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of the innermost planet
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Frequently Asked Questions
The Mission   |  The Journey   |  The Planet   |  The Science

The Planet
General information about Mercury

For additional information, check out this NASA fact sheet.

1. What are some differences between Mercury and Earth?

The major differences between Mercury and Earth are that Mercury is smaller and much closer to the Sun. Its surface temperature is a lot higher than Earth’s during the day and a lot colder than Earth’s during the night. This happens because Mercury’s atmosphere is negligible compared to Earth’s. (That is also why, even though much of the atmosphere on Mercury is made of oxygen, that you wouldn’t be able to breathe there – there just isn’t enough oxygen to breathe!)

Mercury’s day is 176 Earth days long and its year is 88 Earth days; that means a day on Mercury – the time from one sunrise to the next – is two Mercury years long! Since Mercury’s rotation axis does not tilt as much as Earth’s, the planet doesn’t have seasons in the same way as we do on Earth. You probably would not notice the passing of a year as much as you would the rising of the blistering Sun. (Viewed from Mercury, the Sun looks as much as three times as large as it does from Earth and is up to 11 times brighter!)

But the biggest difference, at least as far as we know, is that there is no life on Mercury. Its inhospitable conditions are not conducive to life as we know it.

2. How did Mercury get its name?

The planets are named after Roman deities. In Roman mythology, Mercury is the god of commerce, travel, and thievery. He was the Roman counterpart of the Greek god Hermes, the messenger of the gods. In art, he was depicted as a young man wearing a winged hat and winged sandals, and he carried a herald’s staff (caduceus) crowned with two snakes.

The planet likely received its name because it moves so quickly across the sky compared with the other planets. Many European languages use a derivation of the Latin word Mercurius for the planet – but there are plenty of non-Latin derivative names for Mercury as well. Visit http://www.nineplanets.org/days.html for more information and a complete list of names.

3. When was Mercury discovered?

Mercury has been known since at least the time of the Sumerians (circa 3000 B.C.), whose texts describe the then-known planets and their perceived astrological influences. For example, references to Mercury can be found in the epic of Gilgamesh.

However, since the planet is so close to the Sun and is only visible from Earth just after sunset or just before dawn, it is not certain whether it was recognized in the same way as the much brighter Venus, Jupiter, or Mars. That Mercury appears as either an “evening star” after sunset or a “morning star” just before dawn often confused the ancient cultures, and it was first thought to be two planets. The ancient Greeks named it Hermes when it appeared in the evening, and Apollo at dawn, and it wasn’t until around 350 B.C. that they realized the two “stars” were the same planet.

4. Is there really ice on Mercury?

Earth-based radar observations of Mercury have revealed bright areas inside craters near the planet’s north and south poles, and one explanation for this effect is the presence of water ice. At first it seems silly to even think about ice on a planet where “daytime” surface temperatures can reach a maximum of about 700 K (about 800°F). However, two main arguments support this idea:

  • Mercury’s rotation axis does not tilt, so the Sun never gets very high in the sky at Mercury’s poles;
  • The atmosphere is so thin that heat from the Sun-baked areas does not spread to colder ones – that’s why the temperatures during Mercury’s night are so cold!

This means that the floors of craters near Mercury’s poles never receive sunlight. The temperatures in these permanently shadowed places are thought to be cold enough for ice to exist for long periods. Even though the temperatures on the night side can get cold enough for ice to form, that ice would vanish when morning arrived.

5. Why explore Mercury?

The importance of Mercury, beyond that we know so little about this planet, is that it offers a chance to examine another outcome of the processes that also produced Earth, Venus, and Mars. Learning how Mercury ended up the densest planet (after correcting for internal pressures) will tell us much about planetary formation. Discovering how Mercury has sustained a magnetic field while larger bodies either have lost an earlier field (as Mars did) or have no present field or a record of a past field (Venus) will help us to understand magnetic field generation in our own planet.

