ANCIENT MARS BOMBARDMENT
University of Colorado at Boulder
The bombardment of Mars some 4 billion years ago by comets and
asteroids probably enhanced climate conditions enough to make the
planet more conducive to life, at least for a time. If early Mars
were as barren and cold as it is today, massive asteroid and comet
impacts would have produced enough heat to melt sub-surface ice.
The impacts would have produced regional hydrothermal systems on Mars
similar to those in Yellowstone National Park, which today harbour
chemically powered microbes, some of which can survive boiling in
hot springs or inhabiting water acidic enough to dissolve nails.
Scientists have long known that there was once running water on Mars,
as evidenced by ancient river valleys, deltas and parts of lake beds.
In addition to producing hydrothermal regions in portions of Mars'
fractured and melted crust, a massive impact could temporarily have
increased the planet's atmospheric pressure, heating Mars up enough to
re-start a dormant water cycle. A new study shows that the ancient
bombardment of Mars by comets and asteroids would have been greatly
beneficial to life there, if life were present. But up to now we have
no convincing evidence that life ever existed there.
Much of the action on Mars occurred during a period known as the 'Late
Heavy Bombardment' about 3.9 billion years ago, when the developing
Solar System was a shooting gallery of comets, asteroids, moons and
planets. Unlike the Earth, which has been 'resurfaced' time and again
by erosion and plate tectonics, heavy cratering is still evident on
Mercury, the Moon and Mars. Astronomers used the Janus supercomputer
at the University of Colorado for some of the 3-D modelling used in
the study. They looked at temperatures beneath millions of individual
craters in their computer simulations to assess heating and cooling,
as well as the effects of impacts on Mars at different angles and
velocities. The study showed that the heating of ancient Mars caused
by individual asteroid collisions would probably have lasted only a
few million years before Mars -- about 1.5 times as far from the Sun
as the Earth -- defaulted to today's cold and inhospitable conditions.
While Mars is believed to have spent most of its history in a cold
state, the Earth has probably been habitable over almost its entire
existence. The Late Heavy Bombardment period in the inner Solar
System nearly 4 billion years ago was not so heavy as to extinguish
potential early life on Earth and may even have given it a boost if it
were present then. What really saved the day for the Earth was its
oceans. In order to wipe out life here, the oceans would have had to
have been boiled away. Such extreme conditions in that time period
are beyond the realm of scientific probability.
PLANET 9 TAKES SHAPE
University of Bern
Astrophysicists have modelled the evolution of that putative planet in
the outer Solar System. They estimate that the object has a present-
day radius equal to 3.7 Earth radii and a temperature of -226°C.
How big and how bright is Planet 9 if it really exists? What is
its temperature and what telescope could find it? Those were the
questions that astronomers wanted to answer when they heard about the
possible additional planet suggested by Konstantin Batygin and Mike
Brown at Caltech in Pasadena. Swiss scientists are experts in
modelling the evolution of planets. They usually study the formation
of young exo-planets in discs around other stars light-years away and
the possible direct imaging of such objects with future instruments
such as the James Webb space telescope. For them, candidate Planet 9
is a close object, although it is about 700 times further away than
the distance between the Earth and the Sun. The astrophysicists
assume that Planet 9 is a smaller version of Uranus and Neptune -- a
small ice giant with an envelope of hydrogen and helium. With their
planet-evolution model they calculated how parameters like the
planetary radius or the brightness might have evolved over time since
the Solar System has formed 4.6 billion years ago.
The scientists conclude that a planet with the supposed mass of 10
Earth masses has a present-day radius of 3.7 Earth radii. Its
temperature is -226°C or 47K. That means that the planet's emission
is dominated by the cooling of its core, otherwise the temperature
would be only 10K. Its intrinsic power is about 1000 times greater
than its absorbed power. Therefore, the reflected sunlight
contributes only a minor part to the total radiation that could be
detected. That also means that the planet is much brighter in the
infrared than in the visual. The researchers also checked if their
results explain why planet 9 has not been detected by telescopes so
far. They calculated the brightness of smaller and bigger planets on
various orbits. They conclude that the sky surveys performed in the
past had only a small chance of detecting an object with a mass of 20
Earth masses or less, especially if it is near the farthest point of
its orbit around the Sun. But the 'Wide-field Infrared Survey
Explorer' (WISE) could have observed a planet with a mass of 50 Earth
masses or more. That puts an interesting upper mass limit for the
planet. According to the scientists, future telescopes like the Large
Synoptic Survey Telescope or dedicated surveys should be able to find
or rule out candidate Planet 9. Contrary to recent reports, the
Cassini spacecraft is not experiencing unexplained deviations in its
orbit around Saturn, according to mission managers and orbit experts
at JPL.
