Space Sunday: when neutron stars collide

When neutron stars collide: an artist’s impression of the point when two neutron stars collided in the galaxy NGC 4993, 130 million years ago, and which are now increasing our understanding of neutron stars and the universe. Credit: SF/LIGO/Sonoma State University/A. Simonnet

Around the world on August 17th, 2017, some 70 telescopes and observatories – including the Laser Interferometer Gravitational-Wave Observatory (LIGO), responsible for confirming the existence of gravitational waves (see here and here for more) – quietly turned their attention on the same spot in the constellation Hydra.

“I don’t think it’s out of the question that this is the most observed astronomical event ever. It’s a thrilling notion, and a little overwhelming,” said LIGO spokesperson David Shoemaker. “We’ve got somewhere between a quarter and a third of all the world’s astronomers working with us.”

The reason? Hours earlier, an observatory in Chile had detected gravitational waves followed by a burst of gamma radiation – potentially the signature of two neutron stars colliding far beyond our galaxy. If so, the detection would be the first time gravitational waves have been observed originating from something other than the merger of two black holes. Hence, an alert was issued to observatories around the globe, resulting in the massed focusing on instruments on that single point in space.

Over the coming days, the data revealed that a collision between two neutron stars in what is referred two as a “kilonova”  – which sits between a star going nova and a super-massive star going supernova.  It marks the first confirmation that neutron star mergers can cause gamma ray bursts. However, there is much more to the event.

Neutron stars are the dense remnants of massive stars that long ago exploded as supernovae. The two stars in question are located in galaxy NGC 4993, 130 million light years from Earth. Originally, these stars were each around 10-20 times the mass of our sun; after each went supernova, they collapsed down to bodies around 16 km (10 mi) in diameter, comprised entirely of neutrons so densely packed, that despite their small size, each still had a mass perhaps twice that of our own Sun.

These two neutron stars, located close together, were gradually drawn together over the course of perhaps 11 billion years by their mutual gravities until they collided, venting huge amounts of energy across the spectrum and space-time in what astronomers call a “multi-messenger event”. It was the arrival of the light waves and gravitational waves here on Earth, 130 million years later, that astronomers from around the world were keen to observe, marking the first time a cosmological event of this nature has been observed in both gravitational waves and light, producing a huge amount of data for researchers to study.

How the kilonova was initially observed through the initial days of visible light observation following the first indication of the collision through to the falling off of light from the initial explosive outburst of energy. Credit: Sarah Wilkinson / LCO.

Thanks to the alert sent out by the Chilean observatory, over 3,500 astronomers and more than 100 instruments  – including LIGO and a the Hubble Space Telescope responded, making the event the first to be observed through the detection of visible light and gravitational waves. Their findings are now being made public, and include some remarkable facts.

These include the first confirmation that neutron star mergers can cause gamma ray bursts – although there is some questions over what this might in fact mean. It also marks the first measurement of the universe’s expansion using gravitational waves.In addition, as the collision was recorded in wavelengths right across the electromagnetic spectrum, from radio to gamma rays, it is the first time a cosmological event of this nature has been observed in both gravitational waves and light. A further result of the observations is that astronomers have witnessed heavy elements being formed from the aftermath of the event.

“People have long suspected that heavy elements were made in neutron star mergers, but this is really the first time we’ve nailed that down,” Andrew Levan, an astronomer at the University of Warwick in the UK. “This merger made something like the mass of the Earth in gold, along with other heavy elements such as platinum, lead and uranium.”

The kilonova as seen from the Hubble Space Telescope a few days after the explosion, tracking it as the initial light faded. Credit: NASA and ESA. Acknowledgement: A.J. Levan (U. Warwick), N.R. Tanvir (U. Leicester), and A. Fruchter and O. Fox (STScI)

It was actually the discovery that heavy elements were being formed in the material resulting from the collision which confirmed the event was an actual collision of two neutron stars. The elements would only be formed if neutrons were being ejected from the two stars to collide with lighter atoms in the surrounding space. Material would only be ejected if the objects in collision each had a surface, something black holes don’t have – they only have an event horizon.