Mercury also has the thinnest atmosphere among all the terrestrial planets and an incredibly wide temperature range. In fact, temperatures vary from nearly the highest in the solar system (at the equator) to among the coldest (in the permanently shadowed areas where ice deposits seem to lurk). Documenting the nature of Mercury’s tenuous and changeable atmosphere and the composition of its mysterious polar deposits - thought by many to consist of water ice - will give us new insight into the volatile materials in the inner solar system.

6. What is the temperature on Mercury?

On its sunny side, Mercury can reach a scorching 700 K (about 800°F). But because it has virtually no atmosphere to hold in that heat, temperatures drop to about 90 K (about -300°F) on the dark side.

7. How will MESSENGER deal with the intense heat at Mercury?

MESSENGER's instruments operate at room temperature behind a sunshade made of heat-resistant Nextel fabric. The spacecraft itself is covered in a blanket of the multi-layer insulation. MESSENGER will also pass only briefly over the hottest parts of the surface, limiting exposure to reflected heat.

8. Is Mercury the hottest planet in our solar system?

Believe it or not, no. Venus - which MESSENGER passed twice on its way to Mercury - is the hottest planet in our solar system, thanks largely to the heat-trapping greenhouse effect of its thick, mostly carbon dioxide atmosphere. The surface temperature is about 740 K (about 870°F) - hot enough to melt lead!

9. How big is Mercury?

Mercury’s diameter is 4,880 kilometers (3,032 miles), roughly 38 percent of Earth’s diameter. Mercury is the smallest planet in the solar system. For comparison: If Earth were the size of a baseball, Mercury would be the size of a golf ball.

10. Mercury looks a lot like Earth's Moon. Are they very similar?

The lack of a thick atmosphere leaves solid bodies in the Solar System to a fate of impact battering, so that the surfaces of the Moon and Mercury are highly cratered and have a superficial resemblance. However, with MESSENGER in orbit we are starting to see features in high-resolution images that are unlike anything on the Moon. Even from Mariner 10 it was clear that Mercury's smooth plains are not dark, whereas the Moon's smooth volcanic mare plains are dark.  MESSENGER's geochemical sensors have determined that the composition of the rocks on Mercury's surface is radically different from those of the Moon, being far poorer in iron and far richer in sulfur.  Mariner 10 also revealed Mercury's astounding, distinctly non-lunar, abundance of lobate scarps, formed by global contraction.  MESSENGER is showing us that there are many more lobate scarps than we expected. In addition, Mercury has a huge iron core, whereas the Moon's iron core is tiny. Motions within the liquid portion of Mercury's core generate a global magnetic field; the Moon has no global field at present.  One of MESSENGER’s prime objectives is to provide clues as to why the bulk composition of Mercury is so different from that of the other inner planets. In other words, Mercury is different from the Moon in just about every way that can be measured!

11. What if life was found on Mercury?

Life on Mercury would be really big news but is not likely because there is no atmosphere and, as far as we know, living things cannot develop in a vacuum. Finding life on Mercury is no more likely than finding life on the Moon, and there is no evidence for life there. Even for life in extreme environments on Earth (such as bacteria in oil shale or tube worms near some deep-sea hydrothermal vents), water seems to be necessary, and there appears to be no evidence for there ever being liquid water on Mercury or on the Moon.

12. Why does the sky on Mercury look black?

Though Mercury has an exosphere, it is too thin to scatter sunlight in any significant amount. Without the scattering of light in an atmosphere, the sky on Mercury is just the blackness of space, similar to the view from the Moon. In contrast, Earth's atmosphere scatters sunlight, and the scattering is greatest for light at the shortest wavelengths, resulting in the blue color of the daytime sky.

If you have a question that has not been answered here, we invite you to submit it to us.
We also welcome questions and comments on the MESSENGER Website. Send a note to the Webmaster or check our Contacts page.


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