OLDEST ASTEROID IMPACT MAY HAVE ENDED ICE AGE
BBC Science
Scientists have identified the world's oldest asteroid crater in Australia, adding it may explain how the planet was lifted from an ice age. The asteroid hit Yarrabubba in Western Australia about 2.2 billion years ago -- making the crater about half the age of Earth. Their conclusion was reached by testing minerals found in rocks at the site. The scientists say that the find is exciting because it could account for a warming event during that era. The crater was discovered in the dry outback in 1979, but geologists had not previously tested how old it was. To determine when the asteroid hit Earth, the team examined tiny zircon and monazite crystals in the rocks. They were 'shocked' in the strike and now can be read like tree rings. These crystals hold tiny amounts of uranium. Because uranium decays into lead at a consistent pace, the researchers were able to calculate how much time had passed. It is at least 200 million years older than the next most ancient impact structure -- the Vredefort Dome in South Africa. The timing of the impact could also explain why the world warmed around this time, according to the researchers. Scientists believe the planet was previously in one of its "Snowball Earth" periods, when it was largely covered in ice. At some point, the ice sheets melted and the planet began rapidly to warm. Using computer modelling, the team calculated that the asteroid struck a kilometres-thick ice sheet covering the Earth. The event would have released huge volumes of water vapour, a greenhouse gas, into the atmosphere. This could have helped the planet's warming during the Proterozoic era -- a stage when oxygen had just appeared in the atmosphere and complex life had not yet formed. There is not enough modelling from the time to test the theory comprehensively, but "the rocks tell a story about the massive impact onto the planet". Another theory for the warming event is that volcanic eruptions may have pushed carbon dioxide into the atmosphere.
MARS' WATER WAS MINERAL-RICH AND SALTY
Tokyo Institute of Technology
Presently, Earth is the only known location where life exists in the Universe. This year the Nobel Prize in physics was awarded to three astronomers who proved, almost 20 years ago, that planets are common around stars beyond the solar system. Life comes in various forms, from organisms like humans to the ubiquitous micro-organisms that inhabit almost every square inch of the planet Earth, affecting almost everything that happens on it. It will probably be some time before it is possible to measure or detect life beyond the solar system, but the solar system offers a host of sites that might get a handle on how hard it is for life to start. Mars is at the top of this list for two reasons. First, it is relatively close to Earth compared to the moons of Saturn and Jupiter (which are also considered good candidates for discovering life beyond Earth in the solar system, and are targeted for exploration in the coming decade). Secondly, Mars is extremely observable because it lacks a thick atmosphere like Venus, and so far, there is pretty good evidence that Mars' surface temperature and pressure hover around the point where liquid water -- considered essential for life -- can exist. Further, there is good evidence in the form of observable river deltas, and more recent measurements made on Mars' surface, that liquid water did in fact flow on Mars billions of years ago. Scientists are becoming increasingly convinced that billions of years ago Mars was habitable. Whether it was in fact inhabited, or is still inhabited, remains hotly debated. To constrain these questions better, scientists are trying to understand the kinds of water chemistry that could have generated the minerals observed on Mars today, which were produced billions of years ago.
Salinity (how much salt was present), pH (a measure of how acidic the water was), and redox state (roughly a measure of the abundance of gases such as hydrogen [H2, which are termed reducing environments] or oxygen [O2, which are termed oxidising environments; the two types are generally mutually incompatible]) are fundamental properties of natural waters. As an example, Earth's modern atmosphere is highly oxygenated (containing large amounts of O2), but one need only dig a few inches into the bottom of a beach or lake today on Earth to find environments which are highly reduced. Recent remote measurements on Mars suggest its ancient environments may provide clues about Mars' early habitability. Specifically, the properties of pore water within sediments apparently deposited in lakes in Gale Crater on Mars suggest these sediments formed in the presence of liquid water which was of a pH close to that of Earth's modern oceans. Earth's oceans are of course host to myriad forms of life, thus it seems compelling that Mars' early surface environment was a place where contemporary Earth life could have lived,but it remains a mystery as to why evidence of life on Mars is so hard to find.
