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Today’s solar system is a pretty tidy place.
The eight planets and their dozens of moons trace out orbits
that have remained basically the same for billions of years.
But astronomers believe things were not always this calm.
Models suggest that as the solar system formed four and a half billion years ago,
objects’ movements were way more erratic.
Unfortunately, proving that hypothesis has been difficult,
because the passage of billions of years has done away with most of the evidence.
But recently, scientists found a possible new clue.
In a paper published last week in the Astrophysical Journal Letters,
a team of astronomers describe two unusual asteroids
that may hint at our solar system’s tumultuous past.
The two asteroids, called 203 Pompeja and 269 Justitia,
orbit in the main asteroid belt that lies between the paths of Mars and Jupiter.
Both were actually discovered way back in the late 1800s,
but scientists are still tracking them today.
And during one recent observation of Pompeja,
researchers picked up on something they hadn’t noticed before:
Pompeja is far redder than other asteroids.
In fact, after searching through a catalog of past observations,
the astronomers could only identify one similarly red object
out of all the known asteroids in the main belt: Justitia.
Now, it’s not like these rocks are tomatoes floating through space.
To an astronomer, an asteroid is considered red if it reflects more light
from the red end of the spectrum than from the blue end,
and it’s considered blue if the reverse is true.
It can be a subtle distinction, but it has major implications.
For a solar system object, redness often indicates
the presence of organic material on its surface.
And it’s a trait that is almost always found
in the outer solar system, where objects are icy.
That’s because, as radiation from the Sun strikes ice,
it drives chemical reactions that end with the production of reddish organic matter.
So the process of forming organic molecules starts with an icy surface.
But in the inner solar system, it’s just not cold enough to make icy bodies.
And as a result, there isn’t much redness there either.
So astronomers are pretty certain that Pompeja and Justitia
are not native to the inner solar system.
They look much more like the objects in the neighborhood of Pluto.
And that raises the question of how Pompeja and Justitia
got to the asteroid belt, if they didn’t form there.
And the authors suspect that the answer
comes down to that turbulence in the early solar system.
If the giant outer planets moved in their orbits the way astronomers think they did,
their gravity would have pushed some
outer solar system objects inward, and launched others out.
And today, these two asteroids could be
direct evidence of that upheaval billions of years ago.
There's not yet enough evidence to seal the deal,
but it lends some weight to a hypothesis that has so far been really hard to prove.
In other solar system news, last week planetary scientists also shared
new insights about Jupiter’s largest moon, Ganymede.
Ganymede has long interested astronomers because
it’s pretty exceptional among moons.
It’s bigger than the planet Mercury, it has its own permanent magnetic field,
and its icy surface may hide an ocean of liquid water larger than any on Earth.
But that might not be all.
According to a new paper published in the journal Nature Astronomy,
Ganymede may also have an atmosphere containing water vapor.
Researchers drew this conclusion from observations
by NASA’s Hubble Space Telescope, which has been
keeping an eye on Ganymede since the 1990s.
In fact, last week’s paper incorporated Hubble data going
all the way back to its very first picture of the moon in 1998.
And it helped answer a question that that first picture raised.
See, this was an ultraviolet image that showed evidence of auroras on the moon.
Auroras are emissions of light that form as charged solar particles
following magnetic field lines collide into atmospheric gases,
such as oxygen, in the case of Ganymede.
So this was exciting, because it suggested that Ganymede had a magnetic field.
But as Hubble took more images, astronomers noticed that
the auroras always seemed brighter on one side of the moon than the other.
That asymmetry got them wondering what was going on.
At the time, they thought maybe it was because
there were more single oxygen atoms in some areas than others.
Atoms of oxygen emit light more efficiently at the wavelength
Hubble was observing than molecules of oxygen,
so maybe they were behind the difference in brightness.
But, in 2018, Hubble used a more sensitive instrument
to search for that atomic oxygen and came up mostly empty.
That sent the research team back to the drawing board.
And they found another possible answer: water vapor.
Like atomic oxygen, molecules of water emit light easily
at the wavelength Hubble was observing.
Now the question was just: Where could the water be coming from?
With a temperature always more than a hundred degrees below freezing,
Ganymede’s surface is solid ice.
But, in the hottest part of the day, it can get warm enough near the equator
for molecules of water to sublimate off the surface and into the atmosphere.
And when researchers examined this possibility,
they found that the regions with brighter auroras were
the same regions where water vapor was expected to be in the atmosphere.
So it looks like, not only is there liquid water inside Ganymede,
and solid water on top of Ganymede,
but there may also be gaseous water above Ganymede.
...Which is a lot of water.
And since water is a key ingredient to life as we know it,
this will be an interesting place to explore.
Fortunately, the European Space Agency is planning to do just that
with their mission named JUICE, which will reach Ganymede in the early 2030s.
From there, it will have a front-row seat for Ganymede’s aurora,
water-rich atmosphere, and who knows what else.
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