Exoplanets: How we'll search for signs of life

Whether there is life elsewhere in the universe is a question people have pondered for millennia; and within the last few decades, great strides have been made in our search for signs of life outside of our solar system. NASA missions like the space telescope Kepler have helped us document thousands of exoplanets - planets that orbit around other stars. And current NASA missions like Transiting Exoplanet Survey Satellite (TESS) are expected to vastly increase the current number of known exoplanets. It is expected that dozens will be Earth-sized rocky planets orbiting in their stars' habitable zones, at distances where water could exist as a liquid on their surfaces. These are promising places to look for life. This will be accomplished by missions like the soon-to-be-launched James Webb Space Telescope, which will complement and extend the discoveries of the Hubble Space Telescope by observing at infrared wavelengths. It is expected to launch in 2021, and will allow scientists to determine if rocky exoplanets have oxygen in their atmospheres. Oxygen in Earth's atmosphere is due to photosynthesis by microbes and plants. To the extent that exoplanets resemble Earth, oxygen in their atmospheres may also be a sign of life. Not all exoplanets will be Earth-like, though. Some will be, but others will differ from Earth enough that oxygen doesn't necessarily come from life. So with all of these current and future exoplanets to study, how do scientists narrow down the field to those for which oxygen is most indicative of life?

To answer this question, an interdisciplinary team of researchers, led by Arizona State University (ASU), has provided a framework, called a "detectability index" which may help prioritize exoplanets that require additional study. The details of this index have recently been published in the Astrophysical Journal of the American Astronomical Society.

"The goal of the index is to provide scientists with a tool to select the very best targets for observation and to maximize the chances of detecting life," says lead author Donald Glaser of ASU's School of Molecular Sciences.

The oxygen detectability index for a planet like Earth is high, meaning that oxygen in Earth's atmosphere is definitely due to life and nothing else. Seeing oxygen means life. A surprising finding by the team is that the detectability index plummets for exoplanets not-too-different from Earth.

Although Earth's surface is largely covered in water, Earth's oceans are only a small percentage (0.025%) of Earth's mass. By comparison, moons in the outer solar system are typically close to 50% water ice.

"It's easy to imagine that in another solar system like ours, an Earth-like planet could be just 0.2% water," says co-author Steven Desch of ASU's School of Earth and Space Exploration. "And that would be enough to change the detectability index. Oxygen would not be indicative of life on such planets, even if it were observed. That's because an Earth-like planet that was 0.2% water--about eight times what Earth has--would have no exposed continents or land."

Without land, rain would not weather rock and release important nutrients like phosphorus. Photosynthetic life could not produce oxygen at rates comparable to other non-biological sources.

"The detectability index tells us it's not enough to observe oxygen in an exoplanet's atmosphere. We must also observe oceans and land," says Desch. "That changes how we approach the search for life on exoplanets. It helps us interpret observations we've made of exoplanets. It helps us pick the best target exoplanets to look for life on. And it helps us design the next generation of space telescopes so that we get all the information we need to make a positive identification of life."

Scientists from diverse fields were brought together to create this index. The formation of the team was facilitated by NASA's Nexus for Exoplanetary System Science (NExSS) program, which funds interdisciplinary research to develop strategies for looking for life on exoplanets. Their disciplines include theoretical and observational astrophysics, geophysics, geochemistry, astrobiology, oceanography, and ecology.

"This kind of research needs diverse teams, we can't do it as individual scientists" says co-author Hilairy Hartnett who holds joint appointments at ASU's School of Earth and Space Exploration and School of Molecular Sciences.

In addition to lead author Glaser and co-authors Harnett and Desch, the team includes co-authors Cayman Unterborn, Ariel Anbar, Steffen Buessecker, Theresa Fisher, Steven Glaser, Susanne Neuer, Camerian Millsaps, Joseph O'Rourke, Sara Imari Walker, and Mikhail Zolotov who collectively represent ASU's School of Molecular Sciences, School of Earth and Space Exploration, and School of Life Sciences. Additional scientists on the team include researchers from the University of California Riverside, Johns Hopkins University and the University of Porto (Portugal).

