火星上的一天有多长？

中国科技网12月17日报道（张微 编译）对科学家们来说火星是一个难解之谜。它在很多方面，与地球有着明显的差异。火星上寒冷、干燥，只有稀薄的大气层和微量的降水，而且寸草不生。按照我们的标准衡量，这里是让人难以想象的不毛之地。但是，在其它方面，火星与我们的世界也有很多相似之处。

例如，火星的内部结构由金属核、硅酸盐地幔和地壳组成，火星上也存在大量的水，但它们中的绝大部分集中在极地地区，以水冰的形式存在(在地表下有一个永久的冻土层)。但最让人关注的是，火星上一天的时长与地球几乎相同。

实际上，火星上的一天大约比地球长40分钟。相比于太阳系中的其它天体，要么一天时间非常短(木星自转周期9小时，55分钟，29.69秒)，要么一天很漫长(金星上的昼夜交替是116天18小时)，火星与地球一天时长的相似性非常令人震惊。

在宣布另一个星球上一天时间有多长之前，我们还需要解决一些事情。实际上，有两种方法用来确定一个天体一天的时长，恒星日和太阳日，两者都是天文学家用来确定时间流逝的。

恒星日VS太阳日

根据定义，子午线两次对向同一恒星的时间间隔(某地经线连续两次通过同一恒星与地心连线的时间间隔)叫做恒星日，恒星日是以遥远的恒星为参考系是地球自转360度的周期。在地球上，一个恒星日是23个小时，56分钟，4.1秒。相比之下，在火星上，一个恒星日持续24小时，37分钟，22秒。

太阳日，一个昼夜的周期。天文学上表示时间的单位，即以太阳的视圆面中心作参考点，日地中心连线连续两次与某地经线相交的时间间隔。在地球上，一个平均太阳日的时间是24个小时。在火星上，一个太阳日持续24小时，39分钟，35秒。四舍五入，所以我们说，地球上计算出的一天时间是24小时，而在火星上，每天24小时40分钟。

想知道火星与地球还有其它的相似之处吗?继续往下读。

季节变化

火星上也有与地球类似的季节更替。部分原因是，火星有一个倾斜的轴线，与轨道平面的倾角是25.19°(地球的倾角是23.44°)。还有一个原因，火星是偏心轨道，这意味着它与太阳的距离范围是2.067亿到2.492亿公里。

距离的变化会导致显著的温度变化。火星的平均温度为-46°(51°)，这个温度范围从冬天极地地区的-143°C(225.4°F)到夏季赤道附近正午时分的35°C(95°F)。在高温下，液态水能够在火星表面流动，虽然是间歇性的。

火星上也会下雪。在2008年NASA凤凰号着陆器在这个星球的极地地区发现了水冰。这不是一个令人意外的发现，但是科学家们没有观察到降雪的云层。这次降雪，结合土壤化学实验，让科学家们相信降雪的地点过去曾经有过温暖湿润的气候。

然后在2012年，火星侦察轨道器获得的数据显示，火星南极地区发生了二氧化碳降雪。几十年来，科学家们已经知道，干冰存在于南极极地冰冠地区，也会有季节性的变化。这是首次发现这个有趣的现象，目前这仍然是我们太阳系降二氧化碳雪的唯一例子。

此外，由火星侦察轨道器，火星科学实验室，火星轨道器任务(MOM)，火星大气与挥发演化探测器(MAVEN)以及机遇号和好奇号火星车所做的最新调查，已经揭发了火星遥远的时光中，那些令人惊讶的神秘过往。

首先，土壤样本和轨道观察器已经证实，大约37亿年前，火星表面有大量的水存在，面积超过现在的大西洋。同时，火星表面和空间进行的大气研究也证实了火星大气层的存在，这个大气层后来逐渐被太阳风剥离了。

天气模式

像地球一样，火星上也有极端天气。在这个红色星球上，常年刮各种类型的沙尘暴。这些风暴会蔓延上万公里，偶尔也会有环绕整个星球的巨型沙尘暴，浓重的雾霾灰尘遮天蔽日。当发生巨型沙尘暴的时候，这些沙尘会阻碍对火星表面的直接观测。

