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Title: Discovery of Soft X-Ray Emission from Io, Europa,
and the Io Plasma Torus
Authors: Elsner, Ronald F.; Gladstone, G. Randall;
Waite, J. Hunter; Crary, Frank J.; Howell, Robert R.;
Johnson, Robert E.; Ford, Peter G.; Metzger, Albert E.;
Hurley, Kevin C.; Feigelson, Eric D.; Garmire, Gordon P.;
Bhardwaj, Anil; Grodent, Denis C.; Majeed, Tariq;
Tennant, Allyn F.; Weisskopf, Martin C.
Affiliation: AA(Space Science Department, NASA Marshall Space
Flight Center, SD50, Huntsville, AL 35812.),
AB(Department of Space Science, Southwest Research
Institute, P.O. Drawer 28510, San Antonio, TX
78228.), AC(Department of Atmospheric, Oceanic and
Space Sciences, University of Michigan, Ann Arbor,
MI 48109.), AD(Department of Atmospheric, Oceanic
and Space Sciences, University of Michigan, Ann
Arbor, MI 48109.), AE(Department of Physics and
Astronomy, University of Wyoming, P.O. Box 3905,
University Station, Laramie, WY 82071.),
AF(Department of Engineering Physics, Thornton Hall,
University of Virginia, Charlottesville, VA 22903.),
AG(Center for Space Research, Massachusetts
Institute of Technology, Cambridge, MA 02139.),
AH(Jet Propulsion Laboratory, Pasadena, CA 91109.),
AI(Space Science Laboratory, University of
California, Berkeley, CA 94720.), AJ(Department of
Astronomy and Astrophysics, 525 Davey Laboratory,
Pennsylvania State University, State College, PA
16802.), AK(Department of Astronomy and
Astrophysics, 525 Davey Laboratory, Pennsylvania
State University, State College, PA 16802.),
AL(Space Physics Laboratory, Vikram Sarabhai Space
Centre, Trivandrum, India.), AM(Department of
Atmospheric, Oceanic and Space Sciences, University
of Michigan, Ann Arbor, MI 48109.), AN(Department of
Space Science, Southwest Research Institute, P.O.
Drawer 28510, San Antonio, TX 78228.), AO(Space
Science Department, NASA Marshall Space Flight
Center, SD50, Huntsville, AL 35812.), AP(Space
Science Department, NASA Marshall Space Flight
Center, SD50, Huntsville, AL 35812.)
Journal: The Astrophysical Journal, Volume 572, Issue 2, pp.
1077-1082. (ApJ Homepage)
Publication Date: 06/2002
Origin: UCP
ApJ Keywords: Planets and Satellites: Individual: Io, Planets and
Satellites: Individual: Europa, Planets and
Satellites: Individual: Jupiter, X-Rays: General
Abstract Copyright: (c) 2002: The American Astronomical Society
Bibliographic Code: 2002ApJ...572.1077E
Abstract
We report the discovery of soft (0.25-2 keV) X-ray emission from the
Galilean satellites Io and Europa, probably Ganymede, and from the Io
Plasma Torus (IPT). Bombardment by energetic (greater than 10 keV) H, O,
and S ions from the region of the IPT seems to be the likely source of
the X-ray emission from the Galilean satellites. According to our
estimates, fluorescent X-ray emission excited by solar X-rays, even
during flares from the active Sun, charge-exchange processes, previously
invoked to explain Jupiter's X-ray aurora and cometary X-ray emission,
and ion stripping by dust grains fail to account for the observed
emission. On the other hand, bremsstrahlung emission of soft X-rays from
nonthermal electrons in the few hundred to few thousand eV range may
account for a substantial fraction of the observed X-ray flux from the
IPT.
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Title: Nonsynchronous Rotation Evidence and Fracture
History in the Bright Plains Region, Europa
Authors: Kattenhorn, Simon A.
Journal: Icarus, Volume 157, Issue 2, p. 490-506. (Icarus
Homepage)
Publication Date: 06/2002
Origin: ELSEVIER
Abstract Copyright: (c) 2002 Elsevier Science (USA)
Bibliographic Code: 2002Icar..157..490K
Abstract
A geologic map for the Bright Plains in the Conamara Chaos region of
Europa is presented and is used to unravel a detailed fracture sequence
using cross-cutting relationships and fracture mechanics principles.
