[meteorite-list] Rosetta and Philae Find Comet 67P/Churyumov-Gerasimenko Not Magnetized

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 16 Apr 2015 14:10:51 -0700 (PDT)
Message-ID: <201504162110.t3GLApBk015010_at_zagami.jpl.nasa.gov>


Rosetta and Philae find comet not magnetised
European Space Agency
14 April 2015

Measurements made by Rosetta and Philae during the probe's
multiple landings on Comet 67P/Churyumov-Gerasimenko show that the
comet's nucleus is not magnetised.

Studying the properties of a comet can provide clues to the role
that magnetic fields played in the formation of Solar System
bodies almost 4.6 billion years ago. The infant Solar System was
once nothing more than a swirling disc of gas and dust but, within
a few million years, the Sun burst into life in the centre of this
turbulent disc, with the leftover material going into forming the
asteroids, comets, moons and planets.

The dust contained an appreciable fraction of iron, some of it in
the form of magnetite. Indeed, millimetre-sized grains of magnetic
materials have been found in meteorites, indicating their presence
in the early Solar System.

This leads scientists to believe that magnetic fields threading
through the proto-planetary disc could have played an important
role in moving material around as it started to clump together to
form larger bodies.

But it remains unclear as to how crucial magnetic fields were
later on in this accretion process, as the building blocks grew to
centimetres, metres and then tens of metres across, before gravity
started to dominate when they grew to hundreds of metres and
kilometres in scale.

Some theories concerning the aggregation of magnetic and
non-magnetic dust particles show that the resulting bigger objects
could also remain magnetised, allowing them to also be influenced
by the magnetic fields of the proto-planetary disc.

Because comets contain some of the most pristine materials in the
Solar System, they offer a natural laboratory for investigating
whether or not these larger chunks could have remained magnetised.

However, detecting the magnetic field of comets has proven
difficult in previous missions, which have typically made rapid
flybys, relatively far from comet nuclei.

It has taken the proximity of ESA's Rosetta orbiter to Comet
67P/Churyumov-Gerasimenko, and the measurements made much closer
to and at the surface by its lander Philae, to provide the first
detailed investigation of the magnetic properties of a comet nucleus.

Philae's magnetic field measuring instrument is the Rosetta Lander
Magnetometer and Plasma Monitor (ROMAP), while Rosetta carries a
magnetometer as part of the Rosetta Plasma Consortium suite of
sensors (RPC-MAG).

Changes in the magnetic field surrounding Rosetta allowed RPC-MAG
to detect the moment when Philae was deployed in the morning of 12
November 2014.

Then, by sensing periodic variations in the measured external
magnetic field and motions in its boom arm, ROMAP was able to
detect the touchdown events and determine the orientation of
Philae over the following hours. Combined with information from
the CONSERT experiment that provided an estimate of the final
landing site location, timing information, images from Rosetta's
OSIRIS camera, assumptions about the gravity of the comet, and
measurements of its shape, it was possible to determine Philae's

The mission teams soon discovered that Philae not only touched
down once at Agilkia, but also came into contact with the comet's
surface four times in fact ??? including a grazing collision with a
surface feature that sent it tumbling towards the final touchdown
point at Abydos.

This complex trajectory turned out to be scientifically beneficial
to the ROMAP team.

"The unplanned flight across the surface actually meant we could
collect precise magnetic field measurements with Philae at the
four points we made contact with, and at a range of heights above
the surface," says Hans-Ulrich Auster, co-principal investigator
of ROMAP and lead author of the results published in the journal
Science and presented at the European Geosciences Union General
Assembly in Vienna, Austria, today.

The multiple descents and ascents meant that the team could
compare measurements made on the inward and outward journeys to
and from each contact point, and as it flew across the surface.

ROMAP measured a magnetic field during these sequences, but found
that its strength did not depend on the height or location of
Philae above the surface. This is not consistent with the nucleus
itself being responsible for that field.

"If the surface was magnetised, we would have expected to see a
clear increase in the magnetic field readings as we got closer and
closer to the surface," explains Hans-Ulrich. "But this was not
the case at any of the locations we visited, so we conclude that
Comet 67P/Churyumov-Gerasimenko is a remarkably non-magnetic object."

