UNEXPECTED MINERAL DISCOVERED ON MARS
NASA
Scientists have discovered an unexpected mineral in a rock sample at
Gale Crater on Mars, a finding that may alter our understanding of how
the planet evolved. The Mars Science Laboratory rover, Curiosity, has
been exploring sedimentary rocks within Gale Crater since landing in
2012. In 2015 July the rover collected powder drilled from rock at a
location named 'Buckskin'. Analyzing data from an X-ray diffraction
instrument on the rover that identifies minerals, scientists detected
significant amounts of a silica mineral called tridymite. That
detection was a surprise to the scientists, because tridymite is
generally associated with silicic vulcanism, which is known on Earth
but was not thought to be important or even present on Mars. The
discovery of tridymite might induce scientists to re-think the vol-
canic history of Mars, suggesting that the planet once had explosive
volcanoes that led to the presence of the mineral. Mount St. Helens,
the active volcano in Washington State, and the Satsuma-Iwojima
volcano in Japan, are examples of such volcanoes. The combination of
high silica content and extremely high temperatures in the volcanoes
creates tridymite. The paper will also stimulate scientists to
re-examine the way tridymite forms. The authors looked for evidence
that tridymite could form on the Earth at low temperatures from
geologically reasonable processes and not imply silicic vulcanism, but
they found none. Researchers will need to look for ways in which it
could form at lower temperatures. The discovery now begs the question
of whether Mars experienced a much more violent and explosive volcanic
history during its early evolution than has previously been thought.
HOW MARS' MOONS FORMED
CNRS
The origin of the two Martian moons, Phobos and Deimos, has been
uncertain. Owing to their small sizes and irregular shapes, they
strongly resemble asteroids, but no one has understood how Mars could
have captured them and made them into satellites with almost circular
and equatorial orbits. According to one theory, towards the end of
its formation Mars suffered a collision with a proto-planet; but then
why did the debris from such an impact create two small satellites
instead of one relatively enormous one, like the Earth's? Another
possibility is that Phobos and Deimos formed at the same time as Mars,
which would entail their having the same composition as their planet,
although their low density seems to contradict that idea.
Two independent studies are now considered to have solved the puzzle:
the Martian moons must have arisen from a giant collision 100 to 800
million years after the beginning of the planet's formation.
According to one study, the debris from the collision formed a very
large disc around Mars, made up of a dense inner part composed of
matter in fusion, and a very thin outer part primarily of gas. In the
inner part of the disc was formed a moon a thousand times the size of
Phobos, which has since disappeared. The gravitational interactions
created in the outer disc by that massive moon could have acted as a
catalyst for the gathering of debris to form other smaller, more
distant moons. After a few thousand years, Mars was surrounded by a
group of approximately ten small moons and one enormous one. A few
million years later, once the debris disc had dissipated, the tidal
effects of Mars brought most of the satellites back down onto the
planet, including the very large moon. Only the two most distant
small moons, Phobos and Deimos, remained.
In a second study, researchers ruled out the possibility of a capture
by statistical arguments based on the compositional diversity of the
asteroid belt. They show, moreover. that the spectra of Phobos and
Deimos are incompatible with that of the primordial matter that formed
Mars (meteorites such as ordinary chondrite, enstatite chondrite
and/or angrite). They therefore support the collision scenario. The
spectra indicate that the satellites are made of fine-grained dust
(smaller than a micrometre). Yet the very small size of grains on the
surface of Phobos and Deimos cannot, according to the researchers, be
solely explained as the consequence of erosion from bombardment by
interplanetary dust. That means (they say) that the satellites were
from the beginning made up of very fine grains, which can only form by
gas condensation in the outer part of the debris disc (and not from
the magma present in the inner part). Both studies are in agreement
on thar point. Moreover, the formation of Martian moons from very
fine grains could also be responsible for a high internal porosity,
which would explain their surprisingly low densities.
