Do you regulate the end? Don’t look for it on porous asteroids (Planetary Science)

If the surface of carbonaceous asteroids such as Bennu and Ryugu – destinations respectively of the Osiris-Rex and Hayabusa 2 space missions – is not covered by large quantities of material known as fine regolith, similar to the sand of our beaches, the cause is to be found in the high porosity of their rocks. These are the results obtained by an international team, which sees an important participation of researchers from the National Institute of Astrophysics

When the NASA mission Osiris-Rex reached the asteroid Bennu in 2018, scientists expected to find a surface covered with fine regolith : material formed by small grains or pebbles at most a few centimeters large, similar to the sand found on beaches of our planet. There were also similar expectations for the Hayabusa 2 mission of the Japanese space agency Jaxa, which reached the asteroid Ryugu in the same year . Once in the vicinity of these two small bodies, however, both probes revealed rocky terrain, surprisingly covered with boulders and with very little fine regolith.

A new study, led by the Italian researcher Saverio Cambioni , with the participation of Giovanni Poggiali and John R. Brucato of the National Institute of Astrophysics, has finally solved the mystery: it would be the very porous nature of the rocks of these asteroids to explain the absence of fine regolith on their surface.

“When the first images from Bennu arrived, we noticed some regions where the resolution wasn’t enough to distinguish between small rocks and fine regolith, so we started using our new AI method to recognize fine regolith from rocks. using the infrared emission recorded by the probe “, explains Saverio Cambioni, first author of the article published today in Nature , who conducted this research during his doctorate at the University of Arizona, in the United States, and is now a postdoctoral researcher -doc at the Massachusetts Institute of Technology .

Osiris-Rex collected high-resolution images – up to 3 millimeters per pixel – across Bennu’s surface to map it out and study it in detail: characterizing the asteroid is one of the mission’s main objectives. The absence of fine regolith on the surface posed not only a scientific question, but also a technical challenge: the probe was designed to collect a sample of the asteroid from a surface rich in fine regolith, not among boulders and rocks. Thus, the sampling area on Bennu had to be drastically reduced from a region of about 160 meters, the size of a parking lot for 100 cars, down to 8 meters, comparable to a parking lot for only 5 cars. The operation was successful on 20 October.

Detail of the Bennu surface. Credits: Nasa / University of Arizona / Csa / York University / Open University / Mda

“To distinguish the contribution of fine regolith from that of rock and to make a global study of the surface, it was necessary to put together a large database of regions observed both in the day and in the night,” says Giovanni Poggiali, co-author of the study and researcher. INAF in Florence.

The thermal emission released by the fine regolith, which depends on the size of its grains, is different from the emission produced by larger rocks, which instead depends on their porosity. For this the team simulated the emission produced by different mixtures of fine regolith and rocks of varying porosity, teaching an algorithm how to recognize the different soils and compare them with observations of the Bennu surface.

The analysis of the data showed that the fine regolith is not randomly distributed on Bennu but that, where the rocks are more porous – that is, on most of the asteroid – it is systematically found less. This suggests that the highly porous rocks of the celestial body produce less fine regolith because they are compacted – and not fragmented, as is the case with less porous rocks – as a result of meteoric impacts: the cavities inside the rocks would help to cushion the impact of the meteorites. resulting in less fragment production. Furthermore, the porous rocks would break down more slowly due to the diurnal cycle of heating and cooling of the asteroid, further inhibiting the formation of the fine regolith.

“With artificial intelligence, used for the first time in this type of research, we were able to go much deeper into the analysis of spectroscopic data by highlighting unique properties of the materials that make up primitive asteroids such as Bennu and Ryugu,” comments John Brucato, co-author of the study and INAF researcher in Florence.

Saverio Cambioni, researcher at the Massachusetts Institute of Technology.

The results of Bennu’s analysis are in agreement with findings from independent laboratory experiments. Also for the asteroid Ryugu, Cambioni and colleagues propose that the porosity of its rocks would explain the lack of fine regolith observed by Hayabusa 2. Similarly, the abundance of this material recorded in 2005 by the previous Jaxa mission, Hayabusa, on a another asteroid of a different type, Itokawa, would be due to a lower porosity of its rocks, determined by the researchers using observations from Earth. According to the team, fine regolith would be rare on carbonaceous asteroids such as Bennu and Ryugu, which are the most common type of asteroid and are thought to be formed from very porous rocks. The fine regolith would instead be abundant on S-type asteroids such as Itokawa,

“Asteroids are fossils of the formation of the Solar System, but recent studies – including ours – are showing how in reality some asteroids are very evolved,” Cambioni concludes. «Understanding the evolution processes of asteroids is important for understanding the evolution of the Solar System and our planet. To shed some light on this, we will need to visit more asteroids in the future to collect samples to be reported and analyzed on Earth. Our study will allow us to understand in advance the nature of the asteroid surfaces, and therefore to plan the missions accordingly ».