YOUNG, UNATTACHED 'JUPITER' FOUND IN SOLAR NEIGHBOURHOOD
Carnegie Institution for Science
A team of astronomers has discovered one of the youngest and brightest
free-floating planet-like objects in reasonably close proximity to the
Sun. At an age of only 10 million years, the object identified as
2MASS J1119-1137 is between four and eight times the mass of Jupiter,
and hence falls in the mass range between a large planet and a small
brown-dwarf star. From data obtained by WISE and other ground-based
telescopes, 2MASS J1119-1137 was identified by its unusual colours:
it emits much more light in the infrared part of the spectrum than
would be expected if it had already aged and cooled. The challenge
of identifying such rare objects is distinguishing them from a
multitude of potential interlopers. Much more commonly, distant old
and red stars residing in the far reaches of our Galaxy can display
the same characteristics as nearby planet-like objects. When the
light from the distant stars passes through the large expanses of dust
in our Galaxy on its way to our telescopes, it gets reddened, so those
stars can pose as potentially exciting nearby young planet-like
objects when actually they are not that at all. With knowledge of
such common misidentifications, the team immediately checked its
findings using the FLAMINGOS-2 spectrograph instrument on the Gemini
South telescope in Chile. It promptly confirmed that 2MASS J1119-1137
is in fact a young low-mass object in the solar neighborhood, and not
a distant reddened star.
Next, the team wanted to determine the age of the object. Gemini
observations showed only that the object was younger than about 200
million years. If it were much younger, it could actually be a
free-floating planet -- an analogue of our own Jupiter, yet without a
host star. The final piece of the puzzle was contributed by the FIRE
spectrograph on Carnegie's 6.5-m telescope in Chile. FIRE measured
the line-of-sight velocity of 2MASS J1119-1137. Combining that
measurement with the sky motion of 2MASS J1119-1137, the team
discovered that it belongs to the youngest group of stars in the solar
neighbourhood. That group contains about two dozen 10 million-year-
old stars, all moving together through space, and is collectively
known as the TW Hydrae Association. Being 'nearby', 95 light-years
away, 2MASS J1119-1137 only narrowly misses being the brightest
free-floating planet analogue. That position is held by another
object known as PSO J318.5−22, discovered three years ago. However,
at an age of 23 million years, PSO J318.5−22 is more than twice the
age of 2MASS J1119-1137, and is more massive. Discovering free-
floating planet analogues like 2MASS J1119-1137 and PSO J318.5−22
offers an opportunity to study giant planets outside the Solar System,
because free-floating planet candidates are much easier to scrutinize
than planets orbiting around other stars. Objects like 2MASS J1119-
1137 are drifting in space all alone and our observations are not
overwhelmed by the brightness of an adjacent host star.
SUPERNOVA SHOWERED THE EARTH WITH RADIOACTIVE DEBRIS
Australian National University
An international team of scientists has found evidence of a series of
massive supernova explosions near the Solar System, which showered the
Earth with radioactive debris. The scientists found radioactive
iron-60 in sediment and crust samples taken from the Pacific, Atlantic
and Indian Oceans. The iron-60 was concentrated in a period between
3.2 and 1.7 million years ago -- quite recent in astronomical terms.
The researchers were very surprised that there was debris clearly
spread across 1.5 million years, because that suggests that there was
a series of supernovae, one after another. It is an interesting
coincidence that they correspond with when the Earth cooled and moved
from the Pliocene into the Pleistocene period. The team also found
evidence of iron-60 from an older supernova around eight million years
ago, coinciding with global faunal changes in the late Miocene. Some
theories suggest that cosmic rays from the supernovae could have
increased cloud cover. The scientists believe that the supernovae in
this case were less than 300 light-years away, close enough to be
visible during the day and comparable with the brightness of the Moon.
Although the Earth would have been exposed to an increased cosmic-ray
bombardment, the radiation would have been too weak to cause direct
biological damage or trigger mass extinctions. Supernova explosions
create many heavy elements and radioactive isotopes which are strewn
into the cosmic neighbourhood. One of the isotopes is iron-60, which
decays with a half-life of 2.6 million years. Any iron-60 dating
from the Earth's formation more than four billion years ago has long
since disappeared.