The theory of the giant collision, which is advanced by the two
independent studies, could explain why the northern hemisphere of Mars
has a lower altitude than the southern hemisphere: the Borealis basin
is most probably the site of a giant collision, such as the one that
gave birth to Phobos and Deimos. It also helps to explain why Mars
has two satellites instead of a single one like our Moon, which is
also supposed to have been created by a giant collision. The research
suggests that the satellite systems that were created depended on the
planet's rotational velocity, because at the relevant time the Earth
was rotating very quickly (in less than four hours), whereas Mars
turned six times more slowly. New observations will soon make it
possible to know more about the age and composition of the Martian
moons. Japan's space agency (JAXA) has decided to launch a mission in
2022, named Mars Moons Exploration (MMX), which will bring back
samples from Phobos in 2027. Their analysis could confirm or
invalidate the new proposal. ESA has planned a similar mission in
2024 in association with the Russian space agency (Roscosmos).
NEW DARK SPOT ON NEPTUNE
Space Telescope Science Institute (STScI)
New images obtained by the Hubble space telescope confirm the presence
of a dark vortex in the atmosphere of Neptune. Though similar
features were seen during the Voyager 2 flyby of Neptune in 1989 and
by Hubble in 1994, the vortex is the first one observed on Neptune in
this century. Neptune's dark vortices are high-pressure systems and
are usually accompanied by bright 'companion clouds', which are also
now visible on the planet. The bright clouds form when the flow of
ambient air is perturbed and diverted upwards over the dark vortex,
probably causing methane gas to freeze into ice-like crystals. Dark
vortices coast through the atmosphere like huge, lens-shaped gaseous
mountains, and the companion clouds are similar to the orographic
clouds that appear as pancake-shaped features lingering over mountains
on Earth. Beginning in 2015 July, bright clouds were again seen on
Neptune by several observers, from amateurs to astronomers at the Keck
Observatory in Hawaii. Astronomers suspected that the clouds might be
bright companion clouds following an unseen dark vortex. Neptune's
dark vortices are typically seen only at blue wavelengths, and only
Hubble has sufficiently high resolution to see them on Neptune. Last
September, the Outer Planet Atmospheres Legacy (OPAL) programme, a
long-term Hubble project that annually captures global maps of the
outer planets, revealed a dark spot close to the location of the
bright clouds, which had been tracked from the ground. By viewing the
vortex a second time, the new Hubble images confirm that OPAL really
detected a long-lived feature. The new data enabled the team to
create a higher-quality map of the vortex and its surroundings.
Neptune's dark vortices have exhibited surprising diversity over the
years, in terms of size, shape, and stability (they meander in
latitude, and sometimes speed up or slow down). They also come and go
on much shorter time-scales than similar anticyclones seen on Jupiter;
large storms on Jupiter evolve over decades.
DWARF PLANET MAKEMAKE HAS MOON
Southwest Research Institute
Scientists have discovered a dark moon orbiting Makemake, one of the
'big four' dwarf planets populating the Kuiper-Belt region at the edge
of the Solar System. The moon -- presently called MK2 -- is 1,300
times fainter than the dwarf planet. A nearly edge-on orbital
configuration helped it evade detection, placing it deep within the
glare of the icy dwarf during a substantial fraction of its orbit.
Makemake is one of the largest and brightest known Kuiper Belt Objects
(KBOs), second only to Pluto. The moon is probably less than 100
miles in diameter while Makemake itsef is about 870 miles across.
Discovered in 2005, Makemake is shaped like a football (notice that
this item was drafted in America, where footballs are not spheres) and
sheathed in frozen methane. With a moon, we can calculate Makemake's
mass and density and contrast the orbits and properties of the parent
dwarf and its moon, to understand the origin and history of the
system. We can compare Makemake and its moon to other systems, and
broaden our understanding of the processes that shaped the evolution
of the Solar System. With the discovery of MK2, all four of the
currently designated dwarf planets are known to have one or more
satellites. The fact that Makemake's satellite went unseen despite
previous searches suggests that other large KBOs may have hidden
moons. Before this discovery, the lack of a satellite for Makemake
suggested that it had not suffered a severe impact in the past. Now,
scientists will be looking at its density to determine whether it was
formed by a collision or whether it was grabbed by the parent dwarf's
gravity. The apparent ubiquity of moons orbiting KBO dwarf planets
supports the idea that collisions are a near-universal feature in the
histories of those distant objects.
