Asteroids are small clumps of rock orbiting the sun. But they likely weren’t always that way. Many may have started out as little more than mud. That’s the conclusion of a new analysis.
Most of what’s known about the first solid bodies in the solar system comes from meteorites. These are lumps of rock-and-metal from space that periodically collide with Earth. Researchers typically study a type of these meteorites known as carbonaceous chondrites (KAR-bon-AY-shus KON-dytes). Scientists have suspected these rocky chunks initially were part of asteroids.
Interestingly, their chemical recipe is almost the same as the sun's. After removing all of the lightweight hydrogen and helium from the sun, you'd be left with heavier elements, such as carbon and sodium. And they’d exist in the same ratios seen in these bits of space rock. Some of these elements are radioactive. That means they are unstable, constantly shedding heat and some subatomic particles through a process known as radioactive decay.
The similarity between the recipe of the sun and these meteorites is key. It suggests the first asteroids evolved directly from the disk of gas and dust that existed just before the planets formed. That common recipe also suggests these rocks formed in the presence of water and at relatively low temperatures. How low? Around 150° Celsius (302° Fahrenheit).
It’s hard to explain all of these traits at once. The original asteroids may have been bigger than about 20 kilometers (12 miles) across. There’s no reason to think they weren’t. If so, then the decay of radioactive elements inside them would have made the rock hotter than 150 °C (302 °F).
Some planetary scientists have suggested that the asteroids were porous. That means they had lots of holes, somewhat resembling a sponge. Like a primitive plumbing system, water flowing through those holes would have cooled the rock. But that water also should have pulled some elements out of the rock. Afterward, the rock's recipe no longer would be sunlike.
“It was a paradox,” notes Philip Bland. He is a planetary scientist at Curtin University of Technology in Perth, Australia.
Bland was running computer models on how those original globs of ice and dust could have compressed into solid rock. That’s when it hit him: What if they weren’t rock at all?
“At that moment, nothing has happened to force those grains together to turn it into a rock,” he says. This was just something everyone had assumed would occur.
Bland reasoned that before they had a chance to turn to rock, heat from radioactive decay in these globs of dust would have melted any ice. The resulting celestial body would then become an enormous glob of mud. And that mud could hold small rocky particles. Now the lumps wouldn’t be stripped of some elements. And these early asteroids could be any size — and relatively cool.
Bryan Travis works at the Planetary Science Institute in Los Alamos, N.M. He and Bland used a computer to simulate how these mud balls would have evolved. And currents driven by convection would have played a role.
Convection currents form as a result of the rising and falling of material in a fluid or gas due to uneven temperatures. Such currents move liquid rock within Earth’s blisteringly hot mantle.
The team’s new simulations showed that these currents also would develop in many of the mud balls. The currents would help to transfer heat into space. After several million years, a ball would have hardened completely, yielding the asteroids seen today.
“It nails the paradox,” Bland says. He and Travis published this assessment July 14 in Science Advances.
The new model could solve several puzzles about the recipe of meteorites found on Earth. It also might explain why asteroids are different from comets, he says. Comets are more icy than rocky. They tend to hang out farther from the sun and may simply have formed later in the solar system’s history. If true, then there also would have been less radioactive heat available to melt them.
The new model also showed differences in the asteroids. Some would be muddy all the way through. Others would develop cores of larger grains, with a great mud ocean atop them.
That last scenario could explain not just asteroids but also bodies like the dwarf planet Ceres (SEER-eez). It is the largest object in the asteroid belt (a ring of rock that circles the sun between the orbits of Mars and Jupiter). Researchers recently studied Ceres with NASA’s Dawn spacecraft. It showed that Ceres has a rocky core.
Ceres may once have had an ocean that has since evaporated, says Edward Young. He is a planetary scientist at the University of California, Los Angeles, who was not involved in the new study. Evaporation on Ceres may have been something like what the team is describing for all early asteroids, he says.
