Models and lab tests suggest the asteroid could be venting sodium vapor as
it orbits close to the Sun, explaining its increase in brightness.
As a comet zooms through the inner solar system, the Sun heats it, causing
ices below the surface to vaporize into space. The venting vapor dislodges
dust and rock, and the gas creates a bright tail that can extend millions of
miles from the nucleus like an ethereal veil.
Whereas comets contain lots of different ices, asteroids are mainly rock and
not known for producing such majestic displays. But a new study examines how
near-Earth asteroid Phaethon may in fact exhibit cometlike activity, despite
lacking significant quantities of ice.
Known to be the source of the annual Geminid meteor shower, the
3.6-mile-wide (5.8 kilometer-wide) asteroid brightens as it gets close to
the Sun. Comets typically behave like this: When they heat up, their icy
surfaces vaporize, causing them to become more active and brighten as the
venting gases and dust scatter more sunlight. But what is causing Phaethon
to brighten if not vaporizing ices?
The culprit could be sodium. As the new study’s authors explain, Phaethon’s
elongated, 524-day orbit takes the object well within the orbit of Mercury,
during which time the Sun heats the asteroid’s surface up to about 1,390
degrees Fahrenheit (750 degrees Celsius). With such a warm orbit, any water,
carbon dioxide, or carbon monoxide ice near the asteroid’s surface would
have been baked off long ago. But at that temperature, sodium may be fizzing
from the asteroid’s rock and into space.
“Phaethon is a curious object that gets active as it approaches the Sun,”
said study lead Joseph Masiero, a scientist at IPAC, a research organization
at Caltech. “We know it’s an asteroid and the source of the Geminids. But it
contains little to no ice, so we were intrigued by the possibility that
sodium, which is relatively plentiful in asteroids, could be the element
driving this activity.”
Asteroid-Meteor Connection
Masiero and his team were inspired by observations of the Geminids. When
meteoroids – small pieces of rocky debris from space – streak through
Earth’s atmosphere as meteors, they disintegrate. But before they do,
friction with the atmosphere causes the air surrounding the meteoroids to
reach thousands of degrees, generating light. The color of this light
represents the elements they contain. Sodium, for example, creates an orange
tinge. The Geminids are known to be low in sodium.
Until now, it was assumed that these small pieces of rock somehow lost their
sodium after leaving the asteroid. This new study suggests that the sodium
may actually play a key role in ejecting the Geminid meteoroids from
Phaethon’s surface.
The researchers think that as the asteroid approaches the Sun, its sodium
heats up and vaporizes. This process would have depleted the surface of
sodium long ago, but sodium within the asteroid still heats up, vaporizes,
and fizzes into space through cracks and fissures in Phaethon’s outermost
crust. These jets would provide enough oomph to eject the rocky debris off
its surface. So the fizzing sodium could explain not only the asteroid’s
cometlike brightening, but also how the Geminid meteoroids would be ejected
from the asteroid and why they contain little sodium.
“Asteroids like Phaethon have very weak gravity, so it doesn’t take a lot of
force to kick debris from the surface or dislodge rock from a fracture,”
said Björn Davidsson, a scientist at NASA’s Jet Propulsion Laboratory in
Southern California and a co-author of the study. “Our models suggest that
very small quantities of sodium are all that’s needed to do this – nothing
explosive, like the erupting vapor from an icy comet’s surface; it’s more of
a steady fizz.”
Lab Tests Required
To find out if sodium turns to vapor and vents from an asteroid’s rock, the
researchers tested samples of the Allende meteorite, which fell over Mexico
in 1969, in a lab at JPL. The meteorite may have come from an asteroid
comparable to Phaethon and belongs to a class of meteorites, called
carbonaceous chondrites, that formed during the earliest days of the solar
system. The researchers then heated chips of the meteorite to the highest
temperature Phaethon would experience as it approaches the Sun.
“This temperature happens to be around the point that sodium escapes from
its rocky components,” said Yang Liu, a scientist at JPL and a study
co-author. “So we simulated this heating effect over the course of a ‘day’
on Phaethon – its three-hour rotation period – and, on comparing the
samples’ minerals before and after our lab tests, the sodium was lost, while
the other elements were left behind. This suggests that the same may be
happening on Phaethon and seems to agree with the results of our models.”
The new study supports a growing body of evidence that categorizing small
objects in our solar system as “asteroids” and “comets” is oversimplified,
depending not only on how much ice they contain, but also what elements
vaporize at higher temperatures.
“Our latest finding is that if the conditions are right, sodium may explain
the nature of some active asteroids, making the spectrum between asteroids
and comets even more complex than we previously realized,” said Masiero.
The study, titled “Volatility of Sodium in Carbonaceous Chondrites at
Temperatures Consistent with Low-Perihelia Asteroids,” was published in The
Planetary Science Journal on August 16, 2021.
Reference:
Volatility of Sodium in Carbonaceous Chondrites at Temperatures Consistent
with Low-perihelion Asteroids by Joseph R. Masiero, Björn J. R. Davidsson,
Yang Liu, Kelsey Moore and Michael Tuite, 16 August 2021, The Planetary
Science Journal.
DOI: 10.3847/PSJ/ac0d02
Tags:
Space & Astrophysics