例子：当1971年火星水手9号探测器成为首个环绕火星轨道的航天器，它向地球发回的照片由于雾霾而模糊不清。整个星球被巨大的沙尘暴遮蔽，只能看到火星上的第一大火山，高达24千米的奥林匹斯山高出云层的部分。这场风暴持续了整整一个月，水手9号试图拍摄火星详细影像的任务被迫延迟了。

然后在2001年6月9日，哈伯太空望远镜发现火星上的海拉斯盆地沙尘暴。到七月，风暴已经平息，但随后又发展成为25年来最大的风暴。这次沙尘暴的规模非常大，业余天文学家使用小型望远镜也能够从地球上看到它。云层将寒冷的火星大气温度升高到惊人的30摄氏度。

火星与地球还有一个共同的话题——全球变暖!就像地球上正在发生的变暖趋势一样，火星上的气候变暖是由于空气中的颗粒物质的出现，这些物质吸收了太阳的能量并向外辐射到大气层，这导致了火星平均温度上升。

当火星靠近太阳时，往往会发生这些沙尘暴，这是温度升高引发了空气和土壤变化的结果。由于土壤干燥，它就很容易被气流吹起，气流是由于气温升高导致的压力变化所触发的。而沙尘暴又进一步导致了气温上升，所以你也可以这样认为，火星有自己的“温室效应”。

你可能会从以上列举的事例中得出结论，火星是一个严酷的星球。知道“火星上一天有多长?”的答案，仅仅让你初步了解那里正在发生什么。在一天结束的时候(不是双关语)，火星上发生的很多事情与地球及其相似，许多人正在考虑有一天能够在那里生活。深刻地了解火星与地球的区别，以及我们能利用火星做什么，才能实现移居火星。

英文原文：

How long is a day on Mars?

Mars represents something of a conundrum for scientists. In many respects, it is significantly different from Earth. It's cold, it's dry, there is little atmosphere or precipitation to speak of, and nothing grows there. By our standards, that makes it an incredibly inhospitable place. And yet, in many other respects, it is quite similar to our world.

For instance, Mars' internal structure is differentiated between a metallic core and a silicate mantle and crust. It also has plenty of water, though the majority of it is concentrated in the polar regions as water ice (and as a permanent layer of permafrost under much of the surface). But perhaps most striking of all, a day on Mars is almost the same as a day here on Earth.

In fact, a day on Mars is roughly 40 minutes longer than a day is here on Earth. Compared to other bodies in our solar system where a day is either incredibly short (Jupiter's rotates once on its axis every 9 hours, 55 minutes and 29.69 seconds) or incredibly long (a day on Venus lasts for 116 days and 18 hours), this similarity is quite astounding.

However, there are some things that need to be addressed before we go about declaring just how long a day is on another planet. In fact, there are two ways to determine the length of a day on a celestial body, the sidereal day and the solar day; both of which are used by astronomers for determining the passage on time.

Sidereal vs. Solar:

By definition, a sidereal day on Mars is the length of time that it takes the planet to rotate once on its axis so that stars appear in the same place in the night sky. On Earth, this takes exactly 23 hours, 56 minutes and 4.1 seconds. In comparison, on Mars, a sidereal day lasts 24 hours, 37 minutes, and 22 seconds.

The solar day, by contrast, is the time it takes for the Earth to rotate on its axis so that the sun appears in the same position in the sky. This position changes slightly each day, but on Earth, a mean solar day works out to being 24 hours long. On Mars, a solar day lasts 24 hours, 39 minutes, and 35 seconds. Rounding that out, we say that a day here on Earth works out to an even 24 hours while on Mars, a day lasts 24 hours and 40 minutes.

Want to know about some other interesting similarities Mars has with Earth? Read on!