This in turn indicated the event was far closer that the previous five detections of gravitational waves which have occurred since 2015. These have been the result of pairs of black holes merging, none of which have been closer than 1.3 billion light years away. That the gravitational waves were observed alongside of light waves also gave further confirmation of another of Einstein’s general relativity predictions: that light and gravitational waves travel and more-or-less the same speed.

Observations and data gathering continued after the initial explosion was detected, although the light from the collision faded over the 6-8 days following the event, and astronomers are keen to discover what has been left behind. Currently, the region of NGC 4993 where the kilonova occurred is obscured behind a cloud of matter and heavy elements, leading to questions on whether or not the two stars may have merged to form an even larger neutron star, or whether they collapsed into a black hole. Some of those studying the data gathered believe the gamma ray burst recorded after the initial detection of gravitational waves might be indicative of the latter, the result of matter left over from the event and collapse being drawn into the event horizon.

Summing up the significance of the event, astronomer Tony Piro from the Harvard–Smithsonian Centre for Astrophysics said, “The ability to study the same event with both gravitational waves and light is a real revolution in astronomy. We can now study the universe with completely different probes, which teaches things we could never know with only one or the other.”

Seeking the Fingerprints of Martian Life and Following the Water

The pendulum on the potential for life once having arisen on Mars continues to swing back and forth. Most recently, the pendulum has been swinging back to being in favour of the potential for life to have arisen on the red planet, and two new studies swing it slightly further in that direction.

In Microbes leave “fingerprints” on Martian rocks, published on October 17th, a team of European scientists state that extreme lifeforms that are capable of metabolising metals could have existed on Mars in the past, and that “fingerprints” of their existence might be found by looking at samples of Mars’ red sands.

The researchers created four controlled environments utilising the atmospheric gases and surface minerals and chemicals designed to reflect different surface conditions thought to exist on ancient Mars. They then introduced a strain of chemolithotrophs – bacteria able to metabolise inogranic metals like iron, sulphur and uranium – called Metallosphaera sedula, and then monitored the environments to see if any bio-processing occurred.

In all four cases the team gathered evidence free soluble metals, indicating the bacteria had effectively colonised the environments, metabolising some of the metallic minerals within each of them. If this is in fact the case on Mars, then because of the way the Martian environment developed it is entirely possible that evidence of these free soluble metals could have been preserved within Martian regolith, offering upcoming missions such as NASA’s Mars 2020 rover a potential science target to seek out.

Glaciation on Mars may have caused many of the features seen today on the surface of the planet which have been attributed to the action of liquid water, and could indicate Mars was more “cold and icy” with bouts of “warm and wet”, rather than moving from “warm and wet” to “cold and icy”. Credit: NASA

While the evidence for water having once flowed freely on Mars is abundant, a lingering question has remained as to whether that water flow was regular, making Mars – for a time – a “warm and wet” environment, or was it intermittent, leaving the planet more of a “cold and icy / slushy” place. The issue here is that while the geology of Mars strongly points towards the former, the majority of the atmospheric models developed for the planet lean towards the latter.

In the second of the two studies recently published, entitled Late Noachian Icy Highlands Climate Model: Exploring the Possibility of Transient Melting and Fluvial/Lacustrine Activity Through Peak Annual and Seasonal Temperatures, researchers from Brown University, Rhode Island, reconciles the two differing models of ancient Mars by showing that both situations were likely the case.

Using a mix of climate models, the Brown University team developed a plausible model of ancient Mars which suggests it may have been largely glaciated, but during the summer periods in each hemisphere, peak daily  temperatures would be sufficient to cause melt water to flow freely from these glaciers, which over time could give rise to many of the water-carved features seen on Mars today.

By looking at past studies on Mars, including estimates of the volume of water required to carve many of the networks of water-carved valleys on Mars (and particularly in the southern hemisphere), and studies examining the likely axial tilt present on Mars around 4-3.5 billion years ago, the Brown team found that while the average temperatures of ancient Mars may have remained below freezing, peak temperatures experienced in the southern hemisphere summer period could give rise to a volume of free-flowing water pretty close to the estimates of that needed to carve the valley networks over a period lasting between 21,000 and 550,000 years.

