【推荐1】Scientists say they have found detailed evidence of ancient rivers on Mars. The discovery supported existing evidence that Mars once had water. The researchers said their findings suggested rivers may have flowed on the surface of Mars for hundreds of thousands of years.

These images were captured by a camera on NASA’s Mars Reconnaissance Orbiter. The camera is able to take detailed pictures of the surface while orbiting the planet from about 400 kilometers away. A team of scientists studied the images, which showed a valley network on Mars.

The team was led by Francesco Salese, a geologist at Utrecht University in the Netherlands. Salese said the scientists studied sedimentary (沉淀物) rocks from a 200 meters high rocky cliff. Sedimentary rocks form when sedimented (使沉积) and transported by water or wind.

“These are sedimentary rocks and were formed by rivers that were likely active for over 100,000 years,” Salese said, adding even without the ability to examine the cliff area on Mars, the pictures show strong similarities to sedimentary rocks found on the earth.

William McMahon is another geologist who was part of the investigation team. He said sedimentary rocks have long been studied on the earth to learn what conditions were like on our planet millions or even billions of years ago. Another leader of the team was Joel Davis, a researcher with Britain’s Natural History Museum. He said scientists had never before been able to examine such a rock formation with such great detail. They created 3D images of the area to get a more detailed understanding of it, which suggested some ancient Martian rivers were several meters deep.

【推荐3】The combined quantity of water on Earth has varied over the course of our planet’s geological history, and it still does.

Today, Earth includes some 1,386 million cubic kilometres of water, a volume that includes water in oceans, lakes and rivers, plus ground water, vapour in the atmosphere, and the frozen water of glaciers and ice caps. On the young Earth — some four billion years ago — vast quantities of water were added to the planet by ice-containing comets (a mass of ice and dust that moves around the sun and looks like a bright star with a tail) that struck us, eventually making our world a “blue planet”. But such events became increasingly rare in Earth’s more recent history, and today we are in an age during which Earth is losing water.

The water loss is due to the fact that particles (微粒) sometimes escape Earth’s gravity to travel into space. This is particularly true of the light hydrogen atoms that form part of water molecules (分子) together with oxygen. Every time the atmosphere loses hydrogen, we lose one of the building blocks of water. Scientists estimate that Earth loses about 3kg of hydrogen per second. At this pace, Earth would run out of water in three billion years — but that assumes that we don’t get any new water supplies.

New water need not come from space — it might come from the inside of the Earth. At depths below 50km, minerals contain water that is not reckoned within the water cycle. Some of it dates back to Earth’s formation; the rest is part of slow geological exchange between the planet’s layers. When Earth’s plates sink and melt, deep minerals can release these bound water molecules so that they can subsequently rise to the surface via volcanic eruptions, adding to the planet’s overall water resources.

【推荐1】Here is a guide to the best countries to see the Northern Lights, as well as top tips such as the best time to go.

★Where to go: Greenland

Kangerlussuaq is often regarded as the best spot in Greenland, with over 300 nights of clear skies. Alternatively, you can visit the capital city of Nuuk, where on clear nights, there have been plenty of sightings of the Northern Lights. And in the southern regions, darkness tends to fall earlier in the evening, ideal for those who don't want to stay up all night.

Best time to go: From November through to April

★Where to go: Norway

Head to the northern regions of Norway for the best possible chances of seeing the Northern Lights. Head to Svalbard, one of the world's northernmost inhabited areas, and you'll feel like you've stepped into an ice kingdom. Those who love the great outdoors might want to check out the Lyngenfjord region, where you'll find plenty of tours to see the Northern Lights. whale watching, skiing, ice climbing and so on!

Best time to go: From October through to March when the polar nights make them easier to spot.

★Where to go: Sweden

Head to Abisko National Park where the clear skies and lack of light pollution make for ideal Northern Lights viewing conditions. One of the best spots is in Swedish Lapland, where you can have dinner under the midnight skies. Hikers might also consider trying the Kungsleden hiking trail, which offers about 440km of breathtaking landscapes.

Best time to go from September through to March.

★Where to go: Iceland

You don't need to go to the remote landscapes to spot the Northern Lights. In fact, sightings are possible from Iceland's capital of Reykjavik.Want to escape the crowds? Head to lesser-known spots such as Thingvellir, which is the place where photographers try to capture the perfect Northern Lights photo.

Best time to go: From late August to mid-April, although you will increase your chances if you visit from late September through to March.

【推荐2】There are many ways to get involved and volunteer at the National Museum of American History. Becoming a Museum Ambassador is one of them. To learn more about this volunteer program, please see the descriptions below.

Museum Ambassadors at the American History Museum are a mobile source of information that helps bridge the gap between two other volunteer groups here at the museum — information desk specialists, who provide orientation (方向) assistance when guests arrive, and volunteer docents (讲师), who provide Highlights tours, facilitate hands-on demonstrations and circulate through many of our exhibitions to interact with visitors. Each year, our Museum Ambassadors help over 100,000 people get the most out of their visits by welcoming groups, circulating throughout the museum's public zones to answer basic questions, and directing visitors to programs and exhibitions of interest. The museum has also employed the Museum Ambassadors to assist with special projects and events as needed.

To learn more about what Museum Ambassadors do, check out what Larry has said, “As museum ambassadors, we get to share in the curiosity of a child, the nostalgia (怀旧) of returning visitors, the treasure hunt of students assigned to find important historical objects, and the amazement of those experiencing our eye on the nation's history for the first time. We can help recommend a meaningful museum visit just for you. There's more. We're also traffic cops when elevators and escalators break and extra eyes for museum security."

If you enjoy…

•interacting with diverse audiences, age groups and learning styles.

•working in an often busy and changing environment.

If you have…

•strong interpersonal and communication skills.

•an interest in history and communication.

If you can…

commit to three shifts a month (weekday Ambassadors) or twice a month (weekend Ambassadors).

The museum is open 364 days a year, including weekends and holidays.

•handle frequent walking, standing, and talking.

To Apply

To learn more information about the application process, please contact Ruth Shirley.

NOTE: Acceptance into the program is subject to successful completion of an interview and background investigation.

【推荐3】Necessary things for travel

Personal alarm

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$14.99

When you're traveling alone, particularly to a place that you know little about, it's important to give yourself peace of mind in terms of safety. A wearable personal safety alarm is a great idea.

It hugely adds to your sense of control on many a dark street and crowded public space. You can sleep with it under your pillow no matter what country you are in.

Portable charger

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$27.95

With our phones acting as our lifelines, traveling with one at low battery can make you stressful and, if an emergency （紧急 情况）occurs, even dangerous. Keeping a portable charger on hand to give your phone the juice it needs to be useful is important, and it won' t take up much space in your bag at all. The peace of mind is worth it.

Hand sanitiser (消毒液)

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$12.99

As a traveler, you should always carry hand sanitiser. It's small enough that you can't even notice it in a pocket of your bag, and it's totally fine to take through any airport. When it comes to hygiene-related (卫生相关的) emergencies, you really can't do without it. Believe it or not, alcohol- based hand sanitiser can even help you get a fire started, which could, of course, be important in a survival situation.

Reusable water bottle with Alter (过滤器)

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$36.84

As anyone who's traveled in the past two decades knows, liquids above the 3. 4-ounce limit will be taken away at the security checkpoint. Carry a reusable water bottle that includes a filter. Easy access (接近) to water in case you are short of it, especially in an emergency, is a lifesaver. Again, it's simple and easy to carry without any problems when it is checked.