Astrophysics journal club

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Summary:
This is the homepage of the of the weekly Astrophysics journal club at the Division of Geophysics and Astronomy at the University of Helsinki.
The meetings are of an unofficial nature and the main driver is to get together and discuss recent interesting papers. This meeting differs from the
Seminar series in that people are not expected to talk about their own research and in that no credits will be awarded to students. The meetings are
open to everyone and the aim is to stimulate discussion about recent results and provide the possibility to
 learn about research that is not necessarily
connected to one's own field of expertise.

All topics are welcome ranging from planetary science to larger scales involving stellar
 astrophysics, Milky Way studies, galaxies and cosmology.
The only requirement is that the
 presented papers should be interesting to a wider audience and that they should be presented 
in such a way that also
a non-expert can follow the presentation. In the meetings we discuss one paper each week for about 35-45 minutes.
 All meetings are in English.

Location: The Division Coffee room on the third floor.

Time: Thursdays at 10.15-11.00am during term time.

Speakers: Please contact Peter Johansson, Mikael Granvik or Petri Käpylä if you want to present a paper.

Present Program: Next talk will be on Thursday 10th of May at 10.15am. Two papers will be discussed!

Presenter: Mikael Granvik
Paper title: An Archaean heavy bombardment from a destabilized extension of the asteroid belt
Authors: Bottke, W.F.; Vokrouhlicky, D.; Minton, D.; Nesvorny, D.; Morbidelli, A.; Brasser, R.; Simonson, B.; Levison, H.F.
Reference: 2012, Nature, 485, 78-81

Abstract: The barrage of comets and asteroids that produced many young lunar basins (craters over 300 kilometres in diameter) has frequently been
called the Late Heavy Bombardment (LHB). Many assume the LHB ended about 3.7 to 3.8 billion years (Gyr) ago with the formation of Orientale basin.
Evidence for LHB-sized blasts on Earth, however, extend into the Archaean and early Proterozoic eons, in the form of impact spherule beds: globally distributed
ejecta layers created by Chicxulub-sized or larger cratering events. At least seven spherule beds have been found that formed between 3.23 and 3.47 Gyr ago,
four between 2.49 and 2.63 Gyr ago, and one between 1.7 and 2.1 Gyr ago. Here we report that the LHB lasted much longer than previously thought,
with most late impactors coming from the E belt, an extended and now largely extinct portion of the asteroid belt between 1.7 and 2.1 astronomical units from Earth.
This region was destabilized by late giant planet migration. E-belt survivors now make up the high-inclination Hungaria asteroids. Scaling from
the observed Hungaria asteroids, we find that E-belt projectiles made about ten lunar basins between 3.7 and 4.1 Gyr ago. They also produced about 15 terrestrial
basins between 2.5 and 3.7 Gyr ago, as well as around 70 and four Chicxulub-sized or larger craters on the Earth and Moon, respectively, between 1.7 and 3.7 Gyr ago.
These rates reproduce impact spherule bed and lunar crater constraints.

Presenter: Mikael Granvik
Paper title: Impact spherules as a record of an ancient heavy bombardement of Earth
Authors: Johnson, B.C. & Melosh, H.J.
Reference: 2012, Nature, 485, 75-77

Abstract: Impact craters are the most obvious indication of asteroid impacts, but craters on Earth are quickly obscured or destroyed by surface
weathering and tectonic processes. Earth’s impact history is inferred therefore either from estimates of the present-day impactor flux as determined
by observations of near-Earth asteroids, or from the Moon’s incomplete impact chronology. Asteroids hitting Earth typically vaporize a mass of target
rock comparable to the projectile’s mass. As this vapour expands in a large plume or fireball, it cools and condenses into molten droplets called spherules.
For asteroids larger than about ten kilometres in diameter, these spherules are deposited in a global layer. Spherule layers preserved in the geologic
record accordingly provide information about an impact even when the source crater cannot be found. Here we report estimates of the sizes and impact
velocities of the asteroids that created global spherule layers. The impact chronology from these spherule layers reveals that the impactor flux was
significantly higher 3.5 billion years ago than it is now. This conclusion is consistent with a gradual decline of the impactor flux after the Late Heavy Bombardment.

Past program:

Spring 2012:

Autumn 2011:

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