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ESO’s VLT Finds No Trace Of Water In DART’s Asteroid Impact Cloud – But Unveils Other Surprises

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Astronomers use ESO’s VLT to study DART’s asteroid impact and uncover secrets of asteroid composition

In a controlled experiment to test our asteroid deflection capabilities, the DART spacecraft collided with Dimorphos asteroid on September 26, 2022. This event occurred at a distance of 11 million kilometers from Earth, enabling detailed observation with numerous telescopes.

The aftermath of the impact was observed by all four 8.2-metre telescopes of ESO’s VLT in Chile. Initial findings from these observations have been recently published in two papers.

Brian Murphy, a PhD student at the University of Edinburgh in the UK and co-author of one of the studies, explains that asteroids are fundamental remnants of the formation of all planets and moons in our Solar System. Thus, analyzing the debris cloud resulting from DART’s impact can offer insights into the origins of our Solar System.

Cyrielle Opitom, an astronomer at the same university and lead author of one of the articles, highlights the unique opportunity presented by DART to study a controlled impact similar to a laboratory experiment. While asteroid collisions occur naturally, they are unpredictable.

Using the Multi Unit Spectroscopic Explorer (MUSE) instrument at ESO’s VLT, Cyrielle Opitom and her team monitored the progression of the debris cloud for a month. Their observations revealed that the ejected cloud was bluer than the asteroid’s pre-impact color, suggesting the presence of fine particles.

Over time, the team observed the emergence of additional structures such as clumps, spirals, and a long tail pushed away by the Sun’s radiation. These structures were redder than the initial cloud, suggesting that they may be composed of larger particles.

By dispersing the light from the debris cloud into a spectrum, Cyrielle Opitom’s team employed MUSE to search for chemical signatures of different gases. They specifically sought evidence of oxygen and water released from ice exposed by the impact. However, their investigation yielded no such detections.

”Asteroids are not expected to contain significant amounts of ice, so detecting any trace of water would have been a real surprise,” adds Opitom.

Astronomers use ESO's VLT to study DART's asteroid impact and uncover secrets of asteroid composition
Astronomers use ESO’s VLT to study DART’s asteroid impact and uncover secrets of asteroid composition

Additionally, the team searched for indications of the propellant used by the DART spacecraft but were unable to detect any traces of it.

”We knew it was a long shot,” she points out, “as the amount of gas that would be left in the tanks from the propulsion system would not be huge. Furthermore, some of it would have travelled too far to detect it with MUSE by the time we started observing.”

Stefano Bagnulo, an astronomer at the Armagh Observatory and Planetarium in the UK, headed another research team that examined how the DART impact affected the asteroid’s surface.

“When we observe the objects in our Solar System, we are looking at the sunlight that is scattered by their surface or by their atmosphere, which becomes partially polarised,” adds Bagnulo.

This implies that light waves vibrate in a specific direction instead of being random.

“Tracking how the polarisation changes with the orientation of the asteroid relative to us and the Sun reveals the structure and composition of its surface.”

Asteroid Deflection Test Yields Valuable Insights into Solar System’s Origins

Bagnulo and his team utilized the FOcal Reducer/low dispersion Spectrograph 2 (FORS2) instrument at the VLT to observe the asteroid. They discovered that the level of polarization significantly decreased following the impact while the overall brightness of the system increased. The researchers suggest that the impact may have exposed untouched material from the interior of the asteroid, which could explain these observations.

”Maybe the material excavated by the impact was intrinsically brighter and less polarising than the material on the surface, because it was never exposed to solar wind and solar radiation,” points out Bagnulo.

An alternate theory is that the impact destroyed particles on the asteroid’s surface, leading to the expulsion of smaller fragments into the debris cloud.

Zuri Gray, a PhD student at the Armagh Observatory and Planetarium, clarifies that smaller fragments can be more effective at reflecting light and less effective at polarizing it under specific conditions.

Astronomers Uncover Secrets of Asteroid Dimorphos After DART Impact

The research conducted by Bagnulo and Opitom’s teams demonstrate the capability of the VLT when its various instruments collaborate. Besides MUSE and FORS2, two additional VLT instruments were also utilized to observe the impact’s aftermath, and investigations into this data are currently underway.

“This research took advantage of a unique opportunity when NASA impacted an asteroid,” says Opitom, “so it cannot be repeated by any future facility. This makes the data obtained with the VLT around the time of impact extremely precious when it comes to better understanding the nature of asteroids.”

Source: 10.1051/0004-6361/202345960 & 10.3847/2041-8213/acb261

Image Credit: ESO

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