Rosetta Mission Update
Jets of Comet 67P ― Failed "Explanations" Continue
It seems that the puzzle of cometary jets continues to haunt comet science.
The mystery has been stated and restated for decades.
But it's possible that a resolution is now within reach
through the Rosetta mission to comet 67P Churyumov-Gerasimenko.
As a comet moves into the inner solar system,
surface activity creates a cloud of gas and dust to form a roughly spherical coma.
With a closer look, we see radial jets erupting from the surface
at remarkable velocities to feed gas and dust into the coma.
What is acting on the surface to create the jets?
And what is holding the accelerated gas and dust in its collimated form,
a persistently narrow and filamentary structure
in the vacuum of interplanetary space.
Rosetta investigators have calculated the speed of the 67P jets at their source.
They give the figure as 700 meters per second.
That means gas and dust departing the comet nucleus
at a speed of 7,5 football fields in one second.
Theorists have offered an explanation of a comet's jets.
Uneven warming of an irregular surface
creates pockets of sublimating gases beneath the surface.
These pockets of gas become the reservoirs of geyser-like activity.
"Gas and dust bursts forth like a geyser...
An area below the surface warms up and punches through the surface
with explosive force due to its gas content.
In comets, the emerging gas carries a stream of dust along with it."
But the theory offered cannot be reconciled with data now in hand.
The speed of 67P's jets is significantly greater than the speed of gas
escaping through a small hole in a high-pressure gas line.
But the shallow surface layer of dry dust on 67P,
offers nothing like the resistance necessary to build up such pressure.
And if the jets do indeed point to subsurface pockets
of gas breaking through a dusty surface,
it seems quite clear that the signature of such activity would be readily apparent.
An energy curve suggestive of an explosive release of gas pressure.
And an explosive expansion and dissipation of the released gas
across a short distance in the vacuum of space.
And of course, we would also expect immediate exposure
of the supposedly sublimating ice beneath the dry surface.
Do we see any sign of these expected signatures of jet chambers below the surface?
It's fair to say that to date nothing of the sort has been observed.
And yet, the creation of comet jets, the acceleration away from the nucleus
and the structure maintained across vast distances,
must have a reasonable explanation
even if we have to look beyond standard theory to find it.
It was just a few years ago that the spectacular jets of comet Hartley 2
placed a huge exclamation point on the need for new perspectives.
Powerful jets were seen not just from the sunlit side of the comet
but from the region in shadow
and the persistent energies of the jets offered no support
for the standard idea of exploding pockets of gas.
All we've actually seen at the source of comet jets,
is evidence of electrical activity on the surface,
extending valleys by eroding away cliffs, mesas and ridges.
We saw this on comet Tempel 1
where such activity had eroded a mesa cliff some 35 meters.
Considering the simplicity of the standard model of cometary jet activity,
one wonders why no experimental verification has ever been produced.
One of the simplest experiments conceivable would simply use a laboratory vacuum chamber,
place into the chamber the ideal concoction of ice and dust
just to see if remote warming would produce anything like the jet activity imagined.
We unhesitatingly predict that when the obvious required experiment is performed,
it will leave every comet investigator in a state of profound disillusionment.
Cometary jets are not driven by thermodynamics.
They are electrical phenomena naturally occurring in the transaction
between a charged body, a comet, and the electric field of the Sun.