Fracture orientations in the Bright Plains region rotated with time,
consistently in a clockwise sense. This conclusion agrees with the
observations of other researchers' northern Europan hemisphere
investigations and points strongly toward the fracture sequence being
controlled by the effect of nonsynchronous rotation, whereby the outer
ice crust of Europa rotates slightly faster than the satellite's
interior. This is convincing evidence that Europa's crust has been
decoupled from the interior, possibly due to the presence of a liquid
ocean beneath the crust. Tidal stresses induced in the ice crust by the
combined effects of nonsynchronous rotation and diurnal tidal flexing
can be calculated using the assumption that the crust behaves
elastically over relatively short time scales (i.e., no viscous
relaxation of stresses). The fracture orientations in the Bright Plains
area were compared to a global scale tidal stress field to determine the
longitudes at which each fracture set developed. The fracture sequence
points strongly to the Bright Plains region of the crust having rotated
at least 720° (and perhaps up to 900°) with respect to the
satellite's interior during the visible fracture history. This amount
exceeds previously published estimates of nonsynchronous rotation. The
orientations of the most recent surface fractures are incompatible with
the current state of stress in the Bright Plains region, implying a
period of a few thousand years since the most recent fracturing events
based on existing nonsynchronous rotation rate estimates.
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Title: Sulfuric Acid Production on Europa: The Radiolysis
of Sulfur in Water Ice
Authors: Carlson, R. W.; Anderson, M. S.; Johnson, R. E.;
Schulman, M. B.; Yavrouian, A. H.
Journal: Icarus, Volume 157, Issue 2, p. 456-463. (Icarus
Homepage)
Publication Date: 06/2002
Origin: ELSEVIER
Abstract Copyright: (c) 2002 Elsevier Science (USA)
Bibliographic Code: 2002Icar..157..456C
Abstract
Europa's surface is chemically altered by radiolysis from energetic
charged particle bombardment. It has been suggested that hydrated
sulfuric acid (H2SO4.nH2O) is a major
surface species and is part of a radiolytic sulfur cycle, where a
dynamic equilibrium exists between continuous production and destruction
of sulfur polymers Sx, sulfur dioxide SO2,
hydrogen sulfide H2S, and
H2SO4.nH2O. We measured the rate of
sulfate anion production for cyclo-octal sulfur grains in frozen water
at temperatures, energies, and dose rates appropriate for Europa using
energetic electrons. The measured rate is
GMixture(SO42-)=fSulfur
(r0/r)βG1 molecules (100
eV)-1, where fSulfur is the sulfur weight
fraction, r is the grain radius, r0=50 μm, β~1.9, and
G1=0.4+/-0.1. Equilibrium column densities N are derived for
Europa's surface and follow the ordering N(H2SO4)
» N(S)>N(SO2)>N(H2S). The lifetime of
a sulfur atom on Europa's surface for radiolysis to
H2SO4 is
τ(-S)=120(r/r0)β years. Rapid radiolytic
processing hides the identity of the original source of the sulfurous
material, but Iogenic plasma ion implantation and an acidic or salty
ocean are candidate sources. Sulfate salts, if present, would be
decomposed in <3800 years and be rapidly assimilated into the sulfur
cycle.
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Title: Cometary Delivery of Biogenic Elements to Europa
Authors: Pierazzo, E.; Chyba, C. F.
Affiliation: AA(Lunar and Planetary Laboratory, The University of
Arizona, Tucson, Arizona. betty@lpl.arizona.edu),
AB()
Journal: Icarus, Volume 157, Issue 1, pp. 120-127 (2002).
(Icarus Homepage)
Publication Date: 05/2002
Origin: AP
Abstract Copyright: (c) 2002: Academic Press
Bibliographic Code: 2002Icar..157..120P
Abstract
Jupiter's moon Europa may harbor an ocean beneath its ice cover, but the
composition of that ocean and the overlying ice is nearly entirely
unknown. Regardless of uncertainties in models for Europa's formation,
we estimate lower limits for Europa's inventory of biogenic elements
(such as C, N, O, and P) by investigating the contribution to the
inventory of impact events over Europa's geologic history. A series of
high-resolution hydrocode simulations were carried out over a range of
comet densities (1.1, 0.8, and 0.6 g/cm3, corresponding to
porosities between 0 and 45%) and impact velocities (16, 21.5, 26.5, and
30.5 km/s). We found that at typical impact velocities on Europa most
impactor material reaches escape velocity, and it is assumed to be lost
from Europa. For a nonporous comet, some fraction (20% or higher) of the
projectile is retained by Europa even at the highest impact velocity
modeled, 30.5 km/s. For porous comets, however, a significant fraction
of the projectile (above 25%) is retained only for the lowest impact
velocity modeled, 16 km/s. Integrated over solar system history, this
suggests that 1 to 10 Gt of carbon could have been successfully
delivered to Europa's surface by impacts of large comets (around 1 km in
diameter). This is a few times more carbon than is contained in the
procaryotic biomass of the upper 200 meters of the Earth's oceans, but
about 2 orders of magnitude less if the whole depth of the oceans is
considered. Therefore, regardless of its initial formation conditions,
Europa should have a substantial inventory of ``biogenic'' elements,
with implications for the chemistry of its oceans, ice cover, and the
possibility of life. .
