Topic: Early January Astronomy Bulletin

[Clive]

RECURRING METEOR SHOWER ON MERCURY
NASA/Goddard Space Flight Center

The closest planet to the Sun appears to get hit by a periodic meteor
shower, possibly associated with a comet that produces multiple events
annually on the Earth. The clues pointing to Mercury's shower were
discovered in the very thin halo of gases that make up the planet's
exosphere, which is under study by the MESSENGER (MErcury Surface,
Space ENvironment, GEochemistry*, and Ranging) spacecraft. A meteor
shower occurs when a planet passes through a swath of debris shed by a
comet, or by an asteroid. The smallest bits of dust, rock and ice are
pushed away from the Sun by radiation pressure, creating the comet's
tail. (The force of the radiation is proportional to the cross-
sectional area of the particle, whereas gravity is proportional to the
mass and therefore to the volume: since area goes as size squared and
volume as size cubed, for a sufficiently small particle the radiation
force can become significant or even dominant.) The larger chunks get
deposited as a trail of debris along the comet's orbit -- a field of
tiny meteoroids in the making. The Earth experiences quite a number
of meteor showers each year, including the Perseids in August, the
calling card of Comet Swift-Tuttle, and December's reliable Geminids,
one of the few events associated with an asteroid. Comet Encke has
left several debris fields in the inner Solar System, giving rise to
the Southern and Northern Taurids, meteor showers that peak in October
and November, and the Beta Taurids in June and July.

The suggested hallmark of a meteor shower on Mercury is a regular
surge of calcium in the exosphere. Measurements taken by MESSENGER's
spectrometer have revealed seasonal surges of calcium that have
occurred regularly over the first nine Mercury years since MESSENGER
began orbiting the planet in 2011 March. The suspected cause of the
increase of calcium levels is a shower of small dust particles hitting
the planet and knocking calcium-bearing molecules free from the
surface. That process, called impact vaporization, continually renews
the gases in Mercury's exosphere as interplanetary dust and meteoroids
rain down on the planet. However, the general background of
interplanetary dust in the inner Solar System cannot, by itself,
account for the periodic spikes in calcium. That suggests the
existence of a periodic source of additional dust, for example, a
cometary debris field. Examination of the handful of comets in orbits
that would permit their debris to cross Mercury's orbit indicated that
the probable source of the planet's event is Comet Encke. The
researchers created detailed computer simulations to test the Encke
hypothesis. However, the calcium spikes found in the MESSENGER data
were offset a bit from the expected results. The offset might be due
to changes in the comet's orbit over time, owing to perturbations by
Jupiter and other planets.

* 'MESSENGER' might be regarded at first sight as a clever, if rather
laboriously contrived, acronym, since in ancient mythology Mercury was
a messenger of the gods. But the acronym is *too* contrived, because
the GEo part of it must refer uniquely to the Earth. The classical
experts who know alternative names for the various ancient deities and
have demonstrated their brilliance by promulgating suitable prefixes
for properties of other planets, such as areo- for Mars and cythero-
for Venus, do not seem to have exhibited the same concern for Mercury.
The obvious analogue in the same spirit as those examples would be
hermo- (but its substitution in MESSENGER would ruin the acronym!).
-- ED.


ROSETTA TO SWOOP DOWN ON COMET IN FEBRUARY
NASA

The orbiting Rosetta spacecraft is expected to come within 6 km of the
surface of comet 67P/Churyumov-Gerasimenko next month. The flyby will
be the closest the comet explorer will come during its prime mission.
As the comet becomes more and more active, it will not be prudent to
get so close to it in the future. The low fly-by will be an oppor-
tunity for Rosetta to obtain imagery with a resolution better than a
metre per pixel. The imagery is expected to provide information on
the comet?s porosity and albedo (its reflectance). The fly-by will
also allow the study of the processes by which cometary dust is
accelerated by the comet's gas emission. The Rosetta orbiter deployed
its Philae lander to one spot on the comet's surface in November.
Philae obtained the first images taken from a comet's surface and will
provide analysis of the comet's possible primordial composition.
Comets are time capsules containing primitive material left over from
the epoch when our Sun and its planets formed. Rosetta will be the
first spacecraft to witness at close proximity how a comet changes as
it is subjected to the increasing intensity of the Sun's radiation.
Observations may help scientists learn a bit about the origin and
evolution of the Solar System and the role comets may have played in
seeding the Earth with water, and perhaps even life.


STRETCHED-OUT SOLID EXO-PLANETS
RAS

Astronomers could soon be able to find rocky planets stretched out by
the gravity of the stars they orbit. Since the first discovery in
1992, more than 1800 planets have been found in orbit around stars
other than our Sun. Those 'exo-planets' are incredibly diverse, some
being gaseous like Jupiter and some mostly rocky like the Earth.
They also orbit their stars at very different distances, from less
than a million to nearly 100 billion km away. Planets that are very
close to their stars experience harsh conditions, often with very
high temperatures (>1000 degrees C) and significant stretching from
the tidal forces resulting from the stellar gravitational field. That
is most obvious with planets with a large atmosphere (so-called 'hot
Jupiters') but is harder to see in the rockier objects.

