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Tuesday, November 30, 2021

Asteroid Material Returned to Earth Contains Surface Water

In May of 2003, the Japanese Space Agency launched the Hayabusa mission to an asteroid called 25143 Itokawa.  In November of 2005, the satellite touched down on the surface of Itokawa becoming only the second vehicle in history to land on an asteroid.  But there was more.  Hayabusa was intended not just to land on the asteroid, but to return a sample to Earth.  During the November 2005 landing, after a series of troubles, the ground lost contact with Hayabusa, not regaining contact until March of 2006.

The story of the mission reads like a case study in perseverance through difficulty after difficulty.  When the vehicle started back for Earth, it wasn't known for sure if any of the asteroid was successfully sampled.  The capsule potentially containing - perhaps - one gram of asteroid surface - re-entered the Earth's atmosphere on 13 June 2010.  The capsule experienced peak deceleration of about 25 G and heating rates approximately 30 times those experienced by the Apollo spacecraft. It landed via parachute near Woomera, Australia, and was recovered the next day.

Fast forward 11 years and we find that Itokawa contains significant amounts of water.  It has probably flown below most peoples' radars that NASA has confirmed (on almost the same time line) that there's plenty of water on the surface of the moon, as well.  One mission, crashing a used rocket stage into the surface near the south pole, revealed the lunar regolith (sort of lunar topsoil) was 5% water.  Considering the temperatures on the moon and the exposure to hard vacuum, how is this even possible?  Wouldn't water evaporate or, if ice, sublimate?  One of the explanations offered at the time was that the solar wind sends a steady stream of protons out in the Solar System, and these protons could interact with lunar material to produce water.  It was found the temperatures at the lunar poles, in the perpetual shadows where the rocket stage hit, are so far below freezing that sublimation is very slow and rare.

Here's where the story of Itokawa gets a little more interesting. 

Itokawa is what's called a "rubble pile," as it's made up of small fragments produced by collisions among asteroids and then slowly gathered together by gravity. Asteroids like this may have fragmented and re-formed multiple times over their history, and they could be composed of portions of more than one body.

A large, international research team took some of the fragments returned to Earth and subjected them to a variety of imaging techniques. The researchers determined that the outermost 40 to 180 nanometers [1-1/2 to 7 millionths of inch] of rock were transformed by their time in space due to bombardment by high-energy radiation. This region also had elevated levels of water and hydroxyl ions (OH-). This finding is consistent with the idea that the water was produced by the interaction between protons in the solar wind and silicate-rich materials in the rocks themselves.

Based on the typical depth of the material that was transformed by the solar wind, the researchers could calculate the amount of water in particles of different sizes. And while there's very little here individually, Itokawa has a lot of small, dust-like particles, which have a high surface area relative to their volume. So it all adds up to an estimated 20 liters of water in every cubic meter of the powdery regolith on the asteroid.


Itokawa - ISAS, JAXA photo 

People who study asteroids like this believe that on a rubble pile like Itokawa, all of the dust gets cycled between the surface and interior as the asteroid has collisions over the millennia, and as a body like Itokawa orbits, it sweeps up more dust which has been hit by high energy solar wind.  So even if something is now buried in the interior, it almost certainly was exposed to the solar wind in the past.

It also presents us with somewhat of a way out of a problem.  For years, we've been told that Earth's oceans are probably the result of a massive collision that formed the moon.  Over time, chunks of proto-Earth fell back down, bringing significant amounts of water.  The problem with that is it doesn't agree with observations of the isotopes of the water found on Earth versus other planets.  

But if we go by the elements in our crust, the bodies that arrived on Earth have a different ratio of hydrogen isotopes from the waters in our ocean. Put differently, the oceans have water that (in isotope terms) is somewhat lighter than the water found in the asteroids that have a composition similar to Earth's. The solar wind, in contrast, has hydrogen isotopes that are overall lighter than what we see in our oceans. So the researchers propose that the solar wind has indirectly helped fill our planet's oceans by producing water on dust particles that eventually fell to Earth.

The interesting side here is that the process continues today.  An estimated 30,000 tonnes of dust grains fall from space each year. And these tiny particles will have the highest amount of water per mass of anything exposed to the solar wind. That's not much water in a given year, but it adds up over time, year after year, after thousands, millions or billions of years. 

 

 

7 comments:

  1. All I can think of is Andromeda Strain when I read about this.

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    1. The Strain, in the book, was supposed to be some alien species "Hello out there" message. Ooops.

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  2. Very interesting. Always thought the big-arsed collision theory had some flaws to it.

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  3. I'd love to see the extrapolation of the water-from-space accretion theory.

    Just spitballing, but I suspect all the time from the Big Bang to now wouldn't account for even a fraction of Earth's water content, and then we're back to aliens, or Mickey's Magical Bucket Brigade from The Sorcerer's Apprentice.

    Kind of like how you get from nothing to a single cell, and from a cell to a human eyeball, it's probably mathematically impossible, without recourse to supernatural miracles.

    And then all the underlying scientism becomes just another bad religion, without any visible underpinning.

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    1. And all that water: isn't going to mess with the Earth's mass, over time, as well?
      Is this why I'm heavier now than when I was 20?
      Does it explain rising ocean levels?

      So many good questions, that deserve the sort of proper sarcasm Elijah poured out on the prophets of Baal...

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    2. I'd love to see the extrapolation of the water-from-space accretion theory.

      Just spitballing, but I suspect all the time from the Big Bang to now wouldn't account for even a fraction of Earth's water content, and then we're back to aliens, or Mickey's Magical Bucket Brigade from The Sorcerer's Apprentice.


      For what it's worth, I didn't read that as either water from when the moon was blasted out of the Earth or water from dust accretion. I read it as both. Either way it changes the ratio of the abundance of the various hydrogen isotopes.

      I also have a tendency to not go along with those one or the other arguments like "Nature vs. Nurture" and don't see why it can't be a bit of both.

      And all that water: isn't going to mess with the Earth's mass, over time, as well?
      Is this why I'm heavier now than when I was 20?
      Does it explain rising ocean levels?


      It's not just water. Exactly how many pounds of space dust fall on the earth every day is one of those things that everyone talks about but is just plain damned hard to know. These guys say "Although most of these meteorites weigh just a few grams and on rare occasions surpass one kilogram, according to NASA calculations every day Earth is bombarded with more than 100 tons of dust and sand-sized particles. That is 36,500 tons of space matter landing on Earth every year! Most of the stuff burns up in the Earth’s atmosphere and just a small fraction makes it to the surface." Since 70% of the Earth's surface area is water, that's where most of it ends up. It adds mass to the Earth, though, I just don't think anyone knows a number well enough to estimate how much.

      There are projects on places like Hackaday that have you drag rare earth magnets along the sides of roads to collect meteorites because the rains will wash the dust into those low places.

      Kind of like how you get from nothing to a single cell, and from a cell to a human eyeball, it's probably mathematically impossible, without recourse to supernatural miracles.

      The Information Problem. Natural selection, selection pressure and all those hand-waving answers apply well to things that are already sophisticated but really fall down in the case of the first something from nothing. A eucaryotic cell from non-living things is such an incredible increase in complexity, analogies fall apart. It's like the creating a nuclear power plant from a bunch of teeter totters.

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