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Title: The Rotation of Europa
Authors: Greenberg, Richard; Hoppa, Gregory V.; Geissler, Paul;
Sarid, Alyssa; Tufts, B. R.
Affiliation: AA(Lunar and Planetary Laboratory, University of
Arizona, Tucson, AZ 85721, U.S.A.), AB(Lunar and
Planetary Laboratory, University of Arizona, Tucson,
AZ 85721, U.S.A.), AC(Lunar and Planetary
Laboratory, University of Arizona, Tucson, AZ 85721,
U.S.A.), AD(Lunar and Planetary Laboratory,
University of Arizona, Tucson, AZ 85721, U.S.A.),
AE(Lunar and Planetary Laboratory, University of
Arizona, Tucson, AZ 85721, U.S.A.)
Journal: Celestial Mechanics and Dynamical Astronomy, v. 83,
Issue 1, p. 35-47 (2002). (CeMDA Homepage)
Publication Date: 05/2002
Origin: KLUWER
CeMDA Keywords: Europa, rotation, tides
Abstract Copyright: (c) 2002 Kluwer Academic Publishers
Bibliographic Code: 2002CeMDA..83...35G
Abstract
Theoretical predictions of non-synchronous rotation and of polar wander
on Europa have been tested by comparing tectonic features observed in
Voyager and Galileo spacecraft images with tidal stresses. Evidence for
non-synchronous rotation comes from studying changes in global scale
lineaments formed over time, from the character of strike-slip faults,
and from comparison of distinctively shaped cycloidal cracks with the
longitudes at which such shapes should have formed, in theory. The study
of cycloids constrains the rotation period (relative to the direction of
Jupiter) to less than 250 000 years, while direct comparison of the
orientation of Europa in Voyager and Galileo images shows the rotation
is slow, with a period of >12 000 years. Comparison of strike-slip
faults with their theoretical locations of formation provides evidence
for substantial polar wander, supported by the distribution of various
thermally produced features.
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Title: A Melt-through Model for Chaos Formation on Europa
Authors: O'Brien, David P.; Geissler, Paul;
Greenberg, Richard
Affiliation: AA(Lunar and Planetary Laboratory, University of
Arizona, Tucson, Arizona. obrien@lpl.arizona.edu),
AB(Lunar and Planetary Laboratory, University of
Arizona, Tucson, Arizona), AC(Lunar and Planetary
Laboratory, University of Arizona, Tucson, Arizona)
Journal: Icarus, Volume 156, Issue 1, pp. 152-161 (2002).
(Icarus Homepage)
Publication Date: 03/2002
Origin: AP
Abstract Copyright: (c) 2002: Academic Press
Bibliographic Code: 2002Icar..156..152O
Abstract
The character of chaotic terrain on Europa is consistent with its
formation by the melting of a thin conducting ice shell from below.
Tidal dissipation can provide adequate energy for such a process. For
example, only a few percent of Europa's predicted tidal heat, spread
over a region 200 km in diameter, can lead to large melt regions within
a few tens of thousands of years. Stronger, more localized
concentrations result in melt-through in significantly shorter times
(i.e., a few hundred years). The time scale for melt-through is shorter
than the time scale for the solid-state viscous inflow of ice by several
orders of magnitude. In general, modest concentrations of tidal heat can
melt ice away faster than viscous inflow, leading to melt-through. A
mechanism to transmit these heat concentrations through the ocean is
required for this model. Such heat transport could be the result of
convective plumes in the ocean driven by seafloor volcanism or by the
destabilization of a stratified ocean. .
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Title: Energy Distributions for Desorption of Sodium and
Potassium from Ice: The Na/K Ratio at Europa
Authors: Johnson, R. E.; Leblanc, F.; Yakshinskiy, B. V.;
Madey, T. E.
