火星並不像我們所見的如此乾涸
兩篇英國牛津大學的新研究闡明了火星為何沒有生命
科學家搜索生命時的首要任務便是尋找維持生命的要素之一：淡水。
雖然現在火星的表面一片貧瘠、寒冷而不適合生命居住，但有一絲證據指出火星過去是一顆較為溫暖濕潤且表面有流水的星球。究竟發生了什麼事情讓火星上的水消失？這項謎題長久以來仍然懸而未解。發表在《自然》的最新研究提出這些水分現今是被封存在火星的岩石當中。
牛津大學地球科學系的科學家提出火星表面的岩石跟水反應後將水給吸收了。這個過程造成岩石的氧化程度增加，也讓火星變得不適合生命居住。
過往研究曾提出由於火星磁場的衰敗造成大部分的水分往外太空流失，當時水分被強烈的太陽風帶走，或者是以冰塊的形式封存於地底。然而，這些理論並無法解釋為何所有水分都消失了。
由牛津大學地球科學系的NERC(自然環境研究委員會)研究員Jon
Wade博士領導的團隊確信這道難題的解答就藏在火星的礦物裡面。他們運用平常分析地球岩石組成的模擬方法，來計算火星表面的水分有多少可以透過岩石跟水之間的反應而流失。團隊評估了岩石溫度、地底壓力和火星整體組成對行星表面會有什麼樣的影響。
研究指出火星的玄武岩所能蘊含的水分比地球的玄武岩還多了將近25%，結果就是火星表面的水分有許多都被岩石吸收到火星內部。
Wade博士表示：「雖然科學家從很久以前就一直在思索這項問題，但卻從來沒有人去驗證水單純是因為跟岩石之間發生了反應而被吸收掉的理論。我們將現有的零星證據拼湊之後得到的結果，使我們開始相信要讓火星地函氧化必定需要一種(跟地球)不同的作用。舉例來說，火星隕石相較於其表面的岩石來說在化學上較為還原，而且它們的成分看起來也相差甚遠。這些現象的其中一個原因，以及火星為何會喪失所有水分的答案，可能都跟它的礦物學有關。」
「地球目前的板塊構造系統可以防止地表水的水位發生劇烈變化，這是因為含水的岩石在進到地球相對乾燥的地函以前會充分脫水的緣故。但早期地球和火星都沒有這種可以讓水循環的系統。在火星，形成玄武岩地殼的新鮮岩漿噴發出來時就會跟水反應，造成的效果就跟海綿一樣。火星的水跟岩石反應後形成了許多種類的含水礦物。這種水和岩石之間的反應不但改變了岩石本身的礦物組成，也讓火星的地表逐漸乾涸而變得不適合生物居住。」
那地球為什麼從未經歷此種變化？Wade博士表示：「火星的體積比地球小上許多，而且它的矽質地函的溫度曲線跟地球不同，含鐵量也比較高。雖然這些差異相當微小，但隨著時間經過卻會累積成為重大影響，使得火星表面更容易跟地表水發生反應並形成含水礦物。火星的地球化學因為這些因素自然會將水分帶往地函內部，然而早期地球的含水岩石卻傾向於在脫水之前持續浮在地函上方。」
Wade博士論文中的主旨――行星的組成基本上決定了其未來的適居性――呼應了另一篇同樣發表在《自然》探討地球鹽度的新研究。由牛津大學地球科學系的教授Chris
Ballentine共同著作的研究中，顯示要讓生命形成並繼續生存下去，地球的鹵素(氯、溴、碘)濃度必須要恰到好處才行，太高或者太低都會讓地球變成不毛之地。過往研究對隕石鹵素濃度的估算得出相當高的數值。相較於組成地球的隕石樣品，它們的含鹽比例對地球來說實在是太高了。
有許多科學家提出理論來解釋此種差異是如何形成。這兩篇研究結合起來提升了證據的可信度，同時也支持進一步研究的必要。Wade博士表示：「太陽系內行星的成分大體來說相去不遠，但是差之毫釐卻會失之千里――岩石化學便是一個例子。火星跟地球岩石化學的最大差異是火星地函岩石的鐵多了一些，其來自於火星形成時的環境稍微偏向氧化條件。」
雖然我們知道火星一度擁有水且有機會讓生命繼續發展下去，但是我們對於其他行星的狀況實在所知不多，而研究團隊正在努力改變這種情況。
Wade博士表示：「要發展這項研究我們想先測試其他敏感因子對各個行星造成的影響――比方說我們所知甚少的金星。我們的問題像是：如果地球地函的鐵含量更多或者更少，會對環境造成什麼樣的變化？那如果地球的體積較大或者較小呢？這些答案有助於我們了解岩石化學對一顆行星的宿命會有多大程度的影響。」
「在其他星球上尋找生命時不能只看它的總體化學組成是否合宜，其他像是行星如何組合的細節也會深深影響到其表面是否擁有水分。但是對於其他星球來說，這些細節造成的影響及其後果卻還沒有被真正地好好探討過。」
Mars: Not as
dry as it seems
Two new
Oxford University papers have shed light on why there is no life on Mars.
