MARS ICE DEPOSIT HOLDS AS MUCH WATER AS LAKE SUPERIOR
NASA/Jet Propulsion Laboratory
Researchers using the Mars Reconnaissance Orbiter have discovered
that, frozen beneath a region of cracked and pitted plains on Mars,
lies about as much water as is contained in Lake Superior, largest of
the North-American Great Lakes. Scientists examined part of Mars'
Utopia Planitia region, in the mid-northern latitudes, with the
orbiter's ground-penetrating Shallow Radar (SHARAD) instrument.
Utopia Planitia is a basin with a diameter of about 3,300 km,
resulting from a major impact early in Mars' history and subsequently
filled. Analyses of the radar data from more than 600 overhead passes
reveal an ice deposit more extensive in area than the British Isles.
The deposit ranges in thickness from about 80 to 170 metres, with a
composition that is 50 to 85% water ice, mixed with dust or larger
rocky particles. At the latitude of the deposit -- about 45 degrees
-- ice cannot persist on the surface of Mars today; it sublimes into
water vapour in the thin, dry atmosphere. The Utopia deposit is
shielded from the atmosphere by a covering of soil, estimated to be
about 1 to 10 metres thick. Mars today, with an axial tilt of 25
degrees, accumulates large amounts of water ice at the poles. In
cycles lasting about 120,000 years, the tilt varies to nearly twice
that much, warming the poles and driving ice to middle latitudes.
Climate modelling and previous findings of buried mid-latitude ice
indicate that frozen water accumulates away from the poles during the
long high-tilt periods.
The newly surveyed ice deposit spans latitudes from 39 to 49 degrees
within the plains. It represents less than 1% of all known water
ice on Mars, but it more than doubles the volume of thick, buried
ice sheets known in the northern plains. Ice deposits close to the
surface are being considered as a resource for potential astronauts.
The deposit described here is probably more accessible than most water
ice on Mars, because it is at a relatively low latitude and it lies in
a flat, smooth area where landing a spacecraft would be less hazardous
than in some of the other areas where there is buried ice. The
Utopian water is all frozen now. If there were a melted layer --
which would be significant for the possibility of life on Mars -- it
would have been evident in the radar scans. However, some melting can
not be ruled out during different climate conditions when the planet's
axis was more tilted.
NEW FAMILY OF STARS IN CORE OF MILKY WAY
Liverpool John Moores University
Astronomers have discovered a new family of stars in the core of our
Milky Way galaxy, providing new insights into the early stages of the
Galaxy's formation. The discovery, made from the Sloan Digital Sky
Survey, has shed new light on the origins of globular clusters. One
of the projects of this collaboration is APOGEE (the Apache Point
Observatory Galactic Evolution Experiment), which collects infrared
data for hundreds of thousands of stars in the Milky Way. It was
through observing stars in the infrared towards the Galactic Centre
that the discovery was made of a new population of stars, the like
of which had been seen before only inside globular clusters. That
intriguing new family of stars could possibly have belonged to
globular clusters that were destroyed during the violent initial
formation of the Galactic centre, in which case there would have been
about 10 times more globular clusters in the Milky Way in its early
stages than there are today. That could mean that a substantial
fraction of the old stars inhabiting the inner parts of the Galaxy
today may have been formed initially in globular clusters that were
later destroyed.
The finding helps astronomers address fascinating questions such as
what is the nature of the stars in the inner regions of the Milky Way,
how globular clusters formed, and what role they played in the
formation of the early Milky Way -- and by extension the formation of
other galaxies. The centre of the Milky Way is poorly understood,
because it is blocked from view by intervening dust. Observing in the
infrared, which is less absorbed by dust than visible light, APOGEE
can see the centre of the Galaxy better than other methods. From the
observations the chemical compositions of thousands of stars could be
determined; among them was a considerable number of stars that
differed from the bulk of those in the inner regions of the Galaxy,
owing to their very high abundance of nitrogen. While not certain, it
is suspected that those stars resulted from the destruction of
globular clusters. They could also be the by-products of the first
episodes of star formation taking place at the beginning of the
Galaxy's history.
EXTREMELY FAINT SATELLITE GALAXY OF MILKY WAY
National Astronomical Observatory of Japan
Astronomers have found an extremely faint dwarf satellite galaxy of
the Milky Way. The satellite lies in the direction of the constellation
Virgo and has accordingly been named Virgo I. At an absolute
magnitude of -0.8 in the optical waveband, it may well be the faintest
satellite galaxy yet found. Its discovery suggests the presence of a
large number of yet-undetected dwarf satellites in the halo of the
Milky Way, and provides important insights into galaxy formation
through hierarchical assembly of dark matter. Currently, some 50
satellite galaxies of the Milky Way have been identified. About 40
of them are faint and diffuse and belong to the category of 'dwarf
spheroidal galaxies'. Many recently discovered dwarf galaxies,
especially those found in systematic photometric surveys such as the
Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES), are
very faint, with absolute luminosities in the optical waveband less
than -8 magnitude. They are called 'ultra-faint dwarf galaxies'.
