CALM LAKES ON TITAN
University of Texas at Austin
The lakes of liquid methane on Saturn's moon, Titan, might be good for
paddling but not for surfing (for people who did not mind the cold --
methane boils at -161 C at normal terrestrial atmospheric pressure --
and whose feet were not harmed by liquid methane)! New research has
found that most waves on Titan's lakes reach only about 1 centimetre
high, a finding that indicates a serene environment that could be good
news for future probes sent to the surface of that moon. Titan is the
largest moon of Saturn and one of the locations in the Solar System
that is thought to possess the ingredients for life. In photos taken
by the Cassini orbiter, it appears as a smooth brown orb because of
its thick atmosphere clouded with gaseous nitrogen and hydrocarbons.
However, radar images from the same probe show that it has a surface
crust made of water ice and drenched in liquid hydrocarbons. Methane
and ethane fall from the sky on Titan as rain, fill deep lakes that
dot the surface, and are possibly spewed into the air by icy volcanoes
called cryovolcanoes. The atmosphere of Titan is very complex, and it
does synthesize complex organic molecules. It may act as a laboratory
of sorts, where basic molecules can be transformed into more complex
molecules that could eventually lead to life. On top of that, it is
also thought to have an ocean of liquid water beneath its icy crust.
Scientists developed a technique for measuring surface roughness in
minute detail from radar data. Called radar statistical reconnaiss-
ance, the technique has been used to measure the snow density and
surface roughness in Antarctica and the Arctic, and to assist the
landing-site selection of NASA's Mars lander InSight, which is
scheduled to be launched next year. Researchers at the Jet Propulsion
Laboratory suggested using the technique to measure Titan's waves.
The research refers to the three largest lakes in Titan's northern
hemisphere: Kraken Mare, Ligeia Mare and Punga Mare. Kraken Mare, the
largest of the three, is estimated to be larger than the Caspian Sea.
By analyzing radar data collected by Cassini during Titan's early-
summer season, the team found that waves on the lakes are diminutive,
reaching only about 1 cm high and 20 cm long. The results call into
question the early-summer's classification as the beginning of the
Titan's windy season, because high winds probably would have made for
larger waves. Information on Titan's climate is essential for sending
a probe safely to the surface. Although there are no formal plans for
a mission, there are plenty of concepts being developed by researchers
around the world. The study indicates that if a future mission lands
in early summer, there is a good chance that it will have a smooth
landing.
METHANOL DETECTED AROUND ENCELADUS
RAS
A serendipitous detection around Enceladus, an intriguing moon of
Saturn, of the organic molecule methanol suggests that material spewed
from Enceladus undertakes a complex chemical journey once it is vented
into space. This is the first time that a molecule from Enceladus has
been detected with a ground-based telescope. Enceladus's plumes are
thought to originate in water escaping from a sub-surface ocean
through cracks in the moon's icy surface. Eventually the plumes feed
into Saturn's second-outermost ring, the E-ring. Recent discoveries
that icy moons in the outer Solar System could host oceans of liquid
water and ingredients for life have sparked exciting possibilities for
their habitability. But, in this case, the findings suggest that the
methanol is being created by further chemical reactions once the plume
is ejected into space, making it unlikely that it is an indication of
life on Enceladus. Past studies of Enceladus have involved the
Cassini spacecraft, which detected molecules like methanol by flying
directly into the plumes. Recent work has found that the Earth's
oceans have amounts of methanol similar to that in Enceladus' plumes.
In the reported study, astronomers detected a bright methanol
signature with the IRAM 30-metre radio telescope in the Spanish Sierra
Nevada. The observation was very surprising, since it was not the
main molecule astronomers were originally looking for in Enceladus'
plumes. The team suggests the unexpectedly large quantity of methanol
may have two possible origins: either a cloud of gas expelled from
Enceladus has been trapped by Saturn's magnetic field, or gas has
spread further out into the E-ring. In either case, the methanol has
been greatly enhanced compared to detections in the plumes. Cassini
will end its journey later this year, leaving remote observations
through ground- and space-based telescopes as the only possibilities
for exploring Saturn and its moons --- at least for now.
