New observations suggest the main-belt asteroid Psyche is an intriguing world, possibly one with volcanic flows of iron on its surface.

The massive asteroid 16 Psyche, pictured here in an artist's illustration, is the subject of a new study by Tracy Becker, who observed the object at ultraviolet wavelengths.
Courtesy of Maxar / ASU / P. Rubin / NASA / JPL-Caltech

Walking on asteroid 16 Psyche might feel like walking on the rusty hull of a huge space freighter in a science fiction movie. According to a team led by Tracy Becker (Southwest Research Institute), iron and iron oxides make up parts of the asteroid’s surface.

NASA has picked the main-belt asteroid as the target for its eponymous Psyche mission, set for arrival in 2026, because the rock’s high density suggests it’s basically a lump of metals. The rock is 220 kilometers (140 miles) wide and is classified as M-type, where M stands for metallic.

“The idea is that it’s the iron-and-nickel core of a protoplanet,” says Becker, who presented her results at the virtual meeting of the Division of Planetary Sciences (DPS) on October 26th. “Subsequent collisions would have torn most of the crust and mantle away, revealing the planetary core.”

The high radar reflectivity of Psyche, as measured by the Arecibo telescope, had already suggested the presence of metals on its surface. Other observations also point to the presence of hydrated rocks – maybe leftovers from the stripped mantle. Now, Becker’s ultraviolet observations with the Hubble Space Telescope confirm surface iron and signs of oxidation. At least 10% of Psyche’s surface would need to be iron to explain the UV signal, the team reports in the December Planetary Science Journal.

The new Hubble observations seem to support the view that Psyche is a planetary core. But Lauri Siltala (University of Helsinki) is not so sure. At the same DPS meeting, he presented a new mass measurement for the asteroid. Combining that result with the latest size and shape estimates based on observations from the European Southern Observatory’s Very Large Telescope, he calculates a density of just 3.3 grams per cubic centimeter – much lower than expected for an iron-nickel body and lower even than the density of iron meteorites.

Artist's concept of the Psyche spacecraft, which will conduct a direct exploration of an asteroid thought to be a stripped planetary core.
SSL / ASU / P. Rubin / NASA / JPL-Caltech

Siltala precisely measured the way in which Psyche’s gravity deflects the paths of small main-belt asteroids when they pass by. The technique has been used before to “weigh” the body, but Siltala says, “most earlier studies used a significantly lower number of ‘test asteroids’ than we did in our recent study. Therefore, I believe our results to be the most realistic.”

If the relatively low density is correct, Psyche might be a mix of metals and rocks after all, albeit more iron-rich than most asteroids. A process known as ferrovolcanism could force the core material of a differentiated and solidifying planetesimal into the overlying rocky mantle, according to a study that Brandon Johnson (Purdue University, West Lafayette, Indiana) and colleagues published in the September 16, 2019 Nature Astronomy. If the mantle is relatively thin, iron could even erupt on the surface, in which case Becker may have detected metallic lava flows.

Becker argues that a lower density doesn’t necessarily rule out the planetary core theory. “The core could be relatively porous, or mixed up with other material,” she says. The Psyche mission, to be launched in 2022, will still provide planetary scientists with a unique window on the interiors of planets, she believes. “Anyway, we need to get close to understand it fully.”


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Andrew James

October 27, 2020 at 8:06 pm

Ching! Ching! Ching! Solar System papers these days are more about positioning for commercial mining rights than any science. This isn't an allegation of misconduct here, rather an observation. (A web search under 'psyche asteroid mining' or '16 psyche asteroid mining' says more than me.) NASA and its Science Mission Directorate contracts have strict disclosure agreements, inasmuch that you don't really know the true depth of the research. (These two Psyche paper mentioned is among them.) Whilst not disclosing commercial or governmental information to competing foreign countries is necessary, it sort of rubs against astronomy's purity and gift of equal universal access for all humanity. This one agreed concept IMO becomes more nebulous everyday. Truth or misinformation become powerful tools to achieve undisclosed goals. e.g. By underplaying the true amount of precious metals - Asteroid having"...density of just 3.3 grams per cubic centimeter"

The Planetary Science Journal is noticeably new by the AAS, and is at least open access, whose aim is "We look forward to highlighting the research and exploration into planets and exoplanets that scientists will pursue and uncover in the twenty-first century." (See "Editorial: Introducing the Planetary Science Journal." in the PSJ site.

Lastly. It would be nice if the policy of 'AAS Sky Publishing' disclosed more clearly article affiliations to their stories. e.g. Seemingly promoting the Planetary Science Journal.

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Dennis Kamenitsa

October 28, 2020 at 6:43 pm

Studies of this sort are fascinating and will undoubtedly have long term positive effects that we can't even imagine today. The possibility of the exploitation of mineral wealth will lead to a much more through investigation of the asteroids. This will have the triple advantage of much more in-depth study of the basic structures themselves, a more through cataloging of their abundance and motions (to alert us to possible future collisions with earth), along with the long term possibility of vast wealth and totally new industries for those able and willing to take the physical and financial risks.

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Andrew James

October 29, 2020 at 7:11 pm

Um. "...exploitation". But to whose benefit? The big space players are modifying the rules to position themselves to take advantage by their technological prowess. The Heavens until now had a wonder and purity because humankind goal was to understand it, not to plunder it. All I see are poor countries like Niger or Central African Republic or Chad being left behind, whilst the richer countries or corporations reap the resources for themselves. Humankind has just one final chance here to benefit the peoples of the Earth and the generations to come. Yet we are seeing this dream torn asunder even as we speak, by the same old ways. We've stuffed up the Earth, now we want to stuff up the Heavens too.

Excuses as "to alert us to possible future collisions with earth" just justifies the 'exploitation' by divisively hiding the true agenda and pretending it is a sideline issue. It's superficial, IMO.

As the goddess Urania's children, and among her acolytes, we have a duty to guard her honour and realm. We need speaking out loud with a very clear and eloquent voice - just like dear Calliope. I'm a little disappointed reading statements purveying just usual economic packsaddling.

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October 30, 2020 at 4:20 pm

Wow, that's interesting about the low density. I would have thought that was a pretty firmed up piece of data; after all the metallicity was the reason Psyche was targeted in the first place. Time will tell...

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Andrew James

October 31, 2020 at 6:27 pm

M Viikinkoski in 2018 says 3.99±0.26 g cm^-3, iron meteorites ∼7.8 g cm^-3. Even Google prompts 4.5 g/cm³. D.F. Lupishko in 2006 says: "(1.4 and 1.8 g/cm3) seem to be too low for asteroids of this type." NASA's Lindy Elkins-Tanton July 11, 2018, 'Planning to measure an unknown asteroid.' "Well, we think it’s metal, mostly nickel and iron. Several density estimates of Psyche have been made, including 4,500±1400 kg m^-3 [1], 6,980±580 kg m^-3 [2], 6,490±2,940 kg m^-3 [3,4], and 7,600 ± 3,000 kg m^-3 [5]." [5] is inIcarus 195, 184–205, 2008.

This verifies "Combining that result with the latest size and shape estimates based on observations from the European Southern Observatory’s Very Large Telescope, he calculates a density of just 3.3 grams per cubic centimeter – much lower than expected for an iron-nickel body and lower even than the density of iron meteorites." is plainly suspicious.

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