetical spinning down to the so called
Yarkovsky-O'Keefe-Razdievskii-Paddack effect or YORP.
This effect basically implies that the irregular form of the asteroid
leads to the imbalance in its thermal emission in different directions,
which in turn causes the object to experience some very small torque,
and on long timescales it might alter the rotational period quite a bit,
introduce tilting of the spin axis, wobbling and other things like that.
Well ok, perhaps, but let us look at Bennu now.
Here we see not the hypothetical but actually observed spinning up of the asteroid.
It has been derived from a series of ground- based observations,
the earliest one being made in 1999, and now confirmed by the OSIRIS-REx data.
Bennu is spinning faster and faster,
losing about a second of spin period each century.
That would mean that the doubling of the spin rate would happen in only 1.5 million years
and the rubble-pile model used by the researchers in OSIRIS-REx team
predicts that it would be incompatible with the stability of the asteroid.
At this rate of spinning up,
it should have been torn apart by centrifugal forces very long time ago.
Perhaps the reasonable conclusion that should follow from that
is that the body should be much younger than even ten million years
but the researchers are obviously not happy with that conclusion,
so to somehow cope with this, they invoke additional and not very plausible assumptions
such as some unexplained long-term cycles of spinning up and spinning down etc.
But that's not even the real problem.
The problem is that they're trying to explain the observed spinning up of Bennu,
with the same YORP effect that Hayabusa2 team
is using to explain the hypothetical spinning down of asteroid Ryugu.
It starts to make really little sense from that point.
Now, it is true that theoretically, and I repeat ― theoretically,
YORP might indeed lead to both spinning up and spinning down
and that would depend mostly on the shape and orientation of the object,
but the thing is that Bennu and Ryugu have almost exactly the same shape.
More than that, they have almost the same orientation
Take a look at this picture for example.
Here I've plotted the orbits of inner solar system planets, that is;
Mercury, Venus, Earth, and Mars.
On the next picture, I've added Bennu's orbit in darker blue
and on the third picture, I've also added Ryugu's orbit in red.
So if I now add here the orientations of their spin axes,
you'd immediately understand what I mean.
Numerically speaking, their spin axes differ less than 8 degrees
so they're pointing almost in the same spot in the sky.
Note also that they are both spinning backwards, just like Venus,
here I've plotted all the other planets' spin axes.
So from that I conclude that if YORP is indeed acting upon both of these asteroids,
it should follow that it would have the same effect on both of them
and therefore either Hayabusa2 or OSIRIS-REx team is wrong in interpreting their data
or maybe even both of them,
but since we're already looking at orbital diagrams,
which I personally enjoy doing,
let me note another thing that might not be immediately evident.
Both these asteroids essentially,
at some point of their orbits, cross the orbit of Earth.
I have indicated these crossings with purple circles.
In fact, both these asteroids might potentially impact Earth in the next couple of centuries,
and I'm only mentioning this to support my next statement
that in my opinion, the orbits of these objects
might indicate that they actually originate from this area of the solar system
and they're not some intruders from the inner asteroid belt.
It would only seem logical that if they both originated from the catastrophes
that involved, for example, planet Earth and Mars, or maybe Venus too,
they wouldn't stray too far from the area where this catastrophe occurred.
That would potentially explain why both these asteroids
move roughly between Earth and Mars,
even more so if one looks at the exact arrangement of the orbit of Ryugu,
another fascinating thing would surface,
that its perihelion is really well aligned with Earth's,
if one discards the difference in orbital inclination.
The coincidence is quite incredible,
both Earth's and Ryugu's ecliptic longitude of perihelion is 102 degrees.
Here I've shown the perihelia positions with thin black lines.
Yes, potentially it is possible to explain all of that
through some long-term gravitational interactions,
tidal forces and resonances, and things like that,
but remember that first we need to somehow drag these objects here
from the inner asteroid belt without them being destroyed
or thrown away into the outer system by these gravitational interactions.
So maybe it would be reasonable to assume that these alignments
are the result of these asteroids being born from some catastrophic processes
involving the inner planets in the first place,
maybe not even the Earth but the Moon for example,
as I've noted that the spin axes of Bennu and Ryugu
are almost exactly opposite to the Lunar spin axis
depicted with grey arrow in this picture.
There's only 4 to 5 degrees difference between them.
OK, let's now return to the issue of the spin rate change.
What are the options from the standpoint of electrical excavation
of these bodies from planetary crusts?
Of course, theoretically here YORP effect might also play a role
so that explanation is possible,
but at the same time I wish to repeat the points made in the previous video
following from that paper with liquid droplets,
that perhaps these objects continue to very slowly relax to a more spherical shape
after being squashed by electrical forces during the time of their birth.
In this case, as more and more matter moves closer to the spin axis,
the angular momentum conservation would dictate that the body should spin faster and faster.
If that is true, theoretically if we'd observe the objects long enough,
we would see the very slow shrinking of their equatorial radii
in favor of higher latitudes radii.
Unfortunately however, we haven't been able to determine their shape
with good enough precision previously.
