Topic: Mid March Astronomy Bulletin

[Clive]

PLANETS

MARS, URANUS and NEPTUNE are unfavourably placed for observation.

VENUS has almost ended its brilliant morning apparition.  It is still
shining at mag -4 very low down in the ESE at dawn.  At the start of
civil twilight (when the Sun is 6° below the horizon) at 05:24 March 20
it is at an altitude of only 3°.  Similarly at 04:57 on March 31 it is
only 2° above the horizon.  During that period its phase increases
from 77% to 80% and its angular size diminishes from 14" to 13".
On March 31 it is just 5° directly below a thin crescent Moon.  That
is always a superb sight and it is worth trying to catch it.

SATURN is now well placed for observation, being due South on March 20
at 1am GMT at an altitude of 34°.  The rings are well displayed, being
tilted towards us at just over 10°.  Saturn is at opposition on April
4, being due South at 1am BST at an altitude of 35°.  It is going to
get progressively lower at successive oppositions over the next 7
years so make the most of this one.

JUPITER is an evening object just above the western horizon at dusk,
and like Venus is almost at the end of this apparition, but it is
directly below an excellent evening apparition of Mercury.

MERCURY has its most favourable evening apparition of the year this
month, being at Greatest Elongation East of 19° on March 23.  At the
end of civil twilight, varying from 1841 on March 20 to 1850 on March
25, it maintains an altitude of 11° above the horizon.  Throughout
that period Jupiter is directly below Mercury, at an altitude of 6° on
the 20th but only 1° on the 25th (i.e. 10° below Mercury).  It is
worth trying to catch elusive Mercury.  I would also be interested to
hear of any visual naked-eye sightings (I've only had a handful of
unaided sightings myself over the years).  From March 20 to 25 its
brightness diminishes from magnitude -0.6 to +0.2.

ENIGMA OF THE MISSING SUNSPOTS SOLVED?
NASA

In 2008/9, sunspots almost completely disappeared for two years.
Solar activity dropped to hundred-year lows; the Earth's upper
atmosphere cooled and collapsed; the Sun's magnetic field weakened,
allowing cosmic rays to penetrate the Solar System in record numbers.
It was a big event, and solar physicists openly wondered where had
all the sunspots gone.  For years, solar physicists have hypothesized
about the Sun's 'Great Conveyor Belt'.  Plasma currents called
'meridional flows' (akin to ocean currents on the Earth) travel along
the Sun's surface, plunge inward around the poles, and pop up again
near the Sun's equator.  Those looping currents play a key role in the
11-year solar cycle.  When sunspots begin to decay, surface currents
sweep up their magnetic remains and pull them down inside the star;
300,000 km below the surface, the Sun's magnetic dynamo amplifies the
decaying magnetic fields.  Re-animated sunspots become buoyant and bob
up to the surface like corks in water - and a new solar cycle is born.

For the first time, astronomers believe that they have developed a
computer model that gets the physics right for all three aspects of
the process -- the magnetic dynamo, the conveyor belt, and the buoyant
evolution of sunspot magnetic fields.  According to the model, the
trouble with sunspots actually began in the late 1990s during the
upswing of Solar Cycle 23.  At that time, the conveyor belt sped up.
The fast-moving belt rapidly dragged sunspot corpses down to the Sun's
inner dynamo for amplification.  At first glance, that might seem to
boost sunspot production, but no, when the remains of old sunspots
reached the dynamo, they rode the belt through the amplification zone
too hastily for full re-animation, and sunspot production was stunted.
Later, in the 2000s, according to the model, the conveyor belt slowed
down again, allowing magnetic fields to spend more time in the
amplification zone, but the damage was already done.  New sunspots
were in short supply.  Adding insult to injury, the slow-moving belt
did little to assist re-animated sunspots on their journey back to the
surface, delaying the onset of Solar Cycle 24. The stage was set for
the deepest solar minimum in a century.