Affiliation: AA(Engineering Physics, University of Virginia,
Charlottesville, Virginia. rej@virginia.edu), AB(),
AC(Department of Physics and Astronomy and
Laboratory for Surface Modification, Rutgers
University, Piscataway, New Jersey), AD(Department
of Physics and Astronomy and Laboratory for Surface
Modification, Rutgers University, Piscataway, New
Jersey)
Journal: Icarus, Volume 156, Issue 1, pp. 136-142 (2002).
(Icarus Homepage)
Publication Date: 03/2002
Origin: AP
Abstract Copyright: (c) 2002: Academic Press
Bibliographic Code: 2002Icar..156..136J
Abstract
The sputtering and decomposition of the surface of Europa by fast ions
and electrons lead to the production of an atomosphere containing sodium
and potassium atoms. Here time-of-flight energy distributions are
measured for Na and K sputtered from a vapor-deposited ice by 200-eV
electrons. These data are then used in a Monte Carlo simulation for
alkalis in Europa's atmosphere. Na/K ratios versus distance from Europa
are calculated and compared to the recent observations in the range 6 to
18 Europan radii from the surface. Normalizing to the observations, the
Na/K ratio for the loss rates is ~27 and the ratio for the average
surface source rates is ~20. These ratios are very different from the
Na/K ratio at Io and are larger than the Na/K ratio suggested for
Europa's putative subsurface ocean, consistent with fractionation on
freezing and upwelling of ocean material. .
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Title: Europa as an Abode of Life
Authors: Chyba, Christopher F.; Phillips, Cynthia B.
Affiliation: AA(Center for the Study of Life in the Universe,
SETI Institute, Mountain View, CA, U.S.A. and
Department of Geological and Environmental Sciences,
Stanford University, Stanford, CA, U.S.A.),
AB(Center for the Study of Life in the Universe,
SETI Institute, Mountain View, CA, U.S.A.)
Journal: Origins of Life and Evolution of the Biosphere, v.
32, Issue 1, p. 47-67 (2002).
Publication Date: 02/2002
Origin: KLUWER
OLEB Keywords: ASTROBIOLOGY, EUROPA, EXOBIOLOGY, JUPITER, OCEANS,
RADIATION
Abstract Copyright: (c) 2002 Kluwer Academic Publishers
Bibliographic Code: 2002OLEB...32...47C
Abstract
Life as we know it on Earth depends on liquid water, a suite of
`biogenic' elements (most famously carbon) and a useful source of free
energy. Here we review Europa's suitability for life from the
perspective of these three requirements. It is likely, though not yet
certain, that Europa harbors a subsurface ocean of liquid water whose
volume is about twice that of Earth's oceans. Little is known about
Europa's inventory of carbon, nitrogen, and other biogenic elements, but
lower bounds on these can be placed by considering the role of cometary
delivery over Europa's history. Sources of free energy are challenging
for a world covered with an ice layer kilometers thick, but it is
possible that hydrothermal activity and/or organics and oxidants
provided by the action of radiation chemistry at Europa's surface and
subsequent mixing into Europa's ocean could provide the electron donors
and acceptors needed to power a Europan ecosystem. It is not premature
to draw lessons from the search for life on Mars with the Viking
spacecraft for planning exobiological missions to Europa.
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Title: Ultraviolet emissions from the magnetic footprints
of Io, Ganymede and Europa on Jupiter
Authors: Clarke, J. T.; Ajello, J.; Ballester, G.; Ben Jaffel, L.;
Connerney, J.; Gérard, J.-C.; Gladstone, G. R.;
Grodent, D.; Pryor, W.; Trauger, J.; Waite, J. H.
Affiliation: AA(), AB(), AC(), AD(), AE(), AF(), AG(), AH(),
AI(), AJ(), AK()
Journal: Nature, Volume 415, Issue 6875, pp. 997-1000 (2002).
Publication Date: 02/2002
Origin: NATURE
Abstract Copyright: (c) 2002: Nature
Bibliographic Code: 2002Natur.415..997C
Abstract
Io leaves a magnetic footprint on Jupiter's upper atmosphere that
appears as a spot of ultraviolet emission that remains fixed underneath
Io as Jupiter rotates. The specific physical mechanisms responsible for
generating those emissions are not well understood, but in general the
spot seems to arise because of an electromagnetic interaction between
Jupiter's magnetic field and the plasma surrounding Io, driving currents
of around 1 million amperes down through Jupiter's ionosphere. The other
galilean satellites may also leave footprints, and the presence or
absence of such footprints should illuminate the underlying physical
mechanism by revealing the strengths of the currents linking the
satellites to Jupiter. Here we report persistent, faint, far-ultraviolet
emission from the jovian footprints of Ganymede and Europa. We also show
that Io's magnetic footprint extends well beyond the immediate vicinity
of Io's flux-tube interaction with Jupiter, and much farther than
predicted theoretically; the emission persists for several hours
downstream. We infer from these data that Ganymede and Europa have
persistent interactions with Jupiter's magnetic field despite their thin
atmospheres.