However, previous searches made use of telescopes of apertures 2.5 to
4 metres, so only satellites relatively close to the Sun or those with
brighter magnitudes were identified.
The combination of the large aperture of the 8.2-m Subaru Telescope
and the large field of view of the Hyper Suprime Cam (HSC) instrument
is very powerful for this study. It enables an efficient search to
be made for very faint dwarf satellites over large areas of the sky.
The first step in searching for a new dwarf galaxy is to identify an
over-density of stars in the sky, using photometric data. Next is to
assess that the over-dense appearance is not due to line-of-sight or
accidental juxtapositions of unrelated dense fields, but is really a
stellar system. The standard method for doing that is to look for a
characteristic distribution of stars in the colour-magnitude diagram
(analogous to the Hertzsprung-Russell diagram). Stars in a general
field show no particular patterns in this diagram. The team examined
the early data of the Subaru Strategic Survey with HSC and found an
apparent over-density of stars in Virgo with very high statistical
significance, showing a characteristic pattern of an ancient stellar
system in the colour-magnitude diagram. It is indeed a galaxy,
because it is spatially extended with a radius of 124 light-years --
larger than a globular cluster with comparable luminosity. The
faintest dwarf satellites identified so far are Segue I, discovered by
SDSS (-1.5 mag) and Cetus II in DES (0.0 mag). Cetus II is yet to be
confirmed, as it is too compact as a galaxy, so Virgo I may turn out
to be the faintest one so far discovered. It lies at a distance of
280,000 light-years from the Sun, and such a remote galaxy with such a
low luminosity has not been identified in previous surveys. It is
beyond the reach of SDSS, which has previously surveyed the same area
in the direction of the constellation Virgo.
DO EXTREMELY REDDENED QUASARS EXTINGUISH STAR FORMATION?
University of California - Riverside
Galaxies formed and grew thousands of millions of years ago by
accumulating gas from their surroundings, or colliding and merging
with other young galaxies. Those early stages of galaxy assembly are
believed to be accompanied by episodes of rapid star formation, known
as starbursts, and rapid growth of a single super-massive black hole
in each galactic centre. A popular paradigm for such evolution has
the black holes growing mostly in obscurity, buried deep within the
dusty gas in the centres of the galaxies. Those are rich star-forming
galaxies, until a blowout of gas and dust (outflow) extinguishes the
star formation and halts further growth in the black holes. At that
stage there is revealed the luminous material in the immediate
vicinity of the rapidly growing black hole in the galactic nucleus.
Such objects are known as quasars. Quasars can eject material at
high speeds, possibly helping to drive the blowout and regulate star
formation in their host galaxies. However, many aspects of that
evolutionary scheme are not understood. Quasars that are partially
obscured by dust, which reddens their light in a way that is similar
to the apparent reddening of the Sun as it approaches sunset on
Earth, might provide windows into galactic evolution during the brief
transition stage when the starburst is winding down and the visibly
luminous quasar is first being revealed in the galactic centre. New
research describes the discovery of the new population of extremely
red quasars detected in the Baryon Oscillation Sky Survey (BOSS) of
the Sloan Digital Sky Survey (SDSS).
The main goal of that study was to determine the size of the
population of extremely red quasars, and to characterize its basic
properties in comparison with the much larger population of quasars
in the BOSS--SDSS survey overall. The extremely red quasars were
selected for study because of their extreme colour, but the analysis
reveal a number of peculiar properties consistent with a unique and
possibly young evolutionary stage. In particular, they have an
exceptionally high incidence of powerful quasar-driven outflows that
could be involved in galaxy-wide blowouts of gas and dust. Overall,
the gaseous environments around the black holes appear to be more
extended and more energetic than the environments of normal quasars,
which might occur at specific times when young gas-rich host galaxies
are dumping prodigious amounts of matter into the central black holes,
creating an extreme variety of quasars. More work is needed now to
examine the population of extremely red quasars further and understand
its relationship to the general phenomenon of quasars and, perhaps, to
a particularly violent phase of quasar-galaxy evolution.
LARGE NUMBER OF DWARF GALAXES FOUND IN EARLY UNIVERSE
University of California at Riverside
A team of researchers has found a large population of distant dwarf
galaxies that could reveal important details about a productive period
of star formation in the Universe thousands of millions of years ago.
