Of the thousands of specimens of meteorites found
on Earth and known to science, only about 100 are
igneous; that
is, they have undergone
melting by volcanic action at some time since the planets were
first formed. These igneous meteorites are known as achondrites because
they lack chondrules—small
stony spherules found in the thousands of meteorites (called
“chondrites”) composed primarily of unaltered
minerals that
condensed from dust and gas at
the origin of the solar system. Achondrites are
the only known samples of volcanic
rocksoriginating outside
the Earth-Moon
system. Most are
thought to have been dislodged by interbody impact from asteroids,
with diameters of from 10 to 500 kilometers, in solar
orbit between
Mars and Jupiter.
Shergottites, the name given to three anomalous achondrites so
far discovered on Earth, present scientists with a
genuine
enigma. Shergottites
crystallized from molten rock less
than 1.1 billion years ago (some 3.5 billion years later than
typical
achondrites) and were
presumably ejected into space when an object impacted on a body
similar in chemical composition to Earth.
While most meteorites appear to derive from
comparatively small bodies, shergottites exhibit properties that
indicate that their source was a large planet, conceivably Mars. In
order to account for such an unlikely source, some unusual factor
must be invoked, because the
impact needed to accelerate a fragment of rock to escape the
gravitational field of a body even as small as the Moon is so great
that no meteorites of lunar origin have been discovered.
While some scientists speculate that shergottites
derive from Io
(a volcanically active moon of
Jupiter), recent
measurements suggest that since Io’s surface is rich in sulfur
and sodium, the
chemical composition of its volcanic products would probably be
unlike that of the shergottites. Moreover, any
fragments dislodged from Io by interbody impact would be unlikely
to escape the gravitational pull of Jupiter.
The only other logical source of shergottites
is Mars.
Space-probe
photographs indicate
the existence of giant volcanoes on the Martian surface. From the
small number of impact craters that appear on Martian lava flows,
one can estimate that the planet was volcanically active as
recently as a half-billion years ago—and may be active today. The
great objection to the Martian
origin of
shergottites is the
absence of lunar meteorites on Earth. An impact capable of ejecting
a fragment of the Martian surface into an Earth-intersecting orbit
is even less probable than such an event on the Moon, in
view
of the Moon’s smaller size and closer proximity to
Earth. A
recent study suggests,
however, that
permafrost ices below the surface of Mars may have altered the
effects of impact on it. If the ices had been rapidly vaporized by
an impacting object, the expanding gases might have helped the
ejected fragments reach escape
velocity . Finally,
analyses performed by space probes show a remarkable chemical
similarity between Martian
soil and the shergottites.
在地球上所被发现的并为科学所已知的数以千计的陨石(meteorite)种类中,仅约100种的陨石是火成的(igneous);也就是说,自从诸行星最初形成以来,它们在过去的某个时候已经历过由火山作用而致的溶化过程。这些火成陨石被称为无球粒陨石(achondrite),因为它们缺乏陨石球粒(chondule)——即在数千种陨石(被称作“球粒陨石”,chondrite)中所发现的石质小球粒,主要是由在阳系形成之际从尘埃和气体中凝结而成的且未曾改变过的矿物质构成。无球粒陨石是唯一已知的起源于地球-月球系(Earty-Moon
system)之外的火山岩实例。绝大多数无球粒陨石被认为是由于介体撞击作用(interbody
impact)而从小行星(asteroid)中分离出来的。这些小行星的直径自10至500公里不等,位于火星(Mars)和木星(Jupiter)之间的太阳轨道。
辉熔长无球粒陨石(shrgottite)——一个给迄今为止在地球上发现的三种不规则无球粒陨石所起的名称——对科学家来说构成了一个真正的不解之谜。在不到1.1亿年之前(大概要比典型的无球粒陨石晚3.5亿年),辉熔长无球粒陨石从深化的岩石中晶化成形,并据推测,当一个物体撞击到一个与地球的化学构成相近似的天体上时,这些辉熔长无球粒陨石便被弹射入太空。
尽管大多数陨石似乎都起源于相对较小的天体,但辉熔长无球粒陨石却展现出某些特性,表明它们的来源是某一颗庞大的行星,据信为火星。为了解释这样一种不太可能的来源,必须援引某个非同寻常的因素,因为一块岩石碎片要想逃逸出即使象月亮这么小的一个天体的重力场,使该岩石碎片加速所必不可少的撞击力是如此之大,以致于任何源自月亮的陨石从未被发现过。
虽然某些科学家猜测,辉熔长无球陨石起源于木卫一(Io,木星的一个火山活动频繁的卫星),但最近的测量表明,由于木卫一的表面含有丰富的硫和钠,因此,其火山产物的化学构成可能不会与辉熔长无球粒陨石的化学构成相同。此外,任何由于介体撞击作用而从木卫一分离出来的岩石碎片将不太可能逃逸出木星的重力引力。
辉熔长无球粒陨石其它唯一合乎逻辑的来源就是火星。太空探测器(spaceprobe)拍摄的照片显示出火星表面存在着巨大的火山。从出现在火星溶岩流(lavaflow)上的少量撞击坑(impact
crater),人们可估计出该行星近来一直到五千万年之前仍有活跃的火山活动——甚至今天还仍活跃。辉熔长无球粒陨石火星起因论最大的缺陷在于,地球上不存在任何源自月亮的陨石。考虑到月亮较小的尺寸以及与地球更为接近的空间距离,能将火星表面的一块碎片弹射入地球交叉轨道(Earth-intersecting
orbit)的撞击事件在火星上发生甚至要比这一事件在月亮上发生更不可能。但是,最近的一项研究表明,在火星表面下的永久冻土冰层(permafrost)很有可能改变了陨石对它的撞击作用。假如这些冰层因撞击的物体而迅速汽化蒸发的话,那么膨胀的气体很有可能帮助弹射出去的碎片达到逃逸速度。最后,由太空探测器进行的分析证明,在火星土壤和辉熔长无球粒陨石之间存在着一种令人瞩目的相近性。