Are comets the origin of Earth’s oceans?

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Are comets the origin of Earth’s oceans?

From The Daily Beast:

Water, water everywhere, but where did it come from? One common explanation is that the water in Earth’s oceans was brought by comets, which bombarded the planet during its earliest period. It’s a simple, logical, and testable story.

But that doesn’t mean it’s right. A new study published last week in Science revealed that the water on Comet 67P/Churyumov-Gerasimenko doesn’t match that found on Earth. Specifically, instruments aboard the Rosetta probe measured the relative amount of deuterium in the comet’s water and found it was roughly three times higher than the amount in Earth’s oceans. Comets are chemically pristine, mostly unchanged over the Solar System’s 4.5 billion year history, so a mismatch in the deuterium content complicates the story of Earth’s water. [Read more at The Daily Beast….]

Artist's impression of the ringed asteroid Chariklo. While the asteroid is too small and distant to image directly, astronomers found two narrow rings around it — making it the smallest known object with a ring system. [Credit: ESO/L. Calçada/M. Kornmesser/Nick Risinger (skysurvey.org)]

Artist’s impression of the ringed asteroid Chariklo. While the asteroid is too small and distant to image directly, astronomers found two narrow rings around it — making it the smallest known object with a ring system. [Credit: ESO/L. Calçada/M. Kornmesser/Nick Risinger (skysurvey.org)]

Saturn’s magnificent rings have been known since Galileo observed the planet’s “ears” in his telescope. In the last few decades, researchers found rings (albeit less shiny ones) around the other giant planets — Jupiter, Uranus, and Neptune. And now the small asteroid Chariklo has joined the ring cycle: observations revealed it has two narrow rings, probably composed of water ice. It’s an intriguing discovery, since nothing else we’ve found at intermediate sizes has rings, leading to questions of how they form, how stable they may be, and whether there might be other beringed objects out there.

Beyond size, another challenge is Chariklo’s location between Saturn and Uranus. It orbits in a long ellipse, ranging from 13 to nearly 19 times farther from the Sun than Earth. This position, along with its composition of rock and ice, marks Chariklo as a “centaur.” Just like mythological centaurs are half human and half horse, astronomical centaurs combine features of asteroids and comets. (Centaurs would grow comet-like tails if they fell closer toward the Sun.) Tens of thousands of centaurs may lurk among the giant planets, though most of those are much smaller than Chariklo, the largest known centaur. [Read more…]

All the single centaurs

Artist’s conception of the Kuiper belt. [Credit: Don Dixon]

When we talk about big advances in planetary science, we often are thinking about Mars rovers or the discovery of exoplanets. However, one area where we’ve learned a lot over the last few decades is the Kuiper belt: a region beyond the orbit of Neptune inhabited by small bodies of ice and rock. Before 1992, Pluto was the most distant known Solar System object, but between then and now, astronomers have discovered a wealth of Kuiper belt objects (KBOs).

A new paper (coauthored by Mike Brown of Pluto-killing infamy) describes a puzzle arising from a survey of many KBOs: some of them don’t fit in with the standard model of planet formation:

A new study of large scale surveys of KBOs revealed that those with nearly circular orbits lying roughly in the same plane as the orbits of the major planets don’t fit the Nice model, while those with irregular orbits do. It’s a puzzling anomaly, one with no immediate resolution, but it hints that we need to refine our Solar System formation models. [Read more…]

Some planet-like Kuiper belt objects don’t play “Nice”

The MESSENGER (MErcury Surface Space ENvironment, GEochemistry, and Ranging) spacecraft has found strong evidence both for water ice and organic molecules in shadowed craters near Mercury’s poles. Unlike Earth, Mercury has no seasons: its axis stands perpendicular to its plane of orbit, so deep craters near the north and south pole will have bottoms in permanent shade. Any place else on the surface will eventually be exposed to the Sun’s punishing glare, not only melting ice but boiling away any residual water. However, during the early Solar System, meteorites and comets brought water and organic compounds to the planet’s surface—at least according to planet formation models. This new discovery lends strong support to that theory.

The results from both the reflection and neutron analyses were consistent: several craters in Mercury’s polar regions provide sufficient shadow for stable water ice. The large craters named Prokofiev and Kandinsky were both found to contain significant radar-bright (RB) patches, indicating highly reflective materials. (Craters on Mercury are commonly named for famous artists, authors, composers, and the like. As a fan of both Prokofiev and Kandinsky, I approve.)

The size of the reflective patches matched the total proportion of each crater that lies in permanent shadow. [Read more…]

MESSENGER: Mercury’s craters have ice and organic molecules

The dwarf planet Makemake (pronounced MAHkayMAHkay) is about 2/3 the diameter of Pluto, and farther from the Sun. That makes it hard to observe. Astronomers using a set of telescopes in South America tracked it during an occultation: a brief interval when it passed in front of a faint star. By measuring the light curve—the variation in light as Makemake eclipsed the star—the researchers determined that the dwarf planet has no substantial atmosphere, and probably no sizable moon.

Makemake potentially occults three stars in a typical year, though not all of these are useful, due to the faintness of the background star. The current study involved an occultation visible from South America on April 23, 2011. Just as eclipses may be partial or total, the “shadow” of Makemake passed over a swath of the continent, allowing telescopes in various locations to measure the passage of the star behind different parts of the dwarf planet. The researchers tried to obtain data from 16 telescopes, but only 7 of those returned successful measurements.

Each telescope saw a clear, sharp drop in the background star’s light, a strong indicator that Makemake has no substantial atmosphere. [Read more….]

Makemake has no atmosphere, possibly a partly frosted surface