It is the hope of this team that this detectability index framework will be employed in the search for life. "The detection of life on a planet outside our solar system would change our entire understanding of our place in the universe," says Glaser. "NASA is deeply invested in searching for life, and it is our hope that this work will be used to maximize the chance of detecting life when we look for it."

US Air Force's X-37B preparing for next space flight

The US Air Force's strange little space plane, the experimental X-37B, is preparing for its sixth mission. Dubbed the Orbital Test Vehicle (OTV), little is known about its mission, apart from that the Pentagon claims it exists to test new technology. According to a rocket launch schedule, one of the Air Force's two remotely piloted X-37Bs will blast off on its next mission atop an Atlas V rocket on May 16 from Cape Canaveral Air Force Station on Florida's Atlantic coast. As for the details about its mission, your guess is as good as ours. The last flight, launched in September 2017, lifted the Advanced Structurally Embedded Thermal Spreader II, an experimental heat pipe used for managing temperature flows, for testing. When the last OTV touched down in October 2019, it had been in orbit for 779 days. The box truck-sized space plane operates at altitudes ranging from 110 to 500 miles up. However, part of the X-37B's mission is simply development of reusable space plane technology, according to the Air Force. The tech is being pursued by a variety of nations as well as private firms like Virgin Galactic. "Technologies being tested in the program include advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, advanced propulsion systems, advanced materials and autonomous orbital flight, reentry and landing," the Air Force adds.

The US space agency NASA once operated the Space Shuttle, which the X-37B somewhat resembles and to which it operates similarly, but those craft have all been retired. The X-37 was originally developed by NASA in the late 1990s, but the program was handed over to the Defense Advanced Research Projects Agency (DARPA), a Pentagon tech lab, in 2004, at which point it became classified, according to Space.com. Its first flight was in 2010.

UCF researchers develop groundbreaking new rocket-propulsion system

A University of Central Florida researcher and his team have developed an advanced new rocket-propulsion system once thought to be impossible. The system, known as a rotating detonation rocket engine, will allow upper stage rockets for space missions to become lighter, travel farther, and burn more cleanly. The result were published this month in the journal Combustion and Flame. "The study presents, for the first time, experimental evidence of a safe and functioning hydrogen and oxygen propellant detonation in a rotating detonation rocket engine," said Kareem Ahmed, an assistant professor in UCF's Department of Mechanical and Aerospace Engineering who led the research. The rotating detonations are continuous, Mach 5 explosions that rotate around the inside of a rocket engine, and the explosions are sustained by feeding hydrogen and oxygen propellant into the system at just the right amounts. This system improves rocket-engine efficiency so that more power is generated while using less fuel than traditional rocket energies, thus lightening the rocket's load and reducing its costs and emissions. Mach 5 explosions create bursts of energy that travel 4,500 to 5,600 miles per hour, which is more than five times the speed of sound. They are contained within a durable engine body constructed of copper and brass. The technology has been studied since the 1960s but had not been successful due to the chemical propellants used or the ways they were mixed. Ahmed's group made it work by carefully balancing the rate of the propellants, hydrogen and oxygen, released into the engine.

"We have to tune the sizes of the jets releasing the propellants to enhance the mixing for a local hydrogen-oxygen mixture," Ahmed said. "So, when the rotating explosion comes by for this fresh mixture, it's still sustained. Because if you have your composition mixture slightly off, it will tend to deflagrate, or burn slowly instead of detonating."

Ahmed's team also had to capture evidence of their finding. They did this by injecting a tracer in the hydrogen fuel flow and quantifying the detonation waves using a high-speed camera.

"You need the tracer to actually see that explosion that is happening inside and track its motion," he said. "Developing this method to characterize the detonation wave dynamics is another contribution of this article."

William Hargus, lead of the Air Force Research Laboratory's Rotating Detonation Rocket Engine Program, is a co-author of the study and began working with Ahmed on the project last summer.

"As an advanced propulsion spectroscopist, I recognized some of the unique challenges in the observation of hydrogen-detonation structures," Hargus said. "After consulting with Professor Ahmed, we were able to formulate a slightly modified experimental apparatus that significantly increased the relevant signal strength."