Seasonal Changes:

Mars also has a seasonal cycle that is similar to that of Earth's. This is due in part to the fact that Mars also has a tilted axis, which is inclined 25.19° to its orbital plane (compared to Earth's axial tilt of approx. 23.44°). It's also due to Mars orbital eccentricity, which mean that it ranges in distance from 206.7 million to 249.2 million kilometers from the sun.

This change in distance causes significant variations in temperature. While the planet's average temperature is -46 °C (51 °F), this ranges from a low of -143 °C (-225.4 °F) during the winter at the poles to a high of 35 °C (95 °F) during summer and midday at the equator. This high in temperatures is what allows for liquid water to still flow, albeit intermittently, on the surface of Mars.

It also snows on Mars. In 2008, NASA's Phoenix Lander found water ice in the polar regions of the planet. This was an expected finding, but scientists were not prepared to observe snow falling from clouds. The snow, combined with soil chemistry experiments, led scientists to believe that the landing site had a wetter and warmer climate in the past.

And then in 2012, data obtained by the Mars Reconnaissance Orbiter revealed that carbon-dioxide snowfalls occur in the southern polar region of Mars. For decades, scientists have known that carbon-dioxide ice is a permanent part of Mars' seasonal cycle and exists in the southern polar caps. But this was the first time that such a phenomena was detected, and it remains the only known example of carbon-dioxide snow falling anywhere in our solar system.

In addition, recent surveys conducted by the Mars Reconnaissance Orbiter, the Mars Science Laboratory, the Mars Orbiter Mission (MOM), the Mars Atmosphere and Volatile Evolution (MAVEN) and the Opportunity and Curiosity Rovers have revealed some startling things about Mars' deep past.

For starters, soil samples and orbital observation have demonstrated conclusively that roughly 3.7 billion years ago, the planet had more water on its surface than is currently in the Atlantic Ocean. Similarly, atmospheric studies conducted on the surface and from space have proven that Mars also had a viable atmosphere at that time, one which was slowly stripped away by solar wind.

Weather Patterns:

Like Earth, Mars can have some pretty extreme weather. In the Red Planet's case, this takes the form of dust stormsthat can dominated the surface from time to time. These storms have been known to grow to be thousands of kilometers across, occasionally encircling the entire planet and covering everything in a thick haze of dust. When these storms become that large, they prevent direct observation of the Martian surface.

Case in point: when the Mariner 9 orbiter became the first spacecraft to orbit Mars in 1971, it sent pictures back to Earth of a world consumed in haze. The entire planet was covered by a dust storm so massive that only Olympus Mons, the giant Martian volcano that measures 24 km high, could be seen above the clouds. This storm lasted for a full month, and delayed Mariner 9's attempts to photograph the planet in detail.

And then on June 9th, 2001, the Hubble Space Telescope spotted a dust storm in the Hellas Basin on Mars. By July, the storm had died down, but then grew again to become the largest storm in 25 years. So big was the storm that amateur astronomers using small telescopes were able to see it from Earth. And the cloud raised the temperature of the frigid Martian atmosphere by a stunning 30° Celsius.

Therein lies another thing Mars has in common with Earth – global warming! Much like warming trends here on Earth, warming on Mars is caused by the presence of particulate matter in the air that absorbs energy from the sun and radiates it outward into the atmosphere. causing average temperatures to rise.

These storms tend to occur when Mars is closest to the sun, and are the result of temperatures rising and triggering changes in the air and soil. As the soil dries, it becomes more easily picked up by air currents, which are caused by pressure changes due to increased heat. The dust storms cause temperatures to rise even further, so you could say Mars has a "greenhouse effect" of its own!

As you have probably concluded from all the facts listed above, Mars can be a harsh and volatile planet. Just knowing the answer to "how long is a day on Mars?" only provides a small glimpse of what is going on there. At the end of the day (no pun intended!) there is plenty happening on Mars that makes it similar enough to Earth that many people are actually contemplating living there someday. And knowing exactly what sets Mars apart, and what we can work with, will be intrinsic to making that happen!