| |
ZL2VAL > SETI 29.05.03 14:52l 111 Lines 5915 Bytes #999 (0) @ WW
BID : 7578_ZL2VAL
Read: GUEST DB0FHN
Subj: More evidence of water
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0BEL<ZL2TZE<ZL2TZE<ZL2WA<ZL2AB
Sent: 030529/1130Z @:ZL2AB.#46.NZL.OC #:22480 [New Plymouth] FBB7.00g $:7578_ZL
From: ZL2VAL@ZL2AB.#46.NZL.OC
To : SETI@WW
Other Evidence for Water on Europa
By Cynthia Phillips
from the SETI Institute’s Center for the Study of Life in the Universe
posted: 07:00 am ET
15 May 2003
In three previous articles, we considered the Galilean satellites
<http://www.space.com/searchforlife/seti_galilean_moons_020523.html> and
the fact that tidal flexing, due to their resonant orbits, provides heat
for volcanism on Io and could result in the presence of liquid water
beneath Europa’s icy surface
<http://www.space.com/searchforlife/seti_tidal_europa_021003.html>. We
also summarized the evidence for liquid water at Europa based on
geological evidence from images of Europa taken by the Voyager and
Galileo spacecraft
<http://www.space.com/searchforlife/seti_phillips_europa_030123.html>.
The geological evidence is tantalizing, but incomplete – it suggests
that liquid water could be present, but also allows for the possibility
that the strange features we see on Europa’s surface could all have
formed through the motion of soft ice, without any liquid water at all.
Fortunately, there are other methods available, in addition to
geological techniques, which can provide information about the presence,
or absence, of water at Europa. Thermal models of Europa’s subsurface
are one theoretical way to study what lies beneath Europa’s surface, and
we will consider them in this article.
Models of Europa’s gravitational field show that Europa possesses a
surface layer about 100 km thick of material with the density of water,
on top of a rocky interior and a metallic core. The surface layer is
most likely H2O, but since the densities of solid ice and liquid water
are very close, gravity models cannot distinguish between the two. So we
know that there is about 100 km of some combination of water and/or ice
at Europa’s surface, but other than knowing that the very top of this
water layer is frozen solid, we do not know how thick the surface ice
layer is, or if there is liquid water under it at some depth.
Thermal models of Europa’s subsurface suggest that it is possible, but
not definite, that liquid water could be present. Thermal models include
sources of heat and methods of cooling, and attempt to determine the
thermal gradient and state (solid or liquid) of subsurface materials.
In the case of Europa, these models include heating from tidal
dissipation and radiogenic sources, and cooling due to conduction and
convection of heat. Tidal heating, as discussed in a previous article
<http://www.space.com/searchforlife/seti_tidal_europa_021003.html> comes
from the flexing of Europa by Jupiter’s gravity, as a result of Europa’s
non-circular orbit due to its resonance with Io and Ganymede. As
Europa’s distance from Jupiter changes over the course of its orbit,
Jupiter’s gravitational attraction changes (since gravitational force is
dependent on distance), and thus the tidal bulge raised by Jupiter goes
up and down. This flexing causes heating of Europa’s subsurface.
Radiogenic heating is caused by the decay of long-lived radioactive
isotopes that were incorporated in Europa when it formed, or brought to
it after formation. Conduction is the direct transfer of heat from
warmer to cooler regions, while convection is heat transfer due to
motion of the materials - warmer materials move upwards, and cooler
materials move downwards. Both conduction and convection result in the
transfer of heat from Europa’s warmer interior to its frigid surface,
and the net cooling of Europa as a result.
For Europa, thermal models that include all of these effects have been
inconclusive. Some models have predicted that convection would remove
all the heat from a liquid layer, resulting in Europa being frozen solid
rather quickly. Other models have predicted that it would be difficult
to produce enough heat to melt a solid ice layer into water, but that if
a water layer existed there would be enough heat to maintain it as
liquid indefinitely, due to a balance of cooling and heating sources.
There are still a number of unknown quantities in these models. For
example, tidal heating is the most important heat source at Europa, but
also the most poorly known. It is strongly affected by the rheology of
ice, which is its behavior when pushed or pulled or squeezed. However,
the rheology of ice is difficult to study under conditions similar to
Europa, since Europa’s surface temperature is a chilly 100 K! It is also
hard to study ice being stretched over the long periods associated with
Europa’s tidal cycle - laboratory measurements are easier to make over
time periods of seconds, not days.
We also don’t know enough about the composition of the ice on Europa -
most models assume that it is pure water ice, but a small amount of
other material present in the ice, especially other volatiles such as
ammonia or salts, could dramatically alter the rheology of Europa’s ice.
Most models also assume that the ice layer is solid, but the rheology
could be changed if the ice layer is broken or fractured, or if the
grain size is different than assumed in the models.
So in addition to the geological evidence, thermal models provide one
more tantalizing, yet insufficient, clue to Europa’s subsurface
structure. Water could be present, but is not required. Fortunately,
however, we have some more definite evidence for subsurface water on
Europa. It comes from an unlikely source - magnetic field measurements.
We’ll consider those in a future article.
============================
73 de Alan
*-----------------------------------------------------------------*
| Packet: ZL2VAL@ZL2AB.#46.NZL.OC APRS: =3903.39S/17406.40E] |
| E-mail: zl2val@qsl.net Snail mail address: http://www.qrz.com |
*-----------------------------------------------------------------*
If love is blind, why is lingerie so popular?
Read previous mail | Read next mail
| |