"These research results already are having repercussions across the international research community," Hargus said. "Several projects are now re-examining hydrogen detonation combustion within rotating detonation rocket engines because of these results. I am very proud to be associated with this high-quality research."

Sun is less active than similar stars

The extent to which solar activity (and thus the number of sunspots and the solar brightness) varies can be reconstructed using various methods - at least for a certain period of time. Since 1610, for example, there have been reliable records of sunspots covering the Sun; the distribution of radioactive varieties of carbon and beryllium in tree rings and ice cores allows us to draw conclusions about the level of solar activity over the past 9000 years. For this period of time, scientists find regularly recurring fluctuations of comparable strength as during recent decades. "However, compared to the entire lifespan of the Sun, 9000 years is like the blink of an eye", says MPS scientist Dr. Timo Reinhold, first author of the new study. After all, our star is almost 4.6 billion years old. "It is conceivable that the Sun has been going through a quiet phase for thousands of years and that we therefore have a distorted picture of our star," he adds. Since there is no way of finding out how active the Sun was in primeval times, scientists can only resort to the stars: Together with colleagues from the University of New South Wales in Australia and the School of Space Research in South Korea, the MPS researchers investigated, whether the Sun behaves "normally" in comparison to other stars. This may help to classify its current activity. To this end, the researchers selected candidate stars that resemble the Sun in decisive properties. In addition to the surface temperature, the age, and the proportion of elements heavier than hydrogen and helium, the researchers looked above all at the rotation period. "The speed at which a star rotates around its own axis is a crucial variable", explains Prof. Dr. Sami Solanki, director at MPS and co-author of the new publication.

A star's rotation contributes to the creation of its magnetic field in a dynamo process in its interior. "The magnetic field is the driving force responsible for all fluctuations in activity," says Solanki.

The state of the magnetic field determines how often the Sun emits energetic radiation and hurls particles at high speeds into space in violent eruptions, how numerous dark sunspots and bright regions on its surface are - and thus also how brightly the Sun shines.

A comprehensive catalogue containing the rotation periods of thousands of stars has been available only for the last few years. It is based on measurement data from NASA's Kepler Space Telescope, which recorded the brightness fluctuations of approximately 150000 main sequence stars (i.e. those that are in the middle of their lifetimes) from 2009 to 2013.

The researchers scoured this huge sample and selected those stars that rotate once around their own axis within 20 to 30 days. The Sun needs about 24.5 days for this. The researchers were able to further narrow down this sample by using data from the European Gaia Space Telescope. In the end, 369 stars remained, which also resemble the Sun in other fundamental properties.

The exact analysis of the brightness variations of these stars from 2009 to 2013 reveals a clear picture. While between active and inactive phases solar irradiance fluctuated on average by just 0.07 percent, the other stars showed much larger variation. Their fluctuations were typically about five times as strong.

"We were very surprised that most of the Sun-like stars are so much more active than the Sun," says Dr. Alexander Shapiro of MPS, who heads the research group "Connecting Solar and Stellar Variabilities".

However, it is not possible to determine the rotation period of all the stars observed by the Kepler telescope. To do this, scientists have to find certain periodically re-appearing dips in the star's lightcurve. These dips can be traced back to starspots that darken the stellar surface, rotate out of the telescope's field of view and then reappear after a fixed period of time.

"For many stars, such periodic darkenings cannot be detected; they are lost in the noise of the measured data and in overlying brightness fluctuations," explains Reinhold. Viewed through the Kepler telescope, even the Sun would not reveal its rotation period.

The researchers therefore also studied more than 2500 Sun-like stars with unknown rotation periods. Their brightness fluctuated much less than that of the other group.

These results allow two interpretations. There could be a still unexplained fundamental difference between stars with known and unknown rotation period. "It is just as conceivable that stars with known and Sun-like rotation periods show us the fundamental fluctuations in activity the Sun is capable of," says Shapiro. This would mean that our star has been unusually feeble over the past 9000 years and that on very large time scales phases with much greater fluctuations are also possible.

