EARLY SOLAR SYSTEM ENCOUNTERS BY LARGE PROTOPLANETS
RAS
A study simulating the final stages of terrestrial planet formation shows
that 'hit-and-run' encounters play a significant role in the acquisition of
water by large protoplanets, like those that grew into Mars and the Earth.
Four and a half billion years ago, the inner Solar System was a chaotic
place with around 50-100 protoplanets ranging in size from the Moon to Mars
that were prone to giant collisions. Bodies that formed within what is now
the orbit of Mars contained no water, as the conditions were too hot for
volatile material, like water or methane, to condense. For water to make
its way onto the developing terrestrial planets, it needed to be delivered
from outside their region via a sequence of collisions. Astronomers have
used high-resolution simulations to track the fate of water and other
materials through a series of different impact scenarios. Outcomes of
collisions could include bodies sticking together, material being lost, or
being redistributed between the two objects. The results depend on various
factors like the speed and angle of impact, the difference in mass between
the bodies and their total mass. They found that 'hit-and-run' collisions,
where the impact is off-centre and the bodies have enough speed to separate
again after the encounter, are very common. In those scenarios, tens of
percent of water can be transferred between the colliding bodies or ejected
and lost entirely. The smaller of the colliding pair is often modified down
to the core and effectively stripped of water, while the more-massive body
remains more or less unaltered. The team is now focusing on how long chains
of successive collisions affect the evolution of a disk of planetesimals and
protoplanets. Recent research shows that comets can only account for a
small fraction of the terrestrial planets' water. The great collisions
early in the Solar System's history must also be a major source. The
results strongly suggest that astronomers need to track the water in both
survivors following hit-and-run encounters. That will help to predict the
properties of planets that form as the end-product of a long sequence of
successive collisions.
DUST STORMS OBSERVED ON TITAN
NASA
Data from the Cassini spacecraft have revealed what appear to be giant dust
storms in equatorial regions of Saturn's moon Titan. The discovery makes
Titan the third Solar-System body, after the Earth and Mars, where dust
storms have been observed. The observation is helping scientists to
understand the fascinating and dynamic environment of Saturn's largest moon.
Titan is an intriguing world -- in ways quite similar to the Earth. In
fact, it is the only moon in the Solar System with a substantial atmosphere
and the only celestial body other than our planet where stable bodies of
surface liquid are known still to exist. There is one big difference,
though: on Earth such rivers, lakes and seas are filled with water, while
on Titan it is primarily methane and ethane that constitute the liquid
reservoirs. In that unique cycle, the hydrocarbon molecules evaporate,
condense into clouds and rain back onto the ground. The weather on Titan
varies from season to season as well, just as it does on Earth. In
particular, around the equinox -- the time when the Sun crosses Titan's
equator -- massive clouds can form in tropical regions and cause powerful
methane storms. Cassini observed such storms during several of its Titan
flybys.
When the team first observed three unusual equatorial brightenings in
infrared images taken by Cassini around the moon's 2009 northern equinox,
they thought that they might be the same kind of methane clouds; however, an
investigation revealed that they were something completely different. The
researchers were also able to rule out that the features were actually on
the surface of Titan in the form of frozen methane rain or icy lavas. Such
surface spots would have a different chemical signature and would remain
visible for much longer than the bright features in this study, which were
visible for only 11 hours to five weeks. In addition, modelling showed that
the features must be atmospheric but still close to the surface -- most
likely forming a very thin layer of tiny solid organic particles. Since
they were located right over the dune fields around Titan's equator, the
only remaining explanation was that the spots were actually clouds of dust
raised from the dunes. Organic dust is formed when organic molecules,
formed from the interaction of sunlight with methane, grow large enough to
fall to the surface. A member of the team said that, while this is the
first observation ever made of a dust storm on Titan, the finding is not
surprising. The team believes that the Huygens probe, which landed on the
surface of Titan in 2005 January, raised a small amount of organic dust upon
arrival owing to its powerful aerodynamic wake. But the near-surface wind
speeds required to raise such an amount of dust as we see in these dust
storms would have to be very strong -- about five times as strong as the
average wind speeds estimated by the Huygens measurements near the surface
and with climate models. The existence of such strong winds generating
massive dust storms implies that the underlying sand can be set in motion,
too, and that the giant dunes covering Titan's equatorial regions are still
active and continually changing. The winds could be transporting the dust
raised from the dunes across large distances, contributing to the global
cycle of organic dust on Titan and causing effects similar to those that can
be observed on the Earth and Mars.
