Huge ring-like structure on Ganymede's surface may have been caused by
violent impact

Date:
August 7, 2020
Source:
Kobe University
Summary:
Image data reanalysis has revealed that ancient tectonic troughs
are concentrically distributed across almost the entire surface
of Jupiter's moon Ganymede. Computer simulation results suggest
that this giant crater could have resulted from the impact of
an asteroid with a 150-kilometer radius. If so, this the largest
impact structure identified in the solar system so far.



FULL STORY ========================================================================== Researchers from Kobe University and the National Institute of Technology, Oshima College have conducted a detailed reanalysis of image data
from Voyager 1, 2 and Galileo spacecraft in order to investigate the orientation and distribution of the ancient tectonic troughs found on
Jupiter's moon Ganymede.

They discovered that these troughs are concentrically distributed across
almost the entire surface of the satellite. This global distribution
indicates that these troughs may be actually part of one giant crater
covering Ganymede.


========================================================================== Based on the results of a computer simulation conducted using the
"PC Cluster" at the National Astronomical Observatory of Japan (NAOJ),
it is speculated that this giant crater could have resulted from the
impact of an asteroid with a radius of 150km. If so, the structure is
the largest impact structure identified in the solar system so far.

The European Space Agency's JUICE (Jupiter Icy Moon Explorer) mission,
which will be launched in 2022 and arrive in Jupiter's system in 2029,
aims to increase our knowledge regarding Jupiter's satellites, including Ganymede. It is hoped that this exploration will confirm the results of
this study and further advance our understanding of the formation and
evolution of Jupiter's satellites.

The research team consisted of Kobe University Graduate School of
Science's Assistant Professor HIRATA Naoyuki and Professor OHTSUKI Keiji
(both of the Department of Planetology), and Associate Professor SUETSUGU
Ryo of National Institute of Technology, Oshima College. The paper for
this study was published online in Icarus on July 15.

Main Points
* Many furrows, or trough formations, have been previously observed
on the
surface of Ganymede, one of Jupiter's moons.

* The research group comprehensively reanalyzed image data of
Ganymede's
surface obtained by NASA's Voyager 1, Voyager 2 and Galileo
spacecraft.

* The results revealed that almost all of these furrows on the
surface of
Ganymede appear to be in concentric rings centered at a single
point, indicating that this global multiring structure may be the
remains of a giant crater.

* The radial extent of the multiring measured along the satellite's
surface
is 7800km. Therefore, if it is actually formed by an impact, it
is the largest vestigial impact crater discovered in the solar
system so far.

* The results of the numerical impact simulation indicated that
an asteroid
with a radius of 150km impacting Ganymede at a speed of 20km/s
would explain the observed structures on the satellite's surface.

Research Background Both Voyager 1 and Voyager 2 have closely approached Ganymede in 1979 and 1980 respectively, taking detailed images of the
surface. In addition, the Galileo spacecraft orbited Jupiter from 1995
to 2003, obtaining a large amount of Ganymede image data. Ganymede is
the largest satellite in the solar system and is bigger than both Pluto
and Mercury. The formation and evolution of Jupiter's moons including
Ganymede is strongly connected to the formation and evolution of the
Jupiter system, and by extension, of the solar system. Consequently,
there are various ongoing and planned spacecraft missions to explore
the satellite system, including NASA's JUNO mission that is ongoing, the
Europa Clipper scheduled to perform a detailed investigation of Jupiter's
moon Europa in around 2030, and the aforementioned JUICE mission.



==========================================================================
The study was conducted with the aims of clarifying one aspect of the
formation and evolution of Jupiter's satellites and of contributing
towards these spacecraft missions. The group reanalyzed image data of
Ganymede. In particular, the researchers focused on furrows, tectonic
troughs that are believed to be the oldest surface features on the
satellite. Therefore, the research group hypothesized that they could reconstruct the early history of Ganymede by analyzing these geological formations.

Research Findings Ganymede's surface is categorized into areas of
Dark Terrain and Bright Terrain. Dark Terrain is extremely old and has
many remaining craters, as well as trough formations. Bright Terrain is comparatively recent, with hardly any craters. These two types of terrain
are not coherently arranged and are randomly distributed over Ganymede's entirety. Furrows are believed to be Ganymede's oldest geological features because they are only found on Dark Terrain and many impact craters have
been formed on top of them later on.

This study reanalyzed the distribution of these trough formations over Ganymede's entire surface, revealing for the first time that almost all of these furrows are concentrically aligned around a single point. The study showed that these furrows form giant, concentric rings over the entire satellite. From this, it can be assumed that there was a giant multiring
impact crater which covered the entire surface of Ganymede before the
formation of the Bright Terrain areas. A similar ring structure known as
the Valhalla Crater remains on the surface of Callisto, another satellite
of Jupiter. Until now, the Valhalla Crater has been the largest identified multiring crater in the solar system, with a radius of approximately
1900km. However, the multiring crater on Ganymede has a radial extent
of 7800km measured along the satellite's surface.

The research team conducted a simulation to estimate the scale of the
impact that formed this giant crater. This was carried out using the "PC Cluster" at the National Astronomical Observatory of Japan (NAOJ). The
results indicated that an asteroid with a radius of 150km impacting
Ganymede at a speed of 20km/ s would be sufficient to form the observed structures on the satellite's surface. It is believed that such an impact occurred around 40 billion years ago.

Further Developments The discovery that the aftermath of a large-scale
impact remains on Ganymede's surface is greatly significant in terms of
the satellite's formation process and evolution. For example, Jupiter's satellite Callisto is around the same size as Ganymede, however it
is believed that it doesn't have an internal structure composed of differentiated layers. On the other hand, Ganymede is thought to be
composed of a differentiated layer structure consisting of rock, iron and
ice. An enormous amount of heat is necessary to form these differentiated layers. It is possible that the aforementioned large-scale impact could
have been the source of this heat.

This study's discovery will also have substantial significance for the
Ganymede exploration programs scheduled in the coming decades. The image
data from both Voyager and Galileo missions only provide partial views
of the satellite's surface. It is hoped that future explorations will
be able to confirm or test this study's results by conducting detailed investigations into the multiring formations and whether or not there are
any other remains of large-scale impacts. Hopefully, this will result
in a deeper understanding of the origins and evolution of Ganymede as
well as Jupiter's other moons.


========================================================================== Story Source: Materials provided by Kobe_University. Note: Content may
be edited for style and length.


========================================================================== Related Multimedia:
* Images_of_Ganymede's_surface ========================================================================== Journal Reference:
1. Naoyuki Hirata, Ryo Suetsugu, Keiji Ohtsuki. A global system
of furrows
on Ganymede indicative of their creation in a single impact event.

Icarus, 2020; 352: 113941 DOI: 10.1016/j.icarus.2020.113941 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200807102339.htm

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