The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 407 2015 September 13
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MARS' EARLY ATMOSPHERE
NASA/Jet Propulsion Laboratory
Scientists may be closer to discovering how Mars changed from a world with surface water billions of years ago to the arid planet of today. A new analysis of the largest known deposit of carbonate minerals on Mars suggests that the original Martian atmosphere may already have lost most of its carbon dioxide by the era of valley-network formation. Carbon dioxide makes up most of the Martian atmosphere. That gas can be pulled out of the air and sequestered or pulled into the ground by chemical reactions with rocks to form carbonate minerals. Before the series of successful Mars missions, many scientists expected to find large Martian deposits of carbonates holding much of the carbon from the planet's original atmosphere. Instead, those missions have found low concentrations of carbonate distributed widely, and only a few concentrated deposits. By far the largest known carbonate-rich deposit on Mars covers an area at least 2,500 square miles, possibly as much as 100,000 square miles, in a region called Nili Fossae. The estimate of how much carbon is locked into the Nili Fossae carbonate deposit depends on observations from numerous Mars missions, including the 'Thermal Emission Spectrometer' on the Mars Global Surveyor orbiter, the mineral-mapping 'Compact Reconnaissance Imaging Spectrometer for Mars' and two telescopic cameras on the Mars Reconnaissance Orbiter, and the 'Thermal Emission Imaging System' on the Mars Odyssey orbiter. Scientists compare their tally of sequestered carbon at Nili Fossae with what would be needed to account for an early Mars atmosphere dense enough to sustain surface waters during the period when flowing rivers left their mark by cutting extensive river-valley networks. By their estimate, it would require more than 35 carbonate deposits the size of the one examined at Nili Fossae. [But we were told above that that size is uncertain by a factor of 40. What the text actually said before editing was "an area at least the size of Delaware, and maybe as large as Arizona". I thought that readers outside the USA might not have immediately to mind a comprehensive catalogue of the areas of individual American states, so I looked them up and substituted the relevant approximate areas: Delaware 2489 square miles (but more than 500 of them are water), Arizona 113635 - ED.] They [this refers back now to "Scientists" - ED] deem it unlikely that so many large deposits have been overlooked in numerous detailed orbiter surveys of the planet. While deposits from an even earlier time in Mars history could be deeper and better hidden, they do not help to solve the thin-atmosphere conundrum at the time the river-cut valleys were formed.
The modern Martian atmosphere is too tenuous for liquid water to persist on the surface. A denser atmosphere on ancient Mars could have kept water from immediately evaporating. It could also have allowed parts of the planet to be warm enough to keep liquid water from freezing. But if the atmosphere was once thicker, what happened to it? One possible explanation is that Mars did have a much denser atmosphere during its flowing-rivers period, and then lost most of it to outer space from the top of the atmosphere, rather than by sequestration in minerals. Perhaps the atmosphere was not so thick by the time of valley-network formation. Instead of a Mars that was wet and warm, maybe it was wet and cold with an atmosphere that had already thinned. It would not have needed to have been very warm for the valleys to form. In most locations, there could have been snow and ice instead of rain. It would only have needed temperatures to rise above the freezing point occasionally to get ice and snow to thaw and give flowing water, and that does not require very much atmosphere. The Curiosity Mars rover mission has found evidence of ancient top-of-atmosphere loss, from the modern Mars atmosphere's isotopic carbon ratio. Uncertainty remains about how much of that loss occurred before the period of valley formation; much may have happened earlier. The MAVEN orbiter, examining the outer atmosphere of Mars since late 2014, may help reduce the uncertainty.