HOTTEST PLANET IN MAJOR MELTDOWN
NASA
Massive gas giants called 'hot Jupiters' -- planets that orbit too close to their stars to sustain life -- are some of the strangest worlds found beyond our solar system. New observations show that the hottest of them all is stranger still, prone to planet-wide meltdowns so severe they tear apart the molecules that make up its atmosphere. Called KELT-9b, the planet is an ultra-hot Jupiter, one of several varieties of exoplanets -- planets around other stars -- found in our galaxy. It weighs in at nearly three times the mass of our own Jupiter and orbits a star some 670 light-years away. With a surface temperature of 4,300 degrees Celsius - hotter than some stars - this
planet is the hottest found so far. Now, a team of astronomers using NASA's Spitzer space telescope has found evidence that the heat is too much even for molecules to remain intact. Molecules of hydrogen gas are likely ripped apart on the dayside of KELT-9b, unable to re-form until their disjointed atoms flow around to the planet's nightside. Though still extremely hot, the night-side's slight cooling is enough to allow hydrogen gas molecules to reform -- that is, until they flow back to the dayside, where they're torn apart all over again. KELT-9b will stay firmly categorized among the uninhabitable worlds. Astronomers became aware of its extremely hostile environment in 2017, when it was first detected using the Kilodegree Extremely Little Telescope (KELT) system -- a combined effort involving observations from two robotic telescopes, one in southern Arizona and one in South Africa.
The science team used the Spitzer space telescope to obtain temperature profiles from this infernal giant. Spitzer, which makes observations in infrared light, can measure subtle variations in heat. Repeated over many hours, these observations allow Spitzer to capture changes in the atmosphere as the planet presents itself in phases while orbiting the star. Different halves of the planet roll into view as it orbits around its star. That allowed the team to catch a glimpse of the difference between KELT-9b's dayside and its "night". In this case, the planet orbits its star so tightly that a "year" - once around the star - takes only 1 1/2 days. That means the planet is tidally locked, presenting one face to its star for all time (as our Moon presents only one face to Earth). On the far side of KELT-9b, nighttime lasts forever. But gases and heat flow from one side to the other. A big question for researchers trying to understand exoplanet atmospheres is how radiation and flow balance each other out. Computer models are major tools in such investigations, showing how these atmospheres are likely to behave in different temperatures. The best fit for the data from KELT-9b was a model that included hydrogen molecules being torn apart and reassembled, a process known as dissociation and recombination. KELT-9b turns out not to have huge temperature differences between its day- and night-sides, suggesting heat flow from one to the other. And the "hot spot" on the dayside, which is supposed to be directly under this planet's star, was shifted away from its expected position. Scientists don't know why - yet another mystery to be solved on this strange, hot planet.
LARGE AMOUNTS OF OXYGEN IN ANCIENT STAR
W. M. Keck Observatory
Astronomers have detected large amounts of oxygen in the atmosphere of one of the oldest and most elementally depleted stars known -- a "primitive star" scientists call J0815+4729. This new finding, which was made using W. M. Keck Observatory on Mauna Kea in Hawaii to analyze the chemical makeup of the ancient star, provides an important clue on how oxygen and other
important elements were produced in the first generations of stars in the universe. Oxygen is the third most abundant element in the universe after hydrogen and helium, and is essential for all forms of life on Earth, as the chemical basis of respiration and a building block of carbohydrates. It is also the main elemental component of the Earth's crust. However, oxygen didn't exist in the early universe; it is created through nuclear fusion reactions that occur deep inside the most massive stars, those with masses roughly 10 times the mass of the Sun or greater. Tracing the early production of oxygen and other elements requires studying the oldest stars still in existence. J0815+4729 is one such star; it resides over 5,000 light-years away toward the constellation Lynx. Stars like J0815+4729 are referred to as halo stars owing to their roughly spherical distribution around the Milky Way, as opposed to the more familiar flat disk of younger stars that include the Sun. Halo stars like J0815+4729 are truly ancient stars, allowing astronomers a peek into element production early in the history of the universe. The research team observed J0815+4729 using Keck Observatory's High-Resolution Echelle Spectrometer (HIRES) on the 10m Keck I telescope. The data, which required more than five hours of staring at the star over a single night, were used to measure the abundances of 16 chemical species in the star's atmosphere, including oxygen.
Keck Observatory's HIRES data of the star revealed a very unusual chemical composition. While it has relatively large amounts of carbon, nitrogen, and oxygen -- approximately 10, 8, and 3 percent of the abundances measured in the Sun -- other elements like calcium and iron have abundances around one millionth that of the Sun. Only a few such stars are known in the halo of
our galaxy, but none has such an enormous amount of carbon, nitrogen, and oxygen compared to their iron content. The search for stars of this type involves dedicated projects that sift through hundreds of thousands of stellar spectra to uncover a few rare sources like J0815+4729, then follow-up observations to measure their chemical composition. This star was first identified in data obtained with the Sloan Digital Sky Survey (SDSS), then characterized by the IAC team in 2017 using the Grand Canary Telescope in La Palma, Spain.
