# 46 The Inner Planets

There are nine planets in our solar system. But there are a lot of differences between them. Some people put the planets into two groups. They are called the inner and the outer planets.

There are four inner planets: Mercury, Venus, Earth, and Mars. All have rocky insides, or cores. Mercury is the smallest. It is also the closest to the sun. This planet has a great number of craters on its surface. The craters were caused by huge rocks. They crashed into the planet millions of years ago. Life is not possible on Mercury. The planet is 36 million miles away from the sun.

Venus is the next planet. It is about the same size as Earth. But it is very different from Earth. Venus is very hot, even hotter than Mercury. It is covered with thick clouds. These clouds are made of poison gases. Strong bolts of lightning constantly pass through the clouds. In the sky, Venus looks beautiful. It is often seen in the east just before sunrise.

Our own planet, Earth, comes next. It is 93 million miles from the sun. So far, it is the only place in our solar system where we are sure life exists. Almost three-quarters of its surface is water.

The fourth planet is Mars. It is about half the size of Earth, and much colder. It has two small moons. The surface of Mars is red. (Mars is sometimes called the Red Planet.) On the surface are many grooves. These look like beds of old rivers. Scientists wonder about these beds. Was there once water in them? Was there once life on Mars? They have sent probes to study Mars. One day we will know the answer to these questions.

# 46 內行星

我們的太陽系有九顆行星。但它們之間有很多差異。有些人將行星分為兩組。它們被稱為內行星和外行星。

有四顆內行星：水星、金星、地球和火星。它們都有岩石內部，或稱核心。水星是最小的。它也是最靠近太陽的。這顆行星表面有大量的隕石坑。這些隕石坑是由巨大的岩石造成的。它們在數百萬年前撞擊了這顆行星。水星上不可能存在生命。這顆行星距離太陽3600萬英里。

下一顆行星是金星。它的大小與地球差不多。但它與地球非常不同。金星非常熱，甚至比水星還熱。它被厚厚的雲層覆蓋。這些雲層是由有毒氣體組成的。強烈的閃電不斷地穿過雲層。在天空中，金星看起來很美。它經常在日出前在東方被看到。

接下來是我們自己的星球，地球。它距離太陽9300萬英里。到目前為止，它是我們太陽系中唯一確信有生命存在的地方。其表面近四分之三是水。

第四顆行星是火星。它大約是地球的一半大小，而且更冷。它有兩顆小衛星。火星表面是紅色的。（火星有時被稱為紅色行星。）表面有許多溝槽。這些看起來像古老河流的河床。科學家對這些河床感到好奇。它們曾經有過水嗎？火星上曾經有過生命嗎？他們已經發送了探測器去研究火星。總有一天我們會知道這些問題的答案。