PLUTO PROBE'S NEXT TARGET
The New Horizons spacecraft has a new target to aim for following its historic flyby of Pluto. It is called 2014 MU69, and was one of two comet-like objects that were under consideration by scientists working on the mission. The US space agency will now carry out a review of the plan before officially approving the mission's extension. New Horizons carried out its flyby of Pluto in July, approaching within 12,500 km of the surface. The spacecraft obtained detailed images and other data not only of Pluto, but also of its moons, Charon, Styx, Nix, Kerberos and Hydra. The new target is about a thousand million miles beyond Pluto. It is about 45 km across and is thought to be one of the type of building blocks from which bigger objects such as Pluto were formed. Such objects form a region of the outer Solar System called the Kuiper Belt, containing a deep-frozen sample of what our neighbourhood was like when it formed 4.6 billion years ago. The spacecraft carries enough hydrazine fuel for another fly-by, and scientists say it could continue operating into the late 2020s or beyond. 2014 MU69 costs less fuel to reach than other candidate targets, leaving more fuel for the fly-by, for ancillary science, and greater fuel reserves to protect against the unforeseen. In summer 2014, the Hubble telescope discovered five icy objects, later narrowed to two, within New Horizons' flight path. In late October and early November, the spacecraft will perform a series of engine burns to set its course towards 2014 MU69 for an encounter currently set for 2019 January 1.
SUPERMASSIVE BLACK HOLES IN NEAREST QUASAR
University of Oklahoma
Astrophysicists using observations from the Hubble telescope have found two super-massive black holes in Markarian 231, the quasar nearest to us. The discovery of *two* black holes -- of unequal masses -- is evidence of a binary black hole and may suggest that super-massive black holes assemble their masses through violent mergers. The observers looked at ultraviolet radiation emitted from the centre of Mrk 231; then they applied a model that they developed to the spectrum of the galaxy and recognized the existence of the binary black hole. The observation not only shows the existence of a close binary black hole in Mrk 231, but also paves a way to search for other binary black holes from the nature of their ultraviolet light emission. The denizens of the Universe, such as giant galaxies and clusters of galaxies, grow by the merging of smaller systems into larger ones, and binary black holes are natural consequences of mergers of galaxies. Eventually, the two black holes discovered in Mrk 231 will collide and merge to form a quasar with a super-massive black hole.
CLUES TO STAR BIRTH IN NEIGHBOURING GALAXY
Space Telescope Science Institute (STScI)
In a survey of Hubble images of 2,753 young, blue star clusters in the Andromeda galaxy (M31), astronomers have found that M31 and our own Galaxy have similar percentages of newborn stars in terms of mass. From the distribution of stellar masses within a cluster (the 'Initial Mass Function' or IMF), scientists are more likely to be able to interpret the light from distant galaxies and understand the formation history of the stars. The intensive survey, assembled from 414 mosaic photographs taken by Hubble of M31, was a collaboration between astronomers and 'citizen scientists', volunteers who provided invaluable help in analyzing the mountain of data from Hubble. Measuring the IMF was the primary driver behind Hubble's survey, called the 'Panchromatic Hubble Andromeda Treasury' (PHAT). Nearly 8,000 images [it was 414 six lines above - ED.] of 117 million stars in the galaxy's disc were obtained from viewing Andromeda in near-ultra-violet, visible, and near-infrared wavelengths. Stars are born when a cloud of molecular hydrogen, dust, and trace elements collapses. The cloud fragments into knots of material that each precipitate hundreds of stars. The stars are not all created equally: their masses can range from 1/12th to 200 solar masses. Until now, astronomers have only had IMF measurements made in the local stellar neighbourhood within our own Galaxy, but the M31 survey has offered a good sampling of star clusters that are all at approximately the same distance of about 2.5 million light-years. The survey is diverse because the clusters are scattered across M31; they vary in mass by factors of 10, and they range in age from 4 million to 24 million years.
