4.  Panspermia – is quarantine a moot point?

The shergottite, nakhlite, and chassigny classes of meteorites[1] are collectively known as SNC meteorites.  SNC meteorites are highly anomalous compared to most other meteorites, and it is thought they are igneous rocks from Mars.  It is now recognized that rocks can be blasted from the surface of one planetary body by a large impact and subsequently make their way to the surface of another planetary body.  Because of the orbital dynamics involved and because of the relatively low escape velocities of Mars and the Moon, Martian and Lunar meteorites, albeit rare, are found on the Earth.   Of the approximately 24,000 known meteorites in 2006, 34 of them were from Mars, and 39 of them were from the Moon (Mars Meteorites, 2006).

Did life originally come from a meteorite striking the Earth?  This is the modern version of the panspermia theory of spores naturally passing from one planet to another.  If so, are we already “contaminated” from elsewhere, or at least are we constantly at risk of contamination from other Solar System bodies?  Yes we are!  Approximately 100 kilograms of Martian material on average strikes the Earth everyday.  

Can organisms survive such a journey?  That is unknown, but it is known the interior of a meteorite remains cool during its passage through the Earth’s atmosphere, and any organisms in the interior of a meteoroid in space may be protected from cosmic rays and extremes of temperature.   A contaminant strain of the bacteria Microbispora was aboard the Space Shuttle Columbia when it broke apart during reentry on February 1, 2003.  The bacteria survived the destruction of the Columbia and its fiery fall to the Earth. 

5.  Conclusions - common sense 

Since a live microorganism could arrive on the Earth from Mars at any time, why worry about forward or backward contamination of the Solar System?  There are several reasons.  First, we don’t in fact know that life has arrived here from elsewhere.  Most consider it unlikely.  Second, if life arose from Mars, for example, and was transported here 3 + billion years ago, the present life forms on Mars may closely resemble those on Earth and may be far more virulent and a greater risk for us than if they were a completely different form of life.  The same holds true in a reverse fashion should we accidentally infect Mars.  If the terrestrial invader comes from the same life stock as the native life, it may prove to be especially pathological.  The familiar is often far more dangerous than the unfamiliar.  It is just good common sense and good science to insist on rigorous quarantine measures until we can be absolutely certain there is no risk to us from alien life and no risk to other worlds from terrestrial life.

References

Barengoltz J.  Galileo and Cassini planetary protection report to COSPAR. 35^th COSPAR Scientific Assembly, 18-25 July 2005, Paris, page 545.  

Diamond J. Guns, Germs, and Steel.  W.W. Norton and Co., 1997, New York.  

Gross M. Life on the Edge. Perseus Publishing, 2001, Cambridge, MA. 

Guralnik DB. Webster’s New World Dictionary of the American Language. The World Publishing Co, 1970, New York.

Koike J. Warning! A crucial period of searching for life on Mars-why international criterion for space quarantine is now required.  Biol Sci Space 1996; 10(4): 283-288.  Abstract in English is available through PubMed.  Original article is in Japanese.

Lunine JI. Astrobiology: a Multi-Disciplinary Approach. Addison Wesley, 2004, pages 404-416.

Mars Meteorites web site at:  http://www2.jpl.nasa.gov/snc/. 2006.

NASA.  Planetary Protection.  NASA web site at:  http://planetaryprotection.nasa.gov/pp/index.htm. 2006. 

Oberg J ET. Web comments: http://yarchive.net/space/science/surveyor_strep.html. 1996. 

Rummel JD. Planetary exploration in the time of astrobiology: protecting against biological contamination. PNAS 2001; 98: 2128-2131. 

Washington University in St. Louis. List of Lunar meteorites.  Web site at: Meteorite Information.  January 2008.

Wood JA. Public briefing: the quarantine and certification of Martian samples.  National Academy of Sciences web site at: http://www4.nationalacademies.org/news.nsf/isbn/s0309075718?OpenDocument.  2001.

Posted Sunday February 8, 2009; minor revision June 23, 2014

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[1] A shergottite meteor is named for a fall in Shergotty, India, 25 August 1865; a nakhlite meteor is named for a fall in Nakhala, Egypt, 28 June 1911; a chassignite meteor is named for a fall in Chassigny, France, 3 October 1815. From: http://www.meteorlab.com/METEORLAB2001dev/snclist.htm.