RE-MAKING PLANETS AFTER STAR DEATH
RAS
Astronomers may have found an answer to the 25-year-old question of
how planets form in the aftermath of a supernova explosion. The first
planets outside the Solar System were discovered 25 years ago -- not
around a normal star like the Sun, but instead orbiting a tiny, super-
dense 'neutron star'. Such remnants are left over after a supernova,
the titanic explosion of a star many times as massive as the Sun.
Such 'planets in the dark' have turned out to be very rare, and
astronomers are puzzled over where they come from. The supernova
explosion should destroy any pre-existing planets, so the neutron star
needs to capture more raw materials to form its new companions. The
after-death planets can be detected because their gravitational pull
alters the times of arrival of radio pulses from the neutron star, or
'pulsar', that otherwise pass by us extremely regularly. Astronomers
believe that they have found a way for that to happen. They started
looking for the raw materials soon after the pulsar planets were
announced. They had one target, the Geminga pulsar located 800 light-
years away in the constellation of Gemini. Astronomers thought they
had found a planet there in 1997, but later discounted it because of
glitches in the timing. The scientists observed Geminga with the
James Clerk Maxwell Telescope (JCMT), which operates at sub-millimetre
wavelengths, sited on Hawaii. The light the astronomers detected has
a wavelength of about half a millimetre, so it is invisible to the
human eye, and is only partially transmitted by the Earth's
atmosphere.
What scientists saw was very faint. They went back to it in 2013 with
the new camera SCUBA-2. Combining the two sets of data helped to
ensure they were not just seeing some faint artefacts. Both images
showed a signal towards the pulsar, plus an arc around it. That seems
to be like a bow-wave -- Geminga is moving incredibly fast through our
Galaxy, much faster than the speed of sound in interstellar gas.
Astronomers think that material gets caught up in the bow-wave, and
then some solid particles drift in towards the pulsar. Calculations
suggest that that trapped interstellar 'grit' adds up to at least a
few times the mass of the Earth -- so there could be enough raw
material to make future planets.
MILKY WAY MAY HAVE 100,000 MILLION BROWN DWARFS
Royal Astronomical Society
Our galaxy could have 100,000 million brown dwarfs or more, according
to work by an international team of astronomers. Brown dwarfs are
objects intermediate in mass between stars and planets, with masses
too low to sustain stable hydrogen fusion in their cores (the hallmark
of stars like the Sun).
After the initial discovery of brown dwarfs in 1995, scientists
quickly realised that they are a natural by-product of processes that
primarily lead to the formation of stars and planets. All of the
thousands of brown dwarfs found so far are relatively close to the
Sun, the overwhelming majority within 1500 light-years, simply because
they are faint and therefore difficult to observe. Most of those
detected are located in nearby star-forming regions, which are all
fairly small and have a low density of stars. In 2006 the team began
a new search for brown dwarfs, observing five 'nearby' star-forming
regions. The Substellar Objects in Nearby Young Clusters (SONYC)
survey included the star cluster NGC 1333, 1000 light-years away in
the constellation Perseus. That object had about half as many brown
dwarfs as stars, a higher proportion than had been seen elsewhere.
To establish whether NGC 1333 was unusual, in 2016 the team turned to
another, more distant, star cluster, RCW 38, in the constellation
Vela. RCW 38 has a high density of more massive stars, and very
different conditions from other clusters. It is 5500 light-years
away, so the brown dwarfs are not only faint but are hard to pick out
next to the brighter stars. To get a clear image, the team used the
NACO adaptive-optics camera on ESO's Very Large Telescope, observing
the cluster for a total of 3 hours, and combining that picture with
earlier ones. The researchers found just as many brown dwarfs in
RCW 38 as in NGC 1333 -- about half as many as there are stars --
and tentatively concluded that the environment where the stars form,
whether stars are more or less massive, tightly packed or less
crowded, has only a small effect on how brown dwarfs form. From the
SONYC survey, it is estimated that our Galaxy, the Milky Way, has a
minimum of between 25,000 and 100,000 million brown dwarfs. There are
many smaller, fainter brown dwarfs too, so that could be a significant
underestimate, and the survey confirms that brown dwarfs are
ubiquitous.
