The surface of Ryugu as Hayabusa2 made its approach.
The following series of events is not fiction: fly a probe to an asteroid, bounce off that asteroid while grabbing a piece of it, and fling that sample back to Earth. This series of activities is exactly what JAXA’s Hayabusa2 mission is in the middle of doing. (And by the way, NASA’s OSIRIS-REx mission is working on the exact same thing, just on a timeline a year behind Hayabusa2.)
Hayabusa2 grabbed its first sample from the surface of a near-Earth asteroid named Ryugu in February 2019. (A second sample was collected in July after it blasted a small crater to expose sub-surface material.) A new study published by the team this week details what the probe saw at the sampling site—including remarkable video of the touchdown itself.
As the probe touched Ryugu’s surface, it fired a small projectile into it, catching some of the debris in an open “sampling horn” before bouncing away and re-establishing its orbit. These samples will return to the Earth later this year, but for now, the scientists are establishing what we know about the areas they came from.
Ryugu has a somewhat angular shape and is a little less than a kilometer across, and its equator bulges out in a distinctive ridge that makes the asteroid resemble a spinning top. Up close, it looks like a rubble pile. But there are some interesting color patterns within that jumble.
Ryugu’s surface is a mix of dark material with slightly reddish or bluish tinges. The blue dominates the poles and equator, and the red dominates the mid-latitude regions in between. But zoom in close and you’ll see that both are represented.
Boulders are primarily bluish—including where you see one cracked open—with reddish patches on the surface. And the spot that Hayabusa2 sampled was initially bluish but was coated in red after the dust settled. Putting everything together, the reddish color seems to be the mark of alteration or weathering—like rust forming on the surface of steel.
Oxidizing isn’t a thing in the vacuum of space, but there are other processes that can create this appearance. Interactions with the charged particles of the solar wind can drive chemical reactions on the surface of a body, for example, but that tends to form a microscopic skin—Ryugu’s layer appears thicker. So the conclusion the researchers draw is that this is actually the product of a closer pass with the Sun earlier in Ryugu’s life. That heat metamorphosed a surface layer of the asteroid, which was then pulverized and redistributed by small impact events. And given that the poles and the equator are topographic highs, that material would tend to settle into the mid-latitude lowlands
The team can actually guess at the timing of Ryugu’s flirtation with the Sun by examining its impact craters. By looking at how craters and their ejected debris overlap, you can work out which are older and which are newer. The inside of the older craters are reddish, but there are younger craters that still appear bluish (more evidence that the reddening was a past episode rather than an ongoing and recent process).
Based on established estimates for the average rates of impact collisions over time, the researchers calculate that the reddening episode occurred somewhere between 300,000 years and 8 million years ago (depending on when Ryugu left the asteroid belt for its near-Earth orbit). They also calculate that Ryugu probably formed (from the breakup of some larger body) around 9 million years before that. That’s quite recent compared to some well-studied breakups of major asteroids, which implies Ryugu is a grandchild, at least—the product of progressively smaller breakups.
The team is hopeful that Hayabusa2 caught some of the reddened material as well as the bluer rock in its sample grab, so we’ll hopefully get a much closer look at this stuff when the shipment comes in later this year.
If you simply can't wait that long for another asteroid sampling fix, NASA’s OSIRIS-Rex probe is currently eyeballing its target on the asteroid Bennu. Its touch-and-go sampling lunge is set for August of this year.
TOKYO -- Researchers in Japan have discovered dark red "sunburned" areas on the surface of the asteroid Ryugu, suggesting it previously orbited much closer to the sun.【
The discovery, made by a team of researchers from the Japan Aerospace Exploration Agency (JAXA) and the University of Tokyo, was published May 8 in the U.S. journal Science.
JAXA's space probe Hayabusa2, which is believed to have successfully collected material samples from Ryugu, observed sunlight reflecting off the surface of the asteroid from above, and found areas that had turned a blackish red color, widely dispersed across the surface. The surface of craters that were relatively new, however, did not show such coloring. This and other factors led the team to conclude it is highly likely the asteroid was exposed to extreme solar heat for a short period in the past.
