Big impact on Ceres

Hi there! Today: Ceres. (1) Ceres is the largest object of the main asteroid belt, so large that the International Astronomical Union (IAU) classified it as a dwarf planet in 2006. As many planetary bodies, it is craterized, the largest crater being named Kerwan. This crater has a diameter of 280 km. But this is not the most remarkable one. The crater Occator, which diameter almost reaches 100 km, is particularly interesting since it exhibits bright spots, which are probably the signature of past hydrothermal activity. This raises the interest of the scientific community, since it could reveal a geophysical activity and water below the surface.
The study I present, The various ages of Occator crater, Ceres: Results of a comprehensive synthesis approach, by A. Neesemann et al., tries to be as accurate as possible on the age of Occator, in summarizing the previous studies and in using as many data as possible. These are actually data provided by the spacecraft Dawn. This paper will be published in Icarus soon.

The dwarf planet (1)Ceres

Discovery

The quest for an object between the orbits of Mars and Jupiter was initially motivated by the Titius-Bode law. This empirical law, which is now proven to be absolutely wrong, noticed a arithmetic progression between the orbital radii of the known planets, and was confirmed by the discovery of Uranus in 1781 (however, it is inconsistent with the presence of Neptune). Anyway, this convinced former astronomers that something was there, and it revealed to be true. A group led by Franz Xaver von Zach looked for an object with a semimajor axis close to 2.8 AU (astronomical units, remember that 1 AU is 150 million kilometers, which is the orbital radius of our Earth). But that group did not discover Ceres.

Ceres has been serendipitously discovered in 1801 by the Italian astronomer Giuseppe Piazzi in Palermo, Sicilia. He wanted to observe the star HR 1110, but saw a slowly moving object instead. He noticed that it looked somehow like a comet, but that it was probably better than that. Ceres was found!

Giuseppe Piazzi (1746-1826) pointing at Ceres. © Palermo Observatory
Giuseppe Piazzi (1746-1826) pointing at Ceres. © Palermo Observatory

Later, the group led by von Zach discovered many asteroids. One of them, Heinrich Olbers, is credited for the discoveries of Pallas, Vesta, and the periodic comet 13P/Olbers. He also gave his name to the Olbers paradox, which wonders why the night is so dark while we are surrounded by so many stars.

Properties

You can find below some of the orbital and physical properties of Ceres.

Semimajor axis 2.77 AU
Eccentricity 0.075
Inclination 10.6°
Revolution 4.60 yr
Rotation 9 h 4 min
Diameters (965.2 × 961.2 × 891.2) km
Density 2.161 g/cm3

These orbital elements and its size make it the largest object of the main asteroid belt. You can see a small eccentricity, and a pretty fast rotation period with respect to its orbital one (i.e., the revolution). Moreover, its equatorial section is pretty circular, i.e. if you look at its 3 diameters, the two largest ones of them are very close, and in fact the uncertainties on the measurements are even consistent with a strict equality. However, the polar diameter is much smaller. This is a consequence of its rotation, which flattens the body.

You can also notice a density, which is between the one of the water (1) and the one of silicates (3.3). This means that its composition should be a mixture of both, i.e. silicates and water ice.

Ceres seen by Dawn © NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Ceres seen by Dawn © NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The physical properties and the image above are due to the spacecraft Dawn. This mission is close to its end.

Dawn at Ceres

The spacecraft Dawn has been launched from Cape Canaveral, Florida, in September 2007, and reached the asteroid Vesta in 2011. After a little more than one year in orbit around Vesta, it left it and has been trapped by the gravity field of Ceres in March 2015. This mission will be completed soon.

Dawn consists of three instruments:

  • the Gamma Ray and Neutron Detector (GRaND) Instrument,
  • the Visible and Infrared Spectrometer (VIR) Instrument,
  • and the Framing Camera (FC).

Dawn is essentially an American mission, even if Germany provided the Framing Camera. The German study we discuss today uses FC data.