Featured image: The asteroid Bennu in a mosaic of images taken by the Nasa Osiris-Rex probe. Credits: NASA / Goddard / University of Arizona

To know more:

  • Read in  Nature  the article ” Fine-regolith production on asteroids controlled by rock porosity “, by S. Cambioni, M. Delbo, G. Poggiali, C. Avdellidou, AJ Ryan, JDP Deshapriya, E. Asphaug, R.-L . Ballouz, MA Barucci, CA Bennett, WF Bottke, JR Brucato, KN Burke, E. Cloutis, DN DellaGiustina, JP Emery, B. Rozitis, KJ Walsh and DS Lauretta

Provided by INAF

Late-time Small-body Disruptions Can Protect the Earth (Planetary Science)

If an asteroid is determined to be on an Earth-impacting trajectory, scientists typically want to stage a deflection, where the asteroid is gently nudged by a relatively small change in velocity, while keeping the bulk of the asteroid together.

A kinetic impactor or a standoff nuclear explosion can achieve a deflection. However, if the warning time is too short to stage a successful deflection, another option is to couple a lot of energy to the asteroid and break it up into many well-dispersed fragments. This approach is called disruption and it is often what people think of when they picture planetary defense. While scientists would prefer to have more warning time, they need to be prepared for any possible scenario, as many near-Earth asteroids remain undiscovered.

Now, new research takes a closer look into at how different asteroid orbits and different fragment velocity distributions affect the fate of the fragments, using initial conditions from a hydrodynamics calculation, where a 1-Megaton-yield device was deployed a few meters off the surface of a Bennu-shaped, 100-meter diameter asteroid (1/5 the scale of Bennu, a near-Earth asteroid discovered in 1999). See the video.

The work is featured in a paper published in Acta Astronautica with lead author Patrick King, a former Lawrence Livermore National Laboratory Graduate Scholar Program fellow who worked with LLNL’s Planetary Defense group on this research as part of his Ph.D. thesis. King currently works at the Johns Hopkins University Applied Physics Laboratory (JHUAPL) as a physicist in the Space Exploration Sector. Co-authors of the paper include Megan Bruck Syal, David Dearborn, Robert Managan, Michael Owen and Cody Raskin.

The results highlighted in the paper are reassuring: for all five asteroid orbits considered, carrying out the disruption just two months before the Earth impact date was able to reduce the fraction of impacting mass by factor of 1,000 or more (99.9 percent of the mass misses Earth). For a larger asteroid, the dispersal would be less robust, but even dispersal velocities reduced by an order of magnitude would result in 99 percent of the mass missing Earth, if disruption is staged at least six months ahead of the impact date.

“One of the challenges in assessing disruption is that you need to model all of the fragment orbits, which is generally far more complicated than modeling a simple deflection,” King said. “Nevertheless, we need to try to tackle these challenges if we want to assess disruption as a possible strategy.”

King said the principal finding of the work was that nuclear disruption is a very effective defense of last resort. “We focused on studying ‘late’ disruptions, meaning that the impacting body is broken apart shortly before it impacts,” he said. “When you have plenty of time — typically decade-long timescales — it is generally preferred that kinetic impactors are used to deflect the impacting body.”

Kinetic impactors have many advantages: for one, the technique is well-known and is being tested on real missions, such as the DART mission, and is capable of handling a wide range of possible threats if you have enough time. However, they do have some limitations, so it is important that if an actual emergency does arise that multiple options are available to deal with a threat, including some ways that can handle pretty short warning times.

Owen said this paper is critically important for understanding the consequences and requirements for disrupting a hazardous asteroid approaching Earth. Owen wrote the software, called Spheral, that was used to model the nuclear disruption of the original asteroid, following the detailed physics of shocking and breaking up the original rocky asteroid and capturing the properties of the resulting fragments. From there, the team used Spheral to follow the gravitational evolution of the fragment cloud, accounting for the effects of the fragments on one another as well as the gravitational influence of the sun and planets.

“If we spotted a hazardous object destined to strike the Earth too late to safely divert it, our best remaining option would be to break it up so thoroughly the resulting fragments would largely miss the Earth,” he said. “This is a complicated orbital question though — if you break up an asteroid into pieces, the resulting cloud of fragments will each pursue their own path around the sun, interacting with each other and the planets gravitationally. That cloud will tend to stretch out into a curved stream of fragments around the original path the asteroid was on. How quickly those pieces spread out (combined with how long until the cloud crosses Earth’s path) tells us how many will strike the Earth.”

Bruck Syal said the work addresses a major goal defined in the White House OSTP’s National Near-Earth Object (NEO) Preparedness Strategy and Action Plan: to improve NEO modeling, prediction and information integration.