Astronomers modelled cases where the planets are in orbit close to
small red dwarf stars, much fainter than the Sun but by far the most
common type of star in the Galaxy. The planets' rotation is locked,
so they keep the same face towards the stars they orbit, much like the
Moon does as it moves around the Earth. According to the scientists,
in those circumstances the distortion of the planets should be
detectable in transit events, where the planets move in front of their
stars and block out some of the light. If astronomers are able to
find such extreme exo-planets, it could give them new insights into
the properties of Earth-like planets as a whole. It would be like
taking a planet like the Earth or Mars, placing it near a cool red
star and stretching it out. Analysing the new shape alone should tell
us something about the internal structure of the planet, otherwise
impossible to see. The subtle signals from stretched rocky planets
could be found by some current telescopes, and certainly by much more
powerful observatories like the James Webb Space Telescope (JWST) and
the European Extremely Large Telescope (E-ELT) that are due to enter
service in the next few years.

NEWLY DISCOVERED GALAXY IN 'LOCAL GROUP'
RAS

The Milky Way is part of a cluster of more than 50 galaxies that make
up the 'Local Group', a collection that includes the Andromeda galaxy
and many other far smaller objects. Now a Russian-American team has
added to the Group a tiny and isolated dwarf galaxy almost 7 million
light-years away. The team found the new galaxy, named KKs3, with the
Hubble 'Advanced Camera for Surveys' (ACS) last August. Kks3 is
located in the southern sky in the direction of the constellation
Hydrus, and it has only one ten-thousandth of the mass of the Milky
Way. Kks3 is a 'dwarf spheroidal' or 'dSph' galaxy, lacking features
like the spiral arms found in our own galaxy. Such systems also have
an absence of the raw materials (gas and dust) needed for new
generations of stars to form, leaving behind older and fainter relics.
In almost every case, the raw material seems to have been stripped out
by nearby massive galaxies like Andromeda, so the vast majority of
dSph objects are found near much bigger companions.

Isolated objects must have formed in a different way, with one
possibility being that they had an early burst of star formation that
used up the available gas resources. Astronomers are particularly
interested in finding dSph objects to understand galaxy formation in
the Universe in general, as even HST struggles to see them beyond the
Local Group. The absence of clouds of hydrogen gas in nebulae also
makes them harder to pick out in surveys, so scientists instead try to
find them by picking out individual stars. For that reason, only one
other isolated spheroidal dwarf, KKR 25, has been found in the Local
Group, a discovery made by the same group in 1999. Finding objects
like Kks3 is painstaking work, even with the Hubble telescope, but
with persistence, astronomers are slowly building up a map of our
local neighbourhood, which turns out to be less empty than was
thought. It may be that a huge number of dwarf spheroidal galaxies is
out there, something that would have some consequences for our ideas
about the evolution of the cosmos. The team will continue to look for
more dSph galaxies, a task that will become a little easier in the
next few years, once the James Webb space telescope and the E-ELT
begin service.


VOYAGER FLIES THROUGH 'TSUNAMI WAVE'
NASA/JPL

The Voyager 1 spacecraft has experienced three shock waves. The most
recent one, first observed in 2014 February, still appears to be going
on. One wave, previously reported, helped researchers determine that
Voyager 1 had entered interstellar space. The "tsunami wave" that the
spacecraft began experiencing earlier this year is still propagating
outward and is the longest-lasting shock wave that researchers have
seen in interstellar space. Most people (if they thought about it at
all and imagined that they understood such things) would have thought
that the interstellar medium would be smooth and quiet. But shock
waves seem to be more common than we thought. A 'tsunami wave' occurs
when the Sun emits a coronal mass ejection, throwing out a magnetic
cloud of plasma from its surface. That generates a wave of pressure.
When the wave runs into the inter-stellar plasma -- the charged
particles found in the space between the stars -- the result is a
shock wave that perturbs the plasma. The tsunami causes the ionized
gas that is out there to resonate -- or vibrate like a bell. This is
the third shock wave that Voyager 1 has experienced. The first event
was in 2012 October/November; the second, in 2013 April/May, revealed
an even higher plasma density. The spacecraft has moved outward 400
million kilometres during the third event.

It is not clear to researchers what the unusual longevity of this
particular wave may mean. They do not know, either, how fast the wave
is moving or how broad a region it covers. The second tsunami wave
helped researchers determine in 2013 that Voyager 1 had left the
heliosphere, the bubble created by the solar wind encompassing the Sun
and the planets in the Solar System. Denser plasma 'rings' at a
higher frequency, and that was the medium that Voyager flew through;
that was the evidence for the conclusion that Voyager had entered a
frontier where no spacecraft had gone before: interstellar space. The
density of the plasma is higher the farther Voyager goes. Is that
because the interstellar medium is denser as Voyager moves away from
the heliosphere, or is it from the shock wave itself? We don't know.
It is thought that such shock waves propagate far out into space,
perhaps even to twice the distance between the Sun and where the
spacecraft is now.

Voyager 1 and its twin, Voyager 2, were launched 16 days apart in
1977. Both spacecraft flew by Jupiter and Saturn. Voyager 2 also
flew by Uranus and Neptune. Voyager 2, launched before Voyager 1, is
the longest continuously operating spacecraft and is expected to enter
interstellar space in a few years.