FASTEST STARS IN MILKY WAY ARE FROM ANOTHER GALAXY
RAS
A group of astronomers has shown that the fastest-moving stars in our
Galaxy -- which are travelling so fast that they can escape from the
Milky Way altogether -- are in fact runaways from a much smaller
galaxy in orbit around our own. The researchers used data from the
Sloan Digital Sky Survey, and computer simulations, to demonstrate
that the stars concerned originated in the Large Magellanic Cloud
(LMC), a dwarf galaxy in orbit around the Milky Way. The fast-moving
stars, known as hyper-velocity stars, were able to escape their
original home when the explosion of one star in a binary system caused
the other to fly off with such speed that it was able to escape the
gravity of the LMC and get absorbed into the Milky Way. Astronomers
at first thought that the hyper-velocity stars, which are large blue
stars, may have been expelled from the centre of the Milky Way by a
super-massive black hole. Other scenarios involving disintegrating
dwarf galaxies or chaotic star clusters can also account for the
speeds of the stars, but all three mechanisms fail to explain why they
are found only in a certain part of the sky. To date, roughly 20
hyper-velocity stars have been observed, mostly in the northern
hemisphere, although it is possible that there are many more that
could be observed only from the southern hemisphere.
An alternative explanation of the origin of hyper-velocity stars is
that they are runaways from binary systems. In binary star systems,
the closer the two stars are, the faster they orbit one another. If
one star explodes as a supernova, it can break up the binary and the
remaining star flies off at almost the speed at which it was orbiting.
The escaping star is known as a runaway. Runaway stars originating in
the Milky Way are not fast enough to be 'hyper-velocity' because blue
stars can not orbit sufficiently close together without the two stars
merging. But a fast-moving galaxy could give rise to such speedy
stars. The LMC is the largest and fastest of the dozens of dwarf
galaxies in orbit around the Milky Way. It has only 10% of the mass
of the Milky Way, and so the fastest runaways born in it can easily
escape its gravity. The LMC flies around the Milky Way at 400 km/s,
and the velocity of runaway stars is the velocity at which they were
ejected plus the velocity of the LMC. That can be fast enough for
them to be the hyper-velocity stars. It also explains their position
in the sky, because the fastest runaways are those ejected 'forward'
along the orbit of the LMC towards the constellations Leo and Sextans.
SUPER-MASSIVE BLACK HOLE SUFFERS FROM 'INDIGESTION'
RAS
A multi-wavelength study of a pair of colliding galaxies has revealed
the cause of a super-massive black hole's case of 'indigestion'. Once
every couple of hundred million years, the small galaxy NGC 5195 falls
into the outer arms of its larger companion, NGC 5194, also known as
the Whirlpool Galaxy. The two galaxies are locked in a gravitational
embrace that will ultimately result in ther combining into a single
galaxy. As NGC 5195 plunges into the Whirlpool, matter streams onto
the super-massive black hole at NGC 5195's centre and forms an
accretion disc. The disc grows to a point at which the black hole can
no longer accrete or 'digest' efficiently, and matter is blasted out
into the surrounding interstellar medium. Last year, the Chandra
X-Ray observatory observed arcs of X-ray emission that appeared to
result from such activity. Now, new high-resolution images of the
core of NGC 5195, taken with the e-MERLIN radio array, and archive
images of the surrounding area from the Very Large Array (VLA),
Chandra, and the Hubble space telescope, reveal in detail how the
blasts occur and spread. The study was led by astronomers at Jodrell
Bank.
The black hole at the centre of NGC 5195 has a mass equivalent to 19
million Suns. When the accretion process breaks down, immense forces
and pressures create a shock wave that pushes matter out into the
interstellar medium. Electrons, accelerated close to the speed of
light, interact with the magnetic field of the interstellar medium and
emit energy at radio wavelengths. The shock wave then inflates and
heats up the interstellar medium, which emits in the X-ray band, and
strips the electrons from surrounding neutral hydrogen atoms to make
ionized hydrogen gas. The inflated bubble creates the arcs detected
by Chandra and Hubble. Comparing the VLA images at radio wavelengths
to Chandra's X-ray observations and the hydrogen-emission detected by
Hubble shows that features in the very different wavelength domains
are not only connected, but that the radio outflows are in fact the
progenitors of the structures seen by Chandra and Hubble. The
activity in NGS 5195 is an event of galactic proportions that we can
see right across the electromagnetic spectrum. The age of the arcs in
NGC 5195 is 1-2 million years. To put that into context, the first
traces of matter were being forced out of the black hole in that
system at about the time that our ancestors were learning to make
fire. That we are able to observe the (celestial) event now through
such a range of astronomical facilities is quite remarkable.
