So what has Curiosity been doing on Mars for the past Earth month?
We’ll get to that in a minute, but first we need to understand Mars time. Curiosity’s activities are normally reported in Mars time, not Earth’s.
Time kept by the sun at a given location, whether on Earth or Mars, is called local mean solar time (LMST). In 1976 when we landed the first spacecraft on Mars, we decided to use the same time system on Mars as on Earth — a 24-hour day with 60-minute hours and 60-second minutes. One solar day on Earth is defined as the average time it takes for the Earth to rotate to where the Sun returns to its starting position in the sky.
But because the rotational period of Mars is longer than Earth’s, the Martian solar day is longer. A Martian solar day has an average length of 24 hours 39 minutes 35.244 seconds Earth time. A Martian day is called a “sol.”
There is no standard calendar for Mars, so each mission determines a system to assign a sol number (like an Earth date).
For Curiosity, sol 0 began on the local midnight immediately before landing; in other words, the landing occurred on sol 0. The sol number rises by one at each following midnight. Therefore, event times are expressed by sol number and time in LMST. (Check out a Mars clock program at www.giss.nasa.gov/tools/mars24 or the iPhone app Mars Clock.)
We have now reached sol 100, which began at 11:48 pm PST on Nov. 15 and ends at 12:28 am on Nov. 17. What has Curiosity been doing since sol 61 (Oct. 6-7)?
One of the most important science goals for the mission is to collect and analyze soil samples to determine whether environmental conditions were favorable for microbial life in the past. The composition and structure of rocks and soil tell us the environmental conditions when they formed.
The first step in this process is to use the clamshell-shaped scoop, part of the collection-and-handling Martian rock analysis device on the turret of tools at the end of the rover’s arm, to collect soil samples. These samples can then be delivered for analysis to either the chemistry and mineralogy (CheMin) instrument or the sample analysis at Mars (SAM) instrument.
To test and prepare the sampling system, a sample of soil was collected on sol 61 (Oct. 7). This sample was held and vibrated inside each chamber of the mechanism to remove any material that might remain from Earth (a rover mini-sandblasting). It is important that all material analyzed be from Mars, not Earth. This first scoop was then discarded and a second scoop was collected on sol 66. There were fears that pieces of the spacecraft were contaminating that sample, so it was also discarded, and a third sample was collected to continue the cleaning process.
A fourth sample was collected on sol 74 (Oct. 20), part of which was used to do more cleaning. On sol 77, a sieved portion of this sample was finally delivered to the CheMin instrument for analysis.
This is the first time an instrument like CheMin has been used on Mars. It reveals not only the composition of the soil, but also its mineral structure. This is very important because different structures of the same composition give us information about the environments in which they formed. For example, graphite and diamond have the same composition but totally different mineral structure because of the different environments in which they form.
The CheMin analysis indicates that the Martian soil is similar to weathered basaltic soils of volcanic origin in Hawaii. This soil is different from the several-billion-year-old conglomerate rocks studied a few weeks ago, which indicated flowing water. This soil is more representative of modern processes on Mars, with limited interaction with water. These findings are consistent with the theory that Mars was wet billions of years ago but is now very dry.
This is a great start! Next update: sol 128.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.