[00:00.00]NARRATOR: Listen to part of a lecture in an astronomy class.[00:03.91]MALE PROFESSOR: There's been a lot of talk recently about life on Mars, at the level of microorganisms anyway, mainly because of a few important discoveries and inventions.
[00:13.71]For example, one major discovery was that at one point water was present on Mars. [00:19.13]How do we know?[00:20.19] Well, in 2004, an exploration robot discovered jarosite there.[00:25.59]Jarosite is a yellowish brown mineral with a crystalline structure that's also found on Earth. [00:31.66]It contains iron, potassium and hydroxide.[00:34.83]The interesting thing is that on Earth at least it needs highly acidic water to form.[00:40.18]So we've got water or had it at one point. [00:43.65]And since most planetary scientists believe that water is essential to life, the presence of jarosite means that one prerequisite for life was once present on Mars.[00:52.75]But there's another thing about jarosite. [00:57.33]One step in its formation on Earth involves microorganisms; they actually speed up the formation of jarosite dramatically. [01:05.26]Now, theoretically it is possible for jarosite to form without the help of biological life forms. [01:11.67]But we don't really know for sure if this happens 'cause, well, because every corner of Earth has some form of biological life.
[01:18.17]But jarosite on Earth incorporates all kinds of microorganisms into its crystalline structure. [01:24.84]So it's possible that if the jarosite on Mars was also formed with the help of microorganisms, we might be able to detect remnants of them in the samples we find. [01:35.18]And we have instruments now that will enable us to try to do this. [01:39.15]For example, there's a new instrument called the microfabricated organic analyzer, or M.O.A.[01:46.74]The organic analyzer is an amazing tool. [01:51.34]It will be able to collect soil samples and analyze them right there on Mars, pure, untouched samples.[01:58.91]It will let us eliminate the risk we would take of contaminating the samples if they were brought back to Earth.[02:04.51]And what they'll look for specifically in the soil is amino acids.[02:09.87]Amino acids, as you may know, are the building blocks of proteins. [02:16.03]In fact, there are twenty standard amino acids involved in making proteins and lots more that aren't.
[02:21.79]And here's the important thing. [02:23.84]Amino acids are what we call handed.[02:26.39]They can exist in two forms, which are mirror images of each other like hands. [02:31.74]Right and left hands have the same number of fingers in the same order plus one thumb.[02:36.85]But right and left hands are not the same; they are mirror images. [02:40.81]Well, like hands, amino acids can be right or left-handed. [02:45.02]And the twenty that make up the proteins on Earth are all left-handed.[02:48.98]Now, one reason the M.O.A., the organic analyzer is so impressive is that it tests not just for the presence of amino acids but also for the handedness of amino acids. [03:02.84]If amino acids are found, it would be especially interesting if they show a prevalence of one type of handedness, either left, like amino acids on Earth, or right.
[03:13.06]See, other physical processes in space, processes that don't involve living organisms, can create amino acids.[03:20.94]But the ones synthesized through abiotic processes, which is to say not involving microorganisms, occur in equal numbers of right- and left-handed.
[03:30.92]So, a prevalence of left-handed amino acids would indicate they were biological in origin, which would be amazing! [03:38.93]A prevalence of right-handed ones—well, that would be really amazing!! [03:44.37]Because the organisms that created them would be unlike anything we have on Earth, which produce only left-handed ones.
托福14天冲刺微信战团