To begin with, they will have to find out whether life has ever existed on the planet of Mars in the past.

首先，他们必须搞清楚火星上是否曾经存在过生命。

youdao

If the young Sun was much fainter and cooler than the Sun today, liquid water couldn't have existed on either planet.

如果早期的太阳比现在的太阳暗淡，也没有现在这么炙热的话，那么液态水就不可能在这两个星球上存在。

youdao

Even setting aside the unproven hints of ancient oceans, the extent of the outflow channels suggests that a huge total volume of water existed on Mars in the past.

即使不考虑未经证实的古代海洋迹象，流出渠道的范围表明，过去火星上存在着大量的水资源。

youdao

President Wilson soon learned that opposition to the League of Nations existed on both sides of the Atlantic Ocean.

VOA: special.2010.11.04

In nineteen sixty-five, NASA's Mariner Four spacecraft showed that no waterways existed on Mars.

VOA: special.2010.07.11

When she was only twelve years old, Mary became the first person to find the almost complete skeletons of several animals that no longer existed on Earth.

VOA: special.2009.05.13

Anti-discrimination and anti-vilification laws have already existed on the books in some states in mid 2000s.

FORBES: Brazil Close To Outlawing Discrimination Against Gays

Of the many species that have existed on earth, more than ninety-nine per cent have disappeared.

NEWYORKER: The Sixth Extinction?

Even so, there is still no evidence that life ever existed on Mars, the scientists said.

WSJ: Scientists Say Microbial Life Was Possible on Mars