The findings build on a growing body of knowledge about dwarf
galaxies, the smallest and dimmest galaxies in the Universe. Though
relatively diminutive, they are very important for understanding the
history of the Universe. It is believed that dwarf galaxies played a
significant role during the 're-ionization era', in transforming the
early Universe from being dark, neutral and opaque to one that is
bright, ionized and transparent. Despite their importance, distant
dwarf galaxies remain elusive, because they are extremely faint and
mostly beyond the reach of even the best telescopes, so the current
picture of the early Universe is incomplete. However, gravitational
lensing, which was predicted by Einstein from his general theory of
relativity long before it was actually observed, causes a massive
object such as a galaxy located along the line of sight to another
distant object to act as a natural lens, concentrating the light
coming from the background source. The phenomenon sometimes allows
us to discover distant dwarf galaxies that would otherwise be too
faint to detect.
As a proof of concept, in 2014 the team targeted one cluster of
galaxies that produces the gravitational-lensing effect and got a
glimpse of what appeared to be a large population of distant dwarf
galaxies. The team used the Wide-Field Camera 3 on the Hubble Space
Telescope to take deep images of three clusters of galaxies. They
found the large population of distant dwarf galaxies from a time when
the Universe was between two and six thousand million years old.
That cosmic time is critical, as it was the most productive time
for star formation in the Universe. In addition, the team obtained
spectroscopic data from the Multi-Object Spectrograph for Infrared
Exploration (MOSFIRE) on the Keck telescope, and confirmed that the
galaxies belonged to that important cosmic period. Those dwarf
galaxies are 10 to 100 times fainter than galaxies that had been
previously observed from that period of time. Though faint, the
galaxies are far more numerous than their brighter counterparts.
The study demonstrates that the number of dwarf galaxies changed
during that important time period in such a way that they were
even more abundant at earlier times. In fact, the researchers had
unveiled a population of dwarf galaxies that were the most numerous
galaxies in the Universe during those times. Despite their individual
faintness, the dwarf galaxies produced more than half of all ultra-
violet light during that era. As ultraviolet radiation is produced
by young hot stars, dwarf galaxies evidently hosted a significant
fraction of newly-formed stars at that period of cosmic time. Those
results suggest that dwarf galaxies played a prominent role in the
re-ionization era; they will be among the primary targets of the next
generation of telescopes, particularly the James Webb Space Telescope,
scheduled to be launched in 2018.
MARS PROBE RETURNS FIRST PICTURES
BBC Online
Europe's and Russia's new satellite at Mars has sent back its first
images of the planet. The Trace Gas Orbiter (TGO) arrived on October
19, putting itself into a highly eccentric elliptical parking orbit,
which must be circularized over the coming year before the mission
can become fully operational. Scientists have, however, taken the
opportunity of some close passes to the planet in recent days to check
out the TGO's instrumentation. They are delighted at the quality of
the pictures returned from the camera system, CaSSIS (the Colour and
Stereo Surface Imaging System). Two of TGO's sensors - NOMAD and ACS
- also came through their early tests. Those are the sensors that
will make a detailed inventory of Mars' atmospheric gases. In
particular, they will go after the components that constitute less
than 1% of the planet's air -- chemical species such as methane, water
vapour, nitrogen dioxide, and sulphur dioxide. Methane is the main
focus. From previous measurements, its concentration seems to be low
and sporadic in nature, but the mere fact that it is detected at all
is really fascinating.
Methane (CH4) is the simplest organic molecule, and ought to be
destroyed easily in the harsh Martian environment, so its persistence
-- and the occasional spikes in its signal -- indicate a source that
replenishes the gas. The speculation is that it could be coming from
microbial life somewhere on the planet. It will be CaSSIS's job to
look for possible topographical forms on the surface that might tie
into methane sources. A fourth instrument, FREND, will sense hydrogen
in the near-surface. Those data can be used as proxy for the presence
of water or hydrated minerals. That again is information that could
yield answers to the methane question. TGO was the unspoken success
on the day that ESA's Schiaparelli lander crashed onto Mars. The
surface probe had been dropped off by TGO and was making its ill-fated
descent just as the satellite took up its parking orbit, and the
successful insertion went almost unnoticed in the fuss over
Schiaparelli. TGO is the first phase in a joint venture at Mars that
Europe is undertaking with Russia. The second step in the project,
known as ExoMars, is to put a robot rover on the planet in 2021. It
needs a lot of money from the European side to go forward, however --
just over 400m Euros. Research ministers from ESA member states are
meeting this week in Switzerland to try to resolve the budget problem.
Topic: Early December Astronomy Bulletin (Read 1386 times)