MARS'S WET PAST
NASA
Studies based on data from the Mars Reconnaissance Orbiter indicate
that Mars once had large lakes, flowing rivers, and other wet
environments. One study shows that regions of the ancient highlands
of Mars, which cover about half the planet, contain clay minerals,
which can form only in the presence of water. Volcanic lavas later
buried the clay, but impact craters have exposed it again at thousands
of locations across Mars. The clay-like minerals, called
phyllosilicates, preserve a record of the interaction of water with
rocks dating back to what is called the Noachian period of Mars'
history, approximately 4.6 billion to 3.8 billion years ago. That is
the earliest epoch of the Solar System, when the Earth, Moon and Mars
were bombarded by comets and asteroids. Rocks of that age have
largely been destroyed on the Earth by plate tectonics. They are
preserved on the Moon, but were never exposed there to liquid water.
The phyllosilicate-containing rocks on Mars are the only ones of that
age to preserve a record of liquid-water environments. Another study
finds that in one area, after the clays formed, a river eroded them
out of the highlands and concentrated them in a delta where it emptied
into a crater lake about 25 miles in diameter. The distribution of
clays in the ancient lake bed shows that standing water must have
persisted for thousands of years.
LIQUID LAKE ON TITAN
NASA/JPL
Cassini scientists have concluded that at least one of the large lakes
observed on Saturn's moon Titan contains liquid hydrocarbons, and have
positively identified ethane by its infrared reflectance spectrum.
No other body apart from the Earth is known to have liquid on its
surface. Before Cassini, scientists thought that Titan might have a
global ocean of methane, ethane and other light hydrocarbons. More
than 40 close fly-bys of Titan by Cassini have shown that no global
ocean exists; there are, however, hundreds of dark, lake-like
features, but until now it has not been known whether the features
were actually liquid.
Ethane has been identified in trace quantities in Titan's
atmosphere, which consists of 95% nitrogen with methane making up
most of the rest. Ethane and other hydrocarbons are produced by the
breakdown of methane by sunlight. Some of the hydrocarbons react
further and form fine aerosol particles. Together, all those
substances form a ubiquitous hydrocarbon haze that hinders the view of
the surface and makes for difficulty in identifying materials there.
A lake, that has been named Ontario Lacus and at 20,000 square miles
in area is slightly larger than Lake Ontario in the North American
Great Lakes on Earth, was observed in Titan's south-polar region
during a close Cassini fly-by last December. The ethane in the lake
is in a liquid solution with methane, other hydrocarbons and nitrogen.
At Titan's surface temperature of about -185 C, those substances can
exist as both liquid and gas. Titan shows evidence of evaporation,
rain, and fluid-carved channels draining into what, in this case, is a
liquid-hydrocarbon lake, so it has a 'hydrological' cycle based on
methane, quite analogous to the cycle based on water on the Earth.
Scientists were able to rule out the presence of water ice, ammonia,
ammonia hydrate and carbon dioxide in Ontario Lacus. The observations
also suggest that the lake is evaporating. It is ringed by a dark
beach, where the black lake meets the bright shoreline. Cassini also
observed a shelf and beach being exposed as the lake evaporates.
NEWEST DWARF PLANET
Universe Today
A Kuiper-Belt object discovered three years ago has been given a name
by the IAU. It has also officially been classified as a dwarf planet,
and a 'plutoid'. Formerly known as 2005 FY9 and also as minor-planet
136472, the third-largest known trans-Neptunian object is now Makemake
(pronounced 'maki-maki'). The object was named after the creator god
from Easter-Island mythology.
THE STAR EVERYONE MISSED
ESA
XMM-Newton has discovered an exploding star in the Milky Way. Usually
that would be important in itself, but calculations show that the
explosion must have been clearly visible to the unaided eye, yet it
was missed by all the star-watchers around the planet. On 2007
October 9, as ESA's orbiting X-ray observatory XMM-Newton was turning
from one object to another, it passed across a bright X-ray source
that was not listed in any previous catalogue. The only celestial
object that the XMM-Newton team could find at that location was a
faint star, which astronomers who were alerted to it at the 6.5-m
Magellan-Clay telescope in Chile found to have brightened by more than
600 times. Spectra showed that it was a nova.