Tomokatsu Morota, an associate professor at the University of Tokyo and a member of the research team, said the traces of charring on the surface of Ryugu suggest that the asteroid was exposed to a high temperature of around 600 to 800 degrees Celsius. Although the path of Ryugu's orbit currently lies between Earth and Mars, the team estimates that the asteroid orbited between the sun and Mercury at a certain point between 300,000 and 8 million years ago. The trajectory of an asteroid's orbit can be changed due to gravitational forces from massive planets like Jupiter.
When Hayabusa2 collected rock samples from the surface of Ryugu in February 2019, it landed in a part that contained both the material altered by the sun and the original substance, and it is likely that the probe was able to collect both. Researchers look forward to uncovering the effects of solar heat on the asteroid by closely examining the samples when they are brought back to Earth.
Seiji Sugita, a planetary science professor at the University of Tokyo who is also a member of the team, commented, "Although it has been known that the orbits of asteroids can largely change, this is the first time that material evidence of this has been obtained, and we are astonished."
Quelle: The Mainichi
JAXA: Asteroid Ryugu may have shifted orbit
Japan's space agency says it has found that the asteroid Ryugu may have orbited between the Sun and Mercury in the distant past. That's different from its current orbit, which passes between Earth and Mars, but not between the Sun and Mercury.
The Japan Aerospace Exploration Agency, or JAXA, says it analyzed images of the asteroid taken by a camera on its probe Hayabusa2.
JAXA says much of the sand and rock on the surface of Ryugu turned red when exposed to heat of more than 600 degrees Celsius. It says such high temperatures are inconceivable even when the asteroid is at its closest point to the Sun in its current orbit.
JAXA says the sand and rock in a recent crater on Ryugu is blue, as it has not been heated to high temperatures. It says this is because the asteroid's orbit moved farther away from the Sun in the relatively recent past.
JAXA says these findings suggest that Ryugu's shortest distance from the Sun about 300,000 to 8 million years ago was roughly one-third to one-fifth the current one.
JAXA adds that Ryugu's orbit at the time may have passed between the Sun and Mercury before shifting to the present trajectory. Mercury is the closest planet to the Sun in our solar system.
JAXA says it remains unknown what caused the change in Ryugu's orbit, but that gravity could be one reason.
University of Tokyo Associate Professor Morota Tomokatsu, who conducted the analysis, says he thinks this is the first time a change in an asteroid's orbit has been studied geologically.
He says he is looking forward to examining sand and other particles to be brought back by Hayabusa2.
Hayabusa2 is now on its way back to Earth after completing its mission on Ryugu last November. The asteroid was about 300 million kilometers from Earth when the explorer reached it in 2018.
Thermal images from the TIR suggest
Ryugu boulders may be "fluffy
The first photographs from Hayabusa2 of the surface of asteroid Ryugu revealed a treacherous landscape, with large boulders carpeting the asteroid to form a rugged topology. Yet when the spacecraft turned on its thermal infrared imager (TIR), it saw a surprisingly homogenous surface in the thermographic images.
How effectively a planetary surface conducts and stores heat is measured by a property known as the thermal inertia. A surface with a high thermal inertia will gain and lose heat slowly, resulting in smaller temperature differences between night and day and summer and winter.
Thermal inertia depends on composition and also density. Boulders like those scattered over Ryugu are expected to be dense and should therefore have a high thermal inertia, changing temperature more slowly than the surrounding material. On the dayside of Ryugu, this would make the boulders stand out on the TIR image as cold spots. But this was not what was seen.
