What We Learnt From a Sahara Space Rock Left Over From the Dawn of the Solar System

Subject :Science and technology

Section: Space technology

Context:

In May 2020, some unusual rocks containing distinctive greenish crystals were found in the Erg Chech sand sea, a dune-filled region of the Sahara Desert in southern Algeria.

Erg chech 002:

The rocks turned out to be from outer space: lumps of rubble billions of years old, left over from the dawn of the Solar System.
They were all pieces of a meteorite known as Erg Chech 002, which is the oldest volcanic rock ever found, having melted long ago in the fires of some now-vanished ancient protoplanet.
Researchers have now analyzed lead and uranium isotopes in Erg Chech 002 and calculated it is some 4.56556 billion years old, give or take 120,000 years. The results also cast doubt on some common assumptions about the early Solar System.

Aluminum in the solar system:

Around 4.567 billion years ago, our Solar System formed from a vast cloud of gas and dust.
Among the many elements in this cloud was aluminium, which came in two forms:
First is the stable form, aluminium-27.
Second is aluminium-26, a radioactive isotope mainly produced by exploding stars, which decays over time into magnesium-26.
Importance of Aluminium -26:
- It is useful to understand how the Solar System formed and developed. Because it decays over time, scientists use it to date events – particularly within the first four or five million years of the Solar System’s life.
- The decay of aluminium-26 was the main source of heat in the early Solar System.
- This decay influenced the melting of the small, primitive rocks that later clumped together to form the planets.

Uranium, lead and age

However, to use aluminum-26 to understand the past, we need to know whether it was spread around evenly or clumped together more densely in some places than in others.
To figure that out, we will need to calculate the absolute ages of some ancient space rocks more precisely.
As Aluminum 26 decays relatively quickly (after around 705,000 years, half of a sample of aluminium-26 will have decayed into magnesium-26), so it alone is not useful, but combining aluminum 26 data with uranium and lead data, can give desired results.
There are two important isotopes of uranium (uranium-235 and uranium-238), which decay into different isotopes of lead (lead-207 and lead-206, respectively).
The uranium isotopes have much longer half-lives (710 million years and 4.47 billion years, respectively), which means we can use them to directly figure out how long ago an event happened.

Meteorite groups

Erg Chech 002 is an “ungrouped achondrite”.
Achondrites are rocks formed from melted planetesimals, which are solid lumps in the cloud of gas and debris that formed the Solar System.
The sources of many achondrites found on Earth have been identified.
Most belong to the Howardite-Eucrite-Diogenite clan, which are believed to have originated from Vesta 4, one of the largest asteroids in the Solar System. Another group of achondrites is called angrites, which all share an unidentified parent body.
Still other achondrites, including Erg Chech 002, are “ungrouped”: their parent bodies and family relationships are unknown.

A clumpy spread of aluminum:

Erg Chech 002 was found to contain a high abundance of lead-206 and lead-207, as well as relatively large amounts of undecayed uranium-238 and uranium-235.
Measuring the ratios of all the lead and uranium isotopes helped to estimate the age of the rock with such unprecedented accuracy.
The comparison with a group of achondrites called volcanic angrites showed that the parent body of Erg Chech 002 must have formed from material containing three or four times as much aluminum-26 as the source of the angrites’ parent body.
This shows aluminum-26 was indeed distributed quite unevenly throughout the cloud of dust and gas which formed the solar system.

Significance:

The results contribute to a better understanding of the Solar System’s earliest developmental stages, and the geological history of burgeoning planets.
Further studies of diverse achondrite groups will continue to refine our understanding and enhance our ability to reconstruct the early history of our Solar System.