Astronomy - USA (2021-12)

8 ASTRONOMY • DECEMBER 2021

QUANTUM GRAVITY

The Geminid meteor shower is

best known for the reliable show

it puts on during the winter holiday

season. But the event is also unique

because it stems not from a comet but

from an asteroid: 3200 Phaethon.

Phaethon’s true nature has puzzled

astronomers for more than 10 years,

ever since they discovered that it

brightens dramatically and expels dust

when it nears the Sun. That kind of

behavior is usually reserved for comets.

When a comet’s path brings it through

the inner solar system, the Sun warms

and vaporizes the ices on its surface,

creating a bright tail extending up to

millions of miles behind it. Escaping

vapor can also dislodge some of the

comet’s dust and rock, which is usually

the debris that fuels meteor showers.

But asteroids like Phaethon are made

of rock and metal, with little or no ice,

leaving scientists to search for a differ-

ent explanation for Phaethon’s comet-

like behavior. In a study published Aug.

16 in The Planetary Science Journal,

researchers reported that they may have

finally uncovered the culprit: sodium.

A FIZZY APPROACH

Aptly named after the son of the Sun

god in Greek mythology, Phaethon has

a 524-day orbit that brings it within just

0.14 astronomical units (AU; where 1

AU is the average distance between the

Earth and Sun) of our star, well within

Mercury’s orbit. At that distance, the

Sun heats the asteroid’s surface to about

1,390 degrees Fahrenheit (750 degrees

Celsius). Any water, carbon dioxide, or

carbon monoxide ices just under the

surface would have evaporated long ago,

but sodium — an element abundant in

asteroids — still could be fizzling just

under its surface.

A steady bubbling of sodium would

explain why Phaethon brightens as it

approaches the Sun, as the resulting gas

and dust would scatter more sunlight. It

could also explain how the fuel for the

Geminids breaks off from Phaethon.

“Asteroids like Phaethon have very

weak gravity, so it doesn’t take a lot of

force to kick debris from the surface

SODIUM MAY MAKE ASTEROID

PHAETHON FIZZLE

Bubbly sodium could explain this

asteroid’s cometlike behavior.

SALTY BUBBLES. As Phaethon speeds closer

to the Sun, our star heats it. This causes the

sodium just under the surface to vaporize,

brightening the asteroid and leaving a trail of

loose debris in its wake, as shown in this

artist’s concept. NASA/JPL-CALTECH/IPAC