WEBB TELESCOPE WILL CONTINUE SPITZER'S LEGACY
NASA
As one window to the universe closes, another will open with an even better view. Some of the same planets, stars and galaxies we first saw through the first window will appear in even sharper detail in the one that will soon open. NASA's Spitzer Space Telescope concluded its mission on Jan. 30, 2020, after more than 16 extraordinary years of exploration. The telescope
has made many discoveries beyond the imaginations of its designers, such as planets outside our solar system, called exoplanets, and galaxies that formed close to the beginning of the universe. Many of Spitzer's break-throughs will be studied more precisely with the forthcoming James Webb Space Telescope, which is expected to be launched next year. Both Webb and
Spitzer are specialized for infrared light, which is invisible to human eyes. But with its giant gold-coated beryllium mirror and nine new technologies, Webb is about 1,000 times more powerful. The forthcoming telescope will be able to push Spitzer's science findings to new frontiers, from identifying chemicals in exoplanet atmospheres to locating some of the first galaxies to form after the Big Bang. Beyond its discoveries, Spitzer is also a pathfinder for Webb in terms of how to operate a telescope of this kind. In order to measure infrared light with high sensitivity, a telescope must be very cold. Spitzer has shown engineers how an infrared observatory behaves in the vastness of space and what temperatures mission planners should expect to grapple with for Webb. With more than 8,700 scientific papers published based on Spitzer's discoveries, the telescope has been a tremendous asset to astronomers across a variety of disciplines. Many of these tantalizing results are ripe for revisiting with a more powerful telescope, and Webb is poised to begin looking into them early in its mission. One of Spitzer's most stunning discoveries was that there are not just three, but seven rocky Earth-size planets orbiting a small, faint star
called TRAPPIST-1. TRAPPIST-1 is one of the best-studied planetary systems apart from our own, but there is a lot more to learn about it. The fourth planet from the star, TRAPPIST-1e is especially interesting because it has a density and surface gravity very similar to Earth's and receives enough stellar radiation to have temperatures friendly enough for liquid water. Webb will observe this planet to get a better sense of whether the planet has an atmosphere and, if so, what its chemistry is. The presence of molecules such as carbon dioxide, dominant on Mars and Venus, would have implications for whether a planet could have liquid water and other habitable conditions. Webb will be able to detect atmospheric water, too. Additionally, Webb will search for heat coming from TRAPPIST-1b, the planet closest to its star.
WASP-18b is another intriguing planet that Spitzer examined and that Webb will investigate further in observations early in the mission. This gas giant, with 10 times the mass of Jupiter, is located extremely close to its star, completing an orbit once every 23 hours. Because of its high temperature - a whopping 2,650 degrees Celsius - and large size, it is known as a "hot Jupiter". Using data from Spitzer and Hubble, astronomers figured out in 2017 that this planet has a lot of carbon monoxide in its upper atmosphere, and little water vapour. The planet is particularly interesting because it's so close to its star that it's in danger of being torn apart completely, and it may not survive another million years. Astronomers are interested in using Webb to look at the processes happening in this planet's atmosphere, which will provide insights into hot Jupiters in general. As light travels from distant objects to Earth, its wavelength becomes longer because the universe is expanding and those objects are moving farther from us. That means that stars that give off visible light in the early universe will appear in the infrared by the time their light reaches Earth. This makes infrared light an especially powerful tool for exploring the universe's ancient past. Pinpointing hundreds of billions of galaxies is currently impossible, but Spitzer has made large galaxy catalogs that represent different slices of the universe, containing some of the most distant galaxies we know. The large survey areas of Spitzer and Hubble Space Telescope have allowed astronomers to look efficiently for objects that could be studied in further detail with Webb. For example, Spitzer, together with Hubble, took an image of a galaxy called GN-z11, which holds the record for most distant galaxy measured yet. It is a relic from when the universe was only 400 million years old, just 3% of its current age and less than 10% of its size today. What's more, Webb's higher sensitivity will allow the telescope to look for galaxies dating back even earlier in the universe. And questions still abound about these distant galaxies: Are there a lot of stars forming in them or relatively few? Are they rich in gas or poor? Are there black holes at their centres, and how do those black holes interact with stars? And scientists have pondered a chicken-and-egg problem for decades about which came first: the black hole or the surrounding galaxy?
Topic: Early February Astronomy Bulletin (Read 1875 times)