Topic: Early April astronomy Bulletin

[Clive]

HUBBLE SEES MARS-BOUND COMET SPROUT MULTIPLE JETS
Space Telescope Science Institute

Comet C/2013 A1 Siding Spring is plunging towards the Sun along a
roughly 1-million-year orbit.  The comet, discovered in 2013, was
within the radius of Jupiter's orbit when the Hubble Space Telescope
photographed it on 2014 March 11.  Hubble resolves two jets of dust
coming from the solid icy nucleus.  They seem to be persistent jets,
as they were first seen in Hubble pictures taken on 2013 October 29.
They should allow astronomers to determine the direction of the
nucleus's pole, and hence its rotation axis.  The comet will come to
perihelion on October 25, at a distance of 130 million miles, well
outside the Earth's orbit.

On its inbound passage, Comet Siding Spring will pass within 84,000
miles of Mars on October 19, which is less than half the Moon's
distance from the Earth.  The comet is not expected to become bright
enough to be seen with the naked eye.  A Hubble observation made on
January 21 this year caught the comet as the Earth was crossing its
orbital plane.  That special geometry facilitates the determination of
the speed of the dust coming off the nucleus -- information that helps
astronomers to determine how likely and how much the dust grains in
the coma will impact Mars and Mars spacecraft.


FIRST RING SYSTEM AROUND ASTEROID
ESO

Observations at many sites in South America have made the surprising
discovery that the remote asteroid Chariklo is surrounded by two dense
and narrow rings.  It is by far the smallest object found to have
rings, and only the fifth body in the Solar System, after Jupiter,
Saturn, Uranus and Neptune.  The rings may be the result of a
collision that created a disc of debris.  The rings of Saturn are
among the most spectacular sights in the sky, and less prominent rings
have also been found around the other giant planets.  Despite many
careful searches, no rings had been found around smaller objects in
the Solar System until now, when observations of the distant minor
planet (10199) Chariklo as it passed in front of a star have shown
that it has two fine rings.

Chariklo is the largest member of a class known as the Centaurs and it
orbits between Saturn and Uranus.  Predictions had shown that it would
pass in front of the star UCAC4 248-108672 on 2013 June 3, as seen
from South America.  Astronomers using telescopes at seven different
locations, including the 1.54-m Danish and TRAPPIST telescopes at La
Silla, and saw the star apparently vanish for a few seconds as its
light was occulted by Chariklo.  But they found much more than they
were expecting.  A few seconds before, and again a few seconds after,
the main occultation there were two further very short dips in the
star's apparent brightness.  By comparing what was seen from different
sites the team could reconstruct not only the shape and size of the
object itself but also the shape, width, orientation and other
properties of the newly discovered rings.  The team found that the
ring system consists of two sharply confined rings only seven and
three kilometres wide, separated by a clear gap of nine kilometres;
Chariklo itself has a diameter of about 250 km.  Astronomers think
that such rings are likely to be formed from debris left over after a
collision.  It must be confined into the two narrow rings by small
unseen satellites.  So, as well as the rings, it is likely that
Chariklo has at least one small moon still waiting to be discovered.


SURFACE OF TITAN SEA IS MIRROR SMOOTH
Stanford University

New radar measurements of a large sea on Titan offer insights into
the weather patterns and landscape composition of the Saturnian moon.
The measurements, made in 2013 by the Cassini spacecraft, reveal that
the surface of Ligeia Mare, Titan's second-largest sea, possesses a
mirror-like smoothness, possibly owing to a lack of wind.  Titan has
a dense, planet-like atmosphere and large seas made of methane and
ethane.  Measuring roughly 420 by 350 km, Ligeia Mare is larger than
Lake Superior.  Titan is the best analogue that we have in the Solar
System to a body like the Earth, because it is the only other body
that we know of that has a complex cycle of solid, liquid, and gas
constituents.  Titan's thick cloud cover makes it difficult for
Cassini to obtain clear optical images of its surface, so scientists
must rely on radar, which can see through the clouds, instead of a
camera.

To paint a radar picture of Ligeia Mare, Cassini bounced radio waves
off the sea's surface and then analyzed the echo.  The strength of the
reflected signal indicated how much wave action was happening on the
sea.  The surface of Ligeia Mare seemed eerily still.  One possible
explanation for the sea's calmness is that no winds happened to be
blowing across that region of the moon when Cassini made its fly-by.
Another possibility is that a thin layer of some material is
suppressing wave action.  For example, on Earth, a layer of oil on top
of a sea suppresses small waves.  Cassini also measured microwave
radiation emitted by the materials that make up Titan's surface.
Those measurements confirmed previous findings that the land around
Ligeia Mare is composed of solid organic material, probably the same
methane and ethane that make up the sea.  Like water on the Earth,
methane on Titan can exist as a solid, a liquid, and a gas all at
once.