In this, the Brown team draw a direct comparison between ancient Mars and Antarctica. For much of the year, the ambient conditions in those regions of Antarctica not under permanent snow and ice cover are dry and arid, with temperatures well below freezing. However, in the summer months melt water from glaciers can give rise to streams and lakes in places such as the McMurdo Dry Valleys, offering environments that can support life within the soil and water.

Dawn mission extended at Ceres as Study of Organics Continues

Earlier in 2017, I reported that the joint NASA / ESA Dawn had detected localised organic-rich material on the dwarf planet Ceres. Since that time, the South-west Research Institute (SwRI) has been investigating the findings to try to understand its origins.

SwRI scientists are studying the geology associated with the organic-rich areas on Ceres. Dawn spacecraft data show a region around the Ernutet crater where organic concentrations have been discovered (background image). The colour coding shows the surface concentration of organics, as inferred from the visible and near infra-red spectrometer. The inset shows a higher resolution enhanced colour image of the Ernutet crater acquired by Dawn’s framing camera. Regions in red indicate higher concentration of organics. Credit: NASA/JPL / UCLA / ASI / INAF / MPS / DLR / IDA

“The discovery of a locally high concentration of organics close to the Ernutet crater poses an interesting conundrum,” said Dr. Simone Marchi, a principal scientist at SwRI. “Was the organic material delivered to Ceres after its formation? Or was it synthesised and/or concentrated in a specific location on Ceres via internal processes? Both scenarios have shortfalls, so we may be missing a critical piece of the puzzle. Earlier research that focused on the geology of the organic-rich region on Ceres were inconclusive about their origin. Recently, we more fully investigated the viability of organics arriving via an asteroid or comet impact.”

Ceres originated around 4.5 billion years ago and has composition which includes clays, sodium, and ammonium-carbonates, suggesting it underwent a complex chemical evolution, understanding the role of organics in this evolution, including how they came to be on Ceres could help explain the origin, evolution, and distribution of organic species across the solar system.

The investigations into whether the organics might have been the result of an impact leaving organic deposits revealed that comet-like projectiles, which would have a high impact velocity would lose almost all of their organics due to shock compression. However, impact from local asteroids would be at lower velocities, potentially allowing them to retain perhaps as much as 30% of any organics they carried. The problem here is that if the organics were deposited on Ceres by local collisions with other asteroid bodies, it would not be unreasonable to expect the organics to be more widely dispersed than is the case, leading to the conclusion that the very localised concentration of organics suggests they could have originated on Ceres – which leads to further questions about the dwarf planet requring .

Dawn mission – click for full size. Credit: NASA/JPL

While investigations into the organics continues, it has been confirmed that the Dawn mission has received a final extension through until its projected end in late 2018. At that point in time, the spacecraft’s hydrazine fuel will be almost expended, and it will be moved away from Ceres to remove any risk of an uncontrolled collision with the dwarf planet, potentially contaminating it.

The mission extension will allow the Dawn mission team to reduce the vehicle’s orbit to as close as 200 km (120 mi) above Ceres, where more extensive data on the dwarf planet can be gathered, particularly with the gamma ray and neutron spectrometer in an attempt to better understand the composition of Ceres’ uppermost layer and how much ice it contains. At the same time, further visible-light images of Ceres’ surface geology will be recorded, as will measurements of its mineralogy using the vehicle’s visible and infra-red mapping spectrometer.

The mission extension also means the spacecraft will be operating as Ceres passes through perihelion, the point in its orbit where it is closest to the Sun, in April 2018. At this closer proximity to the Sun, more ice on Ceres’ surface may turn to water vapour, and may in turn increase Ceres’ transient atmosphere, first detected by the European Space Agency’s Herschel Space Observatory before Dawn’s arrival, offering an opportunity to better understand its composition and formation.

 

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