To the researchers' surprise, the IMF was very similar among all the clusters surveyed. Nature apparently cooks up stars with a consistent distribution from massive blue supergiants to small red dwarfs. Curiously, the brightest and most massive stars in the clusters are 25% less abundant than suggested by previous research. Astronomers use the light from the brightest stars to estimate the masses of distant star clusters and galaxies and to measure how rapidly the clusters are forming stars. The new result suggests that mass estimates in previous work were too low because they assumed that there were too few faint, low-mass stars forming along with the bright, massive stars. The evidence also implies that the early Universe did not have as many heavy elements for making planets, because there would be fewer supernovae from massive stars to manufacture heavy elements for planet building. It is critical to know the star-formation rate in the early Universe -- about 10 billion years ago -- because that was the time when most of the Universe's stars formed. The PHAT star-cluster catalogue, which forms the foundation of the study, was assembled with the help of 30,000 volunteers who sifted through the thousands of images taken by Hubble to search for star clusters. The Andromeda Project is one of the many citizen-science efforts hosted by the Zooniverse organization. Over the course of 25 days, the citizen-scientist volunteers submitted 1.82 million individual image classifications (based on how concentrated the stars were, their shapes, and how well the stars stood out from the background), which roughly represents 24 months of constant human attention. Scientists used those classifications to identify a sample of 2,753 star clusters, increasing the number of known clusters by a factor of six in the PHAT survey region.
FARTHEST GALAXY EVER DETECTED
California Institute of Technology
A team of researchers that has spent years searching for the earliest objects in the Universe now reports the detection of what may be the most distant galaxy ever found. There is evidence for a galaxy called EGS8p7 that is more than 13.2 billion years old. The Universe itself is about 13.8 billion years old. Earlier this year, EGS8p7 had been identified as a candidate for further investigation on the basis of data gathered by the Hubble and Spitzer space telescopes. Using the 'multi-object spectrometer for infrared exploration' at the Keck Observatory in Hawaii, the researchers etermined its redshift from the wavelength of the Lyman-alpha line, whose laboratory wavelength is well down in the ultraviolet. A surprising aspect of the observation is the detection of Lyman-alpha in a faint galaxy at a redshift of 8.68, corresponding to a time when the Universe has been thought to be full of absorbing hydrogen clouds. Before the new discovery, the farthest detected galaxy had a redshift of 7.73. One possible reason why the object may be visible despite the hydrogen-absorbing clouds, the researchers say, is that hydrogen re-ionization did not occur in a uniform manner. Evidence from several observations indicates that the re-ionization process was probably patchy. Some objects are so bright that they form a bubble of ionized hydrogen, but the process is not coherent in all directions. The galaxy EGS8p7 is unusually luminous, and may be powered by a population of unusually hot stars, and have special properties that enabled it to create a large bubble of ionized hydrogen much earlier than would have been possible for more typical galaxies at those times. Researchers are currently trying to estimate more thoroughly the chances of finding that galaxy and seeing such emission from it, and to understand whether they need to revise the time-line of the re-ionization, which is one of the major questions to answer in our understanding of the evolution of the Universe.
RUSSIA OFFICIALLY GOING BACK TO MOON
Over 40 years have passed since human beings walked on the Moon, and all of them were American. Russia has some grand plans to change that as the US turns its attention to Mars. Roscosmos, the Russian space agency, wants to set up a high-tech lunar base, complete with human habitats, scientific and technical laboratories, a launching and landing port for spacecraft, and even an astronomical observatory. Roscosmos's concern with the Moon has a complicated past. During the Apollo-era space race, the former Soviet Union landed robotic rovers on the Moon -- but a series of rocket failures and explosions halted progress on any man-carrying missions. Eventually, Roscosmos decided that its Moon programme was not worth the money and the risk and shut it down. But now Roscosmos is reviving that programme and thinking of sending (but not in the near future) a robotic spacecraft, called Luna 25, to the Moon to do some scouting for a future lunar base. The agency has announced that it will land Luna 25 at the Moon's south pole in 2024. Engineers are already building the spacecraft, and the finished product will carry eight cameras to help it navigate, take pictures, and keep an eye on its drill tool as it digs into the lunar surface. The Luna 25 spacecraft will run its electronics with a battery fuelled by plutonium-238. Roscosmos isn't the only agency thinking about a return to the Moon. ESA has already announced plans for its own ambitious Moon colony.
Bulletin compiled by Clive Down
(c) 2015 the Society for Popular Astronomy
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