SETI [Fwd: [META] Cracks Best Evidence Yet for Europan Ocean]

Robert Owen (
Fri, 17 Sep 1999 12:13:04 -0400

Larry Klaes wrote:

> From: Larry Klaes <>
> >From SpaceViews:
> Cracks Best Evidence Yet for Europan Ocean
> Published: 1999 September 16
> 2:42 pm ET (1842 UT)
> An unusual set of cracks on the icy
> surface of Europa may be the most
> convincing evidence yet that the moon
> of Jupiter harbors a liquid water ocean
> under the ice, University of Arizona
> scientists reported this week.
> In a paper published in the September 17 issue of the journal
> Science, four University of Arizona researchers concluded that a
> set of curved cracks called "flexi" on Europa are caused by tidal
> stresses from a subsurface ocean.
> Flexi are cycloidal cracks that appear as a series of arcs,
> joined together at each end to form a long, wavy crack across
> the surface. The cracks, unique to Europa, were first noticed in
> Voyager images in 1979, and have been studied in more detail
> more recently by Galileo, but have defied explanation until
> now.
> "What causes the cycloid to form is that Europa is in a
> slightly eccentric orbit because of Io and Ganymede," two other
> large moons of Jupiter that orbit closer and farther from the
> planet than Europa, explained lead author Gregory V. Hoppa.
> The varying distance of Europa from Jupiter causes tides in
> the hypothetical subsurface ocean to rise and fall as Europa is
> closer and father, respectively, from the planet. These tides can
> rise and fall as much as 30 meters (110 feet), compared to the 1-2
> meters (3.3-6.6 feet) for most terrestrial tides. "This causes
> Europa's ice shell to flex," said Hoppa.
> When the tidal stress exceeds the tensile strength of the ice, a
> crack forms. That crack propagates along a curved path on the
> surface until the stress drops below the strength of the ice, at
> which point the crack stops.
> Each arc in the flexi is 75 to 200 km long, and forms over the
> course of 3.5 days, the orbital period of the planet. "You could
> probably walk along with the advancing tip of a crack as it was
> forming," Hoppa said. "And while there's not enough air to
> carry sound, you would definitely feel vibrations as it formed."
> Many scientists have believed that Europa has an ocean of
> liquid water, perhaps up to 100 km (62 mi.) deep, below its icy
> surface, but have relied on only indirect evidence from other
> surface features, and models of the interior of the moon, to
> reach that conclusion. The Arizona scientists believe this is the
> most convincing evidence yet since no other mechanism can
> explain the formation of the flexi.
> "What amazes me about this is just how long these features
> have been a mystery," Hoppa said. "We've been staring at
> pictures of them for 20 years, since Voyager. We didn't know
> what made them. And it seems what they've been telling us all
> along is that an ocean was there when these things formed."
> A definitive answer may have to wait until next decade,
> however. NASA is planning a Europa Orbiter mission,
> scheduled for launch in 2003, that would include instruments
> such as an ice-penetrating radar that could see through the ice
> to any ocean below. The fate of that mission, though, is
> dependent on what budget cuts Congress enacts in NASA's
> fiscal year 2000 budget.
> Images and animations of the Europan cracks:
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Robert M. Owen
The Orion Institute
57 W. Morgan Street
Brevard, NC 28712-3659 USA

This archive was generated by hypermail 2.0b3 on Sun Oct 10 1999 - 15:46:35 PDT