Planetary scientist Brandon Johnson thinks the new model of mud-ball asteroids will inspire more research. Johnson works at Brown University in Providence, R.I. “I’m interested in it myself,” he says. “It makes a lot of sense and paints a clear picture of what might have been happening.”
asteroid A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.
Ceres The largest known asteroid orbiting the sun. It sits 270 million kilometers (nearly 168 million miles) from Earth in the asteroid belt between Mars and Jupiter. At about 1,000 kilometers (600 miles) across, Ceres is so big that it is classified as a dwarf planet. In 2014, astronomers found it spewing water from two places on its surface.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O.
comet A celestial object consisting of a nucleus of ice and dust. When a comet passes near the sun, gas and dust vaporize off the comet’s surface, creating its trailing “tail.”
convection The rising and falling of material in a fluid or gas due to uneven temperatures. This process occurs in the outer layers of some stars.
core (in geology) Earth’s innermost layer. Or, a long, tube-like sample drilled down into ice, soil or rock. Cores allow scientists to examine layers of sediment, dissolved chemicals, rock and fossils to see how the environment at one location changed through hundreds to thousands of years or more.
decay (for radioactive materials) The process whereby a radioactive isotope — which means a physically unstable form of some element — sheds energy and subatomic particles. In time, this shedding will transform the unstable element into a slightly different but stable element. For instance, uranium-238 (which is a radioactive, or unstable, isotope) decays to radium-222 (also a radioactive isotope), which decays to radon-222 (also radioactive), which decays to polonium-210 (also radioactive), which decays to lead-206 — which is stable. No further decay occurs. The rates of decay from one isotope to another can range from timeframes of less than a second to billions of years.
dwarf planet One of the solar system’s small celestial objects. Like a true planet, it orbits the sun. However, dwarf planets are too small to qualify as true planets. Prime examples of these objects: Pluto and Ceres.
element (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.
evolve (adj. evolving) To change gradually over generations, or a long period of time. In living organisms, the evolution usually involves random changes to genes that will then be passed along to an individual’s offspring. Nonliving things may also be described as evolving if they change over time. For instance, the miniaturization of computers is sometimes described as these devices evolving to smaller, more complex devices.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
helium An inert gas that is the lightest member of the noble gas series. Helium can become a solid at -272 degrees Celsius (-458 degrees Fahrenheit).
hydrogen The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.
Jupiter (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).
mantle (in geology) The thick layer of the Earth beneath its outer crust. The mantle is semi-solid and generally divided into an upper and lower mantle.
Mars The fourth planet from the sun, just one planet out from Earth. Like Earth, it has seasons and moisture. But its diameter is only about half as big as Earth’s.
meteorite A lump of rock or metal from space that passes through Earth’s atmosphere and collides with the ground.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.
molten A word describing something that is melted, such as the liquid rock that makes up lava.
NASA Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.
orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.
paradox An idea or a statement that is true, but that seems logically impossible.
particle A minute amount of something.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
planetary science The science of planets other than Earth.
porous The description of a substance that contains tiny holes, called pores , through which a liquid or gas can pass.
radioactive An adjective that describes unstable elements, such as certain forms (isotopes) of uranium and plutonium. Such elements are said to be unstable because their nucleus sheds energy that is carried away by photons and/or and often one or more subatomic particles. This emission of energy is by a process known as radioactive decay.
ratio The relationship between two numbers or amounts. When written out, the numbers usually are separated by a colon, such as a 50:50. That would mean that for every 50 units of one thing (on the left) there would also be 50 units of another thing (represented by the number on the right).
scenario A possible (or likely) sequence of events and how they might play out.
simulation (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.
sodium A soft, silvery metallic element that will interact explosively when added to water. It is also a basic building block of table salt (a molecule of which consists of one atom of sodium and one atom of chlorine: NaCl). It is also found in sea salt.
solar system The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.
subatomic Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).
sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.
trait A characteristic feature of something.