When searching for life, scientists first look for an
element key to sustaining it: fresh water.
Although today’s Martian surface is barren, frozen
and inhabitable, a trail of evidence points to a once warmer, wetter planet,
where water flowed freely. The conundrum of what happened to this water is long
standing and unsolved. However, new research published in Nature suggests that this water is now locked in the Martian rocks.
Scientists at Oxford’s Department of Earth Sciences,
propose that the Martian surface reacted with the water and then absorbed it,
increasing the rocks oxidation in the process, making the planet uninhabitable.
Previous research has suggested that the majority of
the water was lost to space as a result of the collapse of the planet’s
magnetic field, when it was either swept away by high intensity solar winds or
locked up as sub-surface ice. However, these theories do not explain where all
of the water has gone.
Convinced that the planet’s minerology held the
answer to this puzzling question, a team led by Dr Jon Wade, NERC Research
Fellow in Oxford’s Department of Earth Sciences, applied modelling methods used
to understand the composition of Earth rocks to calculate how much water could
be removed from the Martian surface through reactions with rock. The team
assessed the role that rock temperature, sub-surface pressure and general
Martian make-up, have on the planetary surfaces.
The results revealed that the basalt rocks on Mars
can hold approximately 25 per cent more water than those on Earth, and as a
result drew the water from the Martian surface into its interior.
Dr Wade said: ‘People have thought about this
question for a long time, but never tested the theory of the water being
absorbed as a result of simple rock reactions. There are pockets of evidence
that together, leads us to believe that a different reaction is needed to
oxidise the Martian mantle. For instance, Martian meteorites are chemically
reduced compared to the surface rocks, and compositionally look very different.
One reason for this, and why Mars lost all of its water, could be in its
minerology.
‘The Earth’s current system of plate tectonics
prevents drastic changes in surface water levels, with wet rocks efficiently
dehydrating before they enter the Earth’s relatively dry mantle. But neither
early Earth nor Mars had this system of recycling water. On Mars, (water
reacting with the freshly erupted lavas’ that form its basaltic crust, resulted
in a sponge-like effect. The planet’s water then reacted with the rocks to form
a variety of water bearing minerals. This water-rock reaction changed the rock
mineralogy and caused the planetary surface to dry and become inhospitable to
life.’
As to the question of why Earth has never experienced
these changes, he said: ‘Mars is much smaller than Earth, with a different
temperature profile and higher iron content of its silicate mantle. These are
only subtle distinctions but they cause significant effects that, over time,
add up. They made the surface of Mars more prone to reaction with surface water
and able to form minerals that contain water. Because of these factors the
planet’s geological chemistry naturally drags water down into the mantle,
whereas on early Earth hydrated rocks tended to float until they dehydrate.’
The overarching message of Dr Wade’s paper, that
planetary composition sets the tone for future habitability, is echoed in new
research also published in Nature,
examining the Earth’s salt levels. Co-written by Professor Chris Ballentine of
Oxford’s Department of Earth Sciences, the research reveals that for life to
form and be sustainable, the Earth’s halogen levels (Chlorine, Bromine and
Iodine) have to be just right. Too much or too little could cause
sterilisation. Previous studies have suggested that halogen level estimates in
meteorites were too high. Compared to samples of the meteorites that formed the
Earth, the ratio of salt to Earth is just too high.
Many theories have been put forward to explain the
mystery of how this variation occurred, however, the two studies combined
elevate the evidence and support a case for further investigation. Dr Wade said
‘Broadly speaking the inner planets in the solar system have similar
composition, but subtle differences can cause dramatic differences – for
example, rock chemistry. The biggest difference being, that Mars has more iron
in its mantle rocks, as the planet formed under marginally more oxidising
conditions.’
We know that Mars once had water, and the potential
to sustain life, but by comparison little is known about the other planets, and
the team are keen to change that.
Dr Wade said: ‘To build on this work we want to test
the effects of other sensitivities across the planets – very little is known
about Venus for example. Questions like; what if the Earth had more or less
iron in the mantle, how would that change the environment? What if the Earth
was bigger or smaller? These answers will help us to understand how much of a
role rock chemistry determines a planet’s future fate.
When looking for life on other planets it is not just
about having the right bulk chemistry, but also very subtle things like the way
the planet is put together, which may have big effects on whether water stays
on the surface. These effects and their implications for other planets have not
really been explored.’
原始論文：(1)
Jon Wade, Brendan Dyck, Richard M. Palin, James D. P. Moore, Andrew J. Smye. The divergent fates of primitive
hydrospheric water on Earth and Mars. Nature,
2017; 552 (7685): 391 DOI: 10.1038/nature25031
(2) Patricia L. Clay, Ray Burgess, Henner Busemann, Lorraine
Ruzié-Hamilton, Bastian Joachim, James M. D. Day, Christopher J. Ballentine. Halogens in chondritic meteorites and
terrestrial accretion. Nature,
2017; 551 (7682): 614 DOI: 10.1038/nature24625
引用自：University
of Oxford. "Mars: Not as dry as it seems: Water on Mars absorbed like a
sponge, new research suggests."