Instead, the magnetic field that was measured was consistent with
an external one, namely the influence of the solar wind
interplanetary magnetic field near the comet nucleus. This
conclusion is confirmed by the fact that variations in the field
that were measured by Philae closely agree with those seen at the
same time by Rosetta.

"During Philae's landing, Rosetta was about 17 km above the
surface, and we could provide complementary magnetic field
readings that rule out any local magnetic anomalies in the comet's
surface materials," says Karl-Heinz Glassmeier, principal
investigator of RPC-MAG on board the orbiter and a co-author of
the Science paper.

If large chunks of material on the surface of
67P/Churyumov-Gerasimenko were magnetised, ROMAP would have
recorded additional variations in its signal as Philae flew over them.

"If any material is magnetised, it must be on a scale of less
than one metre, below the spatial resolution of our measurements.
And if Comet 67P/Churyumov-Gerasimenko is representative of all
cometary nuclei, then we suggest that magnetic forces are unlikely
to have played a role in the accumulation of planetary building
blocks greater than one metre in size," concludes Hans-Ulrich.

"It's great to see the complementary nature of Rosetta and
Philae's measurements, working together to answer this simple, but
important 'yes-no' question as to whether the comet is
magnetised," says Matt Taylor, ESA's Rosetta project scientist.

Notes for Editors

"The non-magnetic nucleus of Comet 67P/Churyumov-Gerasimenko,"
by H.-U. Auster et al. is published in Science Express
<http://www.sciencemag.org/lookup/doi/10.1126/science.aaa5102> on
14 April.

The results were also presented on 14 April at the European
Geosciences Union (EGU) General Assembly 2015
<http://www.egu2015.eu/> in Vienna, Austria, during a dedicated
Rosetta mission press briefing <http://client.cntv.at/egu2015/PC1>.

The data were collected by the Rosetta Lander Magnetometer and
Plasma Monitor (ROMAP) on board Philae and the Rosetta Plasma
Consortium fluxgate magnetometer (RPC-MAG) on board Rosetta.

Overall, the data show that the comet has an upper magnetic field
magnitude of less than 2 nT at the cometary surface at multiple
locations, with a specific magnetic moment of < 3.1 ?? 10^-5 Am^2
/kg, less than known values for lunar material and meteorites
measured on Earth.


ROMAP is the Rosetta Lander Magnetometer and Plasma Monitor. The
contributing institutions to ROMAP are: Institut f??r Geophysik und
Extraterrestrische Physik, Technische Universit??t Braunschweig,
Germany; Max-Planck Institut f??r Sonnensystemforschung, G??ttingen,
Germany; Hungarian Academy of Sciences Centre for Energy Research,
Hungary; and Space Research Institute Graz, Austria. The
co-principal investigators are Hans-Ulrich Auster (Technische
Universit??t, Braunschweig) and Istv??n Ap??thy, KFKI, Budapest, Hungary.


RPC-MAG one of six instruments comprising the Rosetta Plasma
Consortium. The fluxgate magnetometer (RPC-MAG) is led by
Karl-Heinz Glassmeier, Technische Universit??t, Braunschweig, Germany.

About Rosetta

Rosetta is an ESA mission with contributions from its Member
States and NASA. Rosetta's Philae lander was provided by a
consortium led by DLR, MPS, CNES and ASI. Rosetta is the first
mission in history to rendezvous with a comet. It is escorting the
comet as they orbit the Sun together. Philae landed on the comet
on 12 November 2014. Comets are time capsules containing primitive
material left over from the epoch when the Sun and its planets
formed. By studying the gas, dust and structure of the nucleus and
organic materials associated with the comet, via both remote and
in situ observations, the Rosetta mission should become the key
to unlocking the history and evolution of our Solar System.

For further information, please contact

Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer at esa.int

Hans-Ulrich Auster
ROMAP principal investigator
Technische Universit??t, Braunschweig
Email: uli.auster at tu-bs.de

Karl-Heinz Glassmeier
RPC-MAG principal investigator
Technische Universit??t, Braunschweig
Email: kh.glassmeier at tu-bs.de

Matt Taylor
ESA Rosetta project scientist
Email: matthew.taylor at esa.int

Last Update: 14 April 2015
Received on Thu 16 Apr 2015 05:10:51 PM PDT

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