NEW DISTANT DWARF PLANET BEYOND NEPTUNE
University of British Columbia
Astronomers have discovered a new dwarf planet orbiting in the disc of
small icy bodies beyond Neptune. The new object is about 700 km in
diameter and has one of the largest orbits for a dwarf planet.
Designated 2015 RR245 by the IAU's Minor Planet Center, it was found
with the CFHT on Mauna Kea, Hawaii, as part of the ongoing Outer Solar
System Origins Survey (OSSOS). The OSSOS project uses powerful
computers to hunt for the images, and the team was presented with a
bright object moving at such a slow rate that it was clearly at least
twice as far away as Neptune. The size of RR245 is not yet exactly
known, as its surface properties need further measurement. The vast
majority of dwarf planets like RR245 were destroyed or thrown out of
the Solar System when the giant planets moved out to their present
positions. RR245 is one of the few dwarf planets that survived to the
present day, along with Pluto and Eris, which are the largest known
ones. RR245 now circles the Sun among the remnant population of tens
of thousands of much smaller trans-Neptunian objects, most of which
orbit unseen. RR245 has been on its highly elliptical orbit for at
least the last 100 million years. After hundreds of years further
than 80 astronomical units (AU) from the Sun, RR245 is travelling
towards its perihelion at 34 AU, which it will reach around 2096. As
RR245 has been observed for only one of the 700 years that it takes to
orbit the Sun, where it came from and how its orbit will slowly evolve
in the far future is still unknown. Its precise orbit will be refined
over the coming years, after which RR245 will be given a name. As
discoverers, the OSSOS team can submit their preferred name for RR245
to the IAU for consideration. RR245 is the largest discovery and the
only dwarf planet found by OSSOS, which has discovered more than 500
new trans-Neptunian objects.
EVIDENCE OF WATER CLOUDS ON BROWN DWARF
University of California, Santa Cruz
Since its detection in 2014, the brown dwarf known as WISE 0855 has
fascinated astronomers. Only 7.2 light-years away, it is the coldest
known solid object outside the Solar System and is just barely visible
at infrared wavelengths with the largest ground-based telescopes. Now
astronomers have succeeded in obtaining an infrared spectrum of WISE
0855 with the Gemini North telescope in Hawaii, providing the first
details of the object's composition and chemistry. Among the findings
is strong evidence for the existence of clouds of water or water ice,
the first such clouds detected outside the Solar System. A brown
dwarf is essentially a failed star, having formed the way stars do
through the gravitational collapse of a cloud of gas and dust, but
without having enough mass to spark the nuclear-fusion reactions that
make stars shine. With about five times the mass of Jupiter, WISE
0855 resembles that gas-giant planet in many respects. Its tempera-
ture is about 250 degrees Kelvin, making it nearly as cold as Jupiter.
Previous observations of it provided tentative indications of water
clouds on the basis of very limited photometric data, but obtaining a
spectrum is the only way to determine an object's molecular
composition. WISE 0855 is too faint for conventional spectroscopy at
optical or near-infrared wavelengths, but thermal emission from the
deep atmosphere at wavelengths in a narrow window around 5 microns
offered an opportunity where spectroscopy would be challenging but not
impossible. The team used the Gemini-North telescope in Hawaii to
observe WISE 0855 over 13 nights for a total of about 14 hours.