The orbital journey of Dawn around Ceres consists of several phases, which are different orbits. This results in variable resolutions of the images. The prime mission considered two mapping orbits, the HAMO (High Altitude Mapping Orbit) and the LAMO (for Low Altitude), at distances of 1,470 and 375 km of the surface, respectively. Since then, the mission has been extended, and the spacecraft is now at only 50 kilometers of the surface. High resolution expected.

This mapping orbits permitted to map comprehensively the surface of Ceres. Unsurprisingly, that survey revealed many craters.
We are today interested in Occator, which is not the largest one, but contains bright spots, possibly signatures of a recent hydrothermal activity.

Occator crater

Occator crater is located in the northern hemisphere of Ceres. Its diameter is some 90 km, which does not make it the largest one, but it is particularly interesting for the bright spots it shows. To be honest, there are bright spots at other locations of Ceres, but anyway Occator is remarkable for that. The spot in the center is a dome called Cerealia Facula, while the small spots are called the Vinalia Faculae. You can see them below, on these high-resolution images due to the extended mission.

Occator Crater on Ceres, with its central bright area called Cerealia Facula. © NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI
Occator Crater on Ceres, with its central bright area called Cerealia Facula. © NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI
Vinalia Faculae © NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI
Vinalia Faculae © NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Yes, there are domes, due to hydrothermal activity! You can find interesting analogies with Earth features here. But basically, the two possible explanations are for now:

  1. either the heat from the impact that formed the crater caused briny liquid or mushy ice to push up on the surface,
  2. or the heat from the impact could have enhanced activity related to pre-existing liquid reservoirs just below the surface.

Anyway, this reveals water! And this makes Ceres and its crater Occator fascinating.

How a crater evolves

This study wants to estimate the age of Occator, or in other words, date the impact that created it. For that, you examine its current state, and guess how long it took from the impact to the observed state.

Because of the elasticity of the surface, after some time (hundreds of millions years, maybe a little more…) the surface relaxes. The consequence is that the crater gets less deep, and its slopes get gentler. A 3-D terrain model will give you the numbers. But the dynamics of the relaxation process is barely constrained.

Another evolution is that the crater is covered by something else. This something could be other, more recent craters. If the new crater is larger than the older one, then the oldest disappears. However, if the new crater is small with respect to the old one, then you see both, and by counting the small craters, you can say “it took this time to get so many craters, so the age is at least…”. OK

But how to constrain this? You calibrate your models with better known bodies, like the Moon, and / or dynamical models of the bombardments. Previous studies have used Lunar Derived Models and Asteroid-flux Models… of course with different outcomes.

In the specific case of Occator, the hydrothermal activity revealed by the bright spots has generated ejecta blankets, as lobate deposits.

Previous estimations of Occator’s age

The quest for the age of Occator crater began with the first data on Ceres, i.e. in 2015. Here are the already published numbers

  • Nathues et al. 2015: 78 ± 5 Ma (million years). This measurement is based on crater counting, and only HAMO data. In particular, the more accurate low-altitude data were missing at that time,
  • Nathues et al. 2016: 6.9 Ma, based on the interior lobate deposits,
  • Jaumann et al. 2016: between 100 and 200 Ma, depending on how you calibrate the dating from craters,
  • Nathues et al. 2017: 34 ± 2 Ma, from the creation of the central dome, i.e. Cerealia Facula,
  • Nathues et al. 2018, stated that the dispersed bright deposits Vinalia Faculae were younger than 2 Ma, in using low-altitude high-resolution images.

The study we now discuss uses almost all of the data, and so should be more accurate.

A young crater anyway

It is interesting that a study points out all of the possible numbers, given the models, the data, and the physical process considered (crater counting, age of ejecta,…). In particular, if the hydrothermal activity has been triggered by the impact which created Occator, then dating the ejecta should tell us something accurate.

The authors find an age of 21.9 ± 0.7 Ma for the crater in using the Lunar Derived Model, and between 1 and 64 Ma in using the Asteroid-flux Derived Model. You see, lots of uncertainties… as they say, the model ages are a matter of perspective. But anyway, this is a very young and interesting crater!

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.

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