“Our group continues to refine our modeling approaches for nuclear deflection and disruption, including ongoing improvements to X-ray energy deposition modeling, which sets the initial blowoff and shock conditions for a nuclear disruption problem,” she said. “This latest paper is an important step in demonstrating how our modern multiphysics tools can be used to simulate this problem over multiple relevant physics regimes and timescales.”

Featured image: The hydro simulation in Spheral that provided the basis for the analysis: 1 Megaton at a few meters standoff distance from a 100-meter diameter asteroid (with Bennu shape). Colors denote velocities. The legend is cm/us, which is equivalent to 10 km/s. © LLNL

Provided by LLNL

Spectrum Reveals Extreme Exoplanet is Even More Exotic (Planetary Science)

Considered an ultra-hot Jupiter – a place where iron gets vaporized, condenses on the night side and then falls from the sky like rain – the fiery, inferno-like WASP-76b exoplanet may be even more sizzling than scientists had realized.

An international team, led by researchers at Cornell, University of Toronto and Queen’s University Belfast, reports the discovery of ionized calcium on the planet – in high-resolution spectra obtained with Gemini North near the summit of Mauna Kea in Hawaii.

Hot Jupiters are so named for their high temperatures, due to proximity to their stars. WASP-76b, discovered in 2016, is a Jupiter-sized planet about 640 light-years from Earth, but so close to its F-type star, which is slightly hotter than the sun, that the giant planet completes one orbit every 1.8 Earth days.

The research results are the first of a multiyear, Cornell-led project, Exoplanets with Gemini Spectroscopy survey, or ExoGemS, that explores the diversity of planetary atmospheres.

“As we do remote sensing of dozens of exoplanets, spanning a range of masses and temperatures,” said co-author Ray Jayawardhana, the Harold Tanner Dean of the College of Arts and Sciences (A&S), and a professor of astronomy, “we will develop a more complete picture of the true diversity of alien worlds – from those hot enough to harbor iron rain to others with more moderate climates, from those heftier than Jupiter to others not much bigger than the Earth.

“It’s remarkable that with today’s telescopes and instruments, we can already learn so much about the atmospheres – their constituents, physical properties, presence of clouds and even large-scale wind patterns – of planets that are orbiting stars hundreds of light-years away,” Jayawardhana said.

The group spotted a rare trio of spectral lines in highly sensitive observations of the exoplanet WASP-76b’s atmosphere, published in the Astrophysical Journal Letters on Sept. 28 and presented on Oct. 5 at the annual meeting of the Division for Planetary Sciences of the American Astronomical Society.

“We’re seeing so much calcium; it’s a really strong feature,” said first author Emily Deibert, a University of Toronto doctoral student, whose adviser is Jayawardhana.

“This spectral signature of ionized calcium could indicate that the exoplanet has very strong upper atmosphere winds,” Deibert said. “Or the atmospheric temperature on the exoplanet is much higher than we thought.”

Since WASP-76b is tidally locked – in that one side of it always faces the star – it has a permanent night side that sports a relatively cool 2,400-degree Fahrenheit average temperature. Its day side, facing toward the star, has an average temperature at 4,400 degrees F.

Deibert and her colleagues examined the moderate temperature zone, on the planet’s limb between day and night. “The exoplanet moves fast on its orbit and that’s how we were able to separate its signal from starlight,” she said. “You can see that the calcium imprint on the spectra is moving quickly along with the planet.”

The ExoGemS survey – intended to study 30 or more planets – is led by Jake Turner, a Carl Sagan Fellow in NASA’s Hubble Fellowship program, who is in Cornell’s Department of Astronomy (A&S) and is also advised by Jayawardhana.

Astronomers continue to delve deeper to understand exoplanets – considered just a dream two decades ago. “Our work, and that of other researchers, is paving the way for exploring the atmospheres of terrestrial worlds beyond our solar system,” Turner said.

Other authors on the paper, “Detection of Ionized Calcium in the Atmosphere of the Ultra-Hot Jupiter WASP-76b,” include Ernst J. W. de Mooij of the Queen’s University Belfast; Luca Fossati of the Austrian Academy of Sciences; Callie E. Hood and Jonathan J. Fortney, both from University of California, Santa Cruz; Romain Allart of the University of Montreal; and David K. Sing of Johns Hopkins University. Cornellians included researchers Andrew Ridden-Harper and Laura Flagg, both in Jayawardhana’s group, and Ryan MacDonald. Portions of this research were funded by NASA.

Gemini North is part of the international Gemini Observatory, a program of National Science Foundation’s NOIRLab.

The image used with this story is provided as a courtesy of the European Southern Observatory.

Featured image: The fiery exoplanet WASP-76b – a so-called hot Jupiter, where it rains iron – may be hotter than previously thought. © ESO/M. Kornmesser

Provided by Cornell University