EVIDENCE OF EARLY PLANETARY SHAKE-UP
Southwest Research Institute
Scientists have studied an unusual pair of asteroids and discovered that
their existence points to an early planetary rearrangement in the Solar
System. Those bodies, called Patroclus and Menoetius, are targets of NASA's
upcoming Lucy mission. They are around 70 miles across and orbit around one
another as they collectively circle the Sun. They constitute the only large
binary known in the population of ancient bodies referred to as the Trojan
asteroids. The two swarms of Trojans orbit at roughly the same distance
from the Sun as Jupiter, one swarm orbiting ahead of, and the other behind,
the planet. The Trojans were probably captured during a dramatic period of
dynamic instability when a skirmish between the Solar System's giant planets
-- Jupiter, Saturn, Uranus and Neptune -- occurred. That shake-up pushed
Uranus and Neptune outwards, where they encountered a large primordial
population of small bodies thought to be the source of today's Kuiper Belt
objects, which orbit at the edge of the Solar System. Many small bodies of
that primordial Kuiper Belt were scattered inwards, and a few of those
became trapped as Trojan asteroids. A key issue with that Solar-System
evolution model, however, has been when it took place. The scientists
demonstrate that the very existence of the Patroclus--Menoetius pair
indicates that the dynamic instability among the giant planets must have
occurred within the first 100 million years of the Solar-System formation.
Recent models of small-body formation suggest that that type of binaries are
left-overs from the very earliest times of the Solar System, when pairs of
small bodies could form directly from a collapsing cloud of 'pebbles'.
Observations of today's Kuiper Belt show that binaries like those were quite
common in ancient times. Only a few of them now exist within the orbit of
Neptune. The question is how to interpret the survivors. Had the
instability been delayed many hundreds of millions of years, as suggested by
some Solar-System evolution models, collisions within the primordial
small-body disc would have disrupted such relatively fragile binaries,
leaving none to be captured in the Trojan population. Earlier dynamical
instabilities would have left more binaries intact, increasing the likeli-
hood that at least one would have been captured in the Trojan population.
The team created new models that show that the existence of the Patroclus--
Menoetius binary strongly indicates an earlier instability. That early
dynamical-instability model has important consequences for the terrestrial
planets, particularly regarding the origin of large impact craters on the
Moon, Mercury and Mars that formed approximately 4 billion years ago. The
impactors that made those craters are less likely to have been flung in from
the outer regions of the Solar System. That could imply that they were made
by small-body leftovers of the terrestrial-planet formation process. This
work underscores the importance of the Trojan asteroids in illuminating the
history of the Solar System. Much more will be learned about Patroclus--
Menoetius binary when NASA's Lucy mission surveys the pair in 2033, at the
conclusion of a 12-year mission to tour both Trojan swarms.
UNUSUAL FEATURES AROUND NEUTRON STAR
NASA/Goddard Space Flight Center
An unusual infrared light emission from a nearby neutron star detected by
the Hubble Space Telescope could indicate new features never before seen.
