ARTICLE IN PRESS
meteoroids: Asymptotic Giant Branch stars (AGB), Young Stellar
Objects (YSOs) and debris disks. Fluxes from AGB are the strongest
ones arriving on Earth, approaching 8 particles yr
À 1
km
À 2
. The
flux of dust and gas of extra-solar origin arriving on top of
terrestrial atmosphere will increase by many orders of magnitude
at each crossing of the sun through a spiral arm. The local density
of stars in the solar neighborhood will also be increased by a
factor up to ten.
An interesting possibility that extends the panspermia concept
is enhanced from the discussion presented in this paper. Microbes
from other places in the galaxy could benefit of the shielding
effect operated by microparticulate material against UV–VUV
radiation of the interplanetary space. As a matter of fact, living
organisms could have more intensively seeded Earth during
crossings of the solar system through dense galactic regions
because of shorter times required for any organism to reach Earth.
This process could thus advantageously overcome the following
issues considered in the lithopanspermia theory: (i) intense
heating during atmospheric entry, (ii) radioactive decay of heavy
elements present in large rocks and (iii) the bystander effects of
HZE particle radiation. Further experiments are currently being
performed in order to address the effects of ionizing radiation
(electromagnetic and charged particles) on radio-resistant mi-
crobes under this micro-shielding scenario.
Authors’ contributions:
I.G.P.L. collected, processed and analyzed data, was involved in
study design and wrote the manuscript; S.P. was involved in the
experimental design, collected and processed data. E.J.P. was
involved in data analysis and proposed the astronomical meaning
to the primary data; A.N.B., J.A.R.G and A.A.C.L. were involved in
advising experimental details and data analysis, provided the
facilities and contributed equally to the study. C.A.S.L. designed
the study, was involved in data analysis and revised the manu-
script. All authors discussed the results and commented on the
manuscript.
Acknowledgements
We thank Dr. Carlos Eduardo Bonacossa de Almeida (IRD/RJ)
for having kindly provided bacterial strains, Dr. Diana Paula
Andrade Pilling Guapyassu
´
de Oliveira for helping data acquisi-
tion, Dr. Douglas Galante for his comments on the manuscript and
Dr. Gordon Imlach from Department of Life Sciences, Open
University, Milton Keynes, United Kingdom for producing the
Scanning Electron Microscopy images. We also thank the Instituto
de Biofı
´
sica Carlos Chagas Filho (IBCCF/UFRJ) for intellectual and
financial contributions, the faculty from the Departamento de
Fı
´
sica from the Pontifı
´
cia Universidade Catolica do Rio de Janeiro
(PUC/RJ) for managing the hydrogen lamp, Conselho Nacional de
Desenvolvimento Cientı
´
fico e Tecnolo
´
gico (CNPq) for providing
I.G. Paulino-Lima’s Ph.D. student fellowship and Fundac-
~
ao de
Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ) for
research grants. This work was also partially supported by the
Brazilian institution Laborato
´
rio Nacional de Luz Sı
´
ncrotron
(LNLS), Brazil, which also provided the facility and staff super-
vision to conduct the main experimental part of this work.
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