Novae occur when a white dwarf accretes gas lost from a close
companion star. When sufficient gas has accumulated on the white
dwarf, a nuclear explosion releases large quantities of energy,
prompting the white dwarf to brighten considerably. However, it does
not immediately release X-rays -- the expanding cloud of debris
created in the detonation temporarily masks them. As it clears, the
X-rays shine through. So for XMM-Newton to see the nova, the actual
explosion must have taken place many days before, yet no-one had
reported seeing it. Usually, dedicated amateur and professional
astronomers find novae by regularly sweeping the night sky for stars
that suddenly brighten; this one, it seemed, had been missed.
Astronomers contacted the robotic survey project ASAS and asked those
in charge of it to check their data. They found that the nova
outburst had taken place on 2007 June 5, and at about fourth
magnitude would have been clearly visible then to the naked eye.
The nova is now officially designated V598 Puppis and is one of the
brightest for almost a decade, increasing the irony that it was not
noticed during its brilliant peak.
STARS SEEM TO BE FORMING NEAR MILKY WAY'S BLACK HOLE
New Scientist
Astronomers have found evidence that there may be some very young
stars in a ring of gas close to the centre of the Milky Way, where a
massive black hole is thought to reside. The supposed proto-stars,
6 to 20 light-years from the Galaxy's centre, are shrouded by gas and
dust, but their existence is implied by strong radio signals from
molecular masers, which often form naturally within dense, collapsing
gas clouds. The presence of masers is therefore held to suggest that
gas is collapsing to form proto-stars, which will eventually become
short-lived stars that are tens of times more massive than the Sun.
The researchers found another line of evidence for young stars in the
gas ring. Newborn stars collect material from the surrounding cloud,
but also blast some gas away from their poles in narrow, high-speed
jets. Radio emission from the region was found to be distorted in a
way that can be explained by such jets. Jets last only a few tens of
thousands of years, suggesting that the ring of gas became dense
enough to form stars not long ago. Older stars exist in the same
region, so clouds may be constantly coming in to form succeeding
generations of stars. There appear to be some stars even closer to
the galaxy's centre, down to only 2 light-years away from the black
hole.
ASTRONOMERS SEE DISCS SURROUNDING BLACK HOLES
Science Daily
The process that astronomers find most appealing to explain quasars,
the brilliant cores of certain remote galaxies, involves matter
falling toward a supermassive black hole and whirling around it in a
disc as it makes its way to the event horizon -- the surface that
marks the boundary of the invisible volume around the black hole. In
the process, friction causes the matter to heat up to such an extent
that it produces light in all wavelengths of the spectrum. Finally,
the matter falls into the black hole and thereby increases its mass.
It is possible to model, in that case, what the spectrum of a quasar
ought to be, but testing the prediction has been impossible until now
because astronomers have not distinguished between the light emanating
from the accretion disc and that from other sources in the vicinity of
the hole. By such an elementary means as a polarizing filter,
researchers have now isolated the light emitted by an accretion disc
from that produced by other matter in the vicinity. The result
supports the accepted explanation of quasars.
DISTANT GALAXIES PROVIDE INSIGHTS INTO EARLY UNIVERSE
Topix
By using the gravitational magnification from six massive lensing
galaxy clusters, the Hubble telescope has provided scientists with the
largest sample (though still only 10) of very distant galaxies seen to
date. Some of the newly found magnified objects are dimmer than the
faintest ones seen in the Hubble 'ultra-deep field', which has been
considered to be the deepest image of the Universe. By combining
visible and near-infrared observations, scientists searched for
galaxies that are only visible in near-infrared light. They found 10
candidates believed to lie about 13 billion light-years away (a red-
shift of approximately 7.5), which means that the light gathered
was emitted by the stars when the Universe was still very young -- a
mere 700 million years old. It has been thought that Universe became
ionized within the first 5-600 million years after the Big Bang, but
there have been conflicting opinions as to whether the ionizing energy
came from a relatively few big galaxies or a more plentiful population
of smaller ones. The number of supposed redshift-7.5 galaxies claimed
in the quoted survey might suggest that most of the ionizing energy
was produced by abundant small galaxies. Such distant galaxies are
terribly faint, and the new result was only made possible with some
cosmic assistance in the form of the gravitational lensing that
concentrated the light from the distant galaxies enough for Hubble to
detect them.
Topic: Early August Astronomy News (a regular fortnightly series) (Read 4359 times)