Newsgroups: sci.astro,alt.sci.planetary,sci.astro.amateur
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Mathilde (was Re: Sky & Telescope News Bulletin - 12/19/97)
Date: Tue, 23 Dec 1997 17:57:52 GMT

In article <67ku3i$ikg$1@pith.uoregon.edu>,
Christopher Michael Jones <cjones@ix.cs.uoregon.edu> wrote:
>However, the multi-spectral camera is a VERY nice instrument and
>provides a LOT of data.  If there were ice on the surface of Mathilde,
>the multi-spectral camera would have spotted it...

Ice on the surface would be unlikely anyway -- I doubt that it's stable at
that distance from the Sun.  Ice in the *interior*, slowly outgassing
through porous overburden, is another story.  There is a serious theory
that the interior of Phobos is exactly that -- Phobos too has an
anomalously low density, and there is some evidence that it is outgassing
something.  (Pity the Phobos 2 mission never got close enough to use its
radar sounder...)

>It is much more likely
>that Mathilde has a low density due to internal voids (like in sand,
>pumice, loose dirt, or gravel)...

Last I heard, getting the density down that low with internal voids was
somewhat difficult and required fairly generous assumptions.

>Water ice is one of the FIRST guesses as a component for any solid low
>density object in the Solar System...
>However, in this case, the evidence rules this out.

Which evidence?

>It is still
>possible, though highly unlikely due to the fact that there would have
>to be lot's of water ice in the interior, but none on the surface.

What exactly is the problem with that?  It's precisely what you would
expect for a partly-ice object in an orbit that is too warm for ice to be
stable on the surface, but not warm enough to completely dry out the
interior in a few billion years.

No, absence of hydration spectral features in the surface materials is
not evidence against ice.  "...there is currently wide agreement that
clay minerals known to exist in asteroids possessing the 2.9um absorption
band are the result of post-accretion heating, and that truly primitive
asteroids ... would *not* contain hydrated silicates, but rather would
contain any water component in the form of ice.  In fact, it is possible
that the depth of the 2.9um band is *anticorrelated* with the *total*
abundance of water in the low-albedo asteroids..."  (Bell, Fanale, and
Cruikshank, "Chemical and Physical Properties of the Martian Satellites",
in Lewis et al (eds), "Resources of Near-Earth Space", UArizona 1993.)
--
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        | henry@zoo.toronto.edu