SETI [Fwd: UA Scientists Say Ocean Tides Create Europa's Unique 'Cycloid' Cracks]

Robert Owen (
Fri, 17 Sep 1999 12:14:01 -0400

Larry Klaes wrote:

> >X-Authentication-Warning: majordom set sender to
> owner-astro using -f
> >Date: Thu, 16 Sep 1999 23:05:54 GMT
> >To:
> >Subject: [ASTRO] UA Scientists Say Ocean Tides Create Europa's Unique
> 'Cycloid' Cracks
> >Sender:
> >Reply-To: Ron Baalke <BAALKE@KELVIN.JPL.NASA.GOV>
> >
> >News Services
> >University of Arizona
> >
> >Contact Information:
> >Gregory V. Hoppa
> >520-621-6520 (pager: 520-446-9127)
> >
> >
> >UA scientists say ocean tides create Europa's unique 'cycloid' cracks
> >
> >September 16, 1999
> >
> >TUCSON, Ariz. -- When Voyager flew by Jupiter's moon Europa in 1979, it
> >photographed geological surface features unlike any others ever seen in the
> >solar system. Near Europa's south pole, chains of scalloped lines joined
> >arc-to-arc at the cusp ran for hundreds of miles across the frozen,
> >fractured surface.
> >
> >Until now, there have been no good ideas as to what formed these bizarre
> >"cycloidal" features, or "flexi," as they were officially dubbed by the
> >International Astronomical Union.
> >
> >Now, planetary scientists at the University of Arizona in Tucson provide a
> >model for how these features are created. It is perhaps the most convincing
> >evidence yet for a global ocean. They report on it in today's issue of
> >Science (Sept. 17).
> >
> >Scientists know that Europa has a 100-mile-thick layer of water -- 20
> >times thicker than the Earth's oceans -- but the visible top layer is frozen.
> >This new strong evidence for a liquid global ocean below the surface makes
> >Europa a prime target in the search for life beyond Earth.
> >
> >Gregory V. Hoppa, B. Randall Tufts, Richard Greenberg and Paul E. Geissler
> >of the UA Lunar and Planetary Laboratory theorize that cycloidal cracks
> >form in Europa's solid-ice surface with the daily rise and fall of tides in
> >the subsurface ocean. They painstaking modeled and scrutinized images of
> >Europa taken by the Galileo spacecraft between 1996 and 1999. The new
> >images show that cycloidal cracks and ridges are widely distributed over
> >all of the moon.
> >
> >Hoppa has posted images and explanatory animation of cycloidal crack
> >formation on the web site:
> >
> >
> >Europa is about the size of our moon. Tidal stresses on its ice-covered
> >ocean ebb and flow as it orbits Jupiter, which is 300 times as massive
> >as Earth. According to the UA researchers' model, Europa's ocean tides
> >rise and fall a distance of 30 meters. By comparison, tides at most ocean
> >beaches on Earth rise and fall 1 to 2 meters, or 4 to 6 feet.
> >
> >"What causes the cycloid to form is that Europa is in a slightly eccentric
> >orbit because of Io and Ganymede (other Jovian moons). Sometimes Europa
> >is a little closer, other times a little farther from Jupiter. When Europa
> >is closer to Jupiter, the tides are higher because Jupiter is pulling on it
> >more. When Europa is farther, the tides fall because Jupiter's force falls.
> >This causes Europa's ice shell to flex."
> >
> >The UA model shows that when tidal stress reaches the tensile strength
> >of ice, the ice begins to crack. It takes very little stress to form the
> >initial crack -- something like the force it takes to break a saltine
> >cracker -- because Europa's surface ice is weakened by countless linear
> >fractures.
> >
> >The crack propagates relatively slowly across the ever-changing stress
> >field. It moves following a curving path until stress drops below the
> >tensile strength of the ice, when it halts. A few hours later, when tidal
> >stress again exceeds the tensile strength of ice, the crack begins a new
> >curve in another direction.
> >
> >"You could probably walk along with the advancing tip of a crack as it
> >was forming -- if you could survive Europa's radiation environment,"
> >Hoppa said. "And while there's not enough air to carry sound, you would
> >definitely feel vibrations as it formed."
> >
> >One of their most striking conclusions is that each arc segment forms
> >in 3.5 days -- the time it takes Europa to make one complete orbit
> >around Jupiter. The cycloids faithfully record the 85-hour daily flexing
> >of Europa's ice shell just as trees faithfully record each growing season
> >in annual rings.
> >
> >"We can look at a crack that has 4 or 5 cusps, each formed every 3.5 days,
> >and know that the entire chain formed in about 2.5 weeks," Hoppa said.
> >
> >Arc segments in the cycloid, each ranging from 75 km to 200 km long,
> >form cracks stretching a thousand kilometers over the ice in a fraction
> >of an instant in geological time. Eventually, cracks evolve into ridges,
> >typically double ridges, according to the UA model.
> >
> >The scientists also can determine which direction the cracks traveled
> >as they formed based on the orientation of the arcs and the hemisphere
> >in which they are found.
> >
> >"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."
> >
> >
> >
> >

Robert M. Owen
The Orion Institute
57 W. Morgan Street
Brevard, NC 28712-3659 USA

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