There is, however, no cause for concern. For the foreseeable future, there is no indication of such solar "hyperactivity". On the contrary: For the last decade, the Sun has been showing itself to be rather weakly active, even by its own low standards. Predictions of activity for the next eleven years indicate that this will not change soon.

Research reveals possibly active tectonic system on the moon

Researchers have discovered a system of ridges spread across the nearside of the Moon topped with freshly exposed boulders. The ridges could be evidence of active lunar tectonic processes, the researchers say, possibly the echo of a long-ago impact that nearly tore the Moon apart. "There's this assumption that the Moon is long dead, but we keep finding that that's not the case," said Peter Schultz, a professor in Brown University's Department of Earth, Environmental and Planetary Sciences and co-author of the research, which is published in the journal Geology. "From this paper it appears that the Moon may still be creaking and cracking - potentially in the present day - and we can see the evidence on these ridges." Most of the Moon's surface is covered by regolith, a powdery blanket of ground-up rock created by the constant bombardment of tiny meteorites and other impactors. Areas free of regolith where the Moon's bedrock is exposed are vanishingly rare. But Adomas Valantinas, a graduate student at the University of Bern who led the research while a visiting scholar at Brown, used data from NASA's Lunar Reconnaissance Orbiter (LRO) to spot strange bare spots within and surrounding the lunar maria, the large dark patches on the Moon's nearside. "Exposed blocks on the surface have a relatively short lifetime because the regolith buildup is happening constantly," Schultz said. "So when we see them, there needs to be some explanation for how and why they were exposed in certain locations."

For the study, Valantinas used the LRO's Diviner instrument, which measures the temperature of the lunar surface. Just as concrete-covered cities on Earth retain more heat than the countryside, exposed bedrock and blocky surfaces on the Moon stays warmer through the lunar night than regolith-covered surfaces. Using nighttime observations from Diviner, Valantinas turned up more than 500 patches of exposed bedrock on narrow ridges following a pattern across the lunar nearside maria.

A few ridges topped with exposed bedrock had been seen before, Schultz says. But those ridges were on the edges of ancient lava-filled impact basins and could be explained by continued sagging in response to weight caused by the lava fill.

But this new study discovered that the most active ridges are related to a mysterious system of tectonic features (ridges and faults) on the lunar nearside, unrelated to both lava-filled basins and other young faults that crisscross the highlands.

"The distribution that we found here begs for a different explanation," Schultz said.

Valantinas and Schultz mapped out all of the exposures revealed in the Diviner data and found an interesting correlation. In 2014, NASA's GRAIL mission found a network of ancient cracks in the Moon's crust. Those cracks became channels through which magma flowed to the Moon's surface to form deep intrusions. Valantinas and Schultz showed that the blocky ridges seemed to line up just about perfectly with the deep intrusions revealed by GRAIL.

"It's almost a one-to-one correlation," Schultz said. "That makes us think that what we're seeing is an ongoing process driven by things happening in the Moon's interior."

Schultz and Valantinas suggest that the ridges above these ancient intrusions arestill heaving upward. The upward movement breaks the surface and enables regolith to drain into cracks and voids, leaving the blocks exposed. Because bare spots on the Moon get covered over fairly quickly, this cracking must be quite recent, possibly even ongoing today. They refer to what they've found as ANTS, for Active Nearside Tectonic System.

The researchers believe that the ANTS was actually set in motion billions of years ago with a giant impact on the Moon's farside. In previous studies, Schultz and a co-worker proposed this impact, which formed the 1500-mile South Pole Aitken Basin, shattered the interior on the opposite side, the nearside facing the Earth.

Magma then filled these cracks and controlled the pattern of dikes detected in the GRAIL mission. The blocky ridges comprising the ANTS now trace the continuing adjustments along these ancient weaknesses.

"This looks like the ridges responded to something that happened 4.3 billion years ago," Schultz said. "Giant impacts have long lasting effects. The Moon has a long memory. What we're seeing on the surface today is testimony to its long memory and secrets it still holds."