Microscopic view on asteroid collisions could help us understand planet formation

Date:
February 24, 2022
Source:
University of Cambridge
Summary:
A new way of dating collisions between asteroids and planetary
bodies throughout our solar system's history could help scientists
reconstruct how and when planets were born.



FULL STORY ==========================================================================
A new way of dating collisions between asteroids and planetary bodies throughout our Solar System's history could help scientists reconstruct
how and when planets were born.


==========================================================================
A team of researchers, led by the University of Cambridge, combined
dating and microscopic analysis of the Chelyabinsk meteorite -- which
fell to Earth and hit the headlines in 2013 -- to get more accurate
constraints on the timing of ancient impact events.

Their study, published in Communications Earth & Environment, looked
at how minerals within the meteorite were damaged by different impacts
over time, meaning they could identify the biggest and oldest events
that may have been involved in planetary formation.

"Meteorite impact ages are often controversial: our work shows that
we need to draw on multiple lines of evidence to be more certain about
impact histories - - almost like investigating an ancient crime scene,"
said Craig Walton, who led the research and is based at Cambridge's
Department of Earth Sciences.

Early in our Solar System's history, planets including the Earth formed
from massive collisions between asteroids and even bigger bodies, called
proto- planets.

"Evidence of these impacts is so old that it has been lost on the planets
- - Earth in particular has a short memory because surface rocks are continually recycled by plate tectonics," said co-author Dr Oli Shorttle,
who is based jointly at Cambridge's Department of Earth Sciences and
Institute of Astronomy.



========================================================================== Asteroids, and their fragments that fall to Earth as meteorites, are in contrast inert, cold and much older -- making them faithful timekeepers
of collisions.

The new research, which was a collaboration with researchers from
the Chinese Academy of Sciences and the Open University, recorded how
phosphate minerals inside the Chelyabinsk meteorite were shattered to
varying degrees in order to piece together a collision history.

Their aim was to corroborate uranium-lead dating of the meteorite,
which looks at the time elapsed for one isotope to decay to another.

"The phosphates in most primitive meteorites are fantastic targets for
dating the shock events experienced by the meteorites on their parent
bodies," said Dr Sen Hu, who carried out the uranium-lead dating at
Beijing's Institute of Geology and Geophysics, Chinese Academy of
Sciences.

Previous dating of this meteorite has revealed two impact ages, one older, roughly 4.5-billion-year-old collision and another which occurred within
the last 50 million years.



==========================================================================
But these ages aren't so clear-cut. Much like a painting fading over
time, successive collisions can obscure a once clear picture, leading
to uncertainty among the scientific community over the age and even the
number of impacts recorded.

The new study put the collisions recorded by the Chelyabinsk meteorite
in time order by linking new uranium-lead ages on the meteorite to
microscopic evidence for collision-induced heating seen inside their
crystal structures. These microscopic clues build up in the minerals
with each successive impact, meaning the collisions can be distinguished,
put in time order and dated.

Their findings show that minerals containing the imprint of the oldest collision were either shattered into many smaller crystals at high
temperatures or strongly deformed at high pressures.

The team also described some mineral grains in the meteorite that were fractured by a lesser impact, at lower pressures and temperatures, and
which record a much more recent age of less than 50 million years. They
suggest this impact probably chipped the Chelyabinsk meteorite off its
host asteroid and sent it hurtling to Earth.

Taken together, this supports a two-stage collision history. "The
question for us was whether these dates could be trusted, could we
tie these impacts to evidence of superheating from an impact?" said
Walton. "What we've shown is that the mineralogical context for dating
is really important." Scientists are particularly interested in the
date of the 4.5-billion-year-old impact because this is about the time
we think the Earth-Moon system came to being, probably as a result of
two planetary bodies colliding.

The Chelyabinsk meteorite belongs to a group of so-called stony
meteorites, all of which contain highly shattered and remelted material
roughly coincident with this colossal impact.

The newly-acquired dates support previous suggestions that many asteroids experienced high energy collisions between 4.48 -- 4.44 billion years
ago. "The fact that all of these asteroids record intense melting at
this time might indicate Solar System re-organisation, either resulting
from the Earth-Moon formation or perhaps the orbital movements of giant planets." Walton now plans to refine dating over the window of the Moon-forming impact, which could tell us how our own planet came to being.

========================================================================== Story Source: Materials provided by University_of_Cambridge. The original
text of this story is licensed under a Creative_Commons_License. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Craig R. Walton, Oliver Shorttle, Sen Hu, Auriol S. P. Rae,
Ji Jianglong,
Ana Černok, Helen Williams, Yu Liu, Guoqiang Tang, Qiuli Li,
Mahesh Anand. Ancient and recent collisions revealed by phosphate
minerals in the Chelyabinsk meteorite. Communications Earth &
Environment, 2022; 3 (1) DOI: 10.1038/s43247-022-00373-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220224091140.htm

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