The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 374 2014 April 20
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'GEOLOGICAL CLOCK' HELPS DETERMINE MOON'S AGE
Southwest Research Institute
An international team of planetary scientists thinks that the Moon
formed nearly 100 million years after the Solar System originated.
Its conclusion is based on measurements from the interior of the Earth
combined with computer simulations of the protoplanetary disc from
which the Earth and other terrestrial planets formed. The team
simulated the growth of the terrestrial planets (Mercury, Venus, the
Earth and Mars) from a disc of thousands of planetary building blocks
orbiting the Sun. By analyzing the growth history of Earth-like
planets from a lot of simulations, the scientists discovered a
relationship between the time the Earth was impacted by a Mars-sized
object to create the Moon and the amount of material added to the
Earth after that impact. Augmenting the computer simulation with
details on the mass of material added to the Earth by accretion after
the formation of the Moon suggested a relationship that works much
like a clock to date the Moon-forming event, and does not rely on
radiometric dating methods. Published literature provided the
estimate for the mass accreted by the Earth after the Moon-forming
impact. Other scientists previously demonstrated that the abundance
in the Earth's mantle of highly siderophile elements, which are atomic
elements that prefer to be chemically associated with iron, is
directly proportional to the mass accreted after the Moon-forming
impact. From such geochemical measurements, the newly proposed clock
dates the Moon to 95 +/- 32 million years after the beginning of the
Solar System. That agrees with some interpretations of radioactive
dating measurements, but not others.
DEEP OCEAN DETECTED INSIDE SATURN'S MOON
NASA
The Cassini spacecraft and Deep Space Network have uncovered evidence
that Saturn's moon Enceladus harbours a large underground ocean.
Researchers theorized the presence of an interior reservoir of liquid
water in 2005 when Cassini discovered water vapour and ice spewing
from vents near the moon's south pole. The gravity measurements
suggest a large, possibly regional, ocean about 10 kilometres deep,
beneath an ice shell about 30 to 40 kilometres thick. Cassini has
flown near Enceladus 19 times. Three flybys, from 2010 to 2012,
yielded precise trajectory measurements. The gravity of a planetary
body, such as Enceladus, alters a spacecraft's flight path.
Variations in the gravity field, such as those caused by mountains on
the surface or differences in underground composition, can be detected
as changes in the spacecraft's velocity, measured from the Earth.
Changes in velocity as small as 90 microns per second can be detected.
With such precision, the flyby data yielded evidence of a zone inside
the southern part of the moon with higher density than other portions
of the interior.
The south-pole area has a surface depression that causes a dip in the
local gravity. However, the magnitude of the dip is less than
expected from the size of the depression, leading researchers to
conclude that the depression's effect is partially offset by denser
material at depth that compensates for the missing volume -- very
likely liquid water, which is 7% denser than ice. The magnitude of
the anomaly indicates the size of the water reservoir. There is no
certainty that the sub-surface ocean supplies the water plume spraying
out of surface fractures near the south pole of Enceladus, but it is a
real possibility. The fractures may lead down to a part of the moon
that is tidally heated by the moon's repeated flexing, as it follows
its eccentric orbit around Saturn.
POSSIBLE EXO-MOON
NASA/Jet Propulsion Laboratory
Are there moons orbiting planets beyond our Sun? Researchers have
observed the first signs of an 'exo-moon', though they say that it is
impossible to confirm its presence. The discovery was made by
watching a chance encounter of objects in our galaxy, which can be
witnessed only once. The technique, called gravitational
micro-lensing, takes advantage of chance alignments between stars.
When a foreground star passes between us and a more distant star, the
closer one can act like a magnifying glass to focus and brighten the
light of the more distant one. Such brightening events usually last
about a month. If the foreground star -- or what astronomers refer to
as the lens -- has a planet, the planet will act as a second lens. By
observing such events, astronomers can estimate the mass of the
foreground star relative to its planet.
In some cases, however, the foreground object could be a free-floating
planet, not a star. Researchers might then be able to measure the
mass of the planet relative to its orbiting companion (a moon). While
astronomers are actively looking for exo-moons -- for example, in data
from the Kepler mission -- so far, they have not found any. In the
new study, the nature of the lensing object is not clear. The mass
ratio of the larger body to the smaller one is 2,000 to 1. That means
the pair could be either a small, faint star circled by a planet about
18 times the mass of the Earth -- or a planet more massive than Jupiter
coupled with a moon with a mass less than the Earth's. The problem is
that astronomers have no way of telling which of those models is
correct, without knowing the distance to the object. In the future
it may be possible to obtain such distance measurements during
lensing events by observations with the Spitzer and Kepler space
telescopes, which are far enough away from the Earth to be be used to
meassure parallaxes.
Meanwhile, surveys like MOA and OGLE are turning up more and more
planets. Those micro-lensing surveys have discovered dozens of exo-
planets so far, in orbit around stars and free-floating. A previous
study led by the MOA team was the first to find strong evidence for
planets the size of Jupiter roaming alone in space, presumably after
they were ejected from planetary systems in course of formation.
MONSTER CLUSTER OF GALAXIES
Space Telescope Science Institute
The Hubble telescope has found the mass of the largest known galaxy
cluster in the distant Universe. By measuring how much the gravity
from the cluster's mass warps images of far-more-distant background
galaxies, a team of astronomers has asserted the cluster's mass to be
as much as 3 times 10 to the power 15 times the mass of the Sun. A
fraction of that mass is in several hundred galaxies that constitute
the cluster, and a larger part is in hot gas that fills the entire
volume of the cluster; the rest is 'dark matter' of indeterminate
character. Though massive clusters, such as the so-called Bullet
Cluster, are found in the nearby Universe, nothing like that has
previously been seen to exist so far back in time, when the Universe
was roughly half of its current age. The unusual size of the cluster
was first reported in 2012. Astronomers estimated its huge mass on
the basis of observations from the Chandra X-ray observatory and
galaxy velocities measured by the VLT in Chile. The estimates are
based on the motions of the galaxies in the cluster and the very high
temperature of the hot gas between the cluster galaxies.
HUBBLE MEASURES DISTANCES TO STARS UP TO 10,000 LIGHT-YEARS AWAY
NASA/Goddard Space Flight Cente
Using the Hubble telescope, astronomers can now measure the distances
of stars up to 10,000 light-years away -- ten times further than
previously possible. They have developed a technique called spatial
scanning, which dramatically improves Hubble's accuracy for making
angular measurements. The technique extends the parallax method for
finding distances by a factor of ten. It has been successfully used
to measure the distances of Cepheid variables approximately 7,500
light-years away in the constellation Auriga. Such measurements will
be used to provide a firmer footing for the so-called cosmic distance
ladder. The ladder's 'bottom rung' relies on measurements to Cepheids
that, because of their known absolute magnitudes, have long been used
to find distances. They are the first step in calibrating more
distant extra-galactic markers such as Type Ia supernovae.
Astronomers have developed a technique to use Hubble to make angular
measurements with errors as small as 20 millionths of a second or arc.
To make a distance measurement, two exposures of the target Cepheid
star were taken six months apart, when the Earth was on opposite sides
of the Sun. A very small shift in the star's position was measured to
an accuracy of 1/1,000 the width of a single image pixel in Hubble's
Wide-Field Camera 3. A third exposure was taken after another six
months to allow the team to account for the effects of the space
motion of the star, with additional exposures used to reduce other
sources of error.
Bulletin compiled by Clive Down
(c) 2014 the Society for Popular Astronomy
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