While a solar maximum is relatively brief, lasting a few years,
punctuated by episodes of violent flaring that are over in days, a
solar minimum can grind on for many years.  The famous Maunder Minimum
of the 17th century lasted 70 years and coincided with the deepest
part of Europe's Little Ice Age.  Researchers are still struggling to
understand the connection, but one thing is clear: during long minima,
strange things happen.  In 2008/9, the Sun's global magnetic field
weakened and the solar wind subsided.  Cosmic rays normally held at
bay by the Sun's magnetism surged into the inner Solar System so,
ironically, during the deepest solar minimum in a century, space
became a more dangerous place to travel.  At the same time, the
heating action of UV rays normally provided by sunspots was absent, so
the Earth's upper atmosphere began to cool and collapse.  The orbits
of space junk did not decay as rapidly as usual and the junk
accumulated in Earth orbit.


MARS EXPRESS SEES IMPACT SCARS
ESA

Impact craters are generally round because the projectiles that create
them penetrate far into the ground before the shock wave of the impact
can explode outward, but Mars Express has returned new images of an
elongated impact crater in Mars' southern hemisphere.  Located just
south of the 450-km Huygens basin, it could have been carved out by a
train of projectiles striking the planet at a shallow angle.  In that
area there are many impact scars, but perhaps none more intriguing
than the elongated craters.  One of them, at 21S/55E, measures 133
by 53 km.  The clue to why it is elongated comes from the surrounding
ejecta blanket, which is shaped like a butterfly's wings, with two
distinct lobes.  That hints that two projectiles, possibly halves of a
once-intact body, slammed into the surface.  In the crater itself,
there are three deeper areas that could be evidence for more than two
projectiles.  In addition, a second elongated crater lies to the
north-northwest.  It is in line with the first one, in agreement with
the notion that the structures were the result of a train of
projectiles.  In the early 1980s, scientists proposed that incoming
chains of orbital debris following trajectories that decayed with time
formed elongated impact craters.  As the debris spiralled downward, it
eventually struck the planet at shallow angles, gouging out the
elongated craters.  This particular ejecta blanket contains many
smaller craters, indicating that the original formed a relatively long
time ago and then itself became a target.


TIDAL EFFECTS CHANGE HABITABLE ZONE CONCEPT
ScienceDaily

Optimists searching for life in outer space focus on those exoplanets
that are located in the 'habitable zone', meaning that they orbit
their sun at a distance where the temperatures on the planet's surface
allow for the presence of liquid water.  Water is believed to be an
essential ingredient for life.  Until now, the two main drivers
thought to determine a planet's temperature were the distance from the
star and the composition of the planet's atmosphere.  It has now
dawned on the enthusiasts, however, that tidal effects modify the
concept of the 'habitable zone' in three different ways.

First, tides can cause the axis of a planet`s rotation to become
perpendicular to its orbit in just a few million years.  Then there
would be no seasonal variations on such planets in the 'habitable
zones' of low-mass stars -- they would have huge temperature
differences between their poles, which would be in perpetual deep
freeze, and their hot equators which in the long run would evaporate
any atmosphere. The temperature difference would itself drive extreme
winds and storms.  The second effect of the tides would be to heat up
the planet, just as Jupiter's tidal heating of Io creates global
vulcanism.  Finally, tides can cause the rotational period of the
planet (the 'day') to synchronize with the orbital period (the
'year'), as has happened to our Moon, which only ever shows the Earth
one face.  As a result, one side of the planet receives extreme
radiation from the star while the other half freezes in eternal
darkness.  So the 'habitable zone' may well be uninhabitable!

Astronomers have applied the theory to GI581g, an exoplanet candidate
that has recently been claimed to be habitable.  They find that GI581g
should not experience any seasons and that its day is synchronized
with its year.  There probably would be no water on the planet's
surface, rendering it uninhabitable.  It seems that the chances for
life existing on exoplanets around low-mass stars, which have so far
been regarded as the most promising candidates for habitable
exoplanets, are pretty bleak.  It now seems that, if you want to find
a second Earth, you need to look for a second Sun.