The mean value of ground-based radar observations gives a diameter of Bennu of 492 meters,
while OSIRIS-REx shows a 490 meters of diameter,
so one could say that indeed the Equator is shrinking,
but the error bars of the first measurement are so big
that they include the second value as possible too.
And finally, at the moment I wouldn't even discount
possible electromagnetic effects of spinning up the asteroid,
especially if it's at least slightly magnetized.
Its interaction with the solar wind plasma might potentially induce a torque
that would spin it up or down depending on the field configuration.
Now, let's discuss the surface properties of these asteroids.
One thing that both Hayabusa2 and OSIRIS-REx teams indicate
is the noticeable lack of small craters.
They assume that it is the consequence of either the dynamic reshaping of the surface
or of the presence of regolith, that is dust, that would quickly bury them.
The problem with that however, is that both Ryugu and Bennu
demonstrate some craters even in the equatorial region,
which suggests that their equatorial bulges
appeared before subsequent alteration of their surfaces
and the surface hasn't completely relaxed ever since.
But researchers themselves acknowledge,
especially in case of Ryugu with its hypothetical spinning down,
that the equatorial bulges should be really old
so why didn't they also reshape during all these supposed hundreds of millions of years?
Large craters also did not relax to the overall shape of their surroundings
and their depth to diameter ratios of about 0.16
are consistent with other asteroids such as Eros and Vesta
which have this parameter of about 0.15 but not in any way represent rubble piles.
That seems not really consistent with a rubble pile hypothesis
and perhaps indicates a higher internal strength of the underlying material
than what is assumed by the researchers.
Conversely, in the electrical scenario, some of the craters
might have appeared due to plasma discharges on the surface
as it was rebalancing its electric potential after the initial catastrophic event
that has led to the formation of the body.
So perhaps the deficit of small craters might be explained
if one assumes that the strength of the surface is underestimated
so that only really energetic processes would leave craters.
And it doesn't really matter if we're talking about discharges or impacts here.
It's quite peculiar that asteroid Ryugu has twice more large boulders
than asteroid Itokawa per unit of area, but the same number of craters,
and in the case of Bennu, the researchers themselves admit
that these boulders and their complexity
"...imply energetic events that far exceed what Bennu can support."
The researchers actually sometimes start contradicting themselves and state that,
"Evidence for structural strength includes Bennuis nonhydrostatic shape,
the high-standing longitudinal ridges, the long linear grooves, and apparent mass wasting.
A fluid-like hydrostatic shape ...
with Bennu's density and rotation rate is not stable."
So how then is, this supposedly rubble-pile object exists?
Maybe it's not a rubble pile after all?
With regards to the regolith, I should note that
it seems that there is much less of it than what was assumed.
For example, MASCOT lander has found no regolith where it landed on the surface of Ryugu
and the same seems true for the photos from other two landers.
It's definitely very rocky out there but also not very dusty.
In fact, OSIRIS-REx team has been complaining in some of the papers
that the abundance of rock and the deficit of pristine flat dusty areas
might compromise the success of the mission, since initially they,
"...hypothesized that, over time, gravel migration had built up the equatorial ridge
that was apparent in the radar shape model.
Even though the equatorial region is the geopotential low,
it is in fact dominated by large concentrations of boulders
with little apparent fine-grained regolith.
Bennu does not contain the extensive patches of fine-grained regolith
according to which we designed the mission."
Other fascinating find is a relatively low values of thermal inertia,
on average 350 for Bennu and 200 to 500 for Ryugu,
where the units are joules, divided by meters squared,
divided by Kelvin, divided by square root of a second.
Compare that to some other similar objects' thermal inertia,
for example asteroid Itokawa has 750
and the lowest thermal inertia documented in a meteorite is 770
in the meteorite Cold Bokkeveld.
Basically, thermal inertia is a measure of how hard it is to heat up or cool down a body,
so in my opinion, low values of thermal inertia of asteroids Bennu and Ryugu
would imply a relatively good heat conduction
and therefore a relatively strong internal structure, at least in the near surface layer
that would allow the heat to be quickly transferred to and from the asteroids.
Now, this is not a definite evidence for that per se,
as the properties of boulders and even small amounts of dust covering the surface
would also play a huge role in the measured value of thermal inertia,
but coupled to some other hints that I've mentioned,
in my opinion it provides a more or less coherent picture.
Speaking of boulders, Hayabusa2 team
acknowledges that they seem to be internally strong.
I quote, "The shallower slope in the small size range suggests
a non-negligible mechanical strength of individual boulders/pebbles on Ryugu."
And also I quote,
"...the boulders on Ryugu have survived impact processes during catastrophic disruption,
the re-accumulation process and more recent impacts on Ryugu;
they are not dust balls with little cohesion."
So if these boulders are supposed to represent some primordial material
and they demonstrate significant internal strength,
why is the rest of these asteroids considered to be so weak?
Maybe they aren't after all.
We'll examine some more evidence for that in the next video
and also talk about the recent finding of dust grains in space around Bennu.
Stay tuned for part 3