ASTRONOMERS GET RARE CHANCE TO SEE GALAXY DEMISE
RAS
A primitive galaxy that could provide clues about the early Universe
has been observed by astronomers as it begins to be consumed by a
gigantic neighbouring galaxy. The Little Cub galaxy -- so called
because it sits in the Ursa Major or Great Bear constellation! -- is
being stripped of the gas that it needs to continue forming stars by
its larger companion. The find gives scientists a rare opportunity
to observe a dwarf galaxy as its gas is removed by the effects of a
nearby giant galaxy, to learn more about how that process happens.
As the Little Cub has remained almost pristine since its formation,
scientists also hope that its elements will reveal more about the
chemical signature of the Universe just minutes after the Big Bang.
The Little Cub and its larger neighbour, a 'grand-design' spiral
galaxy called NGC 3359, are about 200 to 300 thousand light-years
apart, and approximately 50 million light-years away. Gas from the
Little Cub is being stripped away by its interaction with NGC 3359,
which has up to 10,000 times as many stars as the Little Cub and is
similar to our Milky Way. By observing the pair, scientists hope to
understand more about how and when gas is lost from smaller galaxies.
It is rare for such a tiny galaxy still to contain gas and be forming
stars when it is in close proximity to a much larger galaxy, so this
is a great opportunity to see how that process works. Essentially the
larger galaxy is removing the fuel that the Little Cub needs to form
stars, which will eventually shut down star formation and lead to the
smaller galaxy's demise. The researchers also hope to gain an insight
into the make-up of the very early Universe, by studying the hydrogen
and helium atoms that are being illuminated by the small number of
very bright stars within the 'Little Cub' -- which also has the less
romantic name SDSS J1044+6306. Astronomers know from studies of the
chemistry of the Little Cub that it is one of the most primitive
objects currently known in our cosmic neighbourhood. Such galaxies,
which have remained dormant for most of their lives, are believed to
contain the chemical elements forged a few minutes after the Big Bang.
By measuring the relative number of hydrogen and helium atoms in the
Little Cub, we might be able to learn more about what made up the
Universe in the moments after it began 13.7 thousand million years
ago.
The Little Cub was initially identified as a potentially pristine
dwarf galaxy in data from the Sloan Digital Sky Survey (SDSS).
Follow-up observations were conducted with the 3-m Shane Telescope at
Lick Observatory and the 10-m Keck telescope.
HUBBLE PUSHED BEYOND LIMITS
NASA/Goddard Space Flight Center
When it comes to the distant Universe, even the Hubble telescope can
go only so far. Teasing out finer details requires clever thinking
and a little help from a cosmic alignment with a gravitational lens.
By applying a new computational analysis to a galaxy magnified by a
gravitational lens, astronomers have obtained images 10 times sharper
than Hubble could achieve on its own. The results show an edge-on disc
galaxy studded with brilliant patches of newly formed stars. The
galaxy in question is so far away that we see it as it appeared 11,000
million years ago, 'only' 2700 million years after the Big Bang. It
is one of more than 70 strongly lensed galaxies studied by the Hubble
telescope, following up targets selected by the Sloan Giant Arcs
Survey, which discovered hundreds of strongly lensed galaxies by
searching Sloan Digital Sky Survey imaging data covering one-fourth of
the sky. The gravity of a giant cluster of galaxies between the
observed galaxy and the Earth distorts the more distant galaxy's
light, stretching the appearance of the galaxy into an arc and also
magnifying it almost 30 times. The team developed a special computer
code to remove the distortions caused by the gravitational lens and
reveal the disc galaxy as it would normally appear.
The resulting reconstructed image revealed two dozen clumps of newborn
stars, each spanning about 200 to 300 light-years. That flew in the
face of theories suggesting that star-forming regions in the distant,
early Universe were much larger, 3,000 light-years or more in size.
Without the magnification boost of the gravitational lens, the disc
galaxy would appear perfectly smooth and unremarkable to Hubble, and
would give astronomers a very different picture of where stars are
forming. While Hubble highlighted new stars within the lensed galaxy,
the upcoming James Webb space telescope could be expected to uncover
older, redder stars that formed even earlier in the galaxy's history.
It will also see through any obscuring dust within the galaxy.
Topic: Mid July Astronomy Bulletin (Read 2408 times)