The iron-60 atoms reached the Earth in minuscule quantities, so the
team needed extremely sensitive techniques to identify the inter-
stellar iron atoms. Iron-60 from space is fifteen orders of magnitude
less abundant than the iron that exists naturally on Earth. Scientists
were intrigued by the first hints of iron-60 in samplesfrom the
Pacific Ocean floor, found a decade ago by a group at TU Munich.
They assembled an international team to search for interstellar dust
from 120 ocean-floor samples spanning the past 11 million years.
The first step was to extract all the iron from the ocean cores.
The team then separated the tiny traces of interstellar iron-60 from
the other terrestrial isotopes with the 'Heavy-Ion Accelerator' at ANU
and found it occurred all over the globe. The age of the cores was
determined from the decay of other radioactive isotopes, beryllium-10
and aluminium-26, using accelerator mass spectrometry. The dating
showed that the fallout had occurred only in two time periods, 3.2 to
1.7 million years ago and eight million years ago. Current results
from TU Munich are in line with those findings. A possible source of
the supernovae is an ageing star cluster, which has since moved away
from the Earth.
SUPER-EARTH 55 CANCRI e IS INCREDIBLY HOT
Cosmos Up!
Using data from the Spitzer Telescope, a team of astronomers has
produced a temperature map of the 'Super-Earth' 55 Cancri e, a rocky
exo-planet 40 light-years away. To date, the Kepler space telescope
has detected more than 2,000 planets outside our Solar System. Many
of them are planets of extreme natures. 55 Cancri e orbits very close
to its parent star, a main-sequence star not too different from the
Sun. A complete orbit around its star takes less than 18 hours.
Such proximity induces extreme temperatures on the planet's surface.
Scientists found a dramatic difference in temperature between 1100°C
on the night side and 2400°C on the day side, temperatures high enough
to melt lead. The difference between the two hemispheres means either
that the planet has no atmosphere at all or its atmosphere is
curiously bad at transporting heat. 55 Cancri e is relatively close
to us, so astronomers are studying it extensively. At first, it was
believed to be covered in water. Now, we are learning that it is
covered in lava. 55 Cancri e is tidally locked to its host star like
our Moon is to the Earth.
MILKY WAY'S YOUNGEST SUPERNOVA
NASA
Scientists have used data from the Chandra X-ray Observatory and the
Very Large Array to try to identify the trigger for the most recent
supernova in the Milky Way. They applied a new technique that could
have implications for understanding other Type-Ia supernovae, a class
of stellar explosions that scientists use to determine the expansion
rate of the Universe. Astronomers had previously identified G1.9+0.3
as the remnant of the most recent supernova in our Galaxy. It is
estimated to have occurred about 110 years ago in a dusty region of
the Galaxy that blocks visible light from reaching the Earth.
G1.9+0.3 belongs to the Type-Ia category, an important class of
supernovae, exhibiting reliable patterns in their brightness, that
make them valuable tools for measuring the rate at which the Universe
is expanding. Most scientists agree that Type-Ia supernovae occur
when white dwarfs, the dense remnants of Sun-like stars that have run
out of fuel, explode. However, there has been a debate over what
triggers the explosions. Two primary ideas are the accumulation of
material onto a white dwarf from a companion star or the violent
merger of two white dwarfs. The new research with archival Chandra
and VLA data examines how the expanding supernova remnant G1.0+0.3
interacts with the gas and dust surrounding the explosion. The
resulting radio and X-ray emission provide clues as to the cause of
the explosion. In particular, an increase with time in X-ray and
radio brightness of the supernova remnant is expected only if a white
dwarf merger took place.
The team observed that the X-ray and radio brightness did increase
with time, so the data point to a collision between two white dwarfs
as being the trigger for the supernova explosion in G1.9+0.3. The
result implies that Type-Ia supernovae are either all caused by white-
dwarf collisions, or are caused by a mixture of white-dwarf collisions
and the mechanism whereby the white dwarf pulls material from a
companion star. It is important to identify the trigger mechanism for
Type-Ia supernovae because if there is more than one cause, then the
contribution from each may change over time. That could mean that
astronomers might have to recalibrate some of the ways they use them
as 'standard candles' in cosmology. The team also derived a new
estimate for the age of the supernova remnant of about 110 years,
younger than previous estimates of about 150 years. More progress on
understanding the trigger mechanism should come from studying Type-Ia
supernovae in nearby galaxies, using the increased sensitivity
provided by a recent upgrade to the VLA.