The spectrum shows that WISE 0855 is dominated by water vapour and
clouds, with an overall appearance that is strikingly similar to
Jupiter's. The researchers developed atmospheric models of the
equilibrium chemistry for a brown dwarf at 250 K and calculated the
resulting spectra under different assumptions, including cloudy and
cloud-free models. The models predicted a spectrum dominated by
features resulting from water vapour, and the cloudy model yielded the
best fit to the features in the spectrum of WISE 0855. Comparing the
brown dwarf to Jupiter, the team found that their spectra are
strikingly similar with respect to water absorption features. One
significant difference is the abundance of phosphine in Jupiter's
atmosphere. Phosphine forms in the hot interior of the planet and
reacts to form other compounds in the cooler outer atmosphere, so its
appearance in the spectrum is evidence of turbulent mixing in
Jupiter's atmosphere. The absence of a strong phosphine signal in the
spectrum of WISE 0855 implies that it has a less turbulent atmosphere.
A PLANET WITH THREE SUNS
ESO
A team of astronomers has used the SPHERE instrument on the Very Large
Telescope to image the first planet ever found in a large orbit inside
a triple-star system. The orbit of such a planet had been expected to
be unstable, probably resulting in the planet being quickly ejected
from the system. But somehow this one survives. This unexpected
observation suggests that such systems may actually be more common
than has previously been thought. The planet, HD 131399Ab, is about
320 light-years away in the constellation Centaurus, and is about 16
million years old, making it also one of the youngest exo-planets
discovered to date, and one of very few directly imaged. With a
temperature of around 580 C and an estimated mass four times that of
Jupiter, it is also one of the coldest and least-massive directly-
imaged exo-planets. For about half of its orbit, which lasts 550
years, three stars are visible in its sky; the fainter two are always
much closer together, and change in apparent separation from the
brightest star throughout the year. Although repeated and long-term
observations will be needed to determine the planet's trajectory among
its stars, observations and simulations seem to suggest the following:
the brightest star, HD 131399A, is estimated to be 80% more massive
than the Sun, and is itself orbited by a sub-system consisting of the
less massive stars, B and C, at about 300 AU. B and C revolve
together like a spinning dumbbell, separated by about 10 AU, a
distance roughly equal to that between the Sun and Saturn.
The planet HD 131399Ab travels round the star A in an orbit with a
radius of about 80 AU, about twice as large as Pluto's in the Solar
System, and takes the planet to about 1/3 of the separation between
star A and the B/C pair. The authors point out that a range of
orbital possibilities exists, and the verdict on the long-term
stability of the system will have to wait for planned follow-up
observations that will allow the determination of the planet's orbit.
If the planet were further away from the most massive star, it would
be kicked out of the system. Computer simulations have shown that its
type of orbit can be stable, but with only small changes it can become
unstable very quickly. Planets in multi-star systems are of special
interest to astronomers and planetary scientists, because they provide
examples of how the mechanism of planetary formation functions in
those more extreme situations. While multi-star systems seem exotic
to us in our orbit around our solitary star, multi-star systems are in
fact just as common as single stars. It is not clear how the planet
ended up in its large orbit in this extreme system, but it shows that
there is more variety out there than many people would have supposed
possible.
MINOR MERGERS DRIVE STAR FORMATION
RAS
Around half of the star formation in the local Universe arises from
minor mergers between galaxies, according to data from the Sloan
Digital Sky Survey. The patch of sky called Stripe 82 is observed
repeatedly to produce high-quality images of spiral galaxies.
Disruptions to the shapes of those galaxies, caused by interactions
with their smallest neighbours, points to increased star formation.
Gravity is a significant driver of galaxy formation. Gravity makes
galaxies collide, and collisions can affect various properties --
merging drives strong star formation in the galaxies in question,
increases the masses of their constituent black holes and can
significantly alter the internal structures of the galaxies, and
increases the masses of their constituent black holes. Our classical
paradigm has often assumed that mergers between equal-mass progenitors
('major' mergers) have the most transformative impact on galaxies.
However, such events are rare. Much more common are mergers between
massive galaxies and small satellites ('minor' mergers). That is
because small galaxies far outnumber their more-massive counterparts
-- the attractive nature of gravity then ensures that massive galaxies
are constantly being bombarded by satellites. While major mergers are
more spectacular and easier to study because they tend to be brighter,
studying minor mergers requires large surveys which offer long-
exposure imaging which is able to detect the faint tidal features that
are the signatures of minor mergers.