One possibility is that there is a dusty disk surrounding the neutron star;
another is that there is an energetic wind coming off the object and
slamming into gas in the interstellar space that the neutron star is
ploughing through. Although neutron stars are generally studied in radio
and high-energy emissions, such as X-rays, this study demonstrates that new
and interesting information about neutron stars can also be gained by
studying them in infrared light. The observation could help astronomers
understand better the evolution of neutron stars -- the incredibly dense
remnants after a massive star explodes as a supernova. Neutron stars are
also called pulsars because their very fast rotation (typically fractions of
a second, in this case 11 seconds) causes time-variable emission from light-
emitting regions. This particular neutron star belongs to a group of seven
nearby X-ray pulsars -- nicknamed 'the Magnificent Seven' -- that are hotter
than they ought to be considering their ages and available energy reservoir
provided by the loss of rotation energy. Astronomers observed an extended
area of infrared emissions around this neutron star -- named RX J0806.4-4123
-- the total size of which translates into about 200 astronomical units at
the assumed distance of the pulsar. This is the first neutron star in which
an extended signal has been seen only in infrared light. The researchers
suggest two possibilities that could explain the extended infrared signal
seen by Hubble. The first is that there is a disk of material -- possibly
mostly dust -- surrounding the pulsar.
Another theory is that there could be what is known as a 'fallback disk' of
material that coalesced around the neutron star after the supernova. Such a
disk would be composed of matter from the progenitor massive star. Its
subsequent interaction with the neutron star could have heated the pulsar
and slowed its rotation. If confirmed as a supernova fallback disk, this
result could change our general understanding of neutron-star evolution.
The second possible explanation for the extended infrared emission from this
neutron star is a 'pulsar wind' nebula. A pulsar wind nebula would require
the neutron star to exhibit a pulsar wind. A pulsar wind can be produced
when particles are accelerated in the electrical field that is produced by
the fast rotation of a neutron star with a strong magnetic field. As the
neutron star travels through the interstellar medium faster than the speed
of sound, a shock can form where the interstellar medium and the pulsar wind
interact. The shocked particles would then emit synchrotron radiation,
causing the extended infrared signal that we see. Typically, pulsar-wind
nebulae are seen in X-rays and an infrared-only pulsar wind nebula would be
very unusual and exciting.
NEW STAR BORN FROM STELLAR EXPLOSION
Purdue University
The explosions of supernovae can be so bright that they outshine their host
galaxies. They take months or years to fade away, and sometimes the gaseous
remains of the explosion slam into hydrogen-rich gas and temporarily become
bright again -- but could they remain luminous without any outside inter-
ference? As large stars explode, their interiors collapse down to a point
at which all their particles become neutrons. If the resulting neutron star
has a magnetic field and rotates fast enough, it may develop into a pulsar
wind nebula. That is most likely what happened to SN 2012au. We know that
supernova explosions produce such types of rapidly rotating neutron stars,
but we never saw direct evidence of it at this unique time frame. This is a
key moment when the pulsar-wind nebula is bright enough to act like a light
bulb illuminating the explosion's outer ejecta. SN 2012au was already known
to be extraordinary -- and weird -- in many ways. Although the explosion
was not bright enough to be termed a 'superluminous' supernova, it was
extremely energetic and long-lasting, and dimmed in a similarly slow light
curve. Superluminous supernovae are a hot topic in transient astronomy.
They are potential sources of gravitational waves and black holes, and
astronomers think that they might be related to other kinds of explosions,
like gamma-ray bursts and fast radio bursts. Researchers want to understand
the fundamental physics behind them, but they are difficult to observe
because they are relatively rare and happen so far away.
THERE MAY HAVE BEEN THREE MAGELLANIC CLOUDS
International Centre for Radio Astronomy Research
Two of the closest galaxies to the Milky Way -- the Large and Small
Magellanic Clouds -- may have had a third companion. New research describes
how another 'luminous' galaxy was probably engulfed by the Large Magellanic
Cloud some three to five billion years ago. Most stars in the Large
Magellanic Cloud revolve clockwise around the centre of that galaxy. But,
unusually, some stars revolve anti-clockwise. For a while, it was thought
that those stars might have come from its companion galaxy, the Small
Magellanic Cloud. Astronomers used computer modelling to simulate galaxy
mergers. What they found is that in such a merging event, you actually can
get quite strong counter-rotation after a merger takes place. That is
consistent with what we see when we actually observe the galaxies. From the
southern hemisphere the Magellanic Clouds can be seen in the night sky with
the naked eye and have been observed by ancient cultures for thousands of
years. The Large Magellanic Cloud is a relatively small 160,000 light-years
away from us, while the Small Magellanic Cloud is around 200,000 light-years
away. The finding could help to resolve a problem that has perplexed
astronomers for years -- why stars in the Large Magellanic Cloud are
generally either very old or very young. Galaxies often contain large star
clusters, which consist of many stars that are all of quite similar ages and
made in similar environments. In the Milky Way, the star clusters are all
very old, but in the Large Magellanic Cloud, there are very old clusters as
well as ones that are very young -- but nothing in between. That is known
as the 'age-gap' problem. Because in the Large Magellanic Cloud we see star
formation starting again, that could be indicative of a galaxy merger taking
place. The finding could also help to explain why the Large Magellanic
Cloud appears to have a thick disk.