SUPER-MASSIVE BLACK HOLES MAY LURK EVERYWHERE IN UNIVERSE
University of California at Berkeley
A near-record super-massive black hole discovered in a sparse area of
the local Universe indicates that such monster objects -- this one has
a mass of about 17 billion Suns -- may be more common than was
once thought. Until now, the most massive black holes -- those with
masses around 10 billion times that of our Sun -- have been found at
the cores of very large galaxies in regions loaded with other large
galaxies. The current record-holder, discovered in the Coma Cluster
in 2011, may be as much as 21 billion solar masses. The newly discovered
one is in a galaxy, NGC 1600, in the opposite part of the sky from the
Coma Cluster in a relative desert. While finding a gigantic black hole in a
massive galaxy in a crowded area of the Universe is to be expected -- like
finding a skyscraper in Manhattan -- it seemed less likely that they would be
found in less-populated regions. Rich groups of galaxies like the Coma
Cluster are rare, but there are quite a few galaxies the size of NGC 1600
that reside in average-size galaxy groups. While the black hole discovered
in 2011 in the galaxy NGC 4889 in the Coma Cluster was estimated to
have an upper limit of 21 billion solar masses, its range of possible
masses was large -- between 3 billion and 21 billion Suns. The
17-billion-solar-mass estimate for the central black hole in NGC 1600
is much more precise, with a range (1 standard deviation) of 15.5 to
18.5 billion solar masses.
Interestingly, the stars around the centre of NGC 1600 are moving as
if the black hole were a binary. Binary black holes are expected to
be common in large galaxies, since galaxies are thought to grow by
merging with other galaxies, each of which could bring a central black
hole with it. The black holes would be likely to sink to the core of
the new and larger galaxy and, after an orbital dance, merge, with the
emission of gravitational waves. The proposed Evolved Laser Interfer-
ometer Space Antenna, or eLISA, is designed to detect gravitational
waves produced by the merger of massive black holes, while other
instruments are looking for evidence of gravitational waves from
massive-black-hole mergers in nanosecond glitches in the precisely
timed flashes of millisecond pulsars. Because NGC 1600 is an old
galaxy with little new star formation, it is suspected that it may
harbour an ancient quasar that once blazed brightly but is now asleep.
It would be the first discovered in a sparsely populated region of the
local Universe. The brightest quasars, probably hosting the most
massive black holes, do not necessarily have to be in the densest
regions of the Universe. NGC 1600's is the first very massive black
hole discovered outside a rich environment in the local Universe, and
could be the first example of a descendent of a very luminous quasar
that also did not live at a privileged site.
ASTEROID-HUNTING SPACECRAFT'S SECOND YEAR OF DATA
NASA
The Near-Earth Object WIde-field Survey Explorer (NEOWISE) mission has
released its second year of survey data. The spacecraft has now
characterized a total of 439 NEOs since the mission was re-started in
2013 December. Of those, 72 were new discoveries. Near-Earth Objects
(NEOs) are comets and asteroids that have been nudged by the gravi-
tational attraction of the giant planets in the Solar System into
orbits that allow them to enter the Earth's neighbourhood. Eight of
the objects discovered in the past year have been classified as
potentially hazardous asteroids (PHAs), on the basis of their size and
how closely their orbits approach the Earth. With the release of its
second year of data, the NEOWISE spacecraft completed another mile-
stone in its mission to discover, track and characterize the asteroids
and comets that approach closest to the Earth. Since beginning its
survey in 2013, NEOWISE has measured more than 19,000 asteroids and
comets at infrared wavelengths. More than 5.1 million infrared images
of the sky were collected in the last year.
Originally called the Wide-field Infrared Survey Explorer (WISE),
the spacecraft was launched in 2009 December. It was placed in
hibernation in 2011 after its primary mission was completed. In 2013
September it was re-activated, re-named NEOWISE, and assigned a new
mission -- to assist NASA's efforts to identify the population of
potentially hazardous near-Earth objects. NEOWISE also is character-
izing previously known asteroids and comets to provide information
about their sizes and compositions. NEOWISE discovers large, dark,
near-Earth objects, complementing our network of ground-based
telescopes operating at visible-light wavelengths. On average, these
objects are many hundreds of metres across.
Topic: Late April Astronomy Bulletin (Read 1749 times)