Recently, circumstantial evidence is accumulating that suggests that
minor mergers are indeed important drivers of galaxy evolution -- for
example, the observed size growth of galaxies over the last 10-12
billion years is probably due to repeated minor mergers. The
new study is the first to use a deep survey to quantify what fraction
of the star formation in the 'nearby' Universe is likely to be driven
by the minor-merger process. The results are striking: just over half
of the cosmic star-formation budget is directly driven by minor
mergers. In other words, if that process did not take place then
galaxies in today's Universe would be at least a factor of two less
massive. Without a good comprehension of the minor-merger process,
therefore, our understanding of galaxy evolution will remain
incomplete. This paper is a precursor to work that can be done with
future instrumentation like the Large Synoptic Survey Telescope
which will, for the first time, provide deep imaging over about half
the sky, enabling the first statistically robust studies of minor
merging over at least 50% of cosmic time.
FAMILY OF SUPER-BRIGHT GALAXIES IN EARLY UNIVERSE
RAS
Astronomers have identified a family of incredible galaxies that could
shed further light on the transformation of the early Universe, known
as the 'epoch of reionization'. About 150 million years after the Big
Bang, some 13 billion years ago, the Universe was completely opaque
to high-energy ultraviolet light, with neutral hydrogen gas blocking
its passage. Astronomers have long realized that that situation ended
at the 'epoch of reionization', where ultraviolet light from the
earliest stars ionized the hydrogen atoms, and could start to travel
freely through the cosmos. That reionization period marks a key
transition between the relatively simple early cosmos, with normal
matter made up of hydrogen and helium, and the Universe we see today,
transparent on large scales and filled with heavier elements. In 2015
astronomers found the first example of a spectacularly bright galaxy
within the epoch of reionization, named Cosmos Redshift 7 or CR7,
which may harbour first-generation stars. The team also discovered a
similar galaxy, MASOSA, which, together with Himiko, discovered by a
Japanese team, hinted at a larger population of similar objects,
perhaps made up of the earliest stars and/or black holes. Using the
Subaru and Keck telescopes on Hawaii, and the VLT in Chile, the team,
along with a group in the US, has now found more examples of that
population. All of the newly found galaxies seem to have a large
bubble of ionized gas around them.
Stars and black holes in the earliest, brightest galaxies must have
emitted so much ultraviolet light that they quickly broke up hydrogen
atoms in the surrounding Universe. The fainter galaxies seem to have
stayed shrouded from view for a lot longer. Even when they eventually
become visible, they show evidence of plenty of opaque material still
in place around them. Bright galaxies were therefore visible much
earlier in the history of the Universe, allowing us not only to use
them to study reionization itself, but also to study the properties of
the very first galaxies and the black holes that they may contain.
With five bright sources now confirmed, and many more expected to
follow, CR7 may be part of a set of tens to hundreds of thousands of
bright galaxies.
SPECTACULAR SURVEY OF EARLY UNIVERSE
RAS
Astronomers have released spectacular new infrared images of the
distant Universe, providing the deepest view ever obtained over a
large area of sky. The final data release from the Ultra-Deep Survey
(UDS) maps an area four times the size of the Full Moon to unprece-
dented depth. Over 250,000 galaxies have been detected, including
several hundred observed within the first thousand million years after
the Big Bang. Astronomers around the world will use the new images to
study the early stages of galaxy formation and evolution. The release
of the final UDS images represents the culmination of a project that
began taking data in 2005. The scientists used the United Kingdom
Infrared Telescope (UKIRT) on Hawaii to observe the same patch of sky
repeatedly, building up more than 1000 hours of exposure time.