RADIATION RISK FOR MARS ASTRONAUTS
ESA
Astronauts on a mission to Mars would be exposed to at least 60% of the
total radiation dose limit recommended for their career during the journey
itself to and from the planet, according to data from the ESA-Roscosmos
ExoMars Trace Gas Orbiter. The Trace Gas Orbiter began its scientific
mission at Mars in April, and while its primary goals are to provide the
most detailed inventory of Martian atmospheric gases to date -- including
those that might be related to active geological or biological processes --
its radiation monitor has been collecting data since launch in 2016. The
Liulin-MO dosimeter of the 'Fine-Resolution Epithermal Neutron Detector'
(FREND) provided data on the radiation doses recorded during the orbiter's
six-month interplanetary cruise to Mars, and since the spacecraft reached
orbit around the planet. On Earth, a strong magnetic field and a thick
atmosphere protect us from the unceasing bombardment of galactic cosmic
rays, fragments of atoms from outside the Solar System that travel at close
to the speed of light and are highly penetrating for biological material.
In space that has the potential to cause serious damage to people, including
radiation sickness, an increased lifetime risk for cancer, central-nervous-
system effects, and degenerative diseases, which is why ESA is researching
ways to protect astronauts on long space missions. The ExoMars measure-
ments cover a period of declining solar activity, corresponding to a high
radiation dose. Increased activity of the Sun can deflect the galactic
cosmic rays, although very large solar flares and eruptions can themselves
be dangerous to astronauts. Radiation doses accumulated by astronauts in
interplanetary space would be several hundred times larger than the doses
accumulated by people over the same time period on Earth, and several times
larger than the doses of astronauts and cosmonauts working on the Inter-
national Space Station.
IRELAND TO JOIN ESO
ESO
Irish astronomers are about to gain access to the world's most advanced
ground-based astronomical telescopes following the signature of Ireland's
Accession Agreement in Dublin. By joining ESO, Ireland adds to its
already rich astronomical history, stretching back centuries. For several
decades in the 19th century, Ireland hosted the world's largest telescope
-- the Leviathan of Parsonstown -- a 1.8-metre reflecting telescope at
Birr Castle (whose grounds are now home to I-LOFAR, port of a Europe-wide
low-frequency radio telescope). Ireland?s vibrant research community and
high-tech industrial sector have supported ESO membership for many years,
and will now gain access to a range of instrumentation and industrial
opportunities as a result of ESO membership. ESO is the foremost inter-
governmental astronomy organisation in Europe and the world's most
productive ground-based astronomical observatory by far. It has 16 Member
States: Austria, Belgium, the Czech Republic, Denmark, France, Finland,
Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden,
Switzerland and the United Kingdom, along with the host state of Chile and
with Australia as a Strategic Partner. It operates three unique world-class
observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO
operates the Very Large Telescope and its world-leading Very Large Telescope
Interferometer as well as two survey telescopes, VISTA working in the infra-
red and the visible-light VLT Survey Telescope. ESO is also a major partner
in two facilities on Chajnantor, APEX and ALMA, the largest astronomical
project in existence. And on Cerro Armazones, close to Paranal, ESO is
building the 39-metre Extremely Large Telescope, the ELT, which will become
the world's biggest telescope.
Topic: Early October Astronomy Bulletin (Read 1195 times)