RARE OBSERVATION OF COSMIC EXPLOSION
ScienceDaily

Gamma-ray bursts (GRBs) are the most powerful blasts in the Universe,
and are thought to be created in the deaths of the most massive stars.
The brief flashes of gamma radiation are picked up by dedicated
satellites, which then send out alerts to the astronomers who study
them.  A GRB was found on 2010 March 16 by the Swift satellite, and
alerted the Gemini South telescope which found an associated supernova
in a galaxy 820 million light-years away.  X-ray radiation revealed
the explosion breaking out of the star, providing a much-needed
confirmation of the idea that GRBs are indeed linked with the
destruction of massive stars.


OLD GALAXY CLUSTER FOUND IN YOUNG UNIVERSE
ESO

Astronomers have used various telescopes on the ground and in space to
discover and measure the distance to the most remote mature cluster of
galaxies yet found.  Although the cluster is seen as it was when the
Universe was less than one-quarter of its current age, it looks
surprisingly similar to present-day clusters.  Clusters of galaxies
are the largest structures in the Universe that are held together by
gravity.  Astronomers expect clusters to grow through time and, hence,
that massive clusters would be rare in the early Universe.  Although
even more distant clusters have been seen, they appear to be young
clusters in the process of formation and are not settled, mature
systems.

The international team used the Very Large Telescope (VLT) to measure
the distances to some of the blobs in a curious patch of faint red
objects first observed with the Spitzer space telescope.  The grouping
had all the hallmarks of being a remote cluster of galaxies.  The
results showed that it was indeed a galaxy cluster, viewed as it was
when the Universe was about 3 billion years old.  Once the team knew
the distance, they observed the component galaxies with the Space
Telescope and the VLT.  They found evidence suggesting that most
of the galaxies in the cluster were not forming stars, but were
composed of stars that were already about 1 billion years old.  That
makes the cluster a mature object, similar in mass to the Virgo
cluster, the nearest rich cluster to the Milky Way.  Further evidence
that it is a mature cluster comes from XMM-Newton observations of
X-rays from it.  They must be coming from a hot cloud of tenuous gas
filling the space between the galaxies and concentrated towards the
centre of the cluster.  That is another sign of a mature cluster of
galaxies held firmly together by its own gravity, as young clusters
have not had time to trap hot gas in that way.  If further
observations find many more such clusters, astronomers' ideas of the
early Universe will need to be revised.


VOYAGER 1 MANOEUVRE
NASA

The two Voyager spacecraft are travelling through a turbulent area
known as the heliosheath. The heliosheath is the outer shell of a
bubble created by the solar wind around our Solar System, a stream of
ions blowing radially outward from the Sun at a million miles per
hour.  The wind must turn as it approaches the outer edge of the
bubble where it makes contact with the interstellar wind, which
originates in the region between stars and blows past our solar
bubble.  In 2010 June, when Voyager 1 was about 17 billion km away
from the Sun, data from the low-energy charged-particle (LECP)
instrument began to show that the nett outward flow of the solar wind
was zero.  That zero reading has continued since.  The Voyager science
team thinks that the wind could not have disappeared in that area, but
must just have turned a corner, and it is of obvious interest to know
in which direction it has turned.

In order to find out, controllers had to change the orientation of
Voyager 1 so that the LECP instrument could act like a weather vane to
see which way the wind is now blowing.  Knowing the strength and
direction of the wind is useful towards understanding the shape of the
solar bubble and estimating how much farther it is to the edge of
interstellar space.  So on March 7 the spacecraft performed a
manoeuvre that it had not done for 21 years: it rolled 70 degrees from
its normal orientation, and held the position by spinning gyroscopes
for more than two hours.  (The previous time either of the two Voyager
spacecraft rolled and stopped in a gyro-controlled orientation was on
1990 Feb. 14, when Voyager 1 took a family portrait of the planets
strewn like tiny gems around the Sun
(http://photojournal.jpl.nasa.gov/catalog/PIA00451).)

The scientists confirmed that the spacecraft had acquired the kind of
information they needed, and mission planners agreed to Voyager 1
doing more rolls and longer holds.  Five more such manoeuvres took
place over the next seven days, with the longest hold lasting nearly
four hours.  The Voyager team plans to execute a series of weekly
rolls for this purpose every three months.  The solar wind's outward
flow has not yet diminished to zero where Voyager 2 is exploring, but
that may happen as the spacecraft approaches the edge of the bubble in
the years ahead.