[meteorite-list] Life's Rocky Road Between Worlds

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:12 2004
Message-ID: <200106122037.NAA14771_at_zagami.jpl.nasa.gov>

>
> Ron replied:
>
> > The main difference is the amount of time the samples spend in space enroute
> > to Earth. For the typical Mars meteorite, it would have been in space for 15
> > to 30 millions years before it landed on Earth. For a sample returned by
> > spacecraft, the sample would only spend a few months in space enclosed in a
> > container. Any Mars organisms, if present, would have a much higher likelihood
> > of surviving the trip from Mars to Earth in a spacecraft than in a Mars meteorite.
>
> I included the particular quote from the article in part to dispell that
> very argument. They have already calculated the 7% microbe survival rate
> in the 150 kg of hospitable rocks per year to derive their equivalent 10
> kg per year number. So that is not a sufficient reason to worry.
>

You should probably look closely at the assumptions being made for the
7% calculation:

>>Mileikowsky's Table 2 provides calculations for one scenario. They select
>>conditions that optimise the chances of lifeforms surviving the journey.
>>These "hospitable" conditions are:
>>
>> * The radius of ejected rock is between 0.67 and 1 metre (mainly to
>> provide protection from radiation in deep space). [note 1]

None of the Mars meteorites are this large.

>> * The core temperature within the rock during ejection or re-entry did
>> not exceed 100 C (two of the dozen or so Martian meteorites that have
>> been found on Earth meet this criterion)

OK, a small number (11%) of the Mars meteorites meet this criteria.

>> * The journey time between planets was 100,000 years or less

None of the Mars meteorite made the trip in this short of time. 15 million to 30
million years more typical.

Obviously, the assumptions made were very optimistic, and the 7% number does not match
up well with the existing Mars meteorites. The journey time assumption was
off by a couple of magnitudes from the physical data. It would be interesting to see
what the calculation would be if it was redone to better align itself with the Mars
meteorite data.

Ron Baalke
Received on Tue 12 Jun 2001 04:37:22 PM PDT