- (For many years, the looping rilles on Europa were called
"fractures", but the model developed to explain them only highlighted
the failure of the underlying interpretation.)
-
- The picture above shows a series of "cycloidal"
grooves cutting across the surface of the Jovian moon Europa. The longest
extends more than 1600 kilometers and each of its many loops averages about
100 kilometers in length. (To envision a cycloid path, imagine a spot on
a wheel rim as the wheel rolls along a flat surface (see inset). The path
traversed by the spot combines two kinds of simultaneous motion -- rotational
and linear).
-
- Though NASA scientists never doubted that these prominent
features were surface cracks or "flexi", the unique form posed
a great mystery to them. But that situation changed when Randy Tufts, a
geologist at the University of Arizona, proposed an explanation. He suggested
that the oddly replicated curvature of the cracks was caused by changes
in the gravitational tug from Jupiter in the course of Europa's 85-hour
orbit of the gas giant. Because Europa keeps the same face toward Jupiter,
a Europan orbit is the same length as a Europan "day".
-
- Tuft's suspicions inspired orbital dynamicist Gregory
Hoppa, also of the University of Arizona. Assuming an ocean of subsurface
water, he produced a computer model of looping cracks in the Europan ice
sheet. The fractures were caused by tidal forces acting on the water below,
he said.
-
- In September 1999, the journal Science published a report
by Hoppa and his colleagues. The graphic beneath the Galileo image, taken
from the Science article, illustrates the proposed model of cycloidal fracturing.
The arrows represent the amplitude and orientation of the tensile stress;
and the numbers below the arrows indicate the hours of the orbit. You can
also view a brief animation of the "model" at- http://pirlwww.lpl.arizona.edu/~hoppa/science/cycloid_69s_240w.gif
-
- Scientists greeted the paper as an "elegant"
breakthrough. But was it a breakthrough, or another example of computers
simply spitting out the results they were programmed to deliver? Computers
are, after all, the generators of virtual reality.
-
- The repeated loops on Europa have no analog in observed
fracturing patterns of brittle material. When ice cracks, the fracture
typically propagates at the speed of sound. But to match Europa's slow
rotation, the computer model required something never observed -- an ice
fracture propagating at a walking speed of about 3 kilometers per hour.
-
- Fracturing of ice is chaotic and profoundly affected
by variations in thickness, composition and surface relief. But the model
allowed for no such influences. On Europa, everything present on the surface
BEFORE the cycloidal grooves were cut had to be ignored by the computer
-- even the maze of pre-existing channels traversed by the loops. Was this
a reasonable simplification? Consider the way a sheet of glass breaks.
The presence of a shallow groove left by a glass cutter is sufficient to
control the direction of a break. Fracturing ice could hardly ignore preexisting
channels on Europa!
-
- The model assumed a global ocean beneath the ice so that
tidal bulging on Europa could reach nearly 100 meters (330 feet) at maximum.
That may sound like a substantial distance, but in relation to the radius
of Europa, which is larger than our Moon, it is trivial. In Europa's tidal
bulge, tensile stresses on a global ice sheet would be dispersed over hundreds
of kilometers. Ice is quite plastic when stresses are applied slowly. There
is no reason to believe that across such distances a sheet of ice could
not make the imperceptible adjustment required.
-
- Other arbitrary assumptions were needed to make the model
"work", and most strayed far from facts already established.
In order for cracks to form, the computer model required a maximum ice
sheet thickness of about a kilometer. But some of the surface channels
cut by the loops are several kilometers deep. Had the computer program
included this fact it would have broken down immediately. But the manipulated
data gave the desired result, enabling science writers to complete the
circular logic: the model became "a powerful argument that an ocean
of water underlies Europa's crust".
-
- In the model each loop represents a Europan day. On this
reasoning, the longest flexus must have formed in about a half month. But
if this is evidence of an ocean today, why are there so few flexi? By now
all other features should be obliterated by the effects of the daily cycloidal
forces.
-
- To produce the succession of loops, the cracking had
to stop after each loop, then resume from that precise point on the next
orbit. In fact it is extremely difficult to discern the "rules"
the computer was instructed to follow. Delphi Flexus and its neighbor,
Sidon Flexus have opposed curvatures, requiring that the "cracks"
progress in opposite directions. The computer program must have allowed
for this variability even as it required the "tensile stress vectors"
to seamlessly extend the fracture from "day" to "day"
on Europa.
-
- One thing should be obvious. The forces that created
the Europan enigmas are not acting on the Jovian moon now. To simply acknowledge
this fact would go a long way toward freeing science from the influence
of ideology. But will NASA's experts pause long enough to entertain a simple
electrical hypothesis?
-
- We claim that Europa was repeatedly entangled in discharge
streamers emanating from Jupiter. Since ice is more homogeneous than rock,
the surface discharges on Europa tended to produce relatively straight
rilles following a strong electric field along great circles from the sub-Jovian
hemisphere (the hemisphere facing Jupiter) to the opposite hemisphere.
That is the general pattern. However, circular motion of an arc can be
demonstrated in the lab when a magnetic field lies parallel to a cathode
surface. We are confident that something of this sort occurred in Europa's
electrical transactions with Jupiter. Occasionally, the powerful magnetic
fields carried by the discharge streamers draped over the moon's surface,
inducing a near-surface electric current and magnetic field in the ice.
As in the laboratory experiments, this would cause the traveling electric
arcs to rotate in cycloidal fashion, producing the distinctive looping
furrows.
-
- In response to changes in the strength of the ambient
magnetic field, the loops in these cycloidal rilles would, as now observed,
display a slowly changing radius of curvature.
-
- What is needed in the case of Europa's surface mysteries
is not an arbitrary computer-generated "solution", but practical
experiments to test a new hypothesis. Can electrical discharges produce
the rille patterns seen on Europa? The required experimental work is easily
within reach if NASA's investigators will follow evidence already in hand.
The resulting insights could change the direction of planetary science.
They could also inspire something our space program lost under the inertia
of outdated theory -- the passion for discovery.
-
- As background we recommend these previous submissions-
-
- Oct 19, 2004 Europa Caught in the Middle -- http://www.thunderbolts.info/tpod/2004/arch/041019europa.htm
- Mar 10, 2005 Electrical Scars on Europa -- http://www.thunderbolts.info/tpod/2005/arch05/050310europascars.htm
- Mar 11, 2005 Electrical Rilles of Europa -- http://www.thunderbolts.info/tpod/2005/arch05/050311europarilles.htm
- Mar 14, 2005 Europa-Dynamics of Rille Formation -- http://www.thunderbolts.info/tpod/2005/arch05/050314europadynamics.htm
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