Observing in the infrared is vital for studying the distant Universe,
as starlight is red-shifted to longer wavelengths by the cosmological
expansion of the Universe. Because of the finite speed of light, the
most distant galaxies are also observed very far back in time. The
UDS is the deepest of five projects, collectively known as the UKIRT
Infrared Deep Sky Survey (UKIDSS). Earlier releases of data from the
UDS have already produced a wide range of scientific advances,
including studies of the earliest galaxies in the first thousand
million years after the Big Bang, measurements of the build-up of
galaxies through cosmic time, and studies of the large-scale
distribution of galaxies in an effort to quantify the mysterious 'dark
matter' that pervades the cosmos. The added depth from the new
release is expected to produce many new results. Astronomers are
particularly keen to understand the dramatic transformation that many
massive galaxies underwent around 10 billion years ago. At that
time many galaxies appear abruptly to have stopped forming stars, and
they also changed shape to form spheroidal-looking galaxies. It is
not understood why that happened. With the new UDS images large
numbers of such galaxies are expected to have been caught in the act
of transformation, so they can be studied in detail to solve that
important puzzle.
JUNO SPACECRAFT IN ORBIT AROUND JUPITER
NASA/Jet Propulsion Laboratory
After a journey of almost five years to Jupiter, the Juno spacecraft
has successfully entered an orbit round Jupiter. Over the next few
months, Juno's mission teams will do some final testing and calibra-
tion of the spacecraft's instruments. The official scientific
investigation phase is due to begin in October, but data may be
collected sooner. Juno's principal goal is to understand the origin
and evolution of Jupiter. With its suite of nine instruments, it will
investigate the existence of a solid planetary core, map Jupiter's
intense magnetic field, measure the amount of water and ammonia in the
deep atmosphere, and observe the planet's aurorae. The mission may
also help our understanding of how giant planets form and the role
they played in putting together the rest of the Solar System. As our
primary example of a giant planet, Jupiter can also provide critical
knowledge for understanding the planetary systems being discovered
around other stars.
ROSETTA GIVEN TERMINATION DATE
ESA
The Rosetta probe will be crash-landed on Comet 67P on September 30.
The manoeuvre, which is expected to destroy the satellite, will bring
to an end two years of investigations at the 4-km object. Flight
controllers plan to have the cameras taking and relaying pictures
during the final descent. Sensors that indicate the chemical
environment will also be switched on. All other instruments will
probably be off. Flight-dynamics experts have still to work out the
fine details, but Rosetta will be put into a tight elliptical orbit
around the comet and commanded to drop its periapsis (lowest pass)
progressively. A final burn will then put the satellite on a
collision course with the duck-shaped object. Mission managers have
previously talked about bringing Rosetta down in a place dubbed
'Agilkia' -- the location originally chosen to land its surface robot,
Philae, in 2014. In the event, Philae bounced a kilometre away, but
Agilkia's relatively flat terrain is still an attractive option,
although other targets are being studied. Having swept around the Sun
last August, Comet 67P is currently on a trajectory that is taking it
away from the inner Solar System towards the orbit of Jupiter. Today,
the probe is nearly 500 million km from the Sun. The amount of light
falling on Rosetta's solar panels is gradually diminishing, so it has
less power day by day to run its instruments and sub-systems.
Engineers would soon have to put the satellite into hibernation mode
if they wanted to use it long-term -- during 67P's next encounter with
the Sun in a few years' time. But having already spent 12 years in
space, battling huge temperature swings and damaging radiation, not to
mention a much-reduced fuel load -- there is little confidence that
Rosetta would still be operable so far into the future. The crash-
landing on the other hand offers the opportunity to get some very
close-in science to complement the more distant remote sensing it has
been doing. Controllers will try to maintain contact with the
satellite for as long as possible during the final descent. Much will
depend on how well Rosetta copes with the dusty environment around the
comet. Recent months have seen several occasions when the probe's
navigation equipment, which tracks the stars to define its orientation
in space, has got confused in the maelstrom of particles emanating
from 67P's surface. That has tripped the satellite into a 'safe mode'
that shuts down all non-essential operations, including observations.
Topic: Mid July Astronomy Bulletin (Read 1222 times)