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Title: Possible Detection of Volcanic Activity on Europa:
Analysis of An Optical Transient Event
Authors: De La Fuente Marcos, R.; Nissar, A.
Affiliation: AA( Saint Louis University, Madrid Campus, Avda. del
Valle 34, E-28003 Madrid, Spain ), AB( Saint Louis
University, Madrid Campus, Avda. del Valle 34,
E-28003 Madrid, Spain )
Journal: Earth, Moon, and Planets, v. 88, Issue 3, p. 167-175
(2000). (EM&P Homepage)
Publication Date: 00/2002
Origin: KLUWER
EM&P Keywords: Active venting, Europa, satellites of Jupiter, solar
system, general, volcanic activity
Abstract Copyright: (c) 2000 Kluwer Academic Publishers
Bibliographic Code: 2002EM&P...88..167D
Abstract
Europa's low crater density suggests that geological activity has
continued to the present epoch, leading to the possibility that current
resurfacing events might be detectable. CCD observations were carried
out with a ST-6 camera at the 0.5 m Mons Cassegrain telescope
(Izaña Observatory, Tenerife, Canary Islands, Spain) during the
night between 2-3 October 1999. Our images show a transient bright
feature on the Galilean satellite. These images are analyzed here with
the purpose of understanding the nature of the transient phenomena as it
could be the result of explosive venting on the surface of the Jovian
satellite. By comparison, we use NASA Infrared Telescope Facility images
of two Io hot spots taken on 12 October 1990. Although we mainly
restrict our discussion on a possible eruptive nature of the observed
spots, we also consider other alternative mechanisms able to produce
bright events. In particular, an interaction between charged material
being ejected from Europa and the Jovian magnetosphere cannot be
entirely ruled out. If confirmed, this result would lend support for the
existence of active resurfacing in Europa.
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Title: Biogenesis and Early Life on Earth and Europa:
Favored by an Alkaline Ocean?
Authors: Kempe, Stephan; Kazmierczak, Jozef
Journal: Astrobiology, vol. 2, Issue 1, p.123
Publication Date: 00/2002
Origin: WEB
Bibliographic Code: 2002AsBio...2..123K
Abstract
Not Available
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Title: Searching for Liquid Water in Europa by Using
Surface Observatories
Authors: Khurana, Krishan K.; Kivelson, Margaret G.;
Russell, Christopher T.
Journal: Astrobiology, vol. 2, Issue 1, p.93
Publication Date: 00/2002
Origin: WEB
Bibliographic Code: 2002AsBio...2...93K
Abstract
Not Available
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Title: Planetary protection for a Europa surface sample
return: The Ice Clipper mission
Authors: McKay, C. P.
Affiliation: Space Science Division, NASA Ames Research Center,
Moffett Field, CA 94035, USA
Journal: Advances in Space Research, Volume 30, Issue 6, p.
1601-1605. (AdSpR Homepage)
Publication Date: 00/2002
Origin: ELSEVIER
Abstract Copyright: (c) 2002 Elsevier Science B.V. All rights reserved.
Bibliographic Code: 2002AdSpR..30.1601M
Abstract
The surface of Europa may hold biochemical evidence of life in the ocean
below. Plans for the analysis and return of samples containing organics
from the surface of Europa are well developed; for example, the Ice
Clipper Mission. Planetary protection issues must be considered in
planning for a returned sample from Europa. Previous studies for sample
return from Mars and the return of comet dust by the Stardust mission
provide a basis for comparison for a Europa sample return mission. The
extreme radiation environment on the surface of Europa would kill even
the most radiation resistant microorganism present to depths of many
tens of meters in the ice. The Ice Clipper mission would impact sample
the upper 1.2 to 3.4 m of the ice depending on the surface hardness. At
these depths the radiation dose is expected to be 500 and 40 rads/year,
respectively. These dose rates would kill dormant cells in less than
36,000 and 450,000 years even for the most radiation resistant strains.
It is therefore likely that a Europa sample return mission such as Ice
Clipper can be treated using the Stardust mission as a model for
planetary protection, that is, the returned material can be assumed to
pose no biological risk.
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