ESA Rosetta: Earth's Water Came from Asteroids, Not Comets

ESA Rosetta’s navigation camera obtained the four images in this mosaic on Dec. 7, 2014, from a distance of 12.2 miles (19.7 km) from the center of Comet 67P/Churyumov-Gerasimenko.

Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

Asteroids, not comets, may have delivered most of Earth's water to the planet when the solar system was young, new data from the ESA Rosetta probe orbiting comet 67/P suggests.

Comets are some of the solar system's most primitive building blocks, with many dating to soon after its formation.

Scientists think that these dirty snowballs probably helped seed Earth with key ingredients for life, such as organic compounds.

The European Space Agency's (ESA) Rosetta spacecraft is helping scientists learn more about the role these icy nomads have played in the evolution of the solar system and life on Earth by analyzing the composition of Comet 67P/Churyumov–Gerasimenko.

In August, Rosetta became the first spacecraft to orbit a comet, and in November, its Philae lander became the first probe to make a soft touchdown on a comet's surface.

Rosetta is also the first mission to escort a comet as it travels around the sun.

Now, Rosetta has helped solve a mystery about how Earth became the watery world it is today.

Before Rosetta began orbiting Comet 67P/C-G in August, it was using an instrument known as ROSINA (short for Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) to analyze the chemical fingerprint of gases in the comet's fuzzy envelope.

Scientists focused on data from the instrument regarding water to help uncover whether asteroids or comets delivered the water in Earth's oceans.

Rosetta has provided data from Comet 67P/C-G, another Kuiper Belt comet.

However, Rosetta has discovered that this comet possesses an even higher deuterium-to-hydrogen ratio than seen in Oort Cloud comets, three times the amount of heavy water compared to normal water as Earth has.

If Earth's water had come from Kuiper Belt objects, even if most of them were like comet 103P/Hartley 2, and if only a small fraction were like Comet 67P/C-G, Earth's deuterium-to-hydrogen ratio would be significantly higher than it is today.

"This probably rules out Kuiper Belt comets from bringing water to Earth," Altwegg said. Instead, most of Earth's water was probably delivered by asteroids, Altwegg said.

"Today's asteroids have very little water, that's clear," Altwegg added. "But that was probably not always the case. During the Late Heavy Bombardment 3.8 billion years ago, at that time, asteroids could have had much more water than they could now."

The asteroids seen now "have stayed in the vicinity of the sun for 4.6 billion years," Altwegg said.

"They've lost water due to the sun, due to heat. But to start with, they might have had much more water than they have now."

Future analysis of ice-rich bodies in the asteroid belt could shed light on whether Earth's water really did come from there, Altwegg said.

NASA ISS IV-TEPC Instrument: Tissue-equivalent proportional counter

The Tissue Equivalent Proportional Counter (TEPC), in situ on the ISS, consists of a spectrometer and cylindrical detector with which to measure external radiation doses.

The purpose of the TEPC is to collect a record of the International Space Station (ISS) environment to construct exposure history records for the crew.

Credit: NASA, JPL

Description
The Tissue Equivalent Proportional Counter (TEPC) is a gas proportional counter used to characterize the radiation environment.

TEPC will also provide near real-time measurements to ground personnel during radiation events and make survey measurements in different parts of the ISS for shield verifications.

TEPC collects data as a function of time to measure the dose and estimate the dose equivalent by making spectral measurements of the lineal energy loss of the radiation as it passes through the detector volume.

The omni-directional detector is surrounded by a tissue equivalent plastic and the internal gas (propane) provides an energy deposition response similar to human tissue. The detector gas is at a very low pressure such that the mass of the gas is approximately that of a cell.

The 512 channel spectrometer stores the lineal energy data in energy bins ranging from approximately 25 keV/micron through channels exceeding 1000 keV/micron. The crew is able to read the current level through an electronic display and has the capability to telemeter data to the ground every 10 seconds.

TEPC is a portable piece of equipment, integrated with numerous ports in various modules to support the survey function of the equipment.

TEPC is an automatic micro-dosimetry system. Each TEPC consists of two main components, the spectrometer unit and the detector unit.

The spectrometer unit contains a powerful computer that allows real-time analysis of the data and provides calculations of total dose, total dose equivalent and incremental dose, as a function of linear energy transfer (LET) and time, for penetrating radiation in space.

The detector unit is attached directly to the multi-channel analyzer (MCA) card in the spectrometer.

Different size detectors can be attached to the TEPC depending on the desired task.

The radiation data that is measured can be stored inside the spectrometer unit for later analysis or communicated via RS-232 to a host computer.

The TEPC is calibrated in terms of lineal energy, by exposing it to fission neutrons and 137Cesium sources.