A paper published in the journal, Nature, this week (March 16, 2020) led by Tatsuaki Okada (ISAS & University of Tokyo) revealed the first global thermal image of an asteroid during a complete rotation, in addition to high resolution images taken close to the surface. This data captured by the TIR on Hayabusa2 showed little temperature difference between the majority of boulders and the surrounding material on the surface of Ryugu, pointing to a similar thermal inertia value of about 300 Jm-2s-0.5K-1 (300 tiu).
Left: "Ryugoid", the expected thermal distribution of the asteroid before arrival. The assumed thermal inertia was 1600 tiu for the boulders and 300 tiu for the regolith. Right: Observed Ryugu, from the global TIR observations at an altitude of about 5km (~4.5 m / pixel). The derived thermal inertia of both boulders and surroundings is about 300 tiu.
The implication is that Ryugu is covered with low-density, porous boulders that are surrounded by similarly porous fragments greater than 10 cm in size. Since the initial expectation was the asteroid should have dense boulders embedded in a layer of fine regolith, this is a startling discovery. The reverberations have important implications for how planets are formed.
Ryugu is an example of a C-type or Carbonaceous asteroid. C-types are primitive asteroids consisting of material that has experienced little change since the formation of the Solar System 4.6 billion years ago. This makes them important time capsules for the early days of planet formation, revealing the material that built the Solar System.
It is suspected that the carbonaceous chondrite meteorites found on Earth are fragments of C-type asteroids. The accessibility of meteorites makes these much studied objects for understanding the Earth’s earliest days. But these meteorites are significantly less porous that the majority of rocks on Ryugu.
The close-up TIR images captured during the Hayabusa2 descent operations reveal a few colder boulders with thermal inertia values corresponding to 600 - 1,000 tiu; a value more typical of the carbonaceous chondrites. But these dense additions are in the minority on the asteroid surface.
Cold spots discovered during close-up thermal images. These images were captured during the TD1-R1-A operation on the 15 October, 2018 from an altitude of 78.8m. The temperature profile along the yellow line is shown in the adjacent panel. The cold spots are colder by more than 20K, indicated a dense and consolidated boulder with high thermal inertia. Only a few cold spots (boulders) are found on the surface of Ryugu: other boulders have the same temperature as their surroundings. Scale bar is 5m in panel a and 1m in panel c.
One possibility is that C-type asteroids may have a far less consolidated nature than expected, consisting of a loose conglomerate of fluffy dust and pebbles. Ryugu would then be a fragment from such a porous parent, with a handful of denser boulders that were formed in the deepest part of the parent asteroid, or even had a separate origin and were delivered to the asteroid during meteoritic impacts.
These fluffy asteroids would struggle to survive the high temperatures and shockwaves experienced during a descent through the atmosphere of Earth, resulting in meteorites that consist of only the dense minor components of the asteroid.
Formation scenario for Ryugu from a porous parent body. (1) Formation begins with the fluffy dust in the Solar nebula. (2) Porous planetesimals were formedr through accretion of dust or pebbles. (3) The parent body of Ryugu may have remained porous owing to a low degree of consolidation. A clear boundary at the inner core is illustrated here, but a gradual increase of consolidation by depth might also have occurred. (4) Impact fragmentation of the parent body, with a few large fragments forming the boulders on Ryugu. (5) Fragments re-accrete to form Ryugu, with porous boulders and sediment on the surface and a small number of dense boulders originating from the parent’s inner core. (6) Re-shaping during a rapid rotation phase created a double-top-shape.
If this fluffy composition was typical of the material that built the planets, then the growth process that took the Solar System from dust to planets might be very different from models that assume a more rigid building material. For example, the result of impacts between porous planetesimals may prove to result more often in coagulation than fragmentation, allowing for a swifter growth and altering the time-scale for planet formation.
It is also possible that Ryugu's porosity may be due to a composition of completely different materials to the carbonaceous chondrite meteorites. For example, the asteroid could consist of carbon-rich material similar to that discovered on comet-67P/Churymov-Gerasimenko. This will ultimately be resolved when Hayabusa2 returns